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.