ORCID Profile
0000-0002-4584-0885
Current Organisation
Monash University
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Tropospheric and Stratospheric Physics | Atmospheric Dynamics | Atmospheric Sciences | Climate Change Processes
Atmospheric Processes and Dynamics | Climate Change Models |
Publisher: American Geophysical Union (AGU)
Date: 16-04-2019
DOI: 10.1029/2019GL082089
Abstract: Using a high‐resolution global model with explicit representation of convection, the physical processes involved in the abrupt onset of South Asian summer monsoon are investigated within a moist entropy budget framework. The monsoon onset is a two‐stage transition. During the first stage, which starts around 2 months before onset, the source terms of column‐integrated moist entropy gradually increase, while the export by the large‐scale circulation slowly strengthens. The second stage is marked by a sudden increase of radiative heating and surface latent heat flux 10 days prior to the onset, followed by abrupt strengthening of large‐scale export of moist entropy. When either cloud‐radiative or wind‐evaporation feedback is disabled in numerical experiments, the monsoon experiences much smoother and weaker onset. The evolution of the system in a gross moist stability plane demonstrates that these positive feedbacks destabilize the system and are responsible for the abruptness of the transition.
Publisher: American Meteorological Society
Date: 24-06-2019
Abstract: The role of planetary rotation in limiting the extent of the cross-equatorial solsticial Hadley cell (SHC) is investigated using idealized simulations with an aquaplanet general circulation model run under perpetual-solstice conditions. Consistent with previous studies that include a seasonal cycle, the SHC extent increases with decreasing rotation rate, and it occupies the entire globe for sufficiently low planetary rotation rates. A simple theory for the summer-hemisphere extent of the SHC is constructed in which it is assumed that the SHC occupies regions for which a hypothetical radiative–convective equilibrium state is physically unattainable. The theory predicts that the SHC extends farther into the summer hemisphere as the rotation rate is decreased, qualitatively reproducing the behavior of the simulations, but it generally underestimates the simulated SHC extent. A diagnostic theory for the summer-hemisphere SHC extent is then developed based on the assumptions of slantwise convective neutrality and conservation of angular momentum within the Hadley cell. The theory relates the structure of the SHC in the summer hemisphere to the distribution of boundary layer entropy in the dynamically equilibrated simulations. The resultant diagnostic for the SHC extent generalizes the convective quasi-equilibrium-based constraint of Privé and Plumb, in which the position of rain belts is related to maxima in the low-level entropy distribution.
Publisher: Wiley
Date: 04-06-2015
DOI: 10.1002/QJ.2567
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-2469
Abstract: The Australian monsoon's wet season is associated with sequences of wet and dry conditions known as bursts and breaks, which usually have timescales of a week or two. There are several hypotheses for the physical processes involved in monsoon bursts, ranging from the effects of the Madden-Julian Oscillation to extratropical influences.We analyse rainfall bursts in Northern Australia using a moist static energy (MSE) budget framework. First, we separate the bursts into pre-monsoon, monsoon, and post-monsoon based on simple monsoon onset and retreat criteria. We then apply ERA5 data to calculate the MSE budget for each burst and construct composite bursts for each of the three types.We find that the horizontal advection of MSE over the tropical northern Australian convergence zone is the most critical term in the budget for the day-to-day precipitation variation. An analysis of the MSE-related gross moist stability (GMS) reveals that the GMS framework is able to predict periods of convective growth and decay before and after monsoon bursts, with the exception of the pre-monsoon bursts which do not follow the characteristic evolution of tropical convective systems. We hypothesise that this is because pre-monsoon bursts have a stronger extratropical influence. We find that the growth phase of convection in monsoon and post-monsoon bursts is associated with a notable reduction of the advection of dry air into the monsoon region. We show that this is likely the result of a rearrangement of the circulation ahead of the burst. &
Publisher: American Meteorological Society
Date: 12-2011
Abstract: An idealized model of advection and condensation of water vapor is considered as a representation of processes influencing the humidity distribution along isentropic surfaces in the free troposphere. Results are presented for how the mean relative humidity distribution varies in response to changes in the distribution of saturation specific humidity and in the litude of a tropical moisture source. Changes in the tropical moisture source are found to have little effect on the relative humidity poleward of the subtropical minima, suggesting a lack of poleward influence despite much greater water vapor concentrations at lower latitudes. The subtropical minima in relative humidity are found to be located just equatorward of the inflection points of the saturation specific humidity profile along the isentropic surface. The degree of mean subsaturation is found to vary with the magnitude of the meridional gradient of saturation specific humidity when other parameters are held fixed. The atmospheric relevance of these results is investigated by comparison with the positions of the relative humidity minima in reanalysis data and by examining poleward influence of relative humidity in simulations with an idealized general circulation model. It is suggested that the limited poleward influence of relative humidity may constrain the propagation of errors in simulated humidity fields.
Publisher: American Meteorological Society
Date: 15-07-2022
Abstract: A simple steady-state model is constructed for the interaction between moist convection and large-scale ascent in the tropics. The model is based on a bulk-plume representation of convection, and it is coupled to the large-scale circulation using methods developed for limited-area numerical models that are consistent with the weak temperature gradient approximation. Given the midtropospheric temperature anomaly in the ascent region, the model solves for the profiles of temperature, relative humidity, and large-scale vertical velocity in this region, as well as the tropical-mean profiles of temperature and relative humidity, as a function of two parameters representing the importance of entrainment and condensate re-evaporation in moist convection. According to the simple model, the ascent region is characterized by an anomalously moist and stable free troposphere with a top-heavy vertical velocity profile that peaks in the upper troposphere. These results are shown to be consistent with simulations using a cloud system–resolving model in which the large-scale circulation is parameterized. Furthermore, it is shown that, due to the effect of entrainment on the tropospheric lapse rate, the predicted vertical velocity profile is more top-heavy than the first-baroclinic mode profile used in previous reduced-complexity models of tropical dynamics. The simple model therefore provides a framework to link mixing and microphysical processes in moist convection to the large-scale structure of the tropical overturning circulation.
Publisher: American Geophysical Union (AGU)
Date: 12-2019
DOI: 10.1029/2019MS001686
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-3532
Abstract: Radiative-convective equilibrium is the simplest possible way to phrase many questions about a deep-convecting atmosphere and is accessible by a wide range of model types. The radiative-convective equilibrium project (RCEMIP) provides a common configuration, but reveals a large spread in the simulated climate across models, including profiles of temperature and relative humidity. Here we use simple models and theory to understand the intermodel spread in CAPE, relative humidity, and their responses to warming.Across the RCEMIP ensemble, temperature profiles are systematically cooler than a moist adiabat, consistent with theory that they are set by dilute ascent. As horizontal grid spacing is reduced in models with explicit convection from 1 km to 200 m, CAPE and relative humidity increase. Across all models, CAPE increases with warming at a rate (14-19%/K) greater than that expected from& the& Clausius-Clapeyron& relation. We find that there is higher CAPE (greater instability) in models that are on average moister in the mid-troposphere, which is consistent with the simple plume model of Romps (2016) in which both instability and relative humidity depend on entrainment and precipitation efficiency. The sign of the relationship suggests that differences in entrainment drive& the intermodel spread. This relationship is true across both models with explicit and parameterized convection.To more explicitly evaluate the drivers of the intermodel spread, we use the Romps (2016) model to diagnose theory-implied values of entrainment and precipitation efficiency given the simulated values of CAPE and relative humidity. We then decompose the the variability across models in CAPE and relative humidity (and their responses to warming) into contributions from entrainment, precipitation efficiency, and the depth of the convecting layer. Targeted microphysics parameter perturbation experiments with an in idual cloud-resolving model in which precipitation efficiency is varied and explicitly diagnosed provide proof of concept for this decomposition technique.&
Publisher: American Geophysical Union (AGU)
Date: 27-05-2019
DOI: 10.1029/2018JD030092
Publisher: American Meteorological Society
Date: 06-2016
Abstract: The influence of eddy momentum fluxes on the equinoctial Hadley circulation is explored using idealized simulations on an equatorial beta plane in which the sea surface temperature (SST) distribution is fixed. By comparing simulations run in a wide-domain configuration, in which large-scale eddies are present, to simulations in which the model domain is too narrow to permit baroclinic instability, the role of large-scale eddies in determining the characteristics of the Hadley circulation is elucidated. The simulations also include an explicit representation of deep convection, allowing for an evaluation of the influence of convective momentum transport on the zonal-mean circulation. The simulated eddy momentum fluxes are much weaker in the narrow-domain configuration than in the wide-domain case, and convective momentum transport is found to be of secondary importance. As a result, many characteristics of the narrow-domain Hadley circulation are well described by axisymmetric theory and differ from those of the wide-domain case. Nevertheless, the strength of the Hadley circulation is similar irrespective of the domain width. The sensitivity of this result to the strength of the eddy forcing is investigated using narrow-domain simulations forced by artificial sinks of zonal momentum. As the magnitude of the momentum sink increases, the Hadley circulation strengthens, but the increase is relatively modest except at very strong forcing magnitudes. The results suggest that the fixed-SST boundary condition places a strong thermodynamic constraint on the Hadley circulation strength and that one should consider the energy budget as well as the angular momentum budget in order to fully understand the influence of large-scale eddies on the zonal-mean circulation in the tropics.
Publisher: American Meteorological Society
Date: 09-2018
Publisher: American Geophysical Union (AGU)
Date: 27-11-2019
DOI: 10.1029/2019GL085031
Abstract: Precipitation efficiency refers to the fraction of condensate in the atmosphere that reaches the surface as precipitation. A high‐quality data set of radar‐estimated precipitation rates and convective scale vertical velocity near Darwin, Australia, is used to construct the first estimate of precipitation efficiency at convective scales for a long record of observations in the tropics. It is found that precipitation efficiency increases with precipitation rate and midtropospheric humidity and decreases with increasing convective available potential energy and surface temperature. Precipitation efficiency is largest under moist monsoonal conditions and smallest during monsoon break periods, which are characterized by a drier free troposphere. However, these differences in efficiency do not translate to differences in the instantaneous precipitation rate across the synoptic regimes because of a compensating change in the net condensation rate. This is driven by variations in cloud updraft velocity, which is larger in drier environments than in moist environments.
Publisher: Wiley
Date: 27-12-2017
DOI: 10.1002/QJ.3195
Publisher: American Geophysical Union (AGU)
Date: 27-01-2022
DOI: 10.1029/2021JD035388
Abstract: Deep moist convection is responsible for a large fraction of rainfall in the tropics, but the interaction between deep convection and the large‐scale atmosphere remains poorly understood. Here, we apply machine learning techniques to examine relationships between the large‐scale state of the atmosphere and two measures of its convective state derived from radar observations in northern Australia: the total area occupied by deep convection and the degree of deep convective organization. Specifically, we use a neural net to predict convective area and convective organization as a function the large‐scale state, defined as the thermodynamic and dynamic properties of the atmosphere averaged over the radar domain. Building on research into explainable artificial intelligence, we apply so‐called “attribution methods” to quantify the most important large‐scale quantities determining these predictions. We find that the large‐scale vertical velocity is the most important contributor to the prediction of both convective measures, but for convective area, its absolute and relative influence are increased. Thermodynamic quantities like atmospheric moisture also contribute to the prediction of convective area, but they are found to be unimportant for convective organization. Instead, the horizontal wind field appears to be more relevant for the prediction of convective organization. The results highlight unique aspects of the large‐scale state that are associated with organized convection.
Publisher: American Meteorological Society
Date: 25-01-2017
Abstract: The strength of the equinoctial Hadley circulation (HC) is investigated in idealized simulations conducted on an equatorial beta plane in which the zonal width of the domain is varied to either permit or suppress large-scale eddies. The presence of such eddies is found to lify the HC by a factor of 2–3 in simulations with slab-ocean boundary conditions or with a simple representation of ocean heat transport. Additional simulations in which the eddy forcing is prescribed externally indicate that this lification is primarily associated with large-scale eddy momentum fluxes rather than large-scale eddy heat fluxes. These results contrast with results from simulations with fixed distributions of sea surface temperature (SST), in which the HC strength has been found to be relatively insensitive to large-scale eddy momentum fluxes. In both the interactive- and fixed-SST cases, the influence of nonlinear momentum advection by the mean flow complicates efforts to use the angular-momentum budget to constrain the HC strength. However, a strong relationship is found between the HC strength and a measure of the meridional gradient of boundary layer entropy, indicating a possible thermodynamic control on the HC strength. In simulations with interactive SSTs, meridional eddy momentum fluxes affect the boundary layer entropy by inducing a low-level frictional flow that reduces the ability of the HC to transport heat poleward. This allows for the maintenance of a large meridional entropy gradient in the presence of a strong HC. The results highlight the potential utility of a thermodynamic perspective for understanding the HC in flow regimes for which dynamical constraints may be difficult to apply.
Publisher: American Meteorological Society
Date: 06-09-2023
Publisher: American Geophysical Union (AGU)
Date: 09-2020
DOI: 10.1029/2020MS002138
Abstract: The Radiative‐Convective Equilibrium Model Intercomparison Project (RCEMIP) is an intercomparison of multiple types of numerical models configured in radiative‐convective equilibrium (RCE). RCE is an idealization of the tropical atmosphere that has long been used to study basic questions in climate science. Here, we employ RCE to investigate the role that clouds and convective activity play in determining cloud feedbacks, climate sensitivity, the state of convective aggregation, and the equilibrium climate. RCEMIP is unique among intercomparisons in its inclusion of a wide range of model types, including atmospheric general circulation models (GCMs), single column models (SCMs), cloud‐resolving models (CRMs), large eddy simulations (LES), and global cloud‐resolving models (GCRMs). The first results are presented from the RCEMIP ensemble of more than 30 models. While there are large differences across the RCEMIP ensemble in the representation of mean profiles of temperature, humidity, and cloudiness, in a majority of models anvil clouds rise, warm, and decrease in area coverage in response to an increase in sea surface temperature (SST). Nearly all models exhibit self‐aggregation in large domains and agree that self‐aggregation acts to dry and warm the troposphere, reduce high cloudiness, and increase cooling to space. The degree of self‐aggregation exhibits no clear tendency with warming. There is a wide range of climate sensitivities, but models with parameterized convection tend to have lower climate sensitivities than models with explicit convection. In models with parameterized convection, aggregated simulations have lower climate sensitivities than unaggregated simulations.
Publisher: Copernicus GmbH
Date: 28-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-10456
Abstract: & & Tropical modes of variability, such as El Ni& #241 o& #8211 Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), exert a strong influence on the interannual variability of Australian precipitation. Nevertheless, commonly used indices of ENSO and IOD variability display significant co-variability that prevents a robust quantification of the independent contribution of each mode to precipitation anomalies. This co-variability issue is often addressed by statistically removing ENSO or IOD variability from the precipitation field before calculating teleconnection patterns. However, by performing a suite of coupled and uncoupled modelling experiments in which either ENSO or IOD variability is physically removed, we show that ENSO-only-driven precipitation patterns computed by statistically removing the IOD influence significantly underestimate the impact of ENSO on Australian precipitation variability. Inspired by this, we propose a conceptual model that allows& one& to effectively separate the contribution of each mode to Australian precipitation variability.& &
Publisher: American Geophysical Union (AGU)
Date: 31-07-2016
DOI: 10.1002/2016MS000673
Publisher: American Geophysical Union (AGU)
Date: 27-08-2013
DOI: 10.1002/GRL.50796
Publisher: American Geophysical Union (AGU)
Date: 10-2020
DOI: 10.1029/2020MS002086
Abstract: There is currently no theory for the rate of tropical cyclone (TC) formation given a particular climate, so our understanding of the relationship between TC frequency and large‐scale environmental factors is largely empirical. Here, we explore the sensitivity of TC formation and intensification rates to climate warming in a series of highly idealized cloud‐permitting simulations, in which TCs form spontaneously from a base state of rest on an f ‐plane. The simulations reveal a nonmonotonic relationship between the time taken for a TC precursor disturbance (a “seed”) to form and the prescribed sea surface temperature (SST), with moderately long seed emergence times at both ends of the SST range tested (292 and 304 K) and a shorter seed emergence time at the middle value of SST (298 K). Genesis potential indices (GPIs) exhibit a different response to warming: either a monotonic increase if the potential intensity and midtropospheric relative humidity are used or relatively little sensitivity if the saturation deficit is used as the humidity variable. The sensitivity of elapsed time between a TC seed disturbance and TC genesis to surface warming is, however, generally well captured by GPIs, especially those that depend on the saturation deficit. The maximum intensification rate of TCs increases strongly with warming, particularly during the second half of the intensification process. Notably, storms intensify much more rapidly with increasing temperature than is predicted by extant theory based on potential intensity, suggesting that TCs in a warmer climate may intensify even more rapidly than recent studies suggest.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-3078
Abstract: The vertical structure of large-scale tropical circulations is determined by their complex coupling with both deep and shallow convection. This is reflected in our theoretical frameworks for understanding the tropical precipitation distribution, which consist of & #8220 deep& #8221 theories that focus on overturning circulations that extend throughout the depth of the troposphere and & #8220 shallow& #8221 theories that focus on low-level convergence driven by boundary-layer pressure gradients. While both types of theories suggest links between low-level thermodynamic fields and the precipitation distribution, shallow theories highlight the importance of the distribution of surface temperature, while deep theories additionally highlight the importance of the low-level humidity.& Here we use idealised cloud-permitting simulations to elucidate the physical factors that control the vertical structure of tropical circulations. We first demonstrate how the influence of convective entrainment on the lapse rate can act to change the vertical structure of deep tropical circulations, with implications for the behaviour of precipitation in the current and future climate. We further investigate the interaction between deep and shallow tropical circulations & by simulating an idealised overturning circulation over varying surface conditions. By independently varying the sea-surface temperature and moisture availability, the low-level temperature and moisture distributions are manipulated such that the predictions of & #8220 deep" and & #8220 shallow" theories of the circulation may be distinguished. The results provide insight into the relative roles of oceanic SST gradients and land-ocean contrasts in determining the climatological precipitation distribution in the tropics.
Publisher: American Physical Society (APS)
Date: 06-01-2022
Publisher: American Geophysical Union (AGU)
Date: 22-08-2014
DOI: 10.1002/2014GL061222
Publisher: American Geophysical Union (AGU)
Date: 04-05-2022
DOI: 10.1029/2021GL097295
Abstract: Convective aggregation refers to the clustering of convective events and occurs on a wide range of spatial scales. It has been suggested that the behavior of convective aggregation may change under global warming, with potential implications for future changes in precipitation extremes. Here, convective regions of the tropics are defined from a percentile threshold on gridded daily precipitation data and used to quantify large‐scale convective aggregation in an ensemble of global climate models. Applying three separate indices for aggregation, it is found that large‐scale convective aggregation increases in 17 of the 19 analyzed models under future warming. However, aggregation is found not to be correlated with tropical‐mean precipitation extremes, either climatologically or with respect to the sensitivity to warming. The large model spread in aggregation indices across the ensemble suggests the possible utility of large‐scale convective aggregation as a target for model evaluation.
Publisher: AIP Publishing
Date: 07-2022
DOI: 10.1063/PT.3.5038
Abstract: To understand Earth’s climate, think of it as a giant, planetary-scale heat engine that drives the circulation of the oceans and atmosphere.
Publisher: Proceedings of the National Academy of Sciences
Date: 16-10-2017
Abstract: A substantial fraction of the world’s most intense thunderstorms occur in the tropics and subtropics, but the response of such storms to climate change remains uncertain. Here, we show that, in simulations of global warming, a measure of the energy available to thunderstorms increases robustly across the tropics and subtropics. Furthermore, we elucidate an important mechanism contributing to such increases in available energy, and we present observational evidence that this mechanism is present in Earth’s atmosphere. By combining theory, observations, and models, our results provide confidence in climate model projections of future intense thunderstorm potential such model projections are shown to imply large future increases in the frequency of damaging thunderstorm environments in tropical and subtropical regions.
Publisher: American Geophysical Union (AGU)
Date: 03-2020
DOI: 10.1029/2019MS001734
Abstract: Small‐domain cloud‐resolving model and single‐column model simulations have historically applied one of three representations of large‐scale vertical motion, . In simulations of radiative‐convective equilibrium, , and a balance develops between convective heating and radiative cooling. Under the weak‐temperature gradient approximation and related approaches, is diagnosed based on the model's thermodynamic profile. Finally, for real‐case simulations, may be prescribed as a time‐varying field derived from observations. Here, we propose one additional setup, namely, a prescribed but time‐invariant vertical motion. In this case, the atmosphere evolves toward an equilibrium state characterized by a three‐way balance between radiative and adiabatic cooling and convective heating, with the relative contribution of radiation decreasing with increasing . We refer to this state as radiative‐convective‐dynamical equilibrium (RCDE). In this preliminary study we highlight the characteristics of the RCDE state through a suite of simulations performed with a single cloud‐resolving model and single‐column model. An appealing aspect of these simulations is the wide variety of equilibrium states achieved, ranging from dry and strongly unstable for small to approximately moist neutral for large . This makes RCDE a propitious framework for future model intercomparisons.
Publisher: American Geophysical Union (AGU)
Date: 09-05-2013
DOI: 10.1002/GRL.50328
Publisher: American Geophysical Union (AGU)
Date: 2019
DOI: 10.1029/2018MS001451
Publisher: American Meteorological Society
Date: 15-09-2022
Abstract: A cloud-resolving model (CRM) is used to investigate how a prototype tropical circulation driven by a sea surface temperature (SST) contrast changes in a warmer climate. The CRM is used to simulate a region of the atmosphere with a positive SST anomaly, and the large-scale circulation in this region is represented using the weak temperature gradient (WTG) and d ed gravity wave (DGW) parameterizations, where the large-scale vertical velocity within the domain is related to the deviation of the simulated density profile from a reference profile representative of the tropical mean state. The behavior of the circulation in response to an increase in SST of both the domain and reference state (i.e., uniform warming) is examined. While the vertical velocity shows an increase in its maximum strength with warming, its value in the lower to midtroposphere decreases. Since the water vapor concentration is largest in the lower troposphere, this leads to a dynamic weakening of precipitation under warming. To understand these results, a simple model for the thermodynamic structure of a convecting atmosphere based on a bulk entraining plume is employed. The model uses a fixed entrainment rate and the relative humidity profiles from the CRM to predict the temperature profiles of the domain and reference state. The vertical velocity profiles calculated from these predicted temperature profiles reproduce important aspects of those simulated with the CRM. This simple modeling framework reveals that the effect of entrainment is crucial to understanding the dynamic response of precipitation to warming, providing a stepping stone to understanding the factors driving changes to the tropical precipitation distribution in a future warmer climate.
Publisher: American Geophysical Union (AGU)
Date: 23-06-2023
DOI: 10.1029/2022JD038196
Abstract: Cold surges are synoptic weather systems that occur over the Maritime Continent during the boreal winter. They are characterized by the strengthening of prevailing low‐level northerly to north‐easterly winds, temperature falls of a few degrees over several days, and in some cases, heavy prolonged rainfall and flooding. We investigate the synoptic structure and development of cold surges through composites of dry, moderate and wet surges. Each surge category is defined by the distribution of precipitation averaged within a specified domain over the equatorial South China Sea. Dry surges are characterized by cross‐equatorial flow and positive mean sea‐level pressure anomalies that reach the Southern Hemisphere, and enhanced descent or weaker ascent. Wet surges are characterized by limited cross‐equatorial flow, low‐pressure anomalies over the equator, and enhanced moisture and ascent. The local diurnal precipitation patterns are consistent with the convection being controlled by the mid‐tropospheric buoyancy of an idealized entraining plume. This buoyancy diagnostic suggests that wet surges are characterized by a moister free troposphere because this reduces the effect of entrainment and allows deep convection to develop. On the large scale, the environment in which wet or dry cold surges form is closely related to the Madden‐Julian Oscillation (MJO) phase. Dry and moderate surges occur predominantly in the suppressed phases of the MJO, while wet surges often herald the eastward propagation of the MJO convective envelope. Finally, convectively active phases in the Australian monsoon are more likely following dry cold surges, and are sometimes associated with the MJO.
Publisher: American Meteorological Society
Date: 12-2015
DOI: 10.1175/JCLI-D-14-00816.1
Abstract: Cyclones are a key element of extratropical weather and frequently lead to extreme events like wind storms and heavy precipitation. Understanding potential changes of cyclone frequency and intensity is thus essential for a proper assessment of climate change impacts. Here the behavior of extratropical cyclones under strongly varying climate conditions is investigated using idealized climate model simulations in an aquaplanet setup. A cyclone tracking algorithm is applied to assess various statistics of cyclone properties such as intensity, size, lifetime, displacement velocity, and deepening rates. In addition, a composite analysis of intense cyclones is performed. In general, the structure of extratropical cyclones in the idealized simulations is very robust, and changes in major cyclone characteristics are relatively small. Median cyclone intensity, measured in terms of minimum sea level pressure and lower-tropospheric relative vorticity, has a maximum in simulations with global mean temperature slightly warmer than present-day Earth, broadly consistent with the behavior of the eddy kinetic energy analyzed in previous studies. Maximum deepening rates along cyclone tracks behave similarly and are in agreement with linear quasigeostrophic growth rates if the effect of latent heat release on the stratification is taken into account. In contrast to moderate cyclones, the relative vorticity of intense cyclones continues to increase with warming to substantially higher temperatures, and this is associated with enhanced lower-tropospheric potential vorticity anomalies likely caused by increased diabatic heating. Moist processes may, therefore, lead to the further strengthening of intense cyclones in warmer climates even if cyclones weaken on average.
Publisher: American Geophysical Union (AGU)
Date: 13-04-2020
DOI: 10.1029/2019JD031801
Publisher: American Meteorological Society
Date: 12-2012
DOI: 10.1175/JCLI-D-11-00699.1
Abstract: Many features of the general circulation of the atmosphere shift upward in response to warming in simulations of climate change with both general circulation models (GCMs) and cloud-system-resolving models. The importance of the upward shift is well known, but its physical basis and the extent to which it occurs coherently across variables are not well understood. A transformation is derived here that shows how an upward shift of a solution to the moist primitive equations gives a new approximate solution with higher tropospheric temperatures. According to the transformation, all variables shift upward with warming but with an additional modification to the temperature and a general weakening of the pressure velocity. The applicability of the vertical-shift transformation is explored using a hierarchy of models from adiabatic parcel ascents to comprehensive GCMs. The transformation is found to capture many features of the response to climate change in simulations with an idealized GCM, including the mid- and upper-tropospheric changes in lapse rate, relative humidity, and meridional wind. The transformation is less accurate when applied to simulations with more realistic GCMs, but it nonetheless captures some important features. Deviations from the simulated response are primarily due to the surface boundary conditions, which do not necessarily conform to the transformation, especially in the case of the zonal winds. The results allow for a physical interpretation of the upward shift in terms of the governing equations and suggest that it may be thought of as a coherent response of the general circulation of the mid- and upper troposphere.
Publisher: Springer Science and Business Media LLC
Date: 31-07-2020
DOI: 10.1038/S41467-020-17683-7
Abstract: Despite the observed monotonic increase in greenhouse-gas concentrations, global mean temperature displays important decadal fluctuations typically attributed to both external forcing and internal variability. Here, we provide a robust quantification of the relative contributions of anthropogenic, natural, and internally-driven decadal variability of global mean sea surface temperature (GMSST) by using a unique dataset consisting of 30-member large initial-condition ensembles with five Earth System Models (ESM-LE). We present evidence that a large fraction (~29–53%) of the simulated decadal-scale variance in in idual timeseries of GMSST over 1950–2010 is externally forced and largely linked to the representation of volcanic aerosols. Comparison with the future (2010–2070) period suggests that external forcing provides a source of additional decadal-scale variability in the historical period. Given the unpredictable nature of future volcanic aerosol forcing, it is suggested that a large portion of decadal GMSST variability might not be predictable.
Publisher: Wiley
Date: 05-12-2022
Publisher: American Meteorological Society
Date: 08-2023
Abstract: Recent analysis of pantropical interactions suggests that after 1980 the tropical Atlantic Ocean’s (TAO) influence on the tropical Pacific Ocean (TPO) appears to have become much more pronounced while the tropical Indian Ocean’s (TIO) influence appears to have weakened. This study explores whether and how decadal changes in TAO and TPO SSTs modulate these pantropical connections in an attempt to explain the recent dominance of the TAO. To this end, we carry out a series of idealized atmosphere-only experiments using the ACCESS atmospheric general circulation model where the magnitude and sign of the decadal TAO SST signal are varied, presenting various warm and cool Atlantic scenarios. To understand further if these pantropical connections are influenced by changes in TPO SST, we carry out the above TAO experiments with both warm and cool phases of Pacific decadal variability (PDV). We find that an imposed TAO warming leads to increases in TPO atmospheric temperature and stability, which lead to a decrease in average TPO precipitation, with the most prominent changes occurring in June–August. These changes in TPO precipitation induced by TAO warming are largely mirrored when TAO cooling is added, whereas the TPO rainfall response to TAO anomalies remains relatively unchanged for the different phases of PDV. In contrast to the precipitation response, the wind response did display some asymmetries between different phases of TAO SST variability. Specifically, surface winds in the western half of the Niño-4 region exhibited a significantly different response to positive versus negative Atlantic multidecadal variability (AMV), whereas the surface winds in the western equatorial Pacific were significantly stronger (roughly 40% larger) in the positive phase of PDV than in the negative phase. These results suggest that the phases of PDV and AMV may modulate pantropical interactions through their effect on zonal wind stress.
Publisher: Cambridge University Press (CUP)
Date: 10-02-2010
DOI: 10.1017/S0022112009992448
Abstract: Hydraulic equations are derived for a stratified (two-layer) flow in which the horizontal velocity varies continuously in the vertical. Viscosity is included in the governing equations, and the effect of friction in hydraulically controlled flows is examined. The analysis yields Froude numbers which depend upon the integrated inverse square of velocity but reduce to the original layered Froude numbers when velocity is constant with depth. The Froude numbers reveal a critical condition for hydraulic control, which equates to the arrest of internal gravity waves. Solutions are presented for the case of unidirectional flow through a lateral constriction, both with and without bottom drag. In the free-slip lower boundary case, viscosity transports momentum from the faster to the slower layer, thereby shifting the control point downstream and reducing the flux through the constriction. However, while the velocity shear at the interface between the two layers is reduced, the top-to-bottom velocity difference of the controlled solution is increased for larger values of viscosity. This counter-intuitive result is due to the restrictions placed on the flow at the hydraulic control point. When bottom drag is included in the model, the total flux may increase, in some cases exceeding that of the inviscid solution.
Publisher: American Geophysical Union (AGU)
Date: 24-06-2013
DOI: 10.1002/JGRD.50450
Publisher: American Geophysical Union (AGU)
Date: 04-01-2022
DOI: 10.1029/2021GL094295
Abstract: Tropical modes of variability, such as El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), exert a strong influence on the interannual variability of Australian precipitation. Nevertheless, commonly used indices of ENSO and IOD variability display significant co‐variability that prevents a robust quantification of the independent contribution of each mode to precipitation anomalies. This co‐variability issue is often addressed by statistically removing ENSO or IOD variability from the precipitation field before calculating teleconnection patterns. However, by performing a suite of coupled and uncoupled modeling experiments in which either ENSO or IOD variability is physically removed, we show that ENSO‐only‐driven precipitation patterns computed by statistically removing the IOD influence significantly underestimate the impact of ENSO on Australian precipitation variability. Inspired by this, we propose a conceptual model that allows one to effectively separate the contribution of each mode to Australian precipitation variability.
Location: United States of America
Start Date: 06-2019
End Date: 12-2022
Summary: Intense thunderstorms in the tropics and subtropics under global warming. This project aims to determine how the frequency of intense tropical and subtropical thunderstorms will change as a result of future global warming. Climate models project that the energy available to such storms will increase in the future, but the reasons for this increase in available energy, and the implications for thunderstorm activity, remain uncertain. Using observations and high-resolution models, the project expe .... Intense thunderstorms in the tropics and subtropics under global warming. This project aims to determine how the frequency of intense tropical and subtropical thunderstorms will change as a result of future global warming. Climate models project that the energy available to such storms will increase in the future, but the reasons for this increase in available energy, and the implications for thunderstorm activity, remain uncertain. Using observations and high-resolution models, the project expects to generate new knowledge of the mechanisms driving changes in intense thunderstorm activity under climate change, and to provide more accurate projections of the frequency of intense thunderstorms in a warmer climate. This project should deliver benefits to sectors of the economy such as agriculture and transportation, which are significantly exposed to such hazards. Read more Read less
Funder: Australian Research Council
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