ORCID Profile
0000-0002-4458-4592
Current Organisations
CSIRO
,
CSIRO Oceans and Atmosphere
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Publisher: Springer Science and Business Media LLC
Date: 17-11-2020
Publisher: American Meteorological Society
Date: 29-05-2014
DOI: 10.1175/JCLI-D-13-00527.1
Abstract: A feature of the Indian Ocean dipole (IOD) is its positive skewness, with cold IOD east pole (IODE) sea surface temperature anomalies (SSTAs) exhibiting larger litudes than warm SSTAs. Using the coupled Geophysical Fluid Dynamics Laboratory Earth System Model with Modular Ocean Model version 4 (MOM4) component (GFDL-ESM2M), the role of nonlinear feedbacks in generating this positive skewness is investigated and their response to global warming examined. These feedbacks are a nonlinear dynamic heating process, the Bjerknes feedback, wind–evaporation–SST feedback, and SST–cloud–radiation feedback. Nonlinear dynamic heating assists IOD skewness by strongly d ing warm IODE SSTAs and reinforcing cold IODE anomalies. In a warmer climate, the d ing strengthens while the reinforcement weakens. The SST–thermocline relationship is part of the positive Bjerknes feedback and contributes strongly to IOD skewness as it is weak during the development of warm IODE SSTAs, but strong during the development of cold IODE SSTAs. In response to global warming, this relationship displays weaker asymmetry associated with weaker westerly winds over the central equatorial Indian Ocean. The negative SST–cloud–radiation feedback is also asymmetric with cold IODE SSTAs less d ed by incoming shortwave radiation. Under global warming, the d ing of cold IODE SSTAs shows little change but warm IODE SSTAs become more d ed. This stronger d ing is a symptom of negative IODs becoming stronger in litude due to the mean IODE thermocline shoaling. The wind–evaporation–SST feedback does not contribute to IOD asymmetry with cold IODE SSTAs decreasing evaporation, which in turn warms the surface. However, as this study focuses on one model, the response of these feedbacks to global warming is uncertain.
Publisher: American Meteorological Society
Date: 03-2021
Abstract: El Niño–Southern Oscillation (ENSO) is the dominant mode of interannual climate fluctuations with wide-ranging socioeconomic and environmental impacts. Understanding the eastern Pacific (EP) and central Pacific (CP) El Niño response to a warmer climate is paramount, yet the role of internal climate variability in modulating their response is not clear. Using large ensembles, we find that internal variability generates a spread in the standard deviation and skewness of these two El Niño types that is similar to the spread of 17 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) that realistically simulate ENSO ersity. Based on 40 Community Earth System Model Large Ensemble (CESM-LE) and 99 Max Planck Institute for Meteorology Grand Ensemble (MPI-GE) members, unforced variability can explain more than 90% of the historical EP and CP El Niño standard deviation and all of the ENSO skewness spread in the 17 CMIP5 models. Both CESM-LE and the selected CMIP5 models show increased EP and CP El Niño variability in a warmer climate, driven by a stronger mean vertical temperature gradient in the upper ocean and faster surface warming of the eastern equatorial Pacific. However, MPI-GE shows no agreement in EP or CP standard deviation change. This is due to weaker sensitivity to the warming signal, such that when the eastern equatorial Pacific surface warming is faster, the change in upper ocean vertical temperature gradient tends to be weaker. This highlights that in idual models produce a different ENSO response in a warmer climate, and that considerable uncertainty within the CMIP5 ensemble may be caused by internal climate variability.
Publisher: Elsevier
Date: 2021
Publisher: Edward Elgar Publishing
Date: 18-10-2022
Publisher: Springer Science and Business Media LLC
Date: 13-01-2021
DOI: 10.1038/S41598-020-79655-7
Abstract: The westerly wind burst (WWB) is an important triggering mechanism of El Niño and typically occurs in the western Pacific Ocean. The Fourier spectrum of the wind field over the western tropical Pacific is characterised by a large variety of peaks distributed from intra-seasonal to decadal time scales, suggesting that WWBs could be a result of nonlinear interactions on these time scales. Using a combination of observations and simulations with 15 coupled models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we demonstrate that the main drivers initiating WWBs are quantifiable physical processes rather than atmospheric stochastic signals. In this study, ensemble empirical mode decomposition (EEMD) from the Holo-Hilbert spectral analysis (HHSA) is used to decompose daily zonal winds over the western equatorial Pacific into seasonal, interannual and decadal components. The seasonal element, with prominent spectral peaks of less than 12 months, is not ENSO related, and we find it to be strongly associated with the East Asian monsoon (EAM) and cross-equatorial flow (CEF) over the Australian monsoon region. The CEF is directly related to the intensity of the Australian subtropical ridge (STR-I). Both the EAM and CEF are essential sources of these high-frequency winds over the western Pacific. In contrast, the interannual wind component is closely related to El Niño occurrences and usually peaks approximately two months prior to a typical El Niño event. Finally, the decadal element merely represents a long-term trend and thus has little to no relation to El Niño. We identified EAM- and CEF-induced westerly wind anomalies in December–January–February (DJF) and September–October–November (SON). However, these anomalies fade in March–April–May (MAM), potentially undermining the usual absence of WWBs in the boreal spring. Similar results are found in CMIP6 historical scenario data.
Publisher: Springer Science and Business Media LLC
Date: 17-08-2021
Publisher: American Geophysical Union (AGU)
Date: 24-03-2021
DOI: 10.1029/2020GL092223
Abstract: The 2018 Central Pacific (CP) El Niño preceded the 2019 strong positive Indian Ocean Dipole (pIOD). The robustness of a CP El Niño preconditioning a strong pIOD has not been investigated. Here, we show that March‐April‐May (MAM) easterly anomalies over the equatorial eastern Indian Ocean (EIO) induced by a preceding CP El Niño drive a thermocline shallowing conducive to an early equatorial EIO cooling. However, the same winds also generate off‐equatorial downwelling Rossby waves in the southern EIO, which reflect as downwelling Kelvin waves, able to weaken the initial anomalies. Furthermore, zonal winds in June‐July‐August (JJA) can either be beneficial or unfavorable for the developing pIOD. Only when the equatorial easterlies, EIO cooling, and thermocline shallowing are sufficiently lified by JJA does a strong pIOD occur. Despite this, a multi‐century model simulation suggests that development of majority of strong pIOD events is facilitated by a preceding CP El Niño.
Publisher: Springer Science and Business Media LLC
Date: 30-11-2020
Publisher: Springer Science and Business Media LLC
Date: 15-05-2014
DOI: 10.1038/SREP04943
Publisher: Springer Science and Business Media LLC
Date: 09-2022
Publisher: American Association for the Advancement of Science (AAAS)
Date: 03-2019
Abstract: The El Niño-Southern Oscillation (ENSO), which originates in the Pacific, is the strongest and most well-known mode of tropical climate variability. Its reach is global, and it can force climate variations of the tropical Atlantic and Indian Oceans by perturbing the global atmospheric circulation. Less appreciated is how the tropical Atlantic and Indian Oceans affect the Pacific. Especially noteworthy is the multidecadal Atlantic warming that began in the late 1990s, because recent research suggests that it has influenced Indo-Pacific climate, the character of the ENSO cycle, and the hiatus in global surface warming. Discovery of these pantropical interactions provides a pathway forward for improving predictions of climate variability in the current climate and for refining projections of future climate under different anthropogenic forcing scenarios.
Publisher: Springer Science and Business Media LLC
Date: 10-04-2020
Publisher: Springer Science and Business Media LLC
Date: 20-02-2023
DOI: 10.1038/S41558-023-01610-X
Abstract: Antarctic shelf ocean warming affects melt of ice shelf/sheets and sea ice but projected changes vary vastly across climate models. A projected increase in El Niño variability has been found to slow future mid-latitude Southern Ocean warming but how this impacts the Antarctic shelf ocean is unknown. Here we show that a projected increase in El Niño variability accelerates Antarctic shelf ocean warming, hastening ice shelf/sheet melt but slowing sea ice reduction.
Publisher: Wiley
Date: 09-09-2014
DOI: 10.1002/JOC.4148
Publisher: American Geophysical Union (AGU)
Date: 28-05-2019
DOI: 10.1029/2019GL082497
Publisher: Springer Science and Business Media LLC
Date: 10-09-2018
DOI: 10.1038/S41598-018-31842-3
Abstract: The Indian Ocean Dipole (IOD) is the dominant mode of interannual variability over the tropical Indian Ocean (IO) and its future changes are projected to impact the climate and weather of Australia, East Africa, and Indonesia. Understanding the response of the IOD to a warmer climate has been largely limited to studies of in idual coupled general circulation models or multi-model ensembles. This has provided valuable insight into the IOD’s projected response to increasing greenhouse gases but has limitations in accounting for the role of internal climate variability. Using the Community Earth System Model Large Ensemble (CESM-LE), the IOD is examined in thirty-five present-day and future simulations to determine how internal variability influences properties of the IOD and their response to a warmer climate. Despite small perturbations in the initial conditions as the only difference between ensemble members, significant relationships between the mean state of the IO and the IOD arise, leading to a spread in the projected IOD responses to increasing greenhouse gases. This is driven by the positive Bjerknes feedback, where small differences in mean thermocline depth, which are caused by internal climate variability, generate significant variations in IOD litude, skewness, and the climatological zonal sea surface temperature gradient.
Publisher: American Meteorological Society
Date: 27-03-2015
DOI: 10.1175/JCLI-D-14-00661.1
Abstract: The tropical Indian Ocean has experienced a faster warming rate in the west than in the east over the twentieth century. The warming pattern resembles a positive Indian Ocean dipole (IOD) that is well captured by climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), forced with the two main anthropogenic forcings, long-lived greenhouse gases (GHGs), and aerosols. However, much less is known about how GHGs and aerosols influence the IOD asymmetry, including the negative sea surface temperature (SST) skewness in the east IOD pole (IODE). Here, it is shown that the IODE SST negative skewness is more enhanced by aerosols than by GHGs using single-factor forcing experiments from 10 CMIP5 models. Aerosols induce a greater mean zonal thermocline gradient along the tropical Indian Ocean than that forced by GHGs, whereby the thermocline is deeper in the east relative to the west. This generates strong asymmetry in the SST response to thermocline anomalies between warm and cool IODE phases in the aerosol-only experiments, enhancing the negative IODE SST skewness. Other feedback processes involving zonal wind, precipitation, and evaporation cannot solely explain the enhanced SST skewness by aerosols. An interexperiment comparison in one model with strong skewness confirms that the mean zonal thermocline gradient across the Indian Ocean determines the magnitude of the SST–thermocline asymmetry, which in turn controls the SST skewness strength. The findings suggest that as aerosol emissions decline and GHGs increase, this will likely contribute to a future weakening of the IODE SST skewness.
Publisher: American Geophysical Union (AGU)
Date: 18-01-2022
DOI: 10.1029/2021GL096887
Abstract: The tropical Pacific Ocean interacts with the tropical Atlantic Ocean on interannual time scales. However, most coupled climate models misrepresent and/or underestimate these teleconnections, which have important implications on the periodicity of the El Niño‐Southern Oscillation (ENSO). Imposing the observed sea surface temperature over the tropical Atlantic from 1970 onwards, a pacemaker experiment is constructed using the second version of the Australian Community Climate and Earth System Simulator Coupled Model (ACCESS‐CM2). Compared with control runs which reasonably simulate the interaction between ENSO and the north tropical Atlantic variability, the pacemaker experiment significantly improves the impact of the Atlantic Niño/Niña on ENSO. This enhanced teleconnection contributes to a more realistic ENSO periodicity compared with the control runs, thus suggesting that capturing the influence from equatorial Atlantic variability is important for an improved ENSO simulation.
Publisher: Springer Science and Business Media LLC
Date: 18-02-2021
Publisher: Springer Science and Business Media LLC
Date: 08-06-2020
DOI: 10.1038/S41467-020-16676-W
Abstract: The severe drought of the 1930s Dust Bowl decade coincided with record-breaking summer heatwaves that contributed to the socio-economic and ecological disaster over North America’s Great Plains. It remains unresolved to what extent these exceptional heatwaves, hotter than in historically forced coupled climate model simulations, were forced by sea surface temperatures (SSTs) and exacerbated through human-induced deterioration of land cover. Here we show, using an atmospheric-only model, that anomalously warm North Atlantic SSTs enhance heatwave activity through an association with drier spring conditions resulting from weaker moisture transport. Model devegetation simulations, that represent the wide-spread exposure of bare soil in the 1930s, suggest human activity fueled stronger and more frequent heatwaves through greater evaporative drying in the warmer months. This study highlights the potential for the lification of naturally occurring extreme events like droughts by vegetation feedbacks to create more extreme heatwaves in a warmer world.
Publisher: Springer Science and Business Media LLC
Date: 02-09-2020
Publisher: Springer Science and Business Media LLC
Date: 27-06-2022
DOI: 10.1038/S41558-022-01398-2
Abstract: The Southern Ocean is a primary heat sink that buffers atmospheric warming and has warmed substantially, accounting for an outsized portion of global warming-induced excess heat in the climate system. However, its projected warming is highly uncertain and varies substantially across climate models. Here, using outputs from Coupled Model Intercomparison Project phase six models, we show that Southern Ocean warming during the twenty-first century is linked to the change in litude of the El Niño–Southern Oscillation (ENSO). Models simulating a larger increase in ENSO litude systematically produce a slower Southern Ocean warming conversely, a smaller increase in ENSO litude sees a stronger warming. The asymmetry in litude and teleconnection between El Niño and La Niña produce cumulative surface wind anomalies over the southern high latitudes, impacting Southern Ocean heat uptake. The magnitude of inter-model ENSO variations accounts for about 50% of the uncertainty in the projected Southern Ocean warming.
Publisher: Proceedings of the National Academy of Sciences
Date: 31-05-2022
Abstract: The western Pacific subtropical high (WPSH) channels moisture from the tropics that underpins the East Asian summer climate. Interannual variability of the WPSH dominates climate extremes in the densely populated countries of East Asia. In 2020, an anomalously strong WPSH led to catastrophic floods with hundreds of deaths, 28,000 homes destroyed, and tens of billions in economic damage in China alone. How the frequency of such strong WPSH events will change is of great societal concern. Our finding of an increase in future WPSH variability, translating into an increased frequency of climate extreme as seen in the 2020 episode, highlights the increased risks for the billions of people in the densely populated East Asia with profound socioeconomic consequences.
Publisher: American Geophysical Union (AGU)
Date: 11-08-2020
DOI: 10.1029/2020GL089396
Abstract: A positive Indian Ocean Dipole (pIOD) refers to a sea surface temperature anomaly pattern with cold anomalies in the equatorial eastern Indian Ocean and warm anomalies in the west, leading to floods in the eastern African countries and droughts and bushfires in Indonesia and Australia. The pIOD displays strong inter‐event differences, ranging from an extreme event dominated by westward‐extended strong cold anomalies along the equator, to a moderate event with weakened cooling confined to region off Sumatra‐Java. Representation of the extreme pIOD varies vastly across ocean temperature products. Here we show that products generated in a system explicitly involving subsurface oceanic processes capture the nonlinear dynamics of the extreme pIOD, i.e., the equatorial nonlinear zonal and vertical advection, and systematically produce a more realistic extreme pIOD. Thus, our study identifies ocean temperature products that are more suitable for studying extreme pIOD and its climatic impacts.
Publisher: Springer Science and Business Media LLC
Date: 18-05-2023
Publisher: Springer Science and Business Media LLC
Date: 26-06-2015
DOI: 10.1038/SREP11697
Abstract: Under global warming, climate models show an almost three-fold increase in extreme positive Indian Ocean Dipole (pIOD) events by 2100. These extreme pIODs are characterised by a westward extension of cold sea surface temperature anomalies (SSTAs) which push the downstream atmospheric convergence further west. This induces severe drought and flooding in the surrounding countries, but the processes involved in this projected increase have not been fully examined. Here we conduct a detailed heat budget analysis of 19 models from phase 5 of the Coupled Model Intercomparison Project and show that nonlinear zonal and vertical heat advection are important for reinforcing extreme pIODs. Under greenhouse warming, these nonlinear processes do not change significantly in litude, but the frequency of occurrences surpassing a threshold increases. This is due to the projected weakening of the Walker circulation, which leads to the western tropical Indian Ocean warming faster than the east. As such, the magnitude of SSTAs required to shift convection westward is relatively smaller, allowing these convection shifts to occur more frequently in the future. The associated changes in wind and ocean current anomalies support the zonal and vertical advection terms in a positive feedback process and consequently, moderate pIODs become more extreme-like.
Publisher: Springer Science and Business Media LLC
Date: 12-08-2014
DOI: 10.1038/SREP06034
Publisher: Springer Science and Business Media LLC
Date: 31-01-2022
DOI: 10.1038/S41558-022-01282-Z
Abstract: Sea surface temperature (SST) variability of El Niño–Southern Oscillation (ENSO) underpins its global impact, and its future change is a long-standing science issue. In its sixth assessment, the IPCC reports no systematic change in ENSO SST variability under any emission scenarios considered. However, comparison between the 20th and 21st century shows a robust increase in century-long ENSO SST variability under four IPCC plausible emission scenarios.
Publisher: American Geophysical Union (AGU)
Date: 21-09-2020
DOI: 10.1029/2020GL090079
Abstract: The 2019 positive Indian Ocean Dipole (IOD) was the strongest event since the 1960s which developed independently without coinciding El Niño. The dynamics is not fully understood. Here we show that in March–May, westward propagating oceanic Rossby waves, a remnant consequence of the weak 2018 Pacific warm condition, led to anomalous sea surface temperature warming in the southwest tropical Indian Ocean (TIO), inducing deep convection and anomalous easterly winds along the equator, which triggered the initial cooling in the east. In June–August, the easterly wind anomalies continued to evolve through ocean‐atmosphere coupling involving Bjerknes feedback and equatorial nonlinear ocean advection, until its maturity in September–November. This study clarifies the contribution of oceanic Rossby waves in the south TIO in different dynamic settings and reveals a new triggering mechanism for extreme IOD events that will help to understand IOD ersity.
Publisher: American Geophysical Union (AGU)
Date: 15-11-2016
DOI: 10.1002/2016GL071208
Abstract: A prominent feature of the Indian Ocean Dipole (IOD) is its positive skewness, where positive phases tend to be stronger in litude than the negative phase. Positive IOD events are associated with devastating floods over parts of East Africa and India, while Australia and Indonesia experience dry conditions. Under greenhouse warming, climate models project a weakening of the positive IOD skewness, but their simulation of present‐day skewness is too weak. Here we show that this bias and the projected skewness change are related to the simulation of the climatological zonal wind in the central equatorial Indian Ocean. In particular, models with overly weak present‐day westerlies, which is a common model bias, generate overly weak present‐day skewness and a smaller projected reduction in skewness. Improving the ability of models in simulating stronger westerly winds may lead to stronger present‐day simulated skewness and a larger skewness reduction in a warmer climate.
Location: Australia
No related grants have been discovered for Benjamin Ng.