ORCID Profile
0000-0001-9851-8198
Current Organisations
Australian Bureau of Meteorology
,
Institute of Oceanology. PP Shirshov Russian Academy of Sciences
,
University of Melbourne
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Publications
Atmospheric trends explained by changes in frequency of short-term circulation patterns
Publisher: Springer Science and Business Media LLC
Date: 19-04-2023
DOI: 10.1038/S43247-023-00785-7
Abstract: The circulation of the atmosphere is subject to natural and anthropogenic forcings that alter the energy balance of the climate system. In each hemisphere the zonally averaged atmospheric circulation can be represented by a single overturning cell if viewed in isentropic coordinates, highlighting the connections between tropics and extratropics. Here we present clusters of the meridional atmospheric circulation based on reanalysis data. Our results reveal preferred global circulation regimes with two clusters in each solstice season. These clusters show strong trends in their occurrence in the last two decades of the 20th century coincident with the depletion of the low-stratospheric ozone over Antarctica. We hypothesize that a change in the occurrence of short-term circulation regimes may lead to some long-term atmospheric trends. Finally, we show a strong coupling between the atmospheric circulation in boreal and austral winters and propose a mechanism linking anomalies in both seasons to the stratospheric ozone that requires confirmation with modelling experiments.
Variability and changes to the mean meridional circulation in isentropic coordinates
Publisher: Springer Science and Business Media LLC
Date: 03-08-2022
DOI: 10.1007/S00382-021-05903-9
Abstract: We examine the climatology, variability and change in the global mean meridional circulation (MMC) as measured in a dry isentropic coordinate system from 1979–2017 using the ERA-Interim reanalysis. The methodology presents a zonal-mean view of the MMC as a single thermally direct circulation cell in each hemisphere. The circulation is decomposed into 'steady' and 'transient' components which allows us to identify and quantify several MMC features, including the Intertropical Convergence Zone, the descending branches of the Hadley circulation and a 'transient updraft' associated with the extratropical storm track. Large changes were identified in the Southern Hemisphere (SH) in both the Hadley Cell and the extratropical storm track in the late-1990s. These changes intertwine with the Interdecadal Pacific Oscillation that changed from a warm to a cold phase around 2000. Less significant changes were observed in the Northern Hemisphere, although high rates of tropical expansion during boreal summer may have been exacerbated by volcanic eruptions in the 1980s and 1990s. Further to those changes, tropical expansion was observed in autumn, with little change in the extratropical storm track. While potential inhomogeneities in the reanalysis limit the certainty about the magnitude of the identified changes, multiple non-reanalysis-based datasets suggest that large changes did occur in the 1990s in the SH, supporting the presented analysis.
Evaluation of Spectral Wave Models Physics as Applied to a 100‐Year Southern Hemisphere Extra‐Tropical Cyclone Sea State
Publisher: American Geophysical Union (AGU)
Date: 09-2023
DOI: 10.1029/2023JC019751
Abstract: Extreme significant wave height estimates, and their probability of exceedance, are fundamental offshore and coastal engineering design parameters. These estimates are characterised by uncertainty due to an incomplete understanding of the atmosphere‐ocean energy and momentum exchanges during intense storms. This particularly affects extreme wave statistics of ocean regions exposed to large and frequent synoptic disturbances such as Extra‐Tropical Cyclones (ETCs). In this work, we assessed the performance of global phase‐averaged spectral wave models in representing the 1 in 100‐year sea state generated by a Southern Ocean ETC in April 2021. We collected in‐situ and remote sensing observations, from the storm generation region to its decaying phase and the impact on South‐East Australian coastlines. We compared the observations with a suite of reanalysis and hindcast global wave model datasets. While comparing well for wind speed up to 20 m/s, the models presented differences in solving the air‐sea momentum exchange between the atmosphere and the ocean for wind speed velocities between 20 and 35 m/s, which are a distinctive characteristic of ETCs. Despite marked differences in the storm generation region, the models converged to a similar representation of the swell systems impacting the South‐East Australian coastlines, as demonstrated by a comparison with deep‐water buoy observations close to the coastlines. Furthermore, we found that the energy of the ERA5 reanalysis, which assimilates satellite wave height measurements is quickly dispersed and, as such, of little advantage in representing the 9.9 m significant wave heights that impacted the South‐East Australian coastlines on April 10 th 2021.
Antarctic Peninsula warm winters influenced by Tasman Sea temperatures
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.
Climatology of cyclone size characteristics and their changes during the cyclone life cycle
Publisher: American Meteorological Society
Date: 07-2007
DOI: 10.1175/MWR3420.1
Abstract: Climatology of the atmospheric cyclone sizes and their change over the cyclone life cycle is analyzed on the basis of tracking 57 yr of NCEP–NCAR reanalysis sea level pressure data over the Northern Hemisphere. To quantify the atmospheric cyclone sizes a coordinate transform was used, which allows for the collocation of the cyclone center with the virtual pole and for the establishment of a unique coordinate system for the further determination of cyclone geometry. This procedure was incorporated into a numerical cyclone tracking scheme and provided quantitative estimation of cyclone geometry at every stage of the cyclone development. Climatological features of the distribution of the cyclone size characteristics (effective radius, asymmetry) are considered for the cyclones with different central pressure, deepening rate, and lifetime. Mean effective cyclone radius may experience significant changes, ranging from 300–400 km over the continents to more than 900 km over the oceans. There is found to be a strong dependence of the cyclone effective radius on the cyclone lifetime and intensity, implying the largest cyclone sizes for the most intense and long-living transients. Analysis of size changes during the cyclone life cycle implies that the cyclone radius increases during the development stage from 50% to 150%. Size evolution during the cyclone life cycle implies a universal dependence of the normalized cyclone effective radius and the normalized cyclone age. The actual maximum cyclone radius can be determined from these two nondimensional parameters and cyclone central pressure. Further application of the analysis of cyclone size and shape are discussed.
Composite analysis of north atlantic extratropical cyclones in NCEP-NCAR reanalysis data
Publisher: American Meteorological Society
Date: 05-2011
Abstract: Composite analysis of North Atlantic midlatitudinal winter cyclones is performed using NCEP–NCAR reanalysis data for the 60-yr period from 1948 to 2007. The composites were developed using an advanced methodology involving the coordinate transform of cyclones into a nondimensional azimuthal coordinate system and the further collocation of fields. Composite analysis is performed for air–sea turbulent fluxes, heat content, precipitable water, and precipitation for 576 oceanic cyclones generated in the Gulf Stream area in winter (January–March) from 1948 to 2007. For the region of cyclone generation over the Gulf Stream, composites were analyzed for different cyclone intensities. Over the whole North Atlantic, composites were developed throughout the life cycle and for different cyclone types classified by the regions of their migration. These classifications allow the case-to-case variability to be minimized and the robustness of the composite to be boosted. In the region of cyclone generation over the Gulf Stream, characteristics of the composites strongly depend on the cyclone intensity quantified through the radial sea level pressure difference between the cyclone’s edge and its center. Stronger cyclone intensity implies larger turbulent fluxes in the rear of a cyclone and stronger precipitation in the forward part. Cyclones gradually dry with the water content and precipitation rate decreasing by about 40% and 50%–70%, respectively, during the lifetime. Although composites of air–sea turbulent fluxes show locally very strong positive fluxes in the rear part of the cyclone, the total air–sea turbulent fluxes provided by cyclones are not significantly different from the averaged background fluxes. This shows that the formation of extreme air–sea fluxes by cyclones is connected to the larger-scale circulation conditions, particularly to the cyclone–anticyclone transition zones.
Subantarctic cyclones identified by 14 tracking methods, and their role for moisture transports into the continent
Publisher: Stockholm University Press
Date: 2018
Can southern Australian rainfall decline be explained? A review of possible drivers
Publisher: Wiley
Date: 20-01-2023
DOI: 10.1002/WCC.820
Abstract: Southern Australia's rainfall is highly variable and influenced by factors across scales from synoptic weather to large‐scale circulation and remote climate modes of variability. Anthropogenic climate change and natural variability modulate these factors and their interactions. However, studies often focus on changes in selected parts of the climate system with less emphasis on the system as a whole. As such, it is difficult to gain a complete understanding of how southern Australia's rainfall responds to broad‐scale changes in the climate system. We step through the existing literature on long‐term changes in synoptic‐to‐large‐scale atmospheric circulation and drivers of climate variability to form a more complete story of rainfall changes across southern Australia. This process reveals that the most robust change is the observed winter decline in rainfall as it is consistent with several changing climatic factors: decreasing rainfall from weather systems, strengthening subtropical ridge, poleward shifts in the Hadley Cell and the Southern Annular Mode, and increasing frequency of positive Indian Ocean Dipole events. In other seasons, particularly summer, changes in atmospheric circulation and drivers may not agree with observed rainfall changes, highlighting gaps in our knowledge of atmospheric dynamics and climate change processes. Future work should focus on research across temporal‐ and spatial‐scales, better understanding of jet interactions, the influence of stratospheric processes on the troposphere, and instances of contrasting trends in drivers and southern Australian rainfall changes. This article is categorized under: Paleoclimates and Current Trends Modern Climate Change Paleoclimates and Current Trends Detection and Attribution Assessing Impacts of Climate Change Observed Impacts of Climate Change
Sensitivity of extratropical cyclone characteristics to horizontal resolution in the ECMWF model
Publisher: Wiley
Date: 07-2006
DOI: 10.1256/QJ.05.212
Global relationship between fronts and warm conveyor belts and the impact on extreme precipitation
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.
Roles of wave trains and synoptic Rossby waves in creating midlatitude temperature extremes during winter
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-6955
Abstract: & & Rossby waves, found in the westerly flow at the upper troposphere, transfer energy, moisture, and momentum across large distances, being responsible for atmospheric teleconnections. Large- litude waves may contribute to rapid changes in wind and temperature, making them import for creating local temperature or precipitation extremes. Wirth et al (2018) separated Rossby waves into a low-frequency type, referred to as Rossby wave trains, and high-frequency, or synoptic, waves. In this work we explore a relative role of these two types in creating seasonal and synoptic temperature extremes in the midlatitudes.& & & & We identify wave propagation regions at 300 hPa using ERA-Interim dataset for JFM 1980 & #8211 2017. Our analysis is based on the daily data. This time scale allows identification of waveguides at a wide range of latitudes, suggesting possibility of Rossby wave propagation between midlatitudes and polar regions, as well as tropics. We show that winter temperature extremes in the midlatitudes are associated with anomalies in both high and low latitudes, while the relative importance of these areas differs across midlatitude regions. Furthermore, we demonstrate, that warm Arctic regions can create cold outbreaks in Siberia and North America.& & & & Analysis of the evolution of midlatitude synoptic extremes reveals the importance of a pre-existing local temperature anomaly, that triggers lification of large-scale Rossby wave trains and creates a local anomaly in the waveguide. The latter modifies propagation of synoptic scale Rossby waves that further lify the local temperature anomaly.& & & & References:& & & & Wirth, V., M. Riemer, E. K. M. Chang, and O. Martius, 2018: Rossby Wave Packets on the Midlatitude Waveguide& #8212 A Review. Mon. Wea. Rev., & strong& & /strong& , 1965& #8211 . 0.1175/MWR-D-16-0483.1.& &
Extreme subtropical precipitation in Australia: reasons for decline
Publisher: Copernicus GmbH
Date: 28-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-10816
Abstract: & & & & & span& A long-term reduction in southern Australian rainfall has been a focus of many studies. & /span& & span& In south-eastern Australia it has been shown that after the Millennium Drought (199& /span& & span& & #8211 2009), & /span& & span& the average precipitation has not recovered to the pre-drought values. Our analysis reveals a decline not only in the average precipitation but also in daily extreme rainfall amounts in the cold season. This study explores the physical processes leading to changes in extreme rainfall. & /span& & span& & & /span& & & & & & span& High rainfall extremes are related, on the one hand, to a certain combination of weather systems at various height levels through the troposphere and, on the other hand, to moisture availability. We first identify which synoptic conditions lead to extreme rainfall events in south-eastern Australia and backtrack their development for a few days. Australia is believed to be affected by Rossby waves (RWs) propagating from the tropics. However, we show that extreme events in the southern part of the country are associated with breaking synoptic RWs propagating from the extratropical Indian Ocean. Interestingly, we find that the frequency of cut-off lows, that form following the breaking of RWs, have not declined over the recent decades. & This fact highlights that not all & /span& & span& cut-off lows necessarily lead to extreme rainfall. We find that the strongest events occur in the presence of a Tasman High pressure system at the surface and a vertically developed low-pressure system to the west of it. We show that, despite little change in the frequency of cut-off lows in the upper troposphere, vertically developed lows have become less frequent after 1997 and when they occur, a larger moisture influxis required to produce an intense rainfall event.& /span& & span& & & /span& & & & / &
On the relation of the number of extratropical cyclones to their sizes
Publisher: Pleiades Publishing Ltd
Date: 06-2008
The Role of Extratropical Cyclones and Fronts for Southern Ocean Freshwater Fluxes
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.
Comparing Cyclone Life Cycle Characteristics and Their Interannual Variability in Different Reanalyses
Publisher: American Meteorological Society
Date: 23-08-2013
DOI: 10.1175/JCLI-D-12-00777.1
Abstract: Characteristics of Northern Hemisphere extratropical cyclone activity were compared for five concurrent reanalyses: the NCEP–U.S. Department of Energy (DOE) reanalysis (herein NCEP–DOE), the Japanese 25-year Reanalysis Project (JRA-25), the ECMWF Interim Re-Analysis (ERA-Interim), the National Aeronautics and Space Administration's Modern-Era Retrospective Analysis for Research and Applications (NASA-MERRA), and the NCEP Climate Forecast System Reanalysis (NCEP-CFSR), for the period 1979–2010 using a single cyclone tracking algorithm. The total number of cyclones, ranging from 1400 to more than 1800 yr−1, was found to depend strongly on the spatial resolution of the respective reanalysis. The largest cyclone population was identified using NASA-MERRA data, which also showed the highest occurrence of very deep cyclones. Of the reanalyses, two (NCEP–DOE and ERA-Interim) are associated with statistically significant positive trends in the total number of cyclones from 1% to 2% decade−1. These trends result from moderate and shallow cyclones contributing to approximately 90% of the total cyclone count on average. The number of very deep cyclones (& hPa) in the North Atlantic increased in most reanalyses until 1990 and then declined during the last decade. In the North Pacific, the number of these events reached a peak in 2000 and then decreased during the last decade. The winter pattern is characterized by robust trends in cyclone numbers, with an enhancement of the North Atlantic storm track and a weakening of the North Pacific subtropical storm track. In the summer, there is a robust intensification of the Mediterranean storm track and a decrease in counts over the North Atlantic. Interannual variability and decadal-scale variations of the cyclone counts are highly correlated among the reanalyses, with the greatest agreement in moderate and deep cyclones.
Intraseasonal non-stationarity of the leading modes of atmospheric moisture over Europe during summer
Publisher: Springer Science and Business Media LLC
Date: 14-11-2009
Variability and trends of global atmospheric frontal activity and links with large-scale modes of variability
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.
Sub-synoptic-scale features associated with extreme surface gusts during the South Australia Storm of September 2016 – Part I: characteristics of the event
Publisher: Wiley
Date: 11-2018
DOI: 10.1002/WEA.3385
Rainfall stable water isotope variability in coastal southwestern Western Australia and its relationship to climate on multiple timescales
Publisher: Wiley
Date: 02-07-2021
Seasonal aspects of an objective climatology of anticyclones affecting the mediterranean
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.
How can atmospheric trends be explained by changes in frequency of short-term circulation regimes and what is the role of the Antarctic ozone?
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-4557
Abstract: Global circulation patterns are analysed using the mean meridional circulation (MMC) from ERA-Interim for the period of 1979 & #8211 2017. The global isentropic MMC consists of a single overturning cell in each hemisphere with net heat transport from the equator to the pole. Six clusters are identified from daily data that are associated with one of four seasons. Two solstitial MMC clusters represent either stronger or weaker circulation in the winter hemisphere. We show that long-term trends do not reflect a gradual change in the atmospheric circulation, but rather a change in the frequency of preferred short-term circulation regimes. Before the late 1990s the clusters showing a stronger (weaker) winter circulation are becoming less (more) frequent from around year 2000 the trends have paused. These trends are in close agreement with the change in the low-stratospheric Antarctic ozone trends reported by earlier studies. Our findings also reveal a strong coupling between Southern and Northern Hemispheres during boreal winter. Following Hartmann et al. (2022), we hypothesize that anomalous polar vortex over Antarctica leads to anomalies in the sea surface temperatures (SST) in the tropical Pacific that impact the circulation in both hemispheres. Furthermore, we show that consecutive solstice season demonstrates coherent anomalies in the frequency of circulation regimes. We discuss possible reasons for such relationship.References:Hartmann, D. L., Kang, S., Polvani, L. & Xie, S.-P. The Antarctic ozone hole and the pattern effect on climate sensitivity. (2022) doi:10.1073 nas.
The great Arctic cyclone of August 2012
Publisher: American Geophysical Union (AGU)
Date: 15-12-2012
DOI: 10.1029/2012GL054259
Rainfall stable water isotope variability in coastal southwestern Western Australia and its relationship to climate on multiple timescales
Publisher: Wiley
Date: 13-10-2021
Rainfall stable water isotope variability in coastal southwestern Western Australia and its relationship to climate on multiple timescales
Publisher: Wiley
Date: 03-12-2021
Anomalous subtropical zonal winds drive decreases in southern Australian frontal rain
Publisher: Copernicus GmbH
Date: 31-01-2023
Abstract: Abstract. Cold fronts make a significant contribution to cool season rainfall in the extratropics and subtropics. In many regions of the Southern Hemisphere the amount of frontal rainfall has declined in recent decades, but there has been no change in frontal frequency. We show that for southeast Australia this contradiction cannot be explained by changes in frontal intensity or moisture at the latitudes of interest. Rather, declining frontal rainfall in southeast Australia is associated with weakening of the subtropical westerlies in the mid-troposphere, which is part of a hemispheric pattern of wind anomalies that modify the extratropical zonal wave 3. Fronts that generate rainfall are associated with strong westerlies that penetrate well into the subtropics, and the observed decrease in frontal rainfall in southern Australia can be linked to a decrease in the frequency of fronts with strong westerlies at 25∘ S.
Extratropical fronts in the lower troposphere-global perspectives obtained from two automated methods
Publisher: Wiley
Date: 21-11-2015
DOI: 10.1002/QJ.2471
Rainfall Stable Water Isotope Variability in Coastal Southwestern Western Australia and Its Relationship to Climate on Multiple Timescales
Publisher: American Geophysical Union (AGU)
Date: 29-12-2022
DOI: 10.1029/2021JD035433
Abstract: The factors driving variability in rainfall stable water isotopes (specifically δ 18 O and deuterium excess, d = δ 2 H − 8 δ 18 O) were studied in a 13‐year data set of daily rainfall s les from coastal southwestern Western Australia (SWWA). Backwards dispersion modeling, automatic synoptic type classification, and a statistical model were used to establish causes of variability on a daily scale and predictions from the model were aggregated to longer temporal scales to discover the cause of variability on multiple timescales. Factors differ between δ 18 O and d and differ according to temporal scale. Rainfall intensity, both at the observation site and upwind, was most important for determining δ 18 O and this relationship was robust across all time scales (daily, seasonal, and interannual) as well as generalizing to a second observation site. The sensitivity of δ 18 O to rainfall intensity makes annual mean values particularly sensitive to the year's largest events. Projecting the rainfall intensity relationship back through ∼100 years of precipitation observations can explain ∼ –0.4‰ shifts in rainfall δ 18 O. Twentieth century speleothem records from the region exhibit signals of a similar magnitude, indicating that rainfall intensity should be taken into account during the interpretation of regional climate archives. For d , humidity during evaporation from the ocean was the most important driver of variability at the daily scale, as well as explaining the seasonal cycle, but source humidity failed to explain the longer‐term interannual variability.
Are Greenhouse Gas Signals of Northern Hemisphere winter extra-tropical cyclone activity dependent on the identification and tracking algorithm?
Publisher: Schweizerbart
Date: 02-2013
Climatological study of frontal precipitation over the Mediterranean
Publisher: Copernicus GmbH
Date: 28-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-10303
Abstract: & & It is well known that severe weather and heavy precipitation are closely connected to the presence or passage of cold fronts over a region. In this study, the MedFTS_DT scheme, developed recently for the identification of cold fronts, is used to perform an objective climatological analysis of cold frontal activity and precipitation in the Mediterranean region. The MedFTS_DT algorithm has been developed for the automated and objective identification of cold fronts and optimized for the Mediterranean. It is based on a combination of wind-shift and thermal criteria. Wind-shift is applied in 6-hour intervals for the identification of cold fronts, whereas the thermal criteria are used to properly filter out any erroneous frontal identifications.& & & & In this work, the spatial distribution and frequency of cold fronts are calculated over the Mediterranean for the period 2007& #8211 on a monthly, seasonal and annual basis. The spatial distribution of the total precipitation (TP) and the frontal-induced precipitation (FP) are also calculated for the same region and temporal scales in order to determine the contribution of cold fronts to the total precipitation (FP/TP) It is observed that, in general, the local maxima of FP agree well with the corresponding maxima of frontal activity. It also becomes evident that, contrary to the TP regime, the maxima of FP are not found over the main mountain ranges of the Mediterranean regions, suggesting that orography does not play an important role in the formation of FP.& &
Reconstructing sea level pressure variability via a feature tracking approach
Publisher: American Meteorological Society
Date: 2015
Abstract: The aim of this paper is to quantify the contribution of synoptic transients to the full spectrum of space–time variability of sea level pressure (SLP) in middle latitudes. In previous work by the authors it was shown that tracking cyclones and anticyclones in an idealized atmospheric model allows one to reconstruct a surprisingly large fraction of the model’s variability, including not only synoptic components, but also its large-scale low-frequency component. Motivated by this result, the authors performed tracking of cyclones and anticyclones and estimated cyclone and anticyclone size and geometry characteristics in the observed SLP field using the 1948–2008 NCEP–NCAR reanalysis dataset. The reconstructed synoptic field was then produced via superimposing radially symmetrized eddies moving along their actual observed trajectories. It was found that, similar to earlier results for an idealized model, the synoptic reconstruction so obtained accounts for a major fraction of the full observed SLP variability across a wide range of time scales, from synoptic to those associated with the low-frequency variability (LFV). The synoptic reconstruction technique developed in this study helps elucidate connections between the synoptic eddies and LFV defined via more traditional spatiotemporal filtering. In particular, we found that the dominant variations in the position of the zonal-mean midlatitude jet are synonymous with random ultralow-frequency redistributions of cyclone and anticyclone trajectories and, hence, is inseparable of that in the storm-track statistics.
Anomalous subtropical zonal winds drive decreases in southern Australian frontal rain
Publisher: Copernicus GmbH
Date: 02-09-2022
DOI: 10.5194/WCD-2022-50
Abstract: Abstract. Cold fronts make a significant contribution to cool season rainfall in the extratropics and subtropics. In many regions of the Southern Hemisphere the amount of frontal rainfall has declined in recent decades, but there has been no change in frontal frequency. We show that for southeast Australia this contradiction cannot be explained by changes in frontal intensity or moisture at the latitudes of interest. Rather, declining frontal rainfall in southeast Australia is associated with weakening of the subtropical westerlies in the mid troposphere, which is part of a hemispheric pattern of wind anomalies that modifies the extratropical zonal wave 3. Fronts that generate rainfall are associated with strong westerlies that penetrate well into the subtropics, and the observed decrease in frontal rainfall in southern Australia can be linked to a decrease in the frequency of fronts with strong westerlies at 25° S.
IMILAST: A Community Effort to Intercompare Extratropical Cyclone Detection and Tracking Algorithms
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.
Related Organisations
Institute Of Oceanology. PP Shirshov Russian Academy Of Sciences
Location: Russian Federation
Institute Of Oceanology. PP Shirshov Russian Academy Of Sciences
Location: No location found
Related Funding Activities
No related grants have been discovered for Irina Rudeva.