CL4.7
Orals: Thu, 27 Apr | Room 0.31/32
This contribution discusses the link between migrations in the intertropical convergence zone (ITCZ) and changes in the Atlantic meridional overturning circulation (AMOC), Atlantic multidecadal variability (AMV), and Pacific decadal oscillation (PDO). We use a coupled climate model that allows us to integrate over climate noise and assess underlying mechanisms. We use an ensemble of ten 300-yr-long simulations forced by a 50-yr oscillatory North Atlantic Oscillation (NAO)-derived surface heat flux anomaly in the North Atlantic, and a 4000-yr-long preindustrial control simulation performed with GFDL CM2.1. In both setups, an AMV phase change induced by a change in the AMOC’s cross-equatorial heat transport forces an atmospheric interhemispheric energy imbalance that is compensated by a change in the cross-equatorial atmospheric heat transport due to a meridional ITCZ shift. Such linkages occur on decadal time scales in the ensemble driven by the imposed forcing, and internally on multicentennial time scales in the control. Regional precipitation anomalies differ between the ensemble and the control for a zonally averaged ITCZ shift of similar magnitude, which suggests a dependence on timescale. Our study supports observational evidence of an AMV–ITCZ link in the twentieth century and further links it to the AMOC, whose long-time-scale variability can influence the phasing of ITCZ migrations. In contrast to the AMV, our calculations suggest that the PDO does not drive ITCZ migrations, because the PDO does not modulate the interhemispheric energy balance.
How to cite: Moreno-Chamarro, E., Marshall, J., and Delworth, T. L.: Linking ITCZ migrations to the AMOC and North Atlantic/Pacific SST decadal variability, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5305, https://doi.org/10.5194/egusphere-egu23-5305, 2023.
Global Earth System Models at storm-resolving resolutions (SR-ESM, with horizontal resolutions of ~4km) are being developed as part of the nextGEMS collaborative European EU’s Horizon 2020 programme. Through breakthroughs in simulation realism, these models will eventually allow us to understand and reliably quantify how the climate will change on a global and regional scale, and how the weather, including its extreme events, will look like in the future.
As part of the Storms & Ocean theme, we are exploring how resolving convective storms, ocean mesoscale eddies, and air-sea interaction on these scales influences the development of tropical SST anomalies and their influence on the mean climate (ITCZ and circulation biases) and its variability. Existing biases in the SR-ESM simulations in the first two development cycles are interpreted using the vertically integrated atmospheric energy budget to disentangle local and remote influences on tropical precipitation. More specifically, these biases are decomposed in hemispherically symmetric and antisymmetric components, which are linked, respectively, to biases in the atmospheric net energy input near the equator (tropical SST biases, low level clouds, etc) and to the cross-equatorial atmospheric energy flux (driven by inter-hemispheric contrast in net energy input, for instance biases in clouds in the southern ocean). We also explore the role that transient eddies, both of extratropical and tropical origin that are usually neglected in this framework, play in the global energetics and tropical precipitation patterns.
How to cite: Bordoni, S., D'Agostino, R., and Tompkins, A. M.: Tropical precipitation biases in nextGEMS storm-resolving Earth System Models, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5962, https://doi.org/10.5194/egusphere-egu23-5962, 2023.
El Niño-Southern Oscillation (ENSO) is one of the major climate phenomena impacting the globe. When a volcanic eruption happens, how ENSO will respond has still no consensus in proxy data though climate models tend to have an El Niño tendency. In this study, using 100 members of diverse (in locations and magnitude) idealized volcanic forcing ensembles, we found that the ENSO responses to north and south extra-tropical eruptions are related to the energy transport to the cooling hemisphere and involve direct and indirect responses through atmospheric and oceanic transport. The north extratropical forcing leads to more El Niño up to three years after eruptions, which is related to the direct atmospheric responses of the southward movement of ITCZ for transporting more energy to the north. The indirect oceanic transport then takes over afterward, leading to more La Niña due to more upwelling in the equatorial eastern Pacific. The south extra-tropical eruptions have less El Niño tendency due to the northward replacement of ITCZ. As the indirect oceanic transport also results in equatorial mean state changes, which may lead to distinct ENSO responses. The long-term ENSO responses from extra-tropical cooling will also be investigated through the simulations from the Extratropical-Tropical Interaction Model Intercomparison Project (ETIN-MIP) experiment.
How to cite: Fang, S.-W. and Timmreck, C.: An Energy Transport View of ENSO Responses to Volcanic Forcing, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7348, https://doi.org/10.5194/egusphere-egu23-7348, 2023.
Stratospheric Aerosol Injection (SAI) is a contentious geo-engineering proposal to lessen the impacts of global heating via an artificial shade of aerosols. While model results suggest that Earth’s global mean surface temperature (GMST) can indeed be stabilised via SAI, the same does not apply to other parts of the climate system. Especially the fate of a weakening Atlantic Meridional Overturning Circulation (AMOC) under SAI remains unclear.
We simulate two SAI deployment scenarios in the Community Earth System Model 2 (CESM2) to study whether a weakened AMOC can be stabilised or recovered. To obtain a strong AMOC response, both scenarios follow a high GHG emission pathway. At the same time, proportionally chosen aerosol injections stabilise the GMST at 1.5K above pre-industrial conditions. The scenarios only differ in their deployment times: aerosol injections start either early (SAI 2020) or late century (SAI 2080).
Both SAI scenarios reach the target GMST. However, we find that SAI only mitigates rather than decisively reverses AMOC decline. This relatively mild oceanic response stands in contrast to efficient surface cooling. As a result, both deployment scenarios lead to drastically different climate states. In particular, late-century deployment (SAI 2080) creates a striking temperature gradient from a cold northern to a warm southern hemisphere. This phenomenon potentially stems from impaired meridional heat transport of a much weaker AMOC in SAI 2080 compared to SAI 2020.
Our findings mirror recent results on fast negative emission scenarios displaying a similar inter-hemispheric gradient likely connected to slow AMOC recovery. This affirms the need for carefully tailoring SAI deployment strategies should the technology ever be considered as part of the climate action portfolio.
How to cite: Pflüger, D., Wieners, C., and Van Kampenhout, L.: Cool, but Different: Climate Response to Solar Geo-Engineering Mediated by the AMOC, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7566, https://doi.org/10.5194/egusphere-egu23-7566, 2023.
We measure baroclinic eddy heat transport in a differentially heated rotating annulus laboratory experiment to test mesoscale ocean eddy parameterization frameworks. The differentially heated rotating annulus comprises a fluid placed between two upright coaxial cylinders which are maintained at different temperatures, usually with a cooled inner cylinder and a heated outer. The annular tank is placed on a rotating table which provides conditions for baroclinic eddies to develop and equilibrate in different flow regimes, depending upon the imposed conditions. As the rotation speed is increased, the equilibrated flow changes from a steady or periodically varying low wavenumber pattern to a more complex, time-varying flow dominated by higher wavenumbers. With a topographic beta effect produced by conically sloping upper boundary, more complex flow regimes are observed combining zonal jets and eddies forming one or more parallel storm tracks. With this possibility to explore varied flow regimes, our experimental approach combines laboratory calorimetry and visualization measurements along with numerical simulations to derive the eddy heat transport properties. In the following, we focus on the visualisation measurement to test related assumptions and parametric dependencies for eddy transport. We first test the assumptions of a down-gradient temperature flux-gradient relationship, determining coefficients of the eddy transport tensor, and exploring scaling relations for the eddy coefficients. A clear statistical scaling is found between eddy heat fluxes and physical variables such as eddy energy, the beta effect, and the temperature contrast.
How to cite: Qian, C., Read, P., and Marshall, D.: Energetic Constraints on Baroclinic Eddy Heat Transport in a Rotating Annulus, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-103, https://doi.org/10.5194/egusphere-egu23-103, 2023.
In this study, we present a variety of parameterizations for simulating ocean eddy dynamics including novel viscous and kinetic energy backscatter closures. Their effect is analyzed using new diagnostics that allow for application on unstructured meshes.
Ocean mesoscale eddy dynamics play a crucial role for large-scale ocean currents as well as for the variability in the ocean and climate. The interactions between eddies and the mean flow affect strength, position and variations of ocean currents. Mesoscale eddies have a substantial impact on oceanic heat transport and the coupling between the atmosphere and ocean. However, at so-called eddy-permitting model resolutions around ¼°, eddy kinetic energy and variability is often substantially underestimated due to excessive dissipation of energy. Despite ever-increasing model resolutions, eddy-permitting simulations will still be used in uncoupled and coupled climate and Earth system simulations for years to come.
To improve the presentation of eddy dynamics in such resolution regimes, we present and systematically compare a set of viscous and kinetic energy backscatter parameterization with different complexity. These schemes are implemented in the unstructured grid, finite volume ocean model FESOM2 and tested in both idealized channel and global ocean simulations. We show that kinetic energy backscatter and adjusted viscosity parameterizations can alleviate some of the substantial eddy related biases, for example biases in sea surface height variability, mean currents and in water mass properties. We then further analyze the effect of these schemes on energy and dissipation spectra using new diagnostics that can be extended to the unstructured grid used by FESOM2. The rigorous intercomparison allows to make informed decisions on which schemes are the most suitable for a given application, considering the complexity of the schemes, their computational costs, their adaptability to various model resolutions and any simulation improvements related to a specific scheme. We will show that novel viscous and kinetic energy backscatter schemes outperform previously used, classical viscous closures. Furthermore, when compared to higher resolution simulations, they are computationally less expensive but achieve similar results.
How to cite: Juricke, S., Danilov, S., Oliver, M., Kutsenko, A., and Bellinghausen, K.: Simulating oceanic mesoscale eddy dynamics: A comparison of novel parameterizations and energy diagnostics and their impact on the global ocean circulation, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15746, https://doi.org/10.5194/egusphere-egu23-15746, 2023.
Linked to increased sea ice loss, the Arctic region has warmed at least four times faster than the global average over the past 40 years. Mutual links between amplified Arctic warming with changes and variability in midlatitude weather have been discussed in several studies. Nevertheless, the lack of consistent conclusions between observations and model simulations obfuscates the interpretation behind the mechanisms of Arctic-midlatitude teleconnections. To contribute to the understanding of Arctic-midlatitude connections that occur in conditions of amplified Arctic warming, we applied causal discovery to analyse causal and contemporaneous links. Initially, we calculated causal dependencies for monthly mean ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and Hadley Centre Sea Ice and Sea Surface Temperature among local and remote processes. Then, by comparing causal graphs detected in reanalyses data with a number of climate model historical simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we assessed the performance of climate models and evaluated the robustness of the observed Arctic-midlatitude connections in the current climate. By comparing causal graphs from the CMIP6 historical and Scenario Model Intercomparison Project (ScenarioMIP) we estimated future changes in Arctic-midlatitude teleconnections towards the end of the century. In this study, we focus on the differences in the mechanism of Arctic-midlatitude teleconnections that occur during the boreal cold season, i.e. early winter (October-November-December), winter (December-January-February), and late winter (January-February-March). In this study, we will present the major findings of Galytska et al., 2022 discussing how causal model evaluation helps to summarize major differences between causal interdependencies detected in observations and simulated by a number of climate models. Understanding these differences can be the basis for further improvement of the representation of Arctic-midlatitude teleconnections in climate models.
References.
Evgenia Galytska, Katja Weigel, Dörthe Handorf, Ralf Jaiser, Raphael Köhler, Jakob Runge, and Veronika Eyring. Causal model evaluation of Arctic-midlatitude teleconnections in CMIP6. Authorea. October 06, 2022. DOI: 10.1002/essoar.10512569.1
How to cite: Galytska, E., Weigel, K., Handorf, D., Jaiser, R., Köhler, R., Runge, J., and Eyring, V.: Causal model evaluation of Arctic-midlatitude process during the boreal cold season in CMIP6, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-11159, https://doi.org/10.5194/egusphere-egu23-11159, 2023.
Understanding the variability of summer-time Arctic sea ice at interannual to multidecadal time scales in the midst of anthropogenically forced sea ice decline is crucial for better predictions of sea ice conditions in the future climate and rapid changes in sea ice. Here, we apply time-frequency analysis to study the modes of variability, extreme events and the trend in the September Arctic sea ice in 100–150 year datasets. We extract the non-linear trend for the sea ice area and provide an estimate for the anthropogenic-driven sea ice loss. For the used dataset, the anthropogenic-related sea ice loss is found to have a rate of ~-0.25 million km2 per decade in the 1980’s and accelerating to ~-0.47 million km2 per decade in 2010’s. By assuming the same rate of sea ice loss in the future, and without the contribution of the internal variability and feedbacks, we can approximate the occurrence of summer sea-ice free Arctic to be around 2060. Regarding the dominant modes of variability for the September sea ice, we find that they have periods of around 3, 6, 17, 28 and 55 years, and show what drives these modes and how they contribute to sea ice extreme events. The main atmospheric and oceanic drivers of sea ice modes include the Arctic oscillation and Arctic dipole anomaly for the 3-year mode, variability of sea surface temperature (SST) in Gulf Stream region for the 6-year mode, decadal SST variability in the northern North Atlantic Ocean for the 17-year mode, Pacific decadal oscillation (PDO) for the 28-year mode, and Atlantic multidecadal Oscillation (AMO) for the 55-year mode. Results show that changes in the sea ice due to internal variability can be as large as forced changes thus can slow down or accelerate the background anthropogenic-driven sea ice loss. By applying the same method, we also present modes of variability and trend of sea ice in the large ensemble global model simulations of EC-Earth model (SMHI-LENS) for the future climate projections and different climate scenarios.
How to cite: Karami, M. P., Koenigk, T., and Tremblay, B.: Variability of summer-time Arctic sea ice: the drivers and the contribution to the sea ice trend and extremes, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3492, https://doi.org/10.5194/egusphere-egu23-3492, 2023.
On the global scale, the frequency of marine heatwaves (MHWs) is projected to increase further in the twenty-first Century. In our earlier study we demonstrate that the high-impact major marine heatwaves over the northeast Pacific are co-located with a systematically-forced long-term warming pool, which we attribute to forcing by elevated greenhouse gases levels (GHG), and the recent industrial aerosol-load decrease (Barkhordarian et al., 2022). In current study we show that the magnitude of Arctic MHWs has significantly increased since 2006, and has exceeded the pre-industrial climate bounds since then. We here perform statistical attribution methodologies, and provide a quantitative assessment of whether GHG forcing was necessary for the Arctic MHWs to occur, and whether it is a sufficient cause for such events to continue to repeatedly occur in the future.
The probability of necessary causation of Arctic MHWs intensity, increases with increasing the severity of MHWs, and saturate to 1.0 by the time MHWs intensity exceeds the 2°C, indicating that any MHWs over the Arctic with an intensity higher than 2°C is entirely attributable to the inclusion of GHG forcing. These amplified extreme MHWs in the Arctic have each been accompanied by a record decline in Arctic Sea ice, in particular in the years 2007, 2012, 2016 and 2020. Over the last decade, MHWs occur in the Arctic where sea ice melt in June is 4 %/year faster, the ice-free season is ~3 months longer, the ocean heat-uptake is 50 W/m2 higher, and the sea surface temperature is ~2°C warmer, in comparison with the previous decade. In autumn surface evaporation rate is increasing, the increased low clouds favor more sea ice melt via emitting stronger longwave radiation. In summary, prolonged Arctic marine heatwaves, triggered by faster early summer sea ice melt, will accelerate Arctic warming, and cause Arctic Sea ice extent to shrink even faster in the near future.
Barkhordarian, A., Nielsen, D.M. & Baehr, J. Recent marine heatwaves in the North Pacific warming pool can be attributed to rising atmospheric levels of greenhouse gases. Nature Communications Earth & Environment, 3, 131 (2022). https://doi.org/10.1038/s43247-022-00461-2
How to cite: Barkhordarian, A., Nielsen, D., and Baehr, J.: The contribution of Arctic marine heatwaves to the minimum sea ice extent as compound events, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12939, https://doi.org/10.5194/egusphere-egu23-12939, 2023.
The Arctic has warmed substantially over the last decades and will continue to do so owing to global warming in conjunction with polar amplification. The changing mean state poses many challenges to the construction, evaluation and calibration of subseasonal-to-seasonal forecasting systems, because it puts into question the representativeness of the system's retrospective forecasts (reforecasts). Furthermore, any inconsistencies with observed trends degrade the forecast skill and point to deficiencies in either the physical modelling or the initialization methods. Here, we assess the consistency of boreal winter trends of surface air temperature (SAT) in the Eurasian Arctic between the ERA5 reanalysis and ECMWF sub-seasonal reforecasts initialised from ERA5, for the 35-year period 1986-2021. We present methods to quantify robustness and importance of the observed trends, and to quantify the consistency of reforecast trends with these observed trends. We find that, in large parts of the marine Arctic, the reforecasts clearly underestimate the reanalsyis warming trend of about 1 K per decade at lead times beyond two weeks. For longer lead times, the reforecast trend is less than half of the reanalysis trend, with very high statistical significance. We present a series of numerical experiments to investigate potential reasons for the trend underestimation. These concern the sea-ice thermodynamic coupling to the atmosphere, impact of sea surface temperatures, and possible remote atmospheric influences from the North Atlantic and the Tropics. The outcome of these experiments provides guidance for future improvements in the physical forecast model and data assimilation methods needed to faithfully represent and predict Arctic climate variability and change.
How to cite: Tietsche, S., Vitart, F., Mayer, M., Weisheimer, A., and Balmaseda, M.: Underestimation of Arctic warming trends in sub-seasonal forecasts, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14141, https://doi.org/10.5194/egusphere-egu23-14141, 2023.
Posters on site: Thu, 27 Apr, 16:15–18:00 | Hall X5
West Antarctica has been losing their ice mass due to global warming, and the El Niño has delayed the ice mass loss by inducing weakening of the Amundsen Sea Low (ASL), encouraging of poleward moisture flux and consequent extreme precipitation. However, it is not yet revealed whether the delay effect will continue in the future. We analyzed future scenarios from the CMIP6 Earth system models (ESMs) to identify future change and identified that the El Niño-driven mass increase by precipitation will vanish in the high-emission future scenarios. Precipitation anomaly in response to El Niño starts to be negative from the 2050s in the SSP5-8.5 and from the 2060s in the SSP3-7.0, which means that the El Niño-driven delay effect disappears. It is because the moisture transport into West Antarctica is prevented due to east-equatorward migration of El Niño-induced ASL anomaly as global warming intensifies. The strengthened polar jet associated with positive Southern Annular Mode (SAM) trend moves the ASL anomaly east- and equatorward under global warming.
How to cite: Lee, H., Jin, E. K., Kim, B.-H., and Lee, W. S.: Vanishing the El Niño-induced delay effect on the ice mass loss of West Antarctica under global warming, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-972, https://doi.org/10.5194/egusphere-egu23-972, 2023.
Tropical instability waves (TIWs) are the dominant mesoscale variability in the eastern equatorial Pacific Ocean. TIWs have direct impacts on the local hydrology, biochemistry and atmospheric boundary layer, and feedback on ocean circulations and climate variability. In this study, the basic characteristics of Pacific Ocean TIWs simulated by an eddy-resolving ocean model and a coupled general circulation model are evaluated. The simulated TIW biases mainly result from the mean climatology state, as TIWs extract eddy energy from the mean potential and kinetic energy. Both the oceanic and coupled models reproduce the observed westward propagating large-scale Rossby waves between approximately 2-8N, but the simulated TIWs have shorter wavelengths than the observed waves due to the shallower thermocline. Meanwhile, the weak meridional shears of background zonal currents and the less-tilted pycnocline in these two models compared to the observations causes weak barotropic and baroclinic instability, which decreases the intensity of the simulated TIWs. We then contrast the TIWs from these two models and identify the roles of atmospheric feedback in modulating TIWs. The latent heat flux feedback is similar to observation in the coupled model but absent in the ocean model, contributing to the stronger standard deviation (STD) of the TIW SST in the ocean model. The ocean model is not able to capture realistic air-sea interaction processes when forced with prescribed atmospheric forcing. However, the misrepresented atmospheric feedback in the ocean model tends to decrease the sea surface height (SSH) variability, and the current feedback damping effect is stronger in the ocean model than in the coupled model. Combined with weaker barotropic conversion rate and baroclinic conversion rate in the ocean model than in the coupled model, the STD of the TIW SSH in the ocean model is weaker.
How to cite: Tianyan, L. and Yongqiang, Y.: Tropical Instability Waves in a High-Resolution Oceanic and Coupled GCM, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1773, https://doi.org/10.5194/egusphere-egu23-1773, 2023.
Widespread observed and projected increases in warm extremes, along with decreases in cold extremes, have been confirmed as being consistent with global and regional warming. However, based on observational datasets and state-of-the-art CMIP6 model simulations, we disclose that the decadal variation in the frequency of the surface air temperature (SAT) extremes over mid- to high latitudes over Eurasia (MHEA) in winter is primarily dominated by the thermodynamical effect of the surface heat fluxes release over the midlatitude North Atlantic induced by Atlantic multidecadal oscillation (AMO), which even masks the dynamical large-scale Rossby wave propagation. Besides, the stronger Atlantic meridional overturning circulation (AMOC) gives rise to both warm and cold extremes through increasing the variance of winter SAT over MHEA due to thermodynamical heat release and enhanced dynamical Rossby wave propagation.
How to cite: Wang, H.: Frequency of winter temperature extremes over Eurasia dominated by variabilities over the Atlantic Ocean, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3106, https://doi.org/10.5194/egusphere-egu23-3106, 2023.
Between 1998 and 2012, there was a smaller rate of global warming, known as the "global warming hiatus". One of the suggested causes is that during this period additional heat sequestration occurs into the deep ocean layers such that deep layers warm at a greater rate than the upper layers. This research is focused on the origins of changes in ocean heat content during the hiatus period, defined in this case as the last 10 years of adjoint model run, where the cost function is defined. Adjoint sensitivities are used to determine the influence of atmospheric forcing (heat and freshwater fluxes and wind stress) on the ocean heat content.
The MIT General Circulation Model with a resolution of 2° x 2° is used over the period 1978-2008 to determine adjoint sensitivities of the globally and temporally (over the last 10 years, defined as hiatus period) integrated vertical heat fluxes across various depth levels. The contributions of different forcing components to the vertical heat flux anomalies are obtained from the scalar product between sensitivities and the anomalies of the atmospheric forcing. For this, the atmospheric forcing anomalies are computed with respect to the climatology calculated over the period 1948-1968 when there was almost no change in the ocean heat content.
A more pronounced increase in ocean heat uptake during the hiatus period has been evidenced by the forward run of the model. Wind anomalies represent more than half of the contribution to the increase in heat flux across 300m, suggesting that the excess of heat stored by the ocean is transferred adiabatically to the deeper layers and that the zonal wind is one of the major drivers of ocean heat uptake. In the Southern Ocean, the sensitivities to the wind stress change from positive to negative when the hiatus starts. This indicates that, during the hiatus, the rate of change of ocean heat content is opposite to the one of the wind stress. The Southern Ocean presents smaller values of the computed amplitude weighted mean time, meaning that this region has the fastest response to changes in surface atmospheric forcing.
How to cite: Albert Fernández, C., Köhl, A., and Stammer, D.: Sensitivity of ocean heat content to atmospheric forcing in the period of global warming hiatus, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1985, https://doi.org/10.5194/egusphere-egu23-1985, 2023.
Poleward atmospheric energy transport is determined by the overall equator-to-pole radiative imbalance. As this imbalance is projected to remain fairly constant in end-of-century greenhouse gas forcing scenarios, an increase in poleward latent heat transport must be accompanied by a reduction in dry static energy flux. Since midlatitude energy transport is dominated by the eddy component, changes in the energy budget go hand in hand with changes in cyclone characteristics. From a dynamical perspective, the enhanced condensation due to climate change promotes intensification, prolongs lifetime of cyclones, and can increase stationarity of anticyclones. However, it also tends to increase static stability and thereby reduce baroclinicity, which is another important driver of cyclone development. Additionally, baroclinicity is projected to increase at upper levels due to tropical amplification, to decrease at low levels as a result of Arctic amplification, and to be affected by land-sea temperature contrast changes. As these processes are at play simultaneously, isolating the role of moisture is rather complicated. Therefore, in addition to coupled climate model simulations we use idealized aquaplanet simulations to single out the effects of individual physical mechanisms and address the question: if the overall poleward energy transport remains largely unaffected by global warming, how do cyclone characteristics change in the presence of increased moisture in the atmosphere?
For bridging the gap between the global energy flux and synoptic-scale features, we analyse the role of increasing moisture for shaping midlatitude storm tracks in present and future climates from both an Eulerian and a Lagrangian perspective. We apply the moist static energy (MSE) framework that allows partitioning atmospheric energy fluxes into eddy and mean, dry and moist components. Here, eddies are related to cyclones and anticyclones, while the mean energy flux is associated with planetary waves and the mean meridional overturning circulation. The goal is to relate the eddy MSE fluxes to feature-based results including extratropical cyclone number, lifetime, intensity, location, and tilt. By combining results from both global-scale eddy energy fluxes and synoptic-scale feature quantities, we aim to improve the understanding of the role of latent heating in shaping the mean properties of extratropical storm tracks. Therefore, a central question of this project is whether and how changes in cyclone quantities can be linked to changes in latent heat transport and release. Building on what we learn from bringing the two perspectives together, we will proceed to investigating the impact of increased latent heating on midlatitude storm tracks.
How to cite: Zibell, J., Schemm, S., and Hermoso Verger, A.: Combining global-scale atmospheric heat transport and synoptic-scale extratropical cyclone characteristics to understand the role of latent heating for midlatitude storm tracks, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7216, https://doi.org/10.5194/egusphere-egu23-7216, 2023.
In recent theory trying to explain the origin of low-frequency atmospheric variability, the concept of eddy-memory has been suggested. In this view, the effect of synoptic scale heat fluxes on the mean flow depends on the history of the mean meridional temperature gradient. Mathematically, this involves a convolution of an integral kernel with the mean meridional temperature gradient over past times. In atmospheric studies, it has been proposed that the shape of this integral kernel is linked to the baroclinic wave life cycle. However, this hypothesis has yet to be supported by numerical and observational evidence. In this study we use a low-order two layer quasi-geostrophic atmospheric model (Demaeyer et al., 2020). By perturbing the model with a known forcing, linear response theory can be used to estimate the shape of the integral kernel. Using this methodology, we find an integral kernel that resembles the shape of an exponentially decaying oscillation, different from the simple exponentially decaying integral kernel assumed in most previous studies. By computing the energies and performing a sensitivity analysis, we link the shape of the integral kernel to atmospheric dynamical processes.
References:
J. Demaeyer, L. De Cruz, and S. Vannitsem. qgs: A flexible python framework of reduced-order multiscale climate
models. Journal of Open Source Software, 5(56):2597, 2020.
How to cite: Vanderborght, E., Demeayer, J., Dijkstra, H., Manucharyan, G., and Moon, W.: Physics of the Eddy Memory Kernel of a Baroclinic Midlatitude Atmosphere, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3282, https://doi.org/10.5194/egusphere-egu23-3282, 2023.
We show that the most prominent of the work theorems, the Jarzynski equality and the Crooks relation, can be applied to the momentum transfer at the air-sea interface using a hierarchy of local models. In the more idealized models, with and without a Coriolis force, the variability is provided from a Gaussian white-noise which modifies the shear between the atmosphere and the ocean. The dynamics is Gaussian and the Jarzynski equality and Crooks relation can be obtained analytically solving stochastic differential equations. The more involved model consists of interacting atmospheric and oceanic boundary-layers, where only the dependence on the vertical direction is resolved, the turbulence is modeled through standard turbulent models and the stochasticity comes from a randomized drag coefficient. It is integrated numerically and can give rise to a non-Gaussian dynamics. Also in this case the Jarzynski equality allows for calculating a dynamic-beta ßD of the turbulent fluctuations (the equivalent of the thermodynamic-beta ß=(kB T)-1 in thermal fluctuations). The Crooks relation gives the ßD as a function of the magnitude of the work fluctuations. It is well defined (constant) in the Gaussian models and can show a slight variation in the more involved models. This demonstrates that recent concepts of stochastic thermodynamics used to study micro-systems subject to thermal fluctuations can further the understanding of geophysical fluid dynamics with turbulent fluctuations.
How to cite: Wirth, A.: Jarzynski equality and Crooks relation for local models of air-sea interaction, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1066, https://doi.org/10.5194/egusphere-egu23-1066, 2023.
River temperature plays an important role in numerous biological and ecological processes within the Yenisei River basin and it is very sensitive to changes in climatic characteristics and anthropogenic disturbances. Water temperature and river discharge characterize heat or energy flux, which is important in northern latitudes for river freeze-up and ice break-up processes and thermal riverbank erosion. The changes in heat flux in river estuary can also significantly impact various biophysical processes in coastal ocean waters.
We use new water temperature data and river discharge records for 12 observational gauges in the Yenisei River basin to analyze changes in water temperature and heat flux from upstream to downstream over 1950-2018. Preliminary results show significant increases for most gauges in maximum annual water temperature as well as in 10-days mean water temperature during May-June and September-October. There were no significant changes in river temperature during July-August unless the gauges were impacted by reservoir regulations. The river heat flux has significantly increased in central and northern parts of the Yenisei basin and decreased in the south, mainly due to discharge variability.
The gridded hydrological Water Balance Model (WBM) developed at the University of New Hampshire, that takes into account various anthropogenic activities, was used to simulate river discharge and water temperature for entire Yenisei basin with a 5 minute spatial resolution river network using several climate reanalysis products (MERRA2, ERA5 and NCEP-NCAR). The modeled results were verified with observational data and simulations using the MERRA2 climate drivers demonstrated the best match with observations (Nash-Sutcliffe model efficiencies coefficients were greater than 0.5 for both river temperature and discharge). Maps of modeled changes in runoff, river temperature and heat flux show the opposite changes in the southern and northern parts of Yenisei basin. The model simulations correspond well with observational data even for heavily disturbed river reaches. For example, they show unfrozen water with positive temperatures during the winter below large dams and reservoirs.
The WBM was also applied to project changes in water temperature, discharge and heat flux up to 2100 for several SSPs and GCMs from CMIP6. In spite of heterogenous projected changes in these parameters across Yenisei basin, significant increases in discharge and heat flux to the Arctic Ocean are expected.
How to cite: Shiklomanov, A., Lammers, R., Prusevich, A., Panyushkina, I., and Meko, D.: Changes in river temperature, discharge and heat flux based on new observational data for Yenisei basin and modeling, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9914, https://doi.org/10.5194/egusphere-egu23-9914, 2023.
As we observe and expect severe changes in the Earth’ climate, the analyses of past climate state transitions is of major value for improving our Earth system understanding. Under this objective, the last deglaciation (~ 21 ka to 9 ka before present), the transition from the Last Glacial Maximum (LGM) to the Holocene, is an ideal case study. During this transition, the orbital configuration gradually changed and greenhouse gases have risen, which caused a sharp decline in northern hemisphere ice sheets and an increase in the global mean surface temperature.
We create an ensemble of deglaciation simulations with a modified version of the Planet Simulator, an Earth system model of intermediate complexity (EMIC). We produce single and combined forcing simulations for further investigation from a thermodynamic perspective. The response to the transiently changing radiative forcing is investigated in terms of energy and entropy budgets of the atmosphere. Here, we focus on the deglacial evolution of the material entropy production (MEP). Its contributions represent the strength of major climate features such as the kinetic energy generation rate, vertical and horizontal heat transport and the hydrological cycle. Preliminary results show an increase of the global mean MEP from the LGM to the Holocene because of a strengthening of the hydrological contribution. In contrast, the relative importance of kinetic energy dissipation and turbulent heat diffusion in the boundary layer decrease. Our work can provide the basis for investigating the MEP as a diagnostic quantity with other models and for other climate state transitions.
How to cite: Racky, M., Trombini, I., Pfeilsticker, K., Weitzel, N., and Rehfeld, K.: Thermodynamic assessment of simulations of the last deglaciation with an Earth system model of intermediate complexity, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12377, https://doi.org/10.5194/egusphere-egu23-12377, 2023.
Arctic amplification (AA) is largely attributed to the effect of sea ice decline leading to greater surface solar absorption and further ice melt, and the vertical structure of the warming. The latter aspect evokes the positive lapse-rate feedback (LRF), which is commonly understood as an effect of stable stratification: The warming in the Arctic is particularly strong close to the surface, but muted aloft. This limits the outgoing long-wave radiative flux at the top-of-the-atmosphere (TOA) relative to vertically uniform warming.
We estimate the future Arctic LRF in 43 global climate models (GCMs) from the highest emission pathway SSP5 of CMIP6. The GCMs simulate a large spread of future AA (2-8 K above global warming) and Arctic LRF (1-4 K warming contribution) at the end of the century 2070-2099. Our work aims to identify emerging relationships between this spread and observable aspects of the current climate to ultimately narrow down the range of future Arctic climate predictions.
Previous studies have identified an emerging relationship for the surface-albedo feedback based on the observed seasonal cycle of Arctic sea ice. We similarly derive a positive relationship (r=0.70) between future and seasonal LRF, but due to its nature, no direct observation of the LRF exists. However, we find relationships between the future LRF and observable sea ice metrics, namely sea ice concentration, seasonality, extent and area. From these relationships, the sea ice concentration provides the strongest correlation (r=-0.76) for the area-averaged Arctic sea ice cover. This relationship implies a contribution of the LRF to future Arctic warming of approx. 2 K, which further relates to an AA of 4 K above global average at the end of the century.
We further emphasise the physical meaning behind our constraint: The negative emerging relationship implies that models with a lower Pan-Arctic sea ice concentration produce a larger LRF in the future. However, when dividing the entire sea ice area into regions of sea ice retreat (SIR) and persisting sea ice (PSI) in the future prediction, the relationship becomes positive over these two area-averaged regions. Thereby, the negative overall relationship is merely a result of the area-size distribution of SIR vs. PSI across the spread of model simulations. We conclude that while the Pan-Arctic perspective enables the emergent constraint, the physical meaning is hidden: A higher initial sea ice concentration produces a stronger positive Arctic LRF by setting the stage for greater sea ice retreat.
How to cite: Linke, O., Feldl, N., and Quaas, J.: Emergent constraints on the future Arctic lapse-rate feedback, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13091, https://doi.org/10.5194/egusphere-egu23-13091, 2023.
The Horizon 2020 project PolarRES is coordinating a large international consortium of regional climate modelling groups in building a new ensemble of regional climate projections out to 2100. The ensemble is built at very high resolution (~12km) and using common domains, and set-ups to give directly comparable model outputs. At the same time, all regional climate models have been upgraded to a next-generation set-up, producing an ensemble of unprecedented sophistication.
We use a storyline approach, focused on Arctic amplification and cyclones in the northern hemisphere and Southern Annular Mode variability in Antarctica, to select global climate models for forcing on the boundaries. Each regional climate modelling group will downscale ERA5 and multiple global climate models. The data produced from these simulations will be used to improve process understanding under present and future conditions as well as to identify impacts of climate change in the polar regions.
Here, we present the experimental protocol developed in PolarRES and give details of the different regional climate models used, their setup, processes and domains as well as an overview of the outputs and planned applications. We show preliminary analysis of hindcast outputs to assess the performance of the ensemble. We invite other regional climate modelling groups outside the PolarRES consortium to consider using the same CORDEX -compatible model set-up and we are happy to receive suggestions of further spin-off studies or requests for collaboration.
How to cite: Mottram, R., Mooney, P., and Torres, J. A. and the PolarRES Consortium: A first look at the new PolarRES ensemble of polar regional climate model storylines to 2100, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14470, https://doi.org/10.5194/egusphere-egu23-14470, 2023.
Posters virtual: Thu, 27 Apr, 16:15–18:00 | vHall CL
The zonal mean Hadley Cell (HC) has been reported to be expanding poleward in the last few decades. However, there has been no consensus on whether the zonal mean HC is strengthening or weakening. The features of longitudinally averaged HC are collectively modulated by various regional HCs, controlled by the regional differences in land-ocean distribution and topography. However, there have not been many studies exploring the variability and trend of regional HCs in a detailed manner. In this study, we examine the variability and long-term trend of the regional HC over the Asia-Pacific and explore the different factors contributing to the regional HC variability. Moist convection can regulate regional HCs on synoptic time scales through equatorial wave dynamics. The ocean–atmosphere coupled variability associated with the El Niño-Southern Oscillation (ENSO), and the modulation of tropical convection and equatorial waves are considered to exert a dominant control on the regional HC variability in the interannual timescale. In addition to the tropical forcing, the regional HC variability is also affected by fluxes transported by the midlatitude eddies from the subtropics to the tropics. In this study, we will be quantifying the relative role of these tropical and extratropical forcings in modulating the variability of regional HC over Asia-Pacific.
How to cite: Baruah, P. P. and Joseph Mani, N.: Understanding the variability and trend of the regional Hadley Cell over Asia-Pacific, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-273, https://doi.org/10.5194/egusphere-egu23-273, 2023.
The Arctic is warming at a rate faster than any other oceans, a phenomenon known as Arctic amplification that has widespread impact on the global climate. In contrast, the Southern Ocean (SO) and Antarctica have been cooling over the past decades. The projection of these regions under global warming has a non-negligible model spread. Here we show that under a strong warming scenario from 1950 to 2100, comparing a cutting-edge high-resolution climate model to a low-resolution model version, the increase of Arctic amplification is 3 °C more and the SO and Antarctica warming is 2°C less. Previously ice-covered Arctic Ocean will exhibit greater SST variability under future global warming. This is due to an increased SST increase in summer due to sea ice retreat. Extreme warming events in the Arctic and SO, known as marine heat waves (MHW) that influence the ecology, are largely unknown. We find that the MHWs in the Arctic and SO are twice as strong in the high-resolution model version, where the increasing intensity of MHWs in the Arctic corresponds to strong decline (<-6% per decade) of sea ice. In both the high-resolution and low-resolution models, the duration of MHWs in the Arctic and SO shows a declining trend under global warming. The much stronger MHWs in the high-resolution model could be caused by two orders of magnitude more ocean turbulent energy. For example, the spatial patterns of SO MHW intensity correspond to the pattern of SO EKE. We conclude that the Arctic amplification and MHWs at high latitudes might be underestimated by the current generation of climate models with low resolution, and the SO and Antarctica warming might be overestimated. Our eddy- and storm-resolving model is expected to open new frontiers on how the system responds to human activities in a high CO2 world by evaluating the impact on past and future climate and environmental extremes.
How to cite: Gou, R., Lohmann, G., and Wu, L.: Increase in Arctic amplification and high-latitude marine extremes in the 21st century as obtained from high-resolution modelling, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1649, https://doi.org/10.5194/egusphere-egu23-1649, 2023.
Global circulation patterns are analysed using the mean meridional circulation (MMC) from ERA-Interim for the period of 1979 – 2017. The global isentropic MMC consists of a single overturning cell in each hemisphere with net heat transport from the equator to the pole. Six clusters are identified from daily data that are associated with one of four seasons. Two solstitial MMC clusters represent either stronger or weaker circulation in the winter hemisphere. We show that long-term trends do not reflect a gradual change in the atmospheric circulation, but rather a change in the frequency of preferred short-term circulation regimes. Before the late 1990s the clusters showing a stronger (weaker) winter circulation are becoming less (more) frequent; from around year 2000 the trends have paused. These trends are in close agreement with the change in the low-stratospheric Antarctic ozone trends reported by earlier studies. Our findings also reveal a strong coupling between Southern and Northern Hemispheres during boreal winter. Following Hartmann et al. (2022), we hypothesize that anomalous polar vortex over Antarctica leads to anomalies in the sea surface temperatures (SST) in the tropical Pacific that impact the circulation in both hemispheres. Furthermore, we show that consecutive solstice season demonstrates coherent anomalies in the frequency of circulation regimes. We discuss possible reasons for such relationship.
References:
Hartmann, D. L., Kang, S., Polvani, L. & Xie, S.-P. The Antarctic ozone hole and the pattern effect on climate sensitivity. (2022) doi:10.1073/pnas.
How to cite: Rudeva, I., Boschat, G., and Lucas, C.: How can atmospheric trends be explained by changes in frequency of short-term circulation regimes and what is the role of the Antarctic ozone?, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4557, https://doi.org/10.5194/egusphere-egu23-4557, 2023.
