ORCID Profile
0000-0002-4479-3255
Current Organisation
University of Melbourne
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Publisher: Wiley
Date: 10-2010
DOI: 10.1002/ASL.283
Publisher: American Geophysical Union (AGU)
Date: 2008
DOI: 10.1029/2007GL031835
Publisher: American Meteorological Society
Date: 06-2016
Abstract: In Part I of this study, the Ural blocking (UB)-induced lification role of winter warm Arctic–cold Eurasian (WACE) anomalies has been examined. It was found that the long-lived UB together with the positive North Atlantic Oscillation (NAO + ) significantly contributes to the lification of the WACE pattern. The present study examines how the UB variability affects quasi-biweekly WACE (QB-WACE) anomalies and depends on the NAO + and North Atlantic conditions by classifying the UB based on a case study of a cold event that occurred over southern China in January 2008. A composite analysis during 1979–2013 shows that the QB-WACE anomalies associated with the UB that often occur with the NAO + are strong and influenced by the North Atlantic jet (NAJ) and zonal wind strengths over Eurasia. For NAO + -related UB, the QB-WACE anomaly depends strongly on the location of UB, and the UB anomalies lag the NAO + by approximately 4–7 days. The strength of the NAJ determines whether the combined NAO + and UB anomalies exhibit a negative East Atlantic/West Russia (EA/WR − ) pattern, while the region of weak zonal winds over Eurasia and the zonal extent of the NAJ dominate the location of UB. For southward-, eastward-, and westward-displaced UBs associated with a strong NAJ, the NAO + favors the UB with a southward-displaced QB-WACE anomaly through wave train propagation like an EA/WR − pattern. Eastward- and southward-displaced UB anomalies induce similarly displaced cold anomalies with intrusion into southern China. However, for a northward-displaced UB, this happens without pronounced EA/WR − patterns because of a weak NAJ and is accompanied by a northward-displaced QB-WACE anomaly.
Publisher: Wiley
Date: 2006
DOI: 10.1002/JOC.1370
Publisher: Wiley
Date: 07-1992
Publisher: Wiley
Date: 15-07-2002
Publisher: Springer Science and Business Media LLC
Date: 04-01-2011
Publisher: Elsevier BV
Date: 03-2008
Publisher: American Meteorological Society
Date: 24-04-2017
Abstract: In Part I of this study, it was shown that the Eurasian cold anomalies related to Arctic warming depend strongly on the quasi stationarity and persistence of the Ural blocking (UB). The analysis here revealed that under weak mean westerly wind (MWW) and vertical shear (VS) (quasi barotropic) conditions with weak synoptic-scale eddies and a large planetary wave anomaly, the growth of UB is slow and its litude is small. For this case, a quasi-stationary and persistent UB is seen. However, under strong MWW and VS (quasi baroclinic) conditions, synoptic-scale eddies are stronger and the growth of UB is rapid the resulting UB is less persistent and has large litude. In this case, a marked retrogression of the UB is observed. The dynamical mechanism behind the dependence of the movement and persistence of UB upon the background conditions is further examined using a nonlinear multiscale model. The results show that when the blocking has large litude under quasi-baroclinic conditions, the blocking-induced westward displacement greatly exceeds the strong mean zonal-wind-induced eastward movement and hence generates a marked retrogression of the blocking. By contrast, under quasi-barotropic conditions because the UB litude is relatively small the blocking-induced westward movement is less distinct, giving rise to a quasi-stationary and persistent blocking. It is further shown that the strong mid–high-latitude North Atlantic mean zonal wind is the quasi-barotropic condition that suppresses UB’s retrogression and thus is conducive to the quasi stationarity and persistence of the UB. The model results show that the blocking duration is longer when the mean zonal wind in the blocking region or eddy strength is weaker.
Publisher: MDPI AG
Date: 11-11-2019
DOI: 10.3390/CLI7110130
Abstract: The objective of this work is the development of an automated and objective identification scheme of cold fronts in order to produce a comprehensive climatology of Mediterranean cold fronts. The scheme is a modified version of The University of Melbourne Frontal Tracking Scheme (FTS), to take into account the particular characteristics of the Mediterranean fronts. We refer to this new scheme as MedFTS. Sensitivity tests were performed with a number of cold fronts in the Mediterranean using different threshold values of wind-related criteria in order to identify the optimum scheme configuration. This configuration was then applied to a 10-year period, and its skill was assessed against synoptic surface charts using statistic metrics. It was found that the scheme performs well with the dynamic criteria employed and can be successfully applied to cold front identification in the Mediterranean.
Publisher: American Geophysical Union (AGU)
Date: 08-2010
DOI: 10.1029/2010GL044136
Publisher: IOP Publishing
Date: 14-10-2020
Abstract: Two intense heatwaves of July and early August 2018 are found to be associated with a European blocking (EB) event accompanied by a series of consecutive positive North Atlantic Oscillation (NAO + ) events. Further analyses show that the collaborative role of an EB event and its upstream NAO + pattern could increase the frequency, persistence, magnitude and scale of heatwaves over Europe. Compared with NAO + -unrelated EB events, NAO + -related EB events are less movable (quasi-stationary) and more persistent over Europe, which could contribute to an increase in the intensity and persistence of heatwaves. In addition, the blocking high of this type has a northeast–southwest orientation with stronger warm airflow and less precipitation in northern and western Europe, where large scopes of higher temperatures tend to occur. In contrast, NAO + -unrelated EB events without orientation correspond to a trough in the south, which results in increased precipitation and cold air in the southern part of Europe, and thus high temperatures contract to the northern part of Europe. Moreover, considering that the NAO + pattern leads the formation of an EB event, the NAO + pattern might serve as a potential predictor for European heatwaves. Our conclusions are strongly supported by the analysis of CMIP6 historical simulations which also capture the differences of high temperatures and atmospheric circulations between NAO + -related EB events and NAO + -unrelated EB events.
Publisher: American Meteorological Society
Date: 14-03-2012
DOI: 10.1175/JCLI-D-11-00100.1
Abstract: Presented here is an objective approach to identify, characterize, and track Southern Hemisphere mobile fronts in hemispheric analyses of relatively modest resolution, such as reanalyses. Among the principles in its design were that it should be based on broadscale synoptic considerations and be as simple and easily understood as possible. The resulting Eulerian scheme has been applied to the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA)–Interim and a climatology of frontal characteristics, at both the 10-m and 850-hPa levels, derived for the period 1 January 1989–28 February 2009. The knowledge of the character of these features is central to understanding weather and climate over the hemisphere. In both summer and winter the latitude belt 40°–60°S hosts the highest frequency of frontal points, but there are significant zonal asymmetries within this band. The climatology reveals that the longest fronts are in the Indian Ocean where mean lengths exceed 2000 km. The mean frontal intensity over the hemisphere tends to be greater at 850 hPa than at 10 m, and greater in winter than in summer. The frontal intensity also shows its maximum in the Indian Ocean. In the mean, the meridional tilt of these fronts is northwest–southeast over much of the midlatitudes and subtropics, and increases with latitude toward the equator. The tilts are of overwhelmingly opposite sign in the coastal Antarctic and subantarctic regions. Broadly speaking, the number of fronts and their mean length and mean intensity exhibit maxima in winter in the midlatitudes (30°–50°S), but show a sizeable semiannual variation (maxima in fall and spring) during the year at higher latitudes.
Publisher: Meteorological Society of Japan
Date: 1996
Publisher: Springer Science and Business Media LLC
Date: 22-06-2014
DOI: 10.1038/NCLIMATE2271
Publisher: Cambridge University Press (CUP)
Date: 30-11-2004
DOI: 10.1017/S0954102004002226
Abstract: The hemispheric and regional atmospheric circulation influences the Southern Ocean in many and profound ways, including intense air-sea fluxes of momentum, energy, fresh water and dissolved gases. The Southern Ocean ventilates a large fraction of the world ocean and hence these influences are spread globally. We use the NCEP-2 reanalysis data set to diagnose aspects of the large-scale atmospheric structure and variability and explore how these impact on the Southern Ocean. We discuss how the ‘Southern Annular Mode’ and the ‘Pacific-South American’ pattern influence the Southern Ocean, particularly in the eastern Pacific. We review the importance of atmospheric eddies in Southern Ocean climate, and the role they play in the transport of mechanical energy into the ocean. The fluxes of fresh water across the air-sea boundary influence strongly the processes of water mass formation. It is shown that climatological precipitation exceeds evaporation over most of the Southern Ocean. When averaged over the ocean from 50°S to the Antarctic coast the annual mean excess is 0.80 mm day −1 . The magnitude of the flux displays only a small measure of seasonality, and its largest value of 0.92 mm day −1 occurs in summer.
Publisher: American Geophysical Union (AGU)
Date: 07-2009
DOI: 10.1029/2009GL038824
Publisher: Wiley
Date: 1992
Publisher: Cambridge University Press (CUP)
Date: 12-1990
DOI: 10.1017/S0954102090000414
Abstract: Increasingly, many aspects of the study of Antarctica and the high southern latitudes are being aided by various types of numerical models. Among these are the General Circulation Models (GCMs), which are powerful tools that can be used to understand the maintenance of present atmospheric climate and determine its sensitivity to imposed changes. The changes in the ability of GCMs used over the last two decades to simulate aspects of atmospheric climate at high southern latitudes are traced and it is concluded there has been a steady improvement in model products. The task of assessing model climates in high southern latitudes is made difficult by the uncertainties in the data used for the climatological statistics. It is suggested that the quality of the climates produced by most modern GCMs in many aspects cannot be said to be poor, especially considering the uncertainties in ‘observed’ climate. There is obviously need for improvements in both modelling and observations. Finally, some topics are highlighted in which the formulation of models could be improved, with special reference to better treatment of physical processes at high southern latitudes.
Publisher: American Geophysical Union (AGU)
Date: 15-11-1995
DOI: 10.1029/95JC02581
Publisher: Stockholm University Press
Date: 1996
Publisher: American Meteorological Society
Date: 10-2010
Abstract: In this study, an updated and extended climatology of cyclonic tracks affecting the eastern Mediterranean region is presented, in order to better understand the Mediterranean climate and its changes. This climatology includes intermonthly variations, classification of tracks according to their origin domain, dynamic and kinematic characteristics, and trend analysis. The dataset used is the 1962–2001, 2.5° × 2.5°, 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The identification and tracking of the cyclones was performed with the aid of the Melbourne University algorithm. It was verified that considerable intermonthly variations of track density occur in the eastern Mediterranean, consistent with previous studies for the entire Mediterranean, while further interesting new features have been revealed. The classification of the tracks according to their origin domain reveals that the vast majority originate within the examined area itself, mainly in the Cyprus area and the southeastern Aegean Sea, while the tracks that originate elsewhere most frequently enter from the west. Deeper cyclones follow the southwest track originating from the area between Algeria and the Atlas Mountains. A greater size characterizes the westerly tracks (southwest, northwest, and west), while the northwest tracks propagate faster over the study area. A negative trend of the track frequency was found on an annual basis that can be mostly attributed to the winter months, being associated with variations in the baroclinicity. This negative trend is more prominent for the westerly and northeasterly tracks, as well as for those originating in the northern part of the examined area.
Publisher: Springer Science and Business Media LLC
Date: 1997
DOI: 10.1007/BF00865020
Publisher: American Geophysical Union (AGU)
Date: 14-04-2011
DOI: 10.1029/2010JD015358
Publisher: Wiley
Date: 21-11-2015
DOI: 10.1002/QJ.2471
Publisher: Springer Science and Business Media LLC
Date: 09-1994
DOI: 10.1007/BF00228031
Publisher: American Meteorological Society
Date: 08-2017
Abstract: The Southern Ocean (SO) is the region of the World Ocean bordering on Antarctica over which significant exchanges between the atmosphere, the ocean, and the sea ice take place. Here, the strong and nearly unhindered eastward flow of the Antarctic Circumpolar Current plays an important role in mean global climate as it transmits climate anomalies around the hemisphere. Features of interannual variability have been observed to propagate eastward around the SO with the circumpolar flow in the form of a system of coupled anomalies, known as the Antarctic circumpolar wave (ACW). In the present study, the 142-yr series of the Twentieth Century Reanalysis, version 2, dataset (850-hPa geopotential height, sea level pressure, sea surface temperature, surface meridional wind, and surface air temperature) spanning from 1871 to 2012 is used to investigate the presence and variability of ACWs. This examination shows, for the first time, the presence of the ACW before the mid-1950s and interdecadal changes in its characteristics. Modifications in the strength and speed of the circumpolar wave are shown to be linked with large-scale climate changes. Complex empirical orthogonal function analyses confirm that the ACW becomes apparent when the tropical El Niño–Southern Oscillation (ENSO) signal gives rise to the Pacific–South American (PSA) pattern and is a consequence of the constructive combination of the PSA and the subantarctic zonal wavenumber 3. Correlation analyses are also performed to quantify the role played by ENSO teleconnections for the appearance of the ACW, and the impact on the presence of ACWs of three super–El Niño events is investigated.
Publisher: Elsevier BV
Date: 02-2010
Publisher: American Meteorological Society
Date: 15-12-2009
Abstract: Cold events (CEs) are an important feature of southern Australian weather. Unseasonably cold conditions can have a significant impact on Australia’s agricultural industry and other aspects of society. In this study the bottom 0.4% of maximum temperatures in Melbourne and Perth from the 1958–2006 period are defined as CEs, representing the large-scale patterns affecting most of extratropical Australia. Compiling 6-hourly progressions of the tracks of the cyclones and anticyclones that are geostrophically associated with CEs gives for the first time a detailed synoptic climatology over the area. The anticyclone tracks display a “cloud” of high density across the Indian Ocean, which is linked, in the mean, to weak but significant negative SST anomalies in the region. The cyclone tracks display much variability, with system origins ranging from subpolar to tropical. Several CEs are found to involve tropical and extratropical interaction or extratropical transition of originally tropical cyclones (hurricanes). CE-associated systems travel farther and exhibit longer life spans than similar, non-CE systems. Upper-level analyses indicate the presence of a wave train originating more than 120° west of the CE. This pattern greatly intensifies over the affected area in conjunction with a merging of the subpolar and subtropical jets. The upper-level wave train is present up to five days before the CE. The absence of large orographic features in Australia highlights the importance of wave lification in CE occurrence. No consistent trend in CE intensity over the period is found, but a significant negative trend in event frequency is identified for both Melbourne and Perth.
Publisher: Wiley
Date: 27-07-2021
DOI: 10.1111/NYAS.14673
Abstract: We analyze the polar sea ice distribution and the global sea level pressure (SLP) and baroclinicity distributions over the "satellite" period of 1979-2020. In the Arctic, there are statistically significant sea ice extent (SIE) decreases in all calendar months, and the annual mean has lost 2.22 million km
Publisher: Springer Science and Business Media LLC
Date: 30-04-2013
Publisher: Springer Science and Business Media LLC
Date: 10-06-2011
Publisher: IOP Publishing
Date: 05-2017
Publisher: Springer Science and Business Media LLC
Date: 10-2005
Publisher: American Geophysical Union (AGU)
Date: 06-03-2018
DOI: 10.1002/2017JD027749
Publisher: Springer Science and Business Media LLC
Date: 15-03-2012
Publisher: American Geophysical Union (AGU)
Date: 04-01-2003
DOI: 10.1029/2000JC000542
Publisher: American Meteorological Society
Date: 16-08-2016
Abstract: The Pacific–South American (PSA) pattern is an important mode of climate variability in the mid-to-high southern latitudes. It is widely recognized as the primary mechanism by which El Niño–Southern Oscillation (ENSO) influences the southeast Pacific and southwest Atlantic and in recent years has also been suggested as a mechanism by which longer-term tropical sea surface temperature trends can influence the Antarctic climate. This study presents a novel methodology for objectively identifying the PSA pattern. By rotating the global coordinate system such that the equator (a great circle) traces the approximate path of the pattern, the identification algorithm utilizes Fourier analysis as opposed to a traditional empirical orthogonal function approach. The climatology arising from the application of this method to ERA-Interim reanalysis data reveals that the PSA pattern has a strong influence on temperature and precipitation variability over West Antarctica and the Antarctic Peninsula and on sea ice variability in the adjacent Amundsen, Bellingshausen, and Weddell Seas. Identified seasonal trends toward the negative phase of the PSA pattern are consistent with warming observed over the Antarctic Peninsula during autumn, but are inconsistent with observed winter warming over West Antarctica. Only a weak relationship is identified between the PSA pattern and ENSO, which suggests that the pattern might be better conceptualized as a preferred regional atmospheric response to various external (and internal) forcings.
Publisher: Springer Science and Business Media LLC
Date: 06-2002
Publisher: Elsevier BV
Date: 05-2006
DOI: 10.1016/J.AAP.2005.06.025
Abstract: We investigate the impact of rainfall on daily road accidents in the metropolitan area of Melbourne, Australia, over 1987-2002. Our analysis from several viewpoints of the accident count, which has been normalised for variation in traffic volume, indicated that the effect of rainfall is multifaceted. Owing to a large non-linear trend a sub ision into three epochs (1987-1991, 1992-1996 and 1997-2002) was made. Nominal daytime and nighttime as well as 3h raw counts were available for the first two epochs only. Generally, the effect of rainfall across the epochs shows a tendency for larger values in autumn with smaller values in spring. For the daily, daytime and nighttime cases there is an approximate 40% decrease in both the volume-normalised dry and wet means from the first to second epoch. Since the second epoch is wetter than the first, and both dry and wet cases are affected in a similar way, then it appears that a non-weather influence is at work. It is suggested that law enforcement measures may be largely responsible. We obtained a conservative estimate of relative risk of an accident in wet conditions based on a matched-pair analysis of 3h dry and wet periods over the first two epochs (1987-1996). As with other studies we find that the risk is greater than unity in almost all cases suggesting that the presence of rainfall consistently represents a driving hazard. Rainfall occurring after a dry spell has an enhanced effect on the volume-normalised accident count as the spell duration increases. The effect of dry spells is more clearly described when broken down by rain class. Generally, there is an increase in the impact of a dry spell when it first rains as the spell duration and rainfall amount increase.
Publisher: American Meteorological Society
Date: 06-2016
Abstract: In Part I of this study, the impact of Ural blocking (UB) on the warm Arctic–cold Eurasian (WACE) pattern associated with the winter (DJF) arctic sea ice loss during 1979–2013 is examined by iding the arctic sea ice reduction region into two dominant subregions: the Barents and Kara Seas (BKS) and the North American high-latitude (NAH) region (Baffin and Hudson Bay, Davis Strait, and Labrador Sea). It is found that atmospheric response to arctic sea ice loss resembles a negative Arctic response oscillation with a dominant positive height anomaly over the Eurasian subarctic region. Regression analyses of the two subregions further show that the sea ice loss over the BKS corresponds to the UB pattern together with a positive North Atlantic Oscillation (NAO + ) and is followed by a WACE anomaly, while the sea ice reduction in the NAH region corresponds to a negative NAO (NAO − ) pattern with a cold anomaly over northern Eurasia. Further analyses reveal that the UB pattern is more persistent during the period 2000–13 (P2) than 1979–99 (P1) because of the reduced middle-to-high-latitude mean westerly winds over Eurasia associated with the intense BKS warming. During P2 the establishment of the UB becomes a slow process because of the role of the BKS warming, while its decay is slightly rapid. In the presence of the long-lived UB that often occurs with the NAO + , the BKS-warming-induced DJF-mean anticyclonic anomaly is intensified and widened and then expands southward during P2 to lify the WACE pattern and induce the southward displacement of its cold anomaly and the further loss of the BKS sea ice. Thus, midlatitude Eurasian cold events should be more frequent as the sea ice loss continues over the BKS.
Publisher: American Meteorological Society
Date: 2014
Abstract: The identification of extratropical fronts in reanalyses and climate models is an important climate diagnostic that aids dynamical understanding and model verification. This study compares six frontal identification methods that are applied to June and July reanalysis data over the Central Wheatbelt of southwest Western Australia for 1979–2006. Much of the winter rainfall over this region originates from frontal systems. Five of the methods use automated algorithms. These make use of different approaches, based on shifts in 850-hPa winds (WND), gradients of temperature (TGR) and wet-bulb potential temperature (WPT), pattern matching (PMM), and a self-organizing map (SOM). The sixth method was a manual synoptic technique (MAN). On average, about 50% of rain days were associated with fronts in most schemes (although methods PMM and SOM exhibited a lower percentage). On a daily basis, most methods identify the same systems more than 50% of the time, and over the 28-yr period the seasonal time series correlate strongly. The association with rainfall is less clear. The WND time series of seasonal frontal counts correlate significantly with Central Wheatbelt rainfall. All automated methods identify fronts on some days that are classified as cutoff lows in the manual analysis, which will impact rainfall correlations. The front numbers identified on all days by the automated methods decline from 1979 to 2006 (but only the TGR and WPT trends were significant at the 10% level). The results here highlight that automated techniques have value in understanding frontal behavior and can be used to identify the changes in the frequency of frontal systems through time.
Publisher: American Geophysical Union (AGU)
Date: 08-2005
DOI: 10.1029/2005GL023390
Publisher: American Geophysical Union (AGU)
Date: 08-2006
DOI: 10.1029/2006GL026005
Publisher: Springer Science and Business Media LLC
Date: 09-1986
DOI: 10.1007/BF02263136
Publisher: American Meteorological Society
Date: 08-2013
DOI: 10.1175/2013BAMSSTATEOFTHECLIMATE.1
Abstract: Editors note: For easy download the posted pdf of the State of the Climate for 2012 is a very low-resolution file. A high-resolution copy of the report is available by clicking here. Please be patient as it may take a few minutes for the high-resolution file to download.
Publisher: American Geophysical Union (AGU)
Date: 09-12-2009
DOI: 10.1029/2009JD012132
Publisher: IOP Publishing
Date: 07-2016
Publisher: American Geophysical Union (AGU)
Date: 25-08-2012
DOI: 10.1029/2012JD017885
Publisher: Springer Science and Business Media LLC
Date: 23-11-2014
Publisher: Elsevier BV
Date: 10-2013
Publisher: Informa UK Limited
Date: 06-1976
Publisher: American Meteorological Society
Date: 15-05-2011
Abstract: Atmospheric reanalyses can be useful tools for examining climate variability and change however, they must be used cautiously because of time-varying biases that can induce artificial trends. This study explicitly documents a discontinuity in the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) that leads to significantly exaggerated warming in the Arctic mid- to lower troposphere, and demonstrates that the continuing use of ERA-40 to study Arctic temperature trends is problematic. The discontinuity occurs in 1997 in response to refined processing of satellite radiances prior to their assimilation into the reanalysis model. It is clearly apparent in comparisons of ERA-40 output against satellite-derived air temperatures, in situ observations, and alternative reanalyses. Decadal or multidecadal Arctic temperature trends calculated over periods that include 1997 are highly inaccurate, particularly below 600 hPa. It is shown that ERA-40 is poorly suited to studying Arctic temperature trends and their vertical profile, and conclusions based upon them must be viewed with extreme caution. Consequently, its future use for this purpose is discouraged. In the context of the wider scientific debate on the suitability of reanalyses for trend analyses, the results show that a series of alternative reanalyses are in broad-scale agreement with observations. Thus, the authors encourage their discerning use instead of ERA-40 for examining Arctic climate change while also reaffirming the importance of verifying reanalyses with observations whenever possible.
Publisher: Wiley
Date: 04-1978
Publisher: Informa UK Limited
Date: 06-1976
Publisher: IOP Publishing
Date: 10-2014
Publisher: Springer Science and Business Media LLC
Date: 12-0011
DOI: 10.1007/BF00209750
Publisher: American Meteorological Society
Date: 2009
Abstract: Satellite altimetry provides an immensely valuable source of operational significant wave height (Hs) data. Currently, altimeters on board Jason-1 and Envisat provide global Hs observations, available within 3–5 h of real time. In this work, Hs data from these altimeters are validated against in situ buoy data from the National Data Buoy Center (NDBC) and Marine Environmental Data Service (MEDS) buoy networks. Data cover a period of three years for Envisat and more than four years for Jason-1. Collocation criteria of 50 km and 30 min yield 3452 and 2157 collocations for Jason-1 and Envisat, respectively. Jason-1 is found to be in no need of correction, performing well throughout the range of wave heights, although it is notably noisier than Envisat. An overall RMS difference between Jason-1 and buoy data of 0.227 m is found. Envisat has a tendency to overestimate low Hs and underestimate high Hs. A linear correction reduces the RMS difference by 7%, from 0.219 to 0.203 m. In addition to wave height–dependent biases in the altimeter Hs estimate, a wave state–dependent bias is also identified, with steep (smooth) waves producing a negative (positive) bias relative to buoys. A systematic difference in the Hs being reported by MEDS and NDBC buoy networks is also noted. Using the altimeter data as a common reference, it is estimated that MEDS buoys are underestimating Hs relative to NDBC buoys by about 10%.
Publisher: American Geophysical Union (AGU)
Date: 11-05-2004
DOI: 10.1029/2004GL019870
Publisher: IOP Publishing
Date: 26-06-2018
Publisher: Research Square Platform LLC
Date: 27-06-2022
DOI: 10.21203/RS.3.RS-1725836/V1
Abstract: The decline of Arctic sea ice concentration (SIC) plays an important role in Arctic lification which influences Arctic ecosystem, midlatitude weather and climate. The Arctic SIC shows prominent interannual and interdecadal variability. Although the interdecadal variability of the Arctic SIC fluctuations is mainly linked to Atlantic Multidecadal Oscillation, it is unclear what is the origin of the interannual variability of Arctic SIC. Here, we indicate that the interannual variability of Arctic SIC over Barents-Kara Seas (BKS) mainly originates from the remote modulation of El Niño-Southern Oscillation (ENSO) via the Atlantic pathway. It is revealed that La Niña (El Niño) can strengthen (weaken) the positive winter North Atlantic Oscillation (NAO+)-like circulation via enhanced (reduced) Atlantic Hadley cell to result in a notable Atlantic interannual variability (AIV). During the positive phase of AIV the enhanced transport of warm Atlantic water and warm moisture toward the BKS due to intensified winter NAO+-like circulation promotes the SIC decline over BKS through the presence of Ural blocking with NAO+. The reversed is seen during the negative phase of AIV. These new findings can help understand what is the origin of the interannual variability of winter sea ice over the BKS.
Publisher: Springer Science and Business Media LLC
Date: 10-08-2012
Publisher: Elsevier BV
Date: 06-1997
Publisher: IOP Publishing
Date: 08-2022
Abstract: This study explores the consequences of independent and combined effects of blockings on the northeast Asian cold vortex (NACV), and corresponding cold outbreaks over East Asia (EA) during boreal winters of 1979–2019. The results show that the development of NACV is closely associated with blocking over the Ural Mountains (UB) upstream and eastern Siberia to mid-North Pacific (SPB) downstream. Here we focus on the initial periods before the peak day of NACV events. It is found that the strong NACV events are usually induced by the initial-UB situation, leading to the greatest temperature drop in EA. While the weak NACV events may be associated with the initial-SPB condition, which can bring less dramatic outbreaks but longer duration, owing to the Ural ridge that formed by the westward shift of SPB. Furthermore, an SPB-UB relay effect is discovered against the background of a negative Arctic Oscillation pattern. In such cases, UB is formed by the westward shift of downstream SPB after the occurrence of NACV, forming a relay effect that motivates the second NACV process, hence prolonging the duration of cold anomalies in EA. These findings highlight the importance of the combined effect of blockings and NACV in the intraseasonal time scale. Compared to the ‘Initial-UB’ and ‘Initial-SPB’ situation, this ‘SPB-UB relay’ scenario can produce longer-lasting cold extreme in EA, which may be indicative of the short-term weather forecasting of such extreme cold weather.
Publisher: Proceedings of the National Academy of Sciences
Date: 08-05-2013
Publisher: Wiley
Date: 10-1987
Publisher: Springer Science and Business Media LLC
Date: 04-10-2000
Publisher: Springer Science and Business Media LLC
Date: 14-07-2016
DOI: 10.1038/SREP29599
Abstract: This paper highlights some caveats in using composite analyses to form physical hypotheses on the associations between environmental variables. This is illustrated using a specific ex le, namely the apparent links between heat waves (HWs) and sea surface temperatures (SSTs). In this case study, a composite analysis is performed to show the large-scale and regional SST conditions observed during summer HWs in Perth, southwest Australia. Composite results initially point to the importance of the subtropical South Indian Ocean, where physically coherent SST dipole anomalies appear to form a necessary condition for HWs to develop across southwest Australia. However, sensitivity tests based on pattern correlation analyses indicate that the vast majority of days when the identified SST pattern appears are overwhelmingly not associated with observed HWs, which suggests that this is definitely not a sufficient condition for HW development. Very similar findings are obtained from the analyses of 15 coupled climate model simulations. The results presented here have pertinent implications and applications for other climate case studies, and highlight the importance of applying comprehensive statistical approaches before making physical inferences on apparent climate associations.
Publisher: American Geophysical Union (AGU)
Date: 14-11-2015
DOI: 10.1002/2015GL066383
Publisher: American Geophysical Union (AGU)
Date: 28-05-2012
DOI: 10.1029/2012GL051598
Publisher: Springer Science and Business Media LLC
Date: 15-06-2013
Publisher: Wiley
Date: 12-2008
Abstract: Transition mechanisms characterizing changes from hurricanes to midlatitude cyclones and vice-versa (extratropical and tropical transition) have become a topic of increasing interest, partially because of their association with recent unusual storms that have developed in different ocean basins of both hemispheres. The aim of this work is to discuss some recent cases of transition and highly unusual hurricane developments and to address some of their wider implications for climate science. Frequently those dramatic cyclones are responsible for severe weather, potentially causing significant damage to property and infrastructure. An additional manifestation discussed here is their association with cold surges, a topic that has been very little explored in the literature. In the Southern Hemisphere, the first South Atlantic hurricane, Catarina, developed in March 2004 under very unusual large-scale conditions. That exceptional cyclone is viewed as a case of tropical transition facilitated by a well-developed blocking structure. A new index for monitoring tropical transition in the subtropical South Atlantic is discussed. This "South Atlantic index" is used to show that the unusual flow during and prior to Catarina's genesis can be attributed to tropical/extratropical interaction mechanisms. The "Donald Duck" case in Australia and Vince in the North Atlantic have also been examined and shown to belong to a category of hybrid-transitioning systems that will achieve at least partial tropical transition. While clearly more research is needed on the topic of transition, as we gain further insight, it is becoming increasingly apparent that features of large-scale circulation do play a fundamental role. A complex interaction between an extratropical transition case and an extreme summer cold surge affecting southeastern Australia is discussed as an ex le of wider climate implications.
Publisher: International Glaciological Society
Date: 1998
DOI: 10.3189/1998AOG27-1-398-402
Abstract: By consideration of model-generated atmospheric data, dominant anomalies in the synoptic circulation patterns are observed under conditions of high Antarctic precipitation. This is associated with strong moisture advection of marine origin. Examining precipitation at in idual locations reveals a strong relationship between local surface temperature and precipitation amount. Days with & 5 mm of precipitation (which, on average, corresponds to about 8% of days over Antarctica) have surface temperatures that are around 10°C warmer than the mean. This bias suggest that abnormal conditions are captured in the ice-core record and that interpretation or reconstruction of palaeotemperatures will succeed only under the possibly flawed assumption that similar abnormal conditions existed at the time of deposition. Although isotopic analysis of Antarctic ice cores has been used successfully in palaeoclimate studies, a complete understanding of the underlying processes affecting the deposition of the core remains to be found. It is reasoned that by obtaining such an understanding, it may be possible to reconstruct the synoptic conditions under which accumulation occurred.
Publisher: American Geophysical Union (AGU)
Date: 27-07-2017
DOI: 10.1002/2016JD026312
Publisher: American Meteorological Society
Date: 12-2015
Abstract: Southern Hemisphere mid- to upper-tropospheric planetary wave activity is characterized by the superposition of two zonally oriented, quasi-stationary waveforms: zonal wavenumber 1 (ZW1) and zonal wavenumber 3 (ZW3). Previous studies have tended to consider these waveforms in isolation and with the exception of those studies relating to sea ice, little is known about their impact on regional climate variability. A novel approach is taken to quantifying the combined influence of ZW1 and ZW3, using the strength of the hemispheric meridional flow as a proxy for zonal wave activity. The methodology adapts the wave envelope construct routinely used in the identification of synoptic-scale Rossby wave packets and improves on existing approaches by allowing for variations in both wave phase and litude. While ZW1 and ZW3 are both prominent features of the climatological circulation, the defining feature of highly meridional hemispheric states is an enhancement of the ZW3 component. Composites of the mean surface conditions during these highly meridional, ZW3-like anomalous states (i.e., months of strong planetary wave activity) reveal large sea ice anomalies over the Amundsen and Bellingshausen Seas during autumn and along much of the East Antarctic coastline throughout the year. Large precipitation anomalies in regions of significant topography (e.g., New Zealand, Patagonia, and coastal Antarctica) and anomalously warm temperatures over much of the Antarctic continent were also associated with strong planetary wave activity. The latter has potentially important implications for the interpretation of recent warming over West Antarctica and the Antarctic Peninsula.
Publisher: Springer Science and Business Media LLC
Date: 16-03-2012
Publisher: International Glaciological Society
Date: 2015
Abstract: We examine the evolution of sea-ice extent (SIE) over both polar regions for 35 years from November 1978 to December 2013, as well as for the global total ice (Arctic plus Antarctic). Our examination confirms the ongoing loss of Arctic sea ice, and we find significant ( p ˂ 0.001) negative trends in all months, seasons and in the annual mean. The greatest rate of decrease occurs in September, and corresponds to a loss of 3 x 10 6 km 2 over 35 years. The Antarctic shows positive trends in all seasons and for the annual mean ( p ˂0.01), with summer attaining a reduced significance ( p ˂0.10). Based on our longer record (which includes the remarkable year 2013) the positive Antarctic ice trends can no longer be considered ‘small’, and the positive trend in the annual mean of (15.29 ± 3.85) x 10 3 km 2 a– 1 is almost one-third of the magnitude of the Arctic annual mean decrease. The global annual mean SIE series exhibits a trend of (–35.29 ± 5.75) x 10 3 km 2 a -1 (p .01). Finally we offer some thoughts as to why the SIE trends in the Coupled Model Intercomparison Phase 5 (CMIP5) simulations differ from the observed Antarctic increases.
Publisher: Springer Science and Business Media LLC
Date: 08-05-2015
Publisher: American Meteorological Society
Date: 04-2014
Abstract: This study examines the application of three different variations of linear-regression corrections to the surface marine winds from the Australian Bureau of Meteorology’s recently implemented operational atmospheric model. A simple correction over the entire domain is found to inadequately account for geographical variation in the wind bias. This is addressed by considering corrections that vary in space. Further, these spatially varying corrections are extended to vary in time. In an operational environment, the error characteristics of the wind forcing can be expected to change over time with the evolution of the atmospheric model. This in turn requires any applied correction to be monitored and maintained. Motivated by a desire to avoid this manual maintenance, a self-learning correction method is proposed whereby spatially and temporally varying corrections are calculated in real time from a moving window of historical comparisons between observations and preceding forecasts. This technique is shown to effectively remove both global and regionally varying wind speed biases.
Publisher: Wiley
Date: 04-1995
Publisher: American Geophysical Union (AGU)
Date: 08-2006
DOI: 10.1029/2004JC002395
Publisher: Springer Science and Business Media LLC
Date: 09-08-2006
Publisher: American Meteorological Society
Date: 15-11-2008
Abstract: The Arctic region has exhibited dramatic changes in recent times. Many of these are intimately tied up with synoptic activity, but little research has been undertaken on how the characteristics of Arctic cyclones have changed. This paper presents a comprehensive analysis of Arctic (here defined as the domain north of 70°N) cyclones diagnosed with the Melbourne University cyclone tracking scheme applied to the 40-yr ECMWF Re-Analysis (ERA-40) and the NCEP–NCAR (NCEP1) and NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP)-II (NCEP2) reanalysis sets (the last two extending to the end of 2006). A wide variety of cyclone characteristics is presented as befits these complex features. In winter the highest density of cyclones is found between Norway and Svalbard and to the east to the Barents and Kara Seas, and significant numbers are found in the central Arctic. In summer the greatest frequencies are found in the central Arctic. The total number of cyclones identified in the ERA-40 record exceeds those in the two NCEP compilations. The mean size of cyclones shows similar maxima in the central Arctic in both winter and summer. By contrast, the greatest mean system depth in winter (in excess of 8 hPa) is found to the southeast of Greenland, although average depths exceed 6 hPa over a considerable portion of the basin. In summer the deepest cyclones are found in the central portion of the Arctic. The analysis shows that the total number of cyclones in winter exceeds that in summer, a result in contrast to earlier studies. This difference comes about primarily due to the greater numbers of “open strong” systems in winter in all reanalyses. Cyclones in this category are associated with very active synoptic situations it is of importance that they be included in cyclone counts but would not be considered in many cyclone identification schemes. Since 1979 neither the ERA-40 nor the NCEP2 sets show significant trends in any of the cyclone variables considered. However, over the entire record starting in 1958 the NCEP1 reanalysis exhibits a significant increase in summer cyclone frequency (due mainly to the increase in closed strong systems). Both NCEP1 and ERA-40 also reveal significant increases in the number of summer closed strong cyclones, as well as in their mean depth and intensity in that season. Interannual variations in Arctic cyclone numbers are closely related to the Arctic Oscillation (AO) index in the full reanalyses records. An even stronger relationship is found between the AO and the number of deep cyclones. These relationships have still held in the last decade when the AO has returned to more normal values but the summer and fall sea ice extent has continued to decrease.
Publisher: American Meteorological Society
Date: 06-2005
DOI: 10.1175/JAS3444.1
Abstract: The southern annular mode is the leading mode of Southern Hemisphere circulation variability, the temporal evolution of which is characterized by large litudes and significant persistence. Previous investigators have suggested a positive feedback mechanism that explains some of this low-frequency variance. Here, a mechanism is proposed, involving transient nonmodal growths of the anomalies, that is at least as effective as the positive feedback mechanism in increasing the low-frequency variance of the southern annular mode. Using the vector autoregressive modeling technique, a number of linear inverse models of southern annular mode variability from National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) Reanalysis 2 is derived. These models are then analyzed applying the ideas of the generalized stability theory. It is found that, as a consequence of the nonnormality of the system matrices, a significant increase in the low-frequency variance of the southern annular mode occurs through optimal nonmodal growth of the zonal wind anomalies. The nonnormality arises mainly from the relative dominance of the eddy forcing, while the nonmodal growth is caused by the interference of the nonorthogonal eigenvectors of the nonnormal system matrix. These results are demonstrated first in a simple model that retains only the two leading modes of the zonally averaged zonal wind and eddy-forcing variability, and then in a more general model that includes all the important modes. Using the more general model the authors have determined, among other things, the optimal initial perturbation and the time scale over which it experiences the maximum nonmodal growth to evolve into the pattern associated with the southern annular mode.
Publisher: Informa UK Limited
Date: 12-2008
Publisher: American Meteorological Society
Date: 07-08-2014
DOI: 10.1175/JCLI-D-13-00409.1
Abstract: In this study, the important role of extratropical cyclones and fronts for the atmospheric freshwater flux over the Southern Ocean is analyzed. Based on the Interim ECMWF Re-Analysis (ERA-Interim), the freshwater flux associated with cyclones is quantified and it is revealed that the structure of the Southern Hemispheric storm track is strongly imprinted on the climatological freshwater flux. In particular, during austral winter the spiraliform shape of the storm track leads to a band of negative freshwater flux bending toward and around Antarctica, complemented by a strong freshwater input into the midlatitude Pacific, associated with the split storm track. The interannual variability of the wintertime high-latitude freshwater flux is shown to be largely determined by the variability of strong precipitation (& th percentile). Using a novel and comprehensive method to attribute strong precipitation uniquely to cyclones and fronts, it is demonstrated that over the Southern Ocean between 60% and 90% of the strong precipitation events are due to these synoptic systems. Cyclones are the dominant cause of strong precipitation around Antarctica and in the midlatitudes of the Atlantic and the Pacific, while in the south Indian Ocean and the eastern Atlantic fronts bring most of the strong precipitation. A detailed analysis of the spatial variations of intense front and cyclone precipitation associated with the interannual variability of the wintertime frequency of cyclones in the midlatitude and high-latitude branches of the Pacific storm track underpins the importance of considering both fronts and cyclones in the analysis of the interannual variability of freshwater fluxes.
Publisher: SPIE
Date: 05-08-2013
DOI: 10.1117/12.2027584
Publisher: Wiley
Date: 05-1997
DOI: 10.1002/(SICI)1097-0088(199705)17:6<597::AID-JOC173>3.0.CO;2-V
Publisher: International Glaciological Society
Date: 1998
DOI: 10.3189/1998AOG27-1-617-622
Abstract: The characteristics of, and the mechanisms causing, multi-decadal variability are currently receiving much attention. This undertaking is particularly challenging in the sub-Antarctic region because of the paucity of data, and the complexity of the governing physical processes. In this paper we report on aspects of high-southern-latitude variability which appear in the European Centre for Medium-range Weather Forecasts twice-daily analyses for the period 1 January 1980 to 31 August 1996 and in the results of global climate model experiments. We show that the number of cyclone positions in the 50-70°S latitude band exhibits considerable interannual variability, as well as changes on longer time-scales. The seasonal distribution of cyclones is linked with the “semi-annual oscillation". We show that the variability of this phenomenon in a 1000 year run of the GFDL coupled model shows “red” characteristics fand on decadal time-scales is similar to that displayed in the available observationsi. The interaction with the ocean and sea ice is siressed as an important factor in determining the nature of climate variability in sub-Antarctic latitudes.
Publisher: Springer Science and Business Media LLC
Date: 05-04-2014
Publisher: Wiley
Date: 29-09-2019
DOI: 10.1002/WEA.3384
Abstract: An extreme extratropical cyclone (ETC) struck South Australia on 28 September 2016, causing state‐wide blackouts and damage. In the second part of this two‐part study, we examine the extreme surface wind producing mechanisms within the ETC. ETCs have been extensively studied in the Northern Hemisphere (particularly in western Europe), highlighting the gust‐producing mesoscale features within. Before now, no Southern Hemisphere ETC has been examined in this way. There were a number of extreme gust‐producing features within the ETC, comparable to those observed in storms over western Europe. One such feature was a convective line, which caused many of the most extreme gusts and knocked out the state power grid. However, dry slot convection also contributed to the extremes, and this feature rarely causes extreme gusts in ETCs over the UK. Thus, further analysis is warranted to examine whether this is a common extreme‐gust‐producing ETC feature over Southern Australia. The strongest winds recorded throughout the event occurred on 29 September, and these were associated with the cold conveyor belt which spiralled around the low‐pressure centre.
Publisher: Wiley
Date: 1986
Publisher: Wiley
Date: 11-2018
DOI: 10.1002/WEA.3385
Publisher: Elsevier BV
Date: 06-2021
Publisher: Springer Science and Business Media LLC
Date: 25-07-2009
Publisher: American Geophysical Union (AGU)
Date: 03-07-2010
DOI: 10.1029/2009JC005974
Publisher: Wiley
Date: 05-1970
DOI: 10.1002/ASL.253
Publisher: Wiley
Date: 2002
DOI: 10.1002/JOC.728
Publisher: American Geophysical Union (AGU)
Date: 08-2008
DOI: 10.1029/2008GL034511
Publisher: American Meteorological Society
Date: 11-2008
Abstract: The association between Southern Hemisphere cyclones and anticyclones and the El Niño–Southern Oscillation (ENSO), southern annular mode (SAM), Antarctic sea ice extent (SIE), and rainfall in Perth and Melbourne is explored. Those cities are, respectively, located in the southwestern and southeastern corners of Australia, where substantial decreasing rainfall trends have been observed over the last decades. The need for a more unified understanding of large-scale anomalies in storm indicators associated with the climate features itemized above has motivated this study. The main aim is to identify cyclone-anomalous areas that are potentially important in characterizing continental rainfall anomalies from a hemispheric perspective, focusing on midlatitude Australia. The study covers the “satellite era” from 1979 to 2003 and was conducted for the southern winter when midlatitude rainfall is predominantly baroclinic. The results indicate a well-organized hemispheric cyclone pattern associated with ENSO, SAM, SIE, and rainfall anomalies. There is a moderate large-scale, high-latitude resemblance between La Niña, negative SAM, and reduced SIE in some sectors. In particular, there is a suggestion that SIE anomalies over the Indian Ocean and Western Australia sectors are associated with a large-scale pattern of cyclone/anticyclone anomalies that is more pronounced over the longitudes of Australia and New Zealand. Spatial correlation analysis suggests a robust link between cyclone density over the sectors mentioned above and rainfall in Perth and Melbourne. Statistical analyses of rainfall and SIE show modest correlations for Perth and weak correlations for Melbourne, generally corroborating the above. It is proposed that SAM and SIE are part of a complex physical system that is best understood as a coupled mechanism, and that their impacts on the circulation can be seen as partially independent of ENSO. While SAM and SIE have greater influence on the circulation affecting rainfall in the western side of Australia, ENSO is the dominant influence on the eastern half of the country. A contraction of the sea ice seems to be accompanied by a southward shift of high-latitude cyclones, which is also hypothesized to increase downstream cyclone density at midlatitudes via conservation of mass, similarly to what is observed during the extreme positive phase of the SAM. These associations build on previous developments in the literature. They bring a more unified view on high-latitude climate features, and may also help to explain the declining trends in Australian rainfall.
Publisher: American Geophysical Union (AGU)
Date: 04-1996
DOI: 10.1029/95JD03633
Publisher: Stockholm University Press
Date: 1997
Publisher: Wiley
Date: 10-1996
DOI: 10.1256/SMSQJ.53502
Publisher: Elsevier BV
Date: 12-2000
Publisher: Copernicus GmbH
Date: 25-02-2013
Abstract: Abstract. We present here data of mineral dust variability retrieved from an ice core of the central West Antarctic, spanning the last five decades. Main evidence provided by the geochemical analysis is that northerly air mass incursions to the coring site, tracked by insoluble dust microparticles, have declined over the past 50 yr. This result contrasts with dust records from ice cores reported to the coastal West Antarctic that show increases since mid-20th century. We attribute this difference to regional climatic changes due to the ozone depletion and its implications to westerly winds. We found that the diameters of insoluble microparticles in the central West Antarctica ice core are significantly correlated with cyclone depth (energy) and wind intensity around Antarctica.
Publisher: American Meteorological Society
Date: 04-2013
DOI: 10.1175/BAMS-D-11-00154.1
Abstract: The variability of results from different automated methods of detection and tracking of extratropical cyclones is assessed in order to identify uncertainties related to the choice of method. Fifteen international teams applied their own algorithms to the same dataset—the period 1989–2009 of interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERAInterim) data. This experiment is part of the community project Intercomparison of Mid Latitude Storm Diagnostics (IMILAST see www.proclim.ch/imilast/index.html). The spread of results for cyclone frequency, intensity, life cycle, and track location is presented to illustrate the impact of using different methods. Globally, methods agree well for geographical distribution in large oceanic regions, interannual variability of cyclone numbers, geographical patterns of strong trends, and distribution shape for many life cycle characteristics. In contrast, the largest disparities exist for the total numbers of cyclones, the detection of weak cyclones, and distribution in some densely populated regions. Consistency between methods is better for strong cyclones than for shallow ones. Two case studies of relatively large, intense cyclones reveal that the identification of the most intense part of the life cycle of these events is robust between methods, but considerable differences exist during the development and the dissolution phases.
Publisher: Stockholm University Press
Date: 09-1992
Publisher: Elsevier BV
Date: 10-2023
Publisher: American Meteorological Society
Date: 15-10-2010
Abstract: Mesoscale cyclones play an important role in the weather and climate of the Southern Hemisphere (SH) mid-to-high latitudes. However, the relatively small size and short lifetime of these systems, combined with the lack of available conventional data in this region, means that there is a poor understanding of their climatological characteristics. In this study, the University of Melbourne cyclone-finding algorithm was applied to relatively high-resolution scatterometer-derived surface pressure fields, obtained from the Department of Atmospheric Sciences at the University of Washington, to produce a decade-long (1999–2008) seasonal climatology of mesoscale cyclone activity over the ice-free regions of the Southern Ocean. The frequency of mesoscale cyclone activity was found to be highest just to the north of the sea ice zone, reaching a maximum over the Amundsen and Bellingshausen Seas (ABS), while the southern Indian Ocean was associated with systems of the largest depth, intensity, and momentum flux at the ocean surface. These spatial patterns in mean mesoscale cyclone characteristics showed a broad resemblance to those reported in existing synoptic-scale cyclone climatologies. Maximum wind speed data indicated that SH polar lows may be more frequent than the current literature suggests, while strong positive trends identified in mesoscale cyclone frequency over the ABS may represent a contributing factor to the rapid warming observed in that region over recent years. Partial correlation analyses indicated a link between mesoscale cyclone frequency and the southern annular mode.
Publisher: American Geophysical Union (AGU)
Date: 04-08-2011
DOI: 10.1029/2011JD015847
Publisher: Wiley
Date: 07-1993
Publisher: American Geophysical Union (AGU)
Date: 15-05-1999
DOI: 10.1029/1999GL900292
Publisher: American Geophysical Union (AGU)
Date: 28-10-2017
DOI: 10.1002/2017GL075375
Publisher: Elsevier BV
Date: 02-1988
Publisher: American Geophysical Union (AGU)
Date: 20-06-1985
Publisher: Springer Science and Business Media LLC
Date: 04-2010
DOI: 10.1038/NATURE09051
Abstract: The rise in Arctic near-surface air temperatures has been almost twice as large as the global average in recent decades-a feature known as 'Arctic lification'. Increased concentrations of atmospheric greenhouse gases have driven Arctic and global average warming however, the underlying causes of Arctic lification remain uncertain. The roles of reductions in snow and sea ice cover and changes in atmospheric and oceanic circulation, cloud cover and water vapour are still matters of debate. A better understanding of the processes responsible for the recent lified warming is essential for assessing the likelihood, and impacts, of future rapid Arctic warming and sea ice loss. Here we show that the Arctic warming is strongest at the surface during most of the year and is primarily consistent with reductions in sea ice cover. Changes in cloud cover, in contrast, have not contributed strongly to recent warming. Increases in atmospheric water vapour content, partly in response to reduced sea ice cover, may have enhanced warming in the lower part of the atmosphere during summer and early autumn. We conclude that diminishing sea ice has had a leading role in recent Arctic temperature lification. The findings reinforce suggestions that strong positive ice-temperature feedbacks have emerged in the Arctic, increasing the chances of further rapid warming and sea ice loss, and will probably affect polar ecosystems, ice-sheet mass balance and human activities in the Arctic.
Publisher: American Meteorological Society
Date: 24-04-2017
Abstract: Part I of this study examines the relationship among winter cold anomalies over Eurasia, Ural blocking (UB), and the background conditions associated with Arctic warming over the Barents and Kara Seas (BKS) using reanalysis data. It is found that the intensity, persistence, and occurrence region of UB-related Eurasian cold anomalies depend strongly on the strength and vertical shear (VS) of the mean westerly wind (MWW) over mid–high-latitude Eurasia related to BKS warming. Observational analysis reveals that during 1951–2015 UB days are 64% (54%) more frequent during weak MWW (VS) winters, with 26.9 (28.4) days per winter, than during strong MWW (VS) winters. During weak MWW or VS winters, as frequently observed during 2000–15, persistent and large UB-related warming is seen over the BKS together with large and widespread midlatitude Eurasian cold anomalies resulting from increased quasi stationarity and persistence of the UB. By contrast, when the MWW or VS is strong as frequently observed during 1979–99, the cold anomaly is less intense and persistent and confined to a narrow region of Europe because of a rapid westward movement of the strong UB. For this case, the BKS warming is relatively weak and less persistent. The midlatitude cold anomalies are maintained primarily by reduced downward infrared radiation (IR), while the surface heat fluxes, IR, and advection all contribute to the BKS warming. Thus, the large BKS warming since 2000 weakens the meridional temperature gradient, MWW, and VS, which increases quasi stationarity and persistence of the UB (rather than its litude) and then leads to more widespread Eurasian cold events and further enhances the BKS warming.
Publisher: Springer Science and Business Media LLC
Date: 03-02-2020
DOI: 10.1038/S41467-023-36136-5
Abstract: Winter Arctic sea-ice concentration (SIC) decline plays an important role in Arctic lification which, in turn, influences Arctic ecosystems, midlatitude weather and climate. SIC over the Barents-Kara Seas (BKS) shows large interannual variations, whose origin is still unclear. Here we find that interannual variations in winter BKS SIC have significantly strengthened in recent decades likely due to increased litudes of the El Niño-Southern Oscillation (ENSO) in a warming climate. La Niña leads to enhanced Atlantic Hadley cell and a positive phase North Atlantic Oscillation-like anomaly pattern, together with concurring Ural blocking, that transports Atlantic ocean heat and atmospheric moisture toward the BKS and promotes sea-ice melting via intensified surface warming. The reverse is seen during El Niño which leads to weakened Atlantic poleward transport and an increase in the BKS SIC. Thus, interannual variability of the BKS SIC partly originates from ENSO via the Atlantic pathway.
Publisher: Wiley
Date: 02-06-2023
DOI: 10.1002/QJ.4489
Abstract: In this article, the meridional gradient of daily potential vorticity (PV) on the 330 K isentropic surface is used to identify atmospheric blocking events for the period 1979–2019. The associated two‐dimensional index considers not only Rossby wave breaking, but also energy dispersion and nonlinearity of blocking systems, and thus has a solid theoretical foundation. It also has the advantage of automatically excluding subtropical high‐pressure systems in summer and autumn. The index reveals that the Northern Hemisphere exhibits high blocking frequency over Euro‐Atlantic, North Pacific and Greenland in winter, spring and autumn, and over two wide‐extended bands at high latitudes in summer. Two prominent blocking episode (BE) intensity centres are found over the eastern Atlantic and eastern Pacific in all seasons, while the long‐lived BE is primarily situated in regions with high BE frequency. There is more frequent and longer‐lived BE in the Euro‐Atlantic sector than in the North Pacific, whereas the BE in the North Pacific is more intense. Notably, with the same poleward criterion for the four seasons, the BE frequency and duration are supposed to be overestimated in summer. Comparing our blocking detection method with previous blocking indices provides additional information about the long‐term trends for blocking frequency and intensity, which can be useful to our understanding of future extremes on climate time‐scales. It is found that both blocking frequency and intensity exhibit upward linear trends in the Ural region and Barents–Kara Sea in winter, Europe, northern North Pacific and East Siberia in spring, western Greenland in summer, as well as Norwegian Sea, Europe and North Atlantic in autumn, which point to increases in high‐impact weather events in these regions.
Publisher: Springer Science and Business Media LLC
Date: 08-03-2021
DOI: 10.1038/S41467-021-21773-5
Abstract: The Antarctic Peninsula of West Antarctica was one of the most rapidly warming regions on the Earth during the second half of the 20th century. Changes in the atmospheric circulation associated with remote tropical climate variabilities have been considered as leading drivers of the change in surface conditions in the region. However, the impacts of climate variabilities over the mid-latitudes of the Southern Hemisphere on this Antarctic warming have yet to be quantified. Here, through observation analysis and model experiments, we reveal that increases in winter sea surface temperature (SST) in the Tasman Sea modify Southern Ocean storm tracks. This, in turn, induces warming over the Antarctic Peninsula via planetary waves triggered in the Tasman Sea. We show that atmospheric response to SST warming over the Tasman Sea, even in the absence of anomalous tropical SST forcing, deepens the Amundsen Sea Low, leading to warm advection over the Antarctic Peninsula.
Publisher: American Meteorological Society
Date: 15-02-2013
DOI: 10.1175/JCLI-D-12-00063.1
Abstract: Arctic sea ice is declining at an increasing rate with potentially important repercussions. To understand better the atmospheric changes that may have occurred in response to Arctic sea ice loss, this study presents results from atmospheric general circulation model (AGCM) experiments in which the only time-varying forcings prescribed were observed variations in Arctic sea ice and accompanying changes in Arctic sea surface temperatures from 1979 to 2009. Two independent AGCMs are utilized in order to assess the robustness of the response across different models. The results suggest that the atmospheric impacts of Arctic sea ice loss have been manifested most strongly within the maritime and coastal Arctic and in the lowermost atmosphere. Sea ice loss has driven increased energy transfer from the ocean to the atmosphere, enhanced warming and moistening of the lower troposphere, decreased the strength of the surface temperature inversion, and increased lower-tropospheric thickness all of these changes are most pronounced in autumn and early winter (September–December). The early winter (November–December) atmospheric circulation response resembles the negative phase of the North Atlantic Oscillation (NAO) however, the NAO-type response is quite weak and is often masked by intrinsic (unforced) atmospheric variability. Some evidence of a late winter (March–April) polar stratospheric cooling response to sea ice loss is also found, which may have important implications for polar stratospheric ozone concentrations. The attribution and quantification of other aspects of the possible atmospheric response are hindered by model sensitivities and large intrinsic variability. The potential remote responses to Arctic sea ice change are currently hard to confirm and remain uncertain.
Publisher: American Geophysical Union (AGU)
Date: 18-12-2002
DOI: 10.1029/2002JD002262
Abstract: Water isotopes are commonly used as indicators of climate state even though many biases and variations in processes affecting the polar signal have not been quantified. Results from the Melbourne University General Circulation Model suggest the annual cycle explains half of the monthly δ 18 O variance, and a semiannual variation contributes more than 15 in places. Eddy moisture convergence drives gross accumulation, while stationary flux allows sublimation of 25–30% of the precipitation. Part of the monthly anomaly variance is associated with a dominant annular disturbance in the circulation. This oscillatory mode alters the character of the transport processes through changes to the preferred location and strength of baroclinic cyclones. A Rayleigh model indicates that a third of the continental δ 18 O anomaly can be explained by temperature‐dependent fractionation, while changes to the condensation give 3 times too much depletion. The residual is explained by the migration of the zone from which midlatitude air is entrained into the polar environment by cyclonic storms. The positive phase of the annular mode is associated with an increased contribution from the near‐coastal region, which enriches the continental precipitation. Such vacillation introduces bias in reconstruction using modern analogues because the spatial temperature‐isotope slope is modified.
Publisher: Stockholm University Press
Date: 2011
Publisher: American Geophysical Union (AGU)
Date: 07-03-2006
DOI: 10.1029/2004JD005611
Publisher: American Geophysical Union (AGU)
Date: 10-2009
DOI: 10.1029/2009GL039810
Publisher: Wiley
Date: 22-10-2008
DOI: 10.1002/JOC.1757
Publisher: American Meteorological Society
Date: 30-10-2015
Abstract: Extratropical cyclones are responsible for many extreme precipitation events in the midlatitudes. Warm conveyor belts (WCBs) and fronts are known to be related to the uplift and hence the precipitation within cyclones. The authors have investigated the link between WCBs and fronts and how such a link impacts the occurrence of extreme precipitation events. WCB trajectories have been calculated from the ERA-Interim dataset, and low-level (below 790 hPa) and midlevel (790–600 hPa) WCBs have been considered. These have been matched with objectively identified fronts (i.e., characterized by an overlap of WCB and front somewhere along the front). About 10% of cold fronts, 8% of warm fronts (identified using a thermal criterion), and 15% of wind fronts (identified using a wind shift method) are matched with WCBs, while up to 70% of WCBs are matched with fronts. Some WCBs, especially in the Southern Hemisphere, are not matched with either type of front (up to 70% east of Australia). The relationship between WCBs and fronts does not change much between the low levels and midlevels, indicating that the WCBs are already strongly associated with fronts during the lowest part of their ascent, although in the Southern Hemisphere the WCBs are more often related to warm fronts during their midtropospheric ascent. In parts of the midlatitudes, more than 60% of extreme precipitation events match either cold or warm fronts, and up to 90% of these have matched WCBs. Fronts associated with WCBs are found to be between 2 and 10 times more likely to produce extreme precipitation events than fronts without associated WCBs.
Publisher: American Geophysical Union (AGU)
Date: 2009
DOI: 10.1029/2008GL036320
Publisher: Wiley
Date: 2007
DOI: 10.1002/JOC.1477
Publisher: Wiley
Date: 28-09-2011
DOI: 10.1002/JOC.2203
Publisher: American Geophysical Union (AGU)
Date: 13-03-2013
DOI: 10.1002/GRL.50174
Publisher: Elsevier BV
Date: 02-2006
Publisher: American Meteorological Society
Date: 07-04-2015
DOI: 10.1175/JCLI-D-14-00458.1
Abstract: Presented here is a global analysis of frontal activity variability derived from ERA-Interim data over the 34-yr period of January 1979–March 2013 using a state-of-the-art frontal tracking scheme. In December–February over that epoch, there is a northward shift of frontal activity in the Pacific in the Northern Hemisphere (NH). In the Southern Hemisphere (SH), the largest trends are identified in the austral summer and are manifested by a southward shift of frontal activity over the Southern Ocean. Variability of frontal behavior is found to be closely related to the main modes of atmospheric circulation, such as the North Atlantic Oscillation (NAO) for the Atlantic–European sector in the NH and the southern annular mode (SAM) in the middle and high latitudes of the SH. A signal associated with El Niño and hence emanating from the tropics is also apparent in the behavior of frontal systems over the Pacific by a reduction in the number of fronts in the middle South Pacific and intensification of frontal activity in high and low latitudes throughout the year. It is shown in general that the associations of the large-scale modes with frontal variability are much stronger than with cyclones. This indicates that the quantification of the behavior of fronts is an important component of understanding the climate system. At the very high latitudes, it is also shown here that, in the recent years of rapid sea ice reduction in the Arctic, there have been fewer summer fronts observed over the Canadian Arctic.
Publisher: Wiley
Date: 26-05-2021
DOI: 10.1002/JOC.7208
Abstract: The goal of this study is to advance an automated and objective scheme for the identification of cold fronts over the Mediterranean based on wind‐related criteria, MedFTS, by further incorporating thermal criteria, in order to improve the identification of cold fronts on a climatological basis. These thermal criteria are related to cold advection and the thermal front parameter, and their in idual or combined contribution is systematically investigated considering different threshold values. For each combination of thermal criteria and/or threshold values, the results are validated with the aid of statistical indices and metrics, and the optimum scheme configuration for the identification of Mediterranean fronts is selected. The advanced version of MedFTS, named MedFTS_DT, having incorporated both dynamic and thermodynamic criteria, is applied and validated for a decade of fronts over the Mediterranean. It was found that the number of erroneously identified cold fronts over Greece was significantly reduced from MedFTS to MedFTS_DT version by almost 40%, as indicated by the statistical metrics. Therefore, the performance of MedFTS_DT is verified to be superior to MedFTS. As a further attempt to investigate the ability of MedFTS_DT to perform adequate climatological analysis in the Mediterranean, the frequency of the cold fronts was calculated over Greece during the period 2007–2016 on a monthly and seasonal basis, demonstrating that the scheme can capture the intra‐annual variations. Finally, the contribution of the cold fronts to the precipitation over the Mediterranean for the same period was investigated, further supporting the scheme's performance.
Publisher: Springer Science and Business Media LLC
Date: 12-02-2013
Publisher: American Meteorological Society
Date: 10-12-2014
DOI: 10.1175/JCLI-D-14-00186.1
Abstract: An objective climatology of anticyclones over the greater Mediterranean region is presented based on the Interim ECMWF Re-Analysis (ERA-Interim) for a 34-yr period (1979–2012) and the Melbourne University automatic identification and tracking algorithm. The scheme’s robustness and reliability for the transient extratropical propagation of anticyclones, with the appropriate choices of parameter settings, has been established and the results obtained here present new research perspectives on anticyclonic activity affecting the Mediterranean. Properties of Mediterranean anticyclones, such as frequency, generation and dissipation, movement, scale, and depth are investigated. The highest frequency of anticyclones is found over continental areas, while the highest maritime frequency occurs over closed basins exhibiting also maxima of anticyclogenesis. There is a significant seasonality in system density and anticyclogenesis maxima, this being associated with the seasonal variations of the larger-scale atmospheric circulation that affect the greater Mediterranean region.
Publisher: American Geophysical Union (AGU)
Date: 15-12-2012
DOI: 10.1029/2012GL054259
Publisher: American Geophysical Union (AGU)
Date: 15-03-1995
DOI: 10.1029/94JC02206
Publisher: American Geophysical Union (AGU)
Date: 13-04-2012
DOI: 10.1029/2011JD017337
Publisher: American Meteorological Society
Date: 04-2017
Abstract: A composite dataset (comprising geopotential height, sea surface temperature, zonal and meridional surface winds, precipitation, cloud cover, surface air temperature, latent plus sensible heat fluxes, and sea ice concentration) has been investigated with the aim of revealing the dominant time scales of variability from 1982 to 2013. Three covarying climate signals associated with variations in the sea ice distribution around Antarctica have been detected through the application of the multiple-taper method with singular value decomposition (MTM-SVD). Features of the established patterns of variation over the Southern Hemisphere extratropics have been identified in each of these three climate signals in the form of coupled or in idual oscillations. The climate patterns considered here are the southern annular mode (SAM), the Pacific–South American (PSA) teleconnection, the semiannual oscillation (SAO), and the zonal wavenumber-3 (ZW3) mode. It is shown that most of the sea ice temporal variance is concentrated at the quasi-triennial scale resulting from the constructive superposition of the PSA and ZW3 patterns. In addition, the combination of the SAM and SAO patterns is found to promote the interannual sea ice variations underlying a general change in the Southern Ocean atmospheric and oceanic circulations. These two modes of variability are also found to be consistent with the occurrence of the positive SAM/negative PSA (SAM + /PSA − ) or negative SAM ositive PSA (SAM − /PSA + ) combinations, which could have favored the cooling of the sub-Antarctic region and important changes in the Antarctic sea ice distribution since 2000.
Publisher: IOP Publishing
Date: 06-2021
Abstract: We investigate linear trends in Antarctic skin temperatures (temperatures from about the top millimeter of the surface) over the four seasons using ERA5 ensemble mean reanalysis data. During 1950–2020, statistically significant warming occurred over East and West Antarctica in spring, autumn and winter, and over the Antarctic Peninsula in autumn and winter. A surface energy budget analysis revealed that increases in downward longwave radiation related to increases in air temperature and total column integrated cloud had a key role in Antarctic surface warming. There were negative sea level pressure trends around the periphery of Antarctica throughout the year, and the associated circulation contributed to warm advection from the middle latitudes to West Antarctica and the Antarctic Peninsula. Over the interior of East Antarctica, increase in moisture advection from lower latitudes enhanced the low-level cloud cover. A two-dimensional parameter diagram showed that skin temperature trends for time segments longer than 30 years starting before 1960 exhibited statistically significant warming in autumn and winter in East and West Antarctica and the Antarctic Peninsula. In spring, West Antarctica also showed statistically significant warming for long segments. In summer, the Antarctic Peninsula had statistically significant warming trends for long segments and cooling trends for segments less than 30 years. For all the studied time intervals, when skin temperatures had statistically significant positive trends, increases in downward longwave radiation contributed more than 70% of the warming and vice versa. This result demonstrates that on all time and space scales, changes in downward longwave radiation associated with variations in air temperature and atmospheric moisture loading play a dominant role controlling skin temperatures.
Publisher: Wiley
Date: 03-1989
Publisher: Schweizerbart
Date: 02-2013
Publisher: American Geophysical Union
Date: 2000
DOI: 10.1029/GM114P0255
Publisher: American Geophysical Union (AGU)
Date: 10-11-2005
DOI: 10.1029/2005JD006071
Publisher: American Geophysical Union (AGU)
Date: 20-08-1992
DOI: 10.1029/92JD00722
Publisher: American Meteorological Society
Date: 06-2007
DOI: 10.1175/JCLI4135.1
Abstract: The mean characteristics and trends of Southern Hemisphere (SH) winter extratropical cyclones occurring at six levels of the troposphere over the period 1979–2001 have been investigated using the 40-yr ECMWF Re-Analysis (ERA-40) data. Cyclonic systems were identified with the Melbourne University cyclone finding and tracking scheme. This study shows that mean sea level pressure (MSLP) cyclones are more numerous, more intense, smaller, deeper, and slower moving than higher-level cyclones. The novel vertical tracing scheme devised for this research revealed that about 52% of SH winter MSLP cyclones have a vertically well organized structure, extending through to the 500-hPa level. About 80% of these vertically coherent SH cyclones keep their westward tilt until the surface cyclones reach their maximum depths, and the mean distance is 300 km between the surface and the 500-hPa level cyclone centers when the surface cyclones obtain their maturity. According to the authors’ definition of vertical organization, explosively developing cyclones are vertically very well organized systems, whose surface development is antecedent to their 500-hPa level counterpart. Over 1979–2001 cyclones have increased in their system density, intensity, and translational velocity but decreased in their scale at almost all levels. However, some of the trends are not statistically significant. The proportion of vertically well organized systems in the entire population of SH winter extratropical cyclones has considerably increased over the last 23 yr, and the mean distance between the surface and the 500-hPa- level cyclone centers has decreased. Such changes in vertical organization of extratropical cyclones are statistically significant at the 95% confidence level.
Publisher: Elsevier BV
Date: 2005
DOI: 10.1016/J.AAP.2004.07.005
Abstract: An investigation into the effect of weather variables on traffic flow at a site in Melbourne, Australia, for the period 1989-1996 was performed. Rainfall was the strongest correlated weather parameter and it had the greatest impact in winter and spring, when traffic volume is reduced on wet days. There are statistically significant decreases of 1.35 and 2.11% in traffic volume on wet days in winter and spring. The reduction increases to 2-3% over the 2-10mm range, the largest being 3.43% for the 2-5mm class in spring. For the first time, our study considers separately daytime and nighttime periods. We found a reduction of 1.86% in winter and 2.16% in spring during daytime rainfall. The reduction at nighttime is significant over all seasons, ranging from 0.87% in winter to 2.91% in spring. We have explored an application where the traffic volume was used to normalise the road accident count and found the rain effect to increase by 2.4, 1.9 and 5.2% relative to the daily, daytime and nighttime dry mean accident count. Generally, the normalised count is greater than the raw count, with a larger increase for the higher rainfall classes.
Publisher: Springer Science and Business Media LLC
Date: 26-06-2014
Start Date: 2015
End Date: 2018
Funder: Marsden Fund
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