Understanding how greenhouse gases (GHGs) perturb the tropical Pacific warming pattern is crucial due to its impact on circulation and the hydrological cycle. A global CO2 increase is known to initially induce cooling in the tropical Pacific, particularly in the eastern basin, which gradually evolves into equatorially peaked warming amplified in the eastern basin. To disentangle the mechanisms driving the evolution of the CO2-induced equatorial Pacific warming pattern, we construct large-ensemble climate model experiments with CO2 increases confined to discretized latitudinal bands. On a fast timescale (years 1-3), Northern Hemisphere off-equatorial forcing (NH_OFFEQ) induces the basin-wide equatorial Pacific cooling due to intensified trade easterlies associated with a northward ITCZ shift. Local equatorial forcing (EQ) drives eastern equatorial Pacific cooling through enhanced climatological upwelling. In contrast, Southern Hemisphere off-equatorial forcing (SH_OFFEQ) leads to basin-wide equatorial Pacific warming, with an amplified response in the eastern basin due to the weakening of the southern subtropical ocean cell (STC). The effect of NH_OFFEQ and EQ forcing on equatorial Pacific SST changes diminishes over time due to dynamical ocean adjustments. Consequently, the fast strengthening of the zonal SST gradient induced by NH_OFFEQ and EQ forcing transitions into a weakening driven by SH_OFF forcing, which becomes dominant during the slow response. Our results suggest that the recent cooling in the eastern tropical Pacific could be part of the CO2-driven fast response.
How to cite: Kang, S. and Olonscheck, D.: Contrasting responses to hemispheric forcing govern the evolution of the CO2-induced equatorial Pacific warming pattern, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10898, https://doi.org/10.5194/egusphere-egu25-10898, 2025.
The ascending branch of the Hadley circulation forms the band of heavy precipitation, the Intertropical Convergence Zone (ITCZ). Due to its sharp meridional gradient, even a small change in its structure can lead to drastic changes in local precipitation patterns, affecting large populations dependent on agriculture. Consequently, understanding its response to a warming climate has been a major focus of research over the past decades. Previous studies have identified a robust strengthening of the ascending branch of the Hadley cell, often referred to as a “deep tropics squeeze”, characterized by the narrowing of the ITCZ in response to increased CO2. However, much of the research has concentrated on the quasi-equilibrated response, with little attention given to its response on shorter timescales, which are more relevant for near-term climate projections.
To examine the evolution of the ITCZ response, we construct a 30-member ensemble of abrupt 4xCO2 simulations using the MPI-ESM. Our experiments reveal distinct phases in the Hadley cell response, starting with a weakening ascent and widening of the ITCZ in the initial years, followed by a reversal of these trends in subsequent decades. This reversal is primarily driven by a shift in the spatial pattern of sea surface temperature (SST) warming, transitioning from cooling to warming over the eastern equatorial Pacific due to dynamical ocean adjustments. Our findings underscore the value of large-ensemble 4xCO2 experiments, which allowed us to identify the shifting dynamics of the ITCZ, with the fast response distinct from the well-established slow response.
How to cite: Zhang, J. and Kang, S.: Shifting dynamics of the ITCZ: from widening to narrowing in response to abrupt 4xCO2, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18271, https://doi.org/10.5194/egusphere-egu25-18271, 2025.
Tropical and subtropical precipitation impact millions of people via agriculture and rainfall-driven disasters, driving interest in their potential future change. CMIP6 simulations broadly predict an increase in global monsoon precipitation. However, regional projections from individual models vary in magnitude and sign, and projected changes by the end of the century are often small compared with model biases. This motivates an interest in understanding model biases, and how to interpret the future shifts in rainfall.
The Hadley and Walker circulations transport Moist Static Energy in the direction of their upper branches, so that the change in sign of MSE transport acts as a proxy for mass convergence in the tropical rainband. MSE transport can then be interpreted in terms of top-of-atmosphere and surface energy fluxes using the column energy budget.
Recent work attributes contributions to annual- and zonal-mean divergent MSE transport to radiative fluxes, evaporative fluxes and sensible heat, and suggests that evaporative fluxes are key in setting the spatial structure of MSE transport. Here we extend this approach to regional and seasonal scales, and explore inter-model differences in CMIP6 historical simulations, and projected changes under SSP585.
MSE transport attributed to evaporative and radiative fluxes dominate the regional JJA & DJF transport. Empirical Orthogonal Functions (EOFs) are used to express historical intermodel differences in MSE transport in a 2-dimensional space of leading EOFs linked to land-sea thermal contrast and interhemispheric thermal contrast. Changes in MSE transport components under SSP585 are then projected into this space and decomposed into terms attributed to the top of atmosphere and surface fluxes. This reveals energetic signatures of model bias and future change, that illustrate how different processes contribute to the overall differences in energy transport.
Shared energetic signatures of bias are predominantly seen within model families. In contrast, shared signatures of future change emerge across (and differ within) model families, with a shared bias signature not implying a shared change signature. This suggests a set coherent but differing pathways through which climate change affects the energy budget and associated tropical rainfall in particular groups of models.
How to cite: Geen, R.: Energetic signatures of tropical rainband biases & shifts in CMIP6, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7062, https://doi.org/10.5194/egusphere-egu25-7062, 2025.
The Pacific Ocean plays a pivotal role in driving Earth’s climate, as it is the primary region for strong deep convection that forms the ascending branch of the zonally symmetric Hadley circulation (HC) and the zonally asymmetric Walker circulation (WC). The HC is characterized by ascending motion in the equatorial region and descending motion in the subtropics, governed by energy and angular momentum conservation. In contrast, the WC features strong ascent over the western Pacific and descent over the central to eastern equatorial Pacific, driven by the zonal sea surface temperature (SST) gradient. Although the HC and WC share an ascending branch, their coupling remains poorly understood from a theoretical perspective. To investigate their coupling, we use an idealized aquaplanet slab ocean model. By prescribing zonally asymmetric warm and cold surface fluxes in the tropics, we deliberately alter the strength of the WC. The results show an anticorrelated relationship between the two circulations, with the HC weakening in proportion to the WC strengthening. This behaviour is attributed to the dominant cloud radiative effect in the cold patch region compared to the warm patch. These findings are further supported by cloud-locked experiments that isolate the contributions of cloud radiative feedback. While the shared ascending branch in the western Pacific is often considered the determining factor governing the dynamics, our idealized experiments suggest that the interplay between the two circulations may instead be driven by cloud radiative feedback in the eastern Pacific. The results therefore highlight the critical role of the cold tongue region in shaping the pattern effect in the tropical Pacific.
How to cite: Schimmer, R. and Kang, S.: Anti-correlated Hadley and Walker circulations through cloud radiative feedback, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18509, https://doi.org/10.5194/egusphere-egu25-18509, 2025.
Understanding the potential reorganizations of the large-scale atmospheric circulation in the tropics is important in the context of anthropogenic climate change and from a theoretical point of view, but also because they might be connected with warm climates of the past. A particularly spectacular, albeit hypothetical, example of such a reorganization is the case of equatorial superrotation, characterized by strong westerly winds at the equator. While potential dynamical processes underlying a transition to equatorial superrotation have been studied to some extent, the question of how the circulation changes would be coupled to the broader climate features has not yet been addressed. In this work, we adopt this perspective and investigate the consequences of such a circulation change on Earth's surface climate and hydrological cycle.
Using general circulation model (GCM) simulations in an aquaplanet setup with an imposed equatorial torque to force superrotation in the atmosphere, we observe large changes in the surface temperature and precipitation distribution. The results show an important global surface warming, comparable to a doubling of the CO2 concentration, which affects in particular regions outside of the tropics, such as the mid-latitudes. In addition, the meridional structure of the precipitation profile becomes flatter; the tropics become drier and the subtropics wetter. These changes are strongly linked to the effect of the circulation changes on the meridional transport of energy and moisture. We analyze these changes and the associated radiative budget changes using a forcing/feedback framework.
Overall, this study demonstrates that equatorial superrotation can have a significant impact on the surface climate, independently of any external radiative forcing. This provides further evidence that such major changes in the large scale circulation might be relevant for warm climates of the Earth, in the past or in the future.
How to cite: Marino, T., Byrne, M. P., and Herbert, C.: Climate change induced by equatorial superrotation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3716, https://doi.org/10.5194/egusphere-egu25-3716, 2025.
A method is presented which allows one to diagnose and to analyze Rossby wave resonance along a circum-global midlatitude jet in the framework of the inviscid linear barotropic model with beta-plane geometry. Zonally symmetric Gaussian-shaped jets of varying amplitude and width are specified as a basic state. The system is forced by a pseudo-orography which varies sinusoidally in the zonal direction, and which has a very small meridional extent. Solutions are obtained through straightforward numerical methods. The strength of resonance is diagnosed by systematically varying the zonal wavenumber s, plotting the resulting wave amplitude as a function of s and quantifying the sharpness of the peak (if existent). The numerical solutions for jet-like basic states are interpreted by reference to analytical solutions obtained for more idealized model configurations
It is shown that a jet with realistic amplitude and width may be subject to a weak form of resonance provided that the jet is truly circum-global and there are no other forms of damping. Given that the zonal scale of a jet is much larger than its meridional scale, one may expect resonance at no more than one zonal wavenumber s_res. This resonant peak is associated with the gravest meridional mode, which is established through partial reflection of wave activity at the periphery of the jet flanks. These results are reproduced well in the classic Charney-Eliassen model with an appropriate choice for the channel width. A spherical version of our diagnostic tool is expected to provide a reliable method to detect the potential for resonant amplification of Rossby waves in observed episodes
How to cite: Wirth, V. and Harnik, N.: Diagnosing and analysing Rossby wave resonance along a circumglobal jetstream, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4211, https://doi.org/10.5194/egusphere-egu25-4211, 2025.
The jet streams are a crucial part of the global atmospheric circulation. Jet streams are highly sheared regions of the atmosphere, leading to Kelvin–Helmholtz instability and the generation of clear-air turbulence (CAT), which affects flying aircraft. Wind shear and CAT at flight cruising altitudes are projected to increase in response to future climate change, as the meridional temperature gradient across the jet streams strengthens, largely due to amplified warming at low latitudes associated with the tropical upper-tropospheric warming hotspot. However, our understanding of past trends in jet stream wind shear and CAT is currently limited. Here we analyse past trends in jet stream vertical wind shear in three different reanalysis datasets since 1979. We find that the shear at flight cruising altitudes has strengthened by 15%, and we show that this change is attributable to the thermal wind response to the enhanced upper-level meridional temperature gradient. We then analyse CAT trends globally during 1979–2020 in a reanalysis dataset using 21 diagnostics. We find clear evidence of large increases around the globe at aircraft cruising altitudes. For example, at an average point over the North Atlantic, the total annual duration of light-or-greater CAT increased by 17% from 466.5 hours in 1979 to 546.8 hours in 2020, with even larger relative changes for moderate-or greater CAT (increasing by 37% from 70.0 hours to 96.1 hours) and severe-or-greater CAT (increasing by 55% from 17.7 hours to 27.4 hours). Future projections using climate models indicate a 17-29% increase in vertical wind shear in the upper-level jet streams by 2100, as well as a possible tripling in the amount of severe CAT. We conclude that the jet streams are becoming more sheared because of climate change, generating more turbulence, with important implications for the future of air travel.
How to cite: Williams, P., Prosser, M., and Smith, I.: Jet streams in a changing climate: evidence for large increases in shear and turbulence since 1979, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18030, https://doi.org/10.5194/egusphere-egu25-18030, 2025.
Recent studies using mesoscale resolving simulations have shown a stronger and spatially different large-scale atmospheric response to North Atlantic (NA) sea-surface temperature (SST) anomalies compared to more coarse-resolution simulations. An idealized setup proved that moving from a horizontal resolution of 110-km of more classic general circulation models to 28-km and further to 14-km leads to distinct large-scale responses to Gulf Stream SST anomalies.
Here, we investigate a new set of simulations using a variable resolution version of the Community Atmospheric Model (CAM6) and more realistic specified SST anomalies. The horizontal resolution is regionally refined in the NA domain from the global 110-km resolution to 28-km, which is not fully mesoscale resolving, and further to 14-km, capable of resolving weather fronts which are crucial features for ocean-atmosphere coupling.
The specified SST anomaly forcing the simulations is created by regressing the observational North Atlantic Oscillation (NAO) index onto SSTs over the period 1958–2018, resulting in a cold-warm-cold tripole anomaly and enabling a comparison of the NAO - NA SST feedback between the different resolutions.
We find that the resulting NA SST tripole anomaly feeds back positively onto the NAO in the 14-km simulations. This positive NAO–SST–NAO feedback is not present in the 28-km and 110-km simulations which show a distinct spatial structure and generally a weaker response.
With the overall push towards more expensive higher-resolution coupled simulations, our results will provide valuable insights into the required atmospheric resolution needed to correctly represent ocean-atmosphere coupling.
How to cite: Müller, J., Herrington, A., and Jnglin Wills, R. C.: Altered NAO - North Atlantic SST Feedback in Mesoscale Resolving Simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17675, https://doi.org/10.5194/egusphere-egu25-17675, 2025.
Atmospheric blocking events in summer over Greenland promote melting of the Greenland ice sheet, a major contributor to sea level rise. Recent observations indicate that, during the early part of the twenty-first century, summertime atmospheric blocking over Greenland has become markedly more frequent. This increasing trend in blocking activity appears to be missing in climate model simulations. The temporal evolution of Greenland blocking (GB) is assessed here in a larger ensemble of around 500 members from the CMIP6 archive. The observed increase in GB is also not present in the larger ensemble of members considered: the maximum 10-year trend in GB in the reanalysis lies almost outside the distributions of trends in the climate models and a period of such increased GB activity is rarely found in the full historical period of the model simulations.
The climate model simulations do however suggest that variability in GB is partly driven by sea surface temperatures (SSTs) and/or sea ice concentrations (SICs), as well as/or by anthropogenic aerosols, but the response of the models to these forcings may be too weak. To understand if it is forcing from the surface or from aerosols that dominates, a set of climate model experiments is performed with the Met Office climate model. Historical simulations are performed with prescribed SSTs/SICs and both with and without aerosol forcing. Results from the experiments indicate that variability in SSTs/SICs is key for capturing variability in GB. Further work is required to understand why climate models cannot represent a period of increased GB, how SST/SIC variability relates to that in GB, and what implications these have for future projections of Greenland climate change.
How to cite: Maddison, J., Catto, J., Hanna, E., Luu, L., and Screen, J.: Missing Increase in Summer Greenland Blocking in Climate Models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10239, https://doi.org/10.5194/egusphere-egu25-10239, 2025.
Climate change is projected to have wide ranging impacts on the atmospheric waves and mean-flow. However, it remains uncertain as to how exactly the warming climate will influence the mean-waviness of the jet stream and extreme wave-activity events in the midlatitude storm track. An objective identification of this phenomena is important as wave-activity aloft plays an important role in driving the extreme weather events over the continents. Here we use the local wave activity (LWA) metric to quantify stationary and transient wave activity during winter-time from multi-member ensembles of state-of-the-art climate model simulations including, NorESM, CESM-LENS2 and MPI-LE simulations for Historical and various SSP warming scenarios. Our analysis reveals a statistically significant decrease in the mean-waviness of the jet stream and region-specific changes in the probability of extreme wave-activity events in the midlatitudes. These changes are found to be dynamically consistent with the theoretical predictions from the non-acceleration relation and the recently proposed traffic-jam theory of atmospheric blocking.
How to cite: Barpanda, P. and Li, C.: The dynamics of extreme wave-activity events in a warming climate., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19200, https://doi.org/10.5194/egusphere-egu25-19200, 2025.
The winter temperature anomalies are influenced by the joint effects of multiple factors. Using NCEP/NCAR reanalysis data from 1979 to 2022,this paper analyzes the circulation patterns and mechanisms influencing temperature anomalies in the mid-to-high latitudes of the Northern Hemisphere under the joint effects of Interhemispheric Oscillation (IHO) and Arctic Oscillation (AO). It was found that the temperature changes under the joint effects of IHO and AO were mainly influenced by AO, with IHO playing a disruptive role. Specifically, when IHO and AO were in the same phase, the disturbance in the polar region would weaken due to the cancellation of different signals, while the disturbance in the mid latitudes would strengthen due to the superposition of the same signals. There is a significant meridional horizontal propagation of Rossby wave energy in the North Pacific and North Atlantic. Stratospheric planetary waves mainly propagate downward over the Eurasian continent, while areas north of 60 °N in North America exhibit oblique downward propagation of stratospheric planetary waves, and areas south of 60 °N exhibit significant southward propagation of planetary waves in the troposphere. When IHO and AO are in opposite phases, disturbances in the Arctic region are enhanced due to the superposition of the same sign, but disturbances in the mid latitude region are weakened due to the cancellation of opposite signs. There is obvious Rossby meridional propagation in the North Pacific and North Atlantic, with wave energy dispersed upwards at high latitudes and propagating towards low latitudes. In addition, the maximum contribution of IHO and AO to surface air temperature is mainly due to the disturbance of horizontal temperature advection caused by the steady wind field and diabatic heating. These conclusions provide reference for studying temperature anomalies and can better understand the mechanisms and impacts of atmospheric circulation anomalies.
How to cite: Qiao, N., Lu, C., Guan, Z., Hu, Y., and Zhong, L.: The joint effects of Interhemispheric Oscillation and Arctic Oscillation on climate anomlies in winter, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10184, https://doi.org/10.5194/egusphere-egu25-10184, 2025.
As a prominent large-scale mode in the Northern Hemisphere, the Atlantic Multidecadal Oscillation (AMO) can have a profound impact on regional and global climate. This study investigates inter-model spread of AMO-related tropical precipitation anomalies by using 52 models in the historical simulation from Coupled Model Intercomparison Project phase 6 (CMIP6). The results indicate that there is a significant spread in AMO-related precipitation anomalies among models in the tropics, particularly in the Maritime Continent–tropical western Pacific. In addition, the inter-model spread is characterized by a seesaw pattern between the Maritime Continent and tropical western Pacific, identified as the primary mode through inter-model EOF analysis. Furthermore, associated with the differences in AMO-related tropical precipitation anomalies, there are substantial differences in sea surface temperature anomalies in the tropics and surface air temperature anomalies in the Eurasian continent.
How to cite: Su, Q. and Lu, R.: Inter-model spread of AMO-related tropical precipitation in CMIP6, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5070, https://doi.org/10.5194/egusphere-egu25-5070, 2025.
The Hadley cell (HC) is a thermally direct circulation in the tropics that transports heat from the tropics to the mid-latitudes. HC is known as the primary cause of subtropical desert formation, and with the recent observation of poleward shift of both the HC and desert regions, extensive studies has been made to understand its formation. The most foundational theory on the HC was proposed by Held and Hou (1980). This theory pinpointed the importance of angular momentum conservation and energy flux balance, while providing approximations for the edge and intensity of the HC. However, it did not consider the influence of baroclinic eddies. By extending this theory, the present study incorporates eddy heat fluxes and changes of adiabatic processes induced by eddy momentum fluxes in the subtropical upper troposphere into the energy flux balance HC dynamics. It is proposed that HC contracts when losing heat and expands when gaining heat due to thermal interactions with baroclinic eddies. This finding is verified through a series of dynamical core model experiments with varying baroclinicity.
How to cite: Lee, S., Moon, W., Son, S.-W., and Seo, K.-H.: Influence of baroclinic eddies on the Hadley cell edge , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7766, https://doi.org/10.5194/egusphere-egu25-7766, 2025.
Observations reveal an increasing frequency and intensity of climate extremes during summer under global warming. Pronounced warming in the tropical upper troposphere and Arctic amplification play as key contributors to these changes. Such warming patterns affect the thermal gradients between the tropics and high latitudes, leading to changes in midlatitude baroclinicity, a key factor for synoptic dynamics. Investigating a comprehensive understanding of the links between global warming and atmospheric energy cycle is crucial for identifying the mechanisms responsible for extreme events.
The Lorenz energy cycle provides a robust framework for examining the generation, conversion, and distribution of atmospheric energy. It effectively explains the formation of available potential energy resulting from differential heating in the atmosphere and its subsequent conversion into kinetic energy through large-scale dynamical processes. Analyzing the Lorenz energy cycle offers crucial insights into the ways global warming impacts large-scale circulation.
This study revisits and evaluates the Lorenz energy cycle to provide a more comprehensive understanding of how global warming influences the global energy cycle and general circulation. Furthermore, it explores the potential implications of these changes for the development and intensification of extreme weather phenomena.
How to cite: Noh, E. and Kim, J.: Revisiting the Loreanz Energy Cycle: Impacts of Global Warming on Atmospheric Energy Cycle, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19098, https://doi.org/10.5194/egusphere-egu25-19098, 2025.
The east Pacific, dominated by synoptic scale convective activity, presents a unique environment to explore the question of how synoptic scale disturbances can modulate the strength of mean meridional circulation. The role of mixed Rossby-Gravity (MRG) waves in modulating the east Pacific Hadley circulation (EPHC) strength is explored during boreal summer season using ERA5 reanalysis data. Composite analysis of MRG activity for five strong and five weak EPHC seasons identified based on a mass stream function based metric reveal that strong EPHC seasons are associated with pronounced MRG activity while the MRG activity is weak during weak EPHC seasons. While the SST background state over east Pacific is not favourable for thermally driven deep convection, low-level convergence induced by synoptic scale disturbances like the MRG waves can trigger deep convection over the region, and in turn influence the EPHC strength. The question of whether boundary forced convergence can have an impact on the EPHC strength is further investigated using an atmospheric mixed layer model. Surface convergence driven by meridional SST gradients are not found to be significantly different during strong and weak EPHC seasons, implying the dominant role of MRG in modulating the strength of EPHC. The study also reveals a new possible mechanism via which the El Niño Southern Oscillation (ENSO) modulates the strength of EPHC –ENSO induced changes in the mean background state modulates the spatio-temporal characteristics of MRG waves which in turn affect the low-level convergence over the region and impacts the strength of EPHC.
How to cite: Baruah, P. P., Mani, N. J., and Ettammal, S.: The Role of Tropical Synoptic-Scale Disturbances in Modulating the Strength of East Pacific Hadley Circulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-217, https://doi.org/10.5194/egusphere-egu25-217, 2025.
Increasing atmospheric angular momentum can alter the fundamental circulation cells that drive the Earth’s climate system and also slow the Earth’s rotation. Prominent examples include the expansion of the Hadley Cell and increasing Length of Day (LOD). Utilising the hundred ensemble member simulations of CESM2-LE with the SSP3-7.0 scenario, we reveal an equatorial super-rotation state of the earth with increased greenhouse gas emissions. With global warming, the momentum exchange between the solid earth and the atmosphere diminishes with reduced surface torques, suggesting slowing of the earth’s rotation. An accelerating atmosphere decelerates the earth’s rotational speed, bringing about challenges to precise time-keeping through increasing LOD. Our results demonstrate that climate-driven LOD changes due to atmospheric angular momentum variations can start as early as 2050, posing problems to global timekeeping. These findings illustrate that with continued warming along with astronomical tidal forcings and postglacial rebound processes, anthropogenic climate change will influence the earth’s rotational rate.
How to cite: Satpathy, S. S., Franzke, C. L. E., Yuan, N., Maher, N., Park, W., and Lee, S.-S.: Anthropogenic Climate Change-driven Atmospheric Angular Momentum Increases Length of Day, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2418, https://doi.org/10.5194/egusphere-egu25-2418, 2025.
The global monsoon circulation, which governs the subtropical rainband, can be interpreted as a manifestation of the seasonal migration of tropical overturning circulation (Hadley cell). However, the dynamics of regional monsoons is additionally controlled by zonal asymmetries occurring from land sea distribution, zonal gradients in sea surface temperature, and other stationary wave forcings. Despite its importance, the dynamics of regional monsoons remain poorly understood. Here, we demonstrate, using a machine learning guided empirical leading order analysis, emergence of distinct dynamical regimes that describe the complex evolution of regional monsoons. Conservation of angular momentum plays an important role in our understanding of the climatological and zonal mean picture of monsoon. It suggests, during the solstitial seasons the dominant balance in the momentum budget comes from the mean meridional circulation and the advection of mean zonal wind by the divergent wind. However, for regional monsoons the resulting angular momentum budget now includes many terms arising from the drivers mentioned above. We deploy an unsupervised machine learning algorithm to find the dominant balances in the momentum budget. This enables us to find spatio-temporal clusters characterized by distinct balances in the momentum budget and to study how they evolve throughout the seasonal cycle. The inherent stochastic nature of the algorithm is leveraged to find the robustness of the identified clusters. Entropy is used to measure uncertainty for the clusters recognized by the algorithm. The algorithm is successfully applied to idealized simulations with varying complexities ranging from aquaplanet to different distributions of land-sea-topography. Consistent with zonal mean theory, resulting clusters capture the dominant tropical overturning. However, zonal asymmetries result in additional clusters with distinct dynamical regimes
How to cite: Dutta, A., Geen, R., and Sonnewald, M.: Exploring the Dynamics of Climatological Mean Monsoon Using a Machine Learning Based Empirical Leading Order Analysis, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10561, https://doi.org/10.5194/egusphere-egu25-10561, 2025.
Recent studies have identified the Asian–Bering–North American (ABNA) teleconnection as a distinct atmospheric pattern influencing winter climates in Eurasia and North America, independent of the well-known Pacific–North America (PNA) pattern. However, the origins of the winter ABNA remain unclear. This study explores the interannual variability of the winter ABNA during 1979–2022 and examines its associated preceding surface boundary forcings. The ABNA pattern accounts for coherent surface air temperature variations across northern Asia, eastern Siberia–Alaska, and eastern North America, even after removing the influences of the PNA, the Arctic Oscillation, the North Atlantic Oscillation, and the North Pacific Oscillation. Surface boundary conditions linked to the ABNA can be traced back to a Eurasian Snow Cover Dipole Pattern (ESCDP) and a Maritime Continent Sea Surface Temperature (MCSST) anomaly in November. The ESCDP leads to a displacement of the Arctic stratospheric polar vortex via troposphere–stratosphere coupling. This anomalous polar vortex subsequently propagates downward during the following winter, generating the tropospheric ABNA pattern. The MCSST induces a diabatic heating anomaly, which is associated with a Tropical Western Pacific Precipitation (TWPP) anomaly in winter. The TWPP excites a poleward-propagating Rossby wave train across the North Pacific, directly amplifying the winter ABNA. These physical processes are well reproduced by a linear baroclinic model (LBM). Leveraging the ESCDP and MCSST as predictors, an empirical model is developed, demonstrating promising prediction skills for winter ABNA during the hindcast period. This approach provides a valuable strategy for improving seasonal prediction of winter climates in the Northern Hemisphere extratropics.
How to cite: Zhong, W. and Wu, Z.: Interannual variability of winter Asian–Bering–North American teleconnection linked to Eurasian snow cover and Maritime Continent SST, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1887, https://doi.org/10.5194/egusphere-egu25-1887, 2025.
Understanding the response of large-scale atmospheric circulation to radiative forcing agents is important for climate prediction. The radiative forcing from volcanic stratospheric aerosol is one of the most important natural climate forcings, with impacts on surface temperature and atmospheric dynamics. In this study, we explore changes in the energetic properties of the Hadley and Ferrel systems under the influence of radiative forcing associated with large volcanic eruptions in multi-model simulations performed as part of the Model Intercomparison Project on the Climatic Response to Volcanic Forcing (VolMIP) within the Coupled Model Intercomparison Project Phase 6 (CMIP6). In the Earth’s atmosphere, the Hadley and Ferrel systems are examples of thermally direct (warm air rises and cold air sinks) and indirect (cold air rises and warm air sinks) circulations, respectively. Being the part of Lorenz cycle of energy transformation in the atmosphere, the direct circulation converts zonal-mean available potential energy into zonal-mean kinetic energy. The indirect circulation in the midlatitude, however, converts some of the zonal-mean kinetic energy back into zonal-mean available potential energy. Averaged over the 4 models that provided the required model output from the VolMIP Pinatubo simulations, the mean power associated with the Hadley system in preindustrial simulations is 235.6 TW. The mean decrease of the power in VolMIP simulations of the 1991 Pinatubo eruption is 7.58 TW (3.22%) for the first post-eruption northern-hemisphere (NH) winter and 6.59 TW (2.80%) for the second one. For the Ferrel system, the preindustrial mean DJF power is 326.10 TW, and post-volcanic anomalies are 16.3 TW (5.00%) and 18.3 TW (5.61%) in NH winters 1 and 2, showing a stronger anomaly in the second NH winter than the first one. In additional VolMIP experiments, we also explore the response of the Hadley and Ferrel cells to the relatively strong forcing associated with the 1815 Tambora eruption and find the Hadley system weakening by 15.3 TW (6.48%) and 11.5 TW (4.90%) for the first two NH winters. We explore how post-eruption changes in the meridional atmospheric circulation strength and the cells' location can be explained with simple theoretical models of atmospheric thermodynamics.
How to cite: Poroshenko, A. and Toohey, M.: Simulated Changes in Large-scale Atmospheric Circulation Energetics from Volcanic Aerosol Forcing, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7437, https://doi.org/10.5194/egusphere-egu25-7437, 2025.
We analyse trends in annual mean wind speed from 1980 to 2010. Observational data (HadISD3) indicate a decline in wind speed across North America, Europe, and central and eastern Asia, a phenomenon referred to as global wind speed stilling. However, a suite of reanalyses fails to accurately reproduce this trend, often showing inconsistencies in the direction of the trend. Additionally, the reanalyses significantly underestimate wind speed variability. We further investigate the sources of discrepancies between the observations and reanalyses. Our findings suggest that the erroneous trends in the reanalyses primarily stem from inaccuracies in simulating high-frequency wind speed variability. In contrast, lower-frequency variability remains consistent between observations and reanalyses. These errors in simulating wind speed trends have significant implications for understanding and predicting wind power variability.
How to cite: Monerie, P.-A., Schiemann, R., Robson, J., and Brayshaw, D.: Historical trends in windspeed over the Northern Hemisphere in observations and reanalyses , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7090, https://doi.org/10.5194/egusphere-egu25-7090, 2025.
Conceptual models of the midlatitude atmospheric circulation have added greatly to understanding its behavior. Here, we present a new conceptual model for the spatial structure of the Ferrel cell. The poleward heat flux resulting from the baroclinic growth of eddies leads to a decrease in the meridional temperature gradient, which is parameterized through a down-gradient eddy diffusion coefficient D. Similarly, the eddy momentum flux, influenced by barotropic wave breaking, is assumed to be proportional to a factor M>0 to the horizontal shear of the zonal mean zonal wind,
thereby enhancing the intensity of the zonal mean zonal wind at upper levels. By incorporating the parameterization of turbulent eddies into the zonal-mean quasi-geostrophic potential vorticity equation, a balance is achieved, resulting in eddy-driven circulations in mid-latitudes akin to the Ferrel cell.
The meridional structure of the temperature exhibits two primary features. The first feature is a linear decline in anomalous potential temperature,
inducing westerly winds in mid-latitudes. The second feature corresponds to jet streams generated by eddy momentum fluxes. Along with the jet streams, the eddy driven circulations exhibit the downward (upward) motion at the southern (northern) flank of the jets. The meridional structure of the circulation is influenced by three key factors. The first factor is a structural number denoted as D/SM, where S is the dry static stability affecting the life cycle of synoptic eddies in mid-latitudes. The second factor relates to the planetary size and the third factor is the vertical structure of the atmosphere, associated with eigenvalues of the vertical mode in the heat equation. The combination of these three factors within the characteristic equation also
determines the location and number of eddy-driven jets in mid-latitudes.
How to cite: Moon, W., Lee, S., Vanderborght, E., Manucharyan, G., and Dijkstra, H.: An idealized model for the spatial structure of the eddy-driven Ferrel cell in mid-latitudes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7843, https://doi.org/10.5194/egusphere-egu25-7843, 2025.
Regional trends in heat extremes are significantly influenced by large-scale atmospheric circulation changes across the Northern Hemisphere, with circulation-induced changes contributing up to one-third of the observed warming in regions like Western Europe. Understanding whether these trends are driven by external forcing or internal variability is key for improving model evaluation, trend detection, attribution, and reducing uncertainties in future climate projections. Here, we present a novel methodological framework to isolate the forced dynamic components of heat extremes. We nudge tropospheric winds in CESM2 pi-control simulations towards transient climate conditions, which provides an estimate of the forced thermodynamic component. By subtracting these thermodynamic contributions from the large ensemble mean, we effectively isolate the forced dynamic contributions to heat extremes. Our results reveal distinct regional patterns. Previously identified heatwave hotspots such as the Pacific Northwest, Central Europe, South Siberia, and North China/Mongolia exhibit substantial warming of up to 1°C since 1950, which is attributable to forced circulation changes. In contrast, forced circulation chnages induces cooling of up to 1°C in regions such as the northeastern United States, parts of Asia, and central Africa. We have rigorously tested the sensitivity of our approach through various experiments and nudging strategies. This framework provides a valuable tool for disentangling forced thermodynamic and dynamic signals from internal variability, offering critical insights to reduce uncertainties in future climate projections.
How to cite: Singh, J., Sippel, S., Gu, L., Knutti, R., and Fischer, E.: A Methodological Framework to Isolate Forced Dynamic Responses in Heat Extremes Using Nudged Climate Simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18003, https://doi.org/10.5194/egusphere-egu25-18003, 2025.
Observed summer temperature trends differ strongly around the northern hemispheric latitudes. Besides changes in aerosol emissions, changes in atmospheric circulation patterns - whether forced or not - are expected to contribute to considerable variation in summer temperature trends. Different statistical and machine learning methods have been developed and applied to quantify this contribution of circulation changes to summer temperatures. Here we test the accuracy of multiple methods by applying them to historical climate simulations and comparing the circulation contribution obtained by different methods to trends found in nudged circulation simulations with wind fields of the historical simulations but pre-industrial control forcing. After validating the methods we apply them to ERA5 and over the entire northern hemispheric mid-latitudes (over land). Our results consistently suggest that especially over Europe circulation changes have contributed to an increase in summer temperatures. In parts of central Asia and eastern North America, circulation changes have contributed to a cooling in summer temperatures.
While providing a systematic overview of circulation contributions to local temperature trends in the northern hemispheric mid-latitudes we also show how nudging experiments can help to validate and consolidate methods. We argue that such method evaluation studies become increasingly important with the ongoing expansion of applications of statistical and machine learning analyses on the observational record.
How to cite: Pfleiderer, P., Merrifield, A., Dunkl, I., and Sippel, S.: Multi-method quantification of the contribution of circulation changes to summer temperature trends in the northern hemispheric mid-latitudes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4119, https://doi.org/10.5194/egusphere-egu25-4119, 2025.
It is generally thought that the climate responses of El Niño-Southern Oscillation (ENSO) variability and precipitation variability are tightly linked. The reasoning is that ENSO variability leads to sea-surface temperatures and surface heat flux variability, affecting the net-energy input (NEI) and thereby precipitation. We show that equally important are changes in the gross moist stability (GMS), which is the sensitivity of the atmospheric circulation to changes in the NEI. We analyze the variance of vertical velocity in monthly outputs of the Community Earth System Model 2 Large Ensemble. Under the SSP3-7.0 scenario, we find that variance of vertical velocity changes result from the competition of spatially varying responses of NEI variance and GMS. While NEI variance changes are complex and influenced by many mechanisms such as ENSO and radiative feedbacks, GMS increases in the sub-tropics can be explained by the rising tropopause and GMS decreases near the equator can be can be explained by the enhanced heating and moistening of the near-surface eastern equatorial Pacific. Together, these changes have important implications for future changes in precipitation variability in the tropics.
How to cite: Xuan, Z., Kroll, C., and Jnglin Wills, R.: Competing influences of energy budget variability and gross moist stability on tropical circulation variance under climate change, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15963, https://doi.org/10.5194/egusphere-egu25-15963, 2025.
In the last decades, there has been an ongoing discussion whether the winter North Atlantic Oscillation (NAO) is a zonally-symmetric hemispheric mode of variability driven by transient eddies and strongly coupled to the stratosphere [Arctic Oscillation/Northern Annular Mode (AO/NAM)]; or a regional mode of variability forced locally by transient eddies (NAO) but with associated hemispheric anomalies related to stationary eddies in the Circumglobal Waveguide Pattern (CWP). We revisit this question using zonal wavenumber decomposition of NAO-related circulation anomalies in reanalysis (ERA5, NCEP-NCAR). At upper-tropospheric levels, the NAO exhibits a wave-like structure that resembles the CWP, where wavenumber 3 seems to dominate at subpolar latitudes and wavenumber 5 is more prominent at suptropical latitudes. Wavenumber 4 does not significantly contribute to the NAO pattern in the total field. The wave activity flux of NAO-related variability reveals downstream propagation and splitting of wave energy which appears to be consistent with the meridional component of the wind and with theoretical arguments of stationary wavenumber based on the background flow. These results support the relevance of large-scale stationary waves in the hemispheric signature of the NAO. To further diagnose the tropospheric propagation of NAO-related anomalies, additional analysis using ray tracing and experiments with a linear barotropic model are performed.
How to cite: Brotons, M., García-Serrano, J., and Haarsma, R. J.: Large-scale tropospheric wave activity involved in the winter NAO-related variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19675, https://doi.org/10.5194/egusphere-egu25-19675, 2025.
Future projections of the European climate suffer from uncertainties in the changes of the North Atlantic jet stream. Previous studies of multi-model ensembles have suggested that the jet will shift poleward, and that the shift is anticorrelated with the simulated present-day latitude of the jet. Model basic state biases are a possible cause for the uncertainty, but their effect is difficult to assess because the spread in simulations may be caused by any inter-model differences. Here, we isolate the effect of model biases on future projections by modifying the basic state of a single atmospheric model with a run-time correction method aiming to adjust the model climatology towards those of three other models and a reanalysis. The effect of model biases was found to be strongly seasonal. In winter, changes in the frequencies of two of the three preferred positions of the jet were found to be sensitive to the model biases, but in summer the impact of biases is small relative to the magnitude of the changes. The results demonstrate that even though the anticorrelation of jet latitude and shift is only partly caused by biases, there is potential to reduce uncertainty in jet stream changes by improving model basic states.
How to cite: Koskentausta, J., Karpechko, A., Köhler, R., Levine, X., Wijngaard, R., and Sinclair, V.: The effect of model biases on the simulated future changes of the North Atlantic jet stream, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10703, https://doi.org/10.5194/egusphere-egu25-10703, 2025.
