CL4.6
Orals: Thu, 27 Apr | Room F1
El Niño-Southern Oscillation (ENSO) teleconnections exhibit a strong dependency on seasonally and intraseasonally varying mean states, which leads to impactful short-term variations in regional climate. The North Atlantic Oscillation (NAO)-ENSO relation is a typical example, in that its phase relationship reverses systematically between the early and late winter. However, the details and underlying mechanisms of this relationship transition are not well understood yet.
Here based on observations and an ensemble of atmosphere-only climate model simulations, we first reveal that this NAO phase reversal occurs synchronously in early January, which indicates strong abruptness. We demonstrate that this abrupt NAO phase reversal is caused by the change in ENSO-induced Rossby wave-propagating direction from northeastward to southeastward over the northeastern North American region, which is largely governed by a climatological alteration of the local jet meridional shear. We also provide evidence that the North Atlantic intrinsic eddy–low-frequency flow feedback further facilitates and amplifies the NAO responses. This abrupt NAO phase reversal signal is strong enough during the ENSO winter to be useful for intraseasonal climate forecasting in the Euro-Atlantic region.
How to cite: Geng, X., Zhao, J., and Kug, J.-S.: ENSO-driven abrupt phase shift in North Atlantic Oscillation in early January, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1944, https://doi.org/10.5194/egusphere-egu23-1944, 2023.
Increased weather and climate extreme events are often attributed solely to either human-induced climate change or internal variability, under the assumption that external forcing does not influence the internal variability. However, with the development of single-model initial-condition large ensembles, recent research shows the impact of global warming on internal variability. This study investigates how global warming influences the North Atlantic Oscillation (NAO) and the East Atlantic (EA) pattern, which are the dominant large-scale circulation/teleconnection modes in the North Atlantic sector.
The study analyzes the geopotential height data of the Max Planck Institute Grand Ensemble (MPI-GE) with 100 ensemble members. The internal variability is quantified as the deviation from the ensemble mean. The influence of global warming on the internal variability is checked with a 1pcCO2 experiment, where the concertation is increased by 1% every year. This experiment provides a scenario for relatively strong global warming based on increasing greenhouse gas concentration alone. The extreme NAO and EA are defined as those years where the indexes are above (positive extremes) or below (negative extremes) 2 standard deviations.
The results show increases in extreme events, especially negative extremes, for both NAO and EA during wintertime, in a warmer climate. While NAO extremes increase consistently across the whole troposphere, EA extremes increase more at higher altitudes (500hpa-200hpa) than at lower altitudes. The warming effect of positive extreme NAO over northern Eurasia gets weaker, while the cooling effect of negative extreme NAO over northern Eurasia gets stronger. The effects of both, positive and negative extremes of EA, extend eastward till Eastern Asia. Overall, this study underlines the impact of global warming onto the internal variability of NAO and EA.
How to cite: Liu, Q., Jungclaus, J., Matei, D., and Bader, J.: Global warming induces more internally generated extremes of North Atlantic Oscillation and East Atlantic pattern, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5176, https://doi.org/10.5194/egusphere-egu23-5176, 2023.
This paper reports the structure of an interhemispheric atmosphere–ocean coupling pattern, which occurs over the Atlantic Ocean from January to February, and refers to it as the Atlantic symmetric pattern (ASP). The ASP occurs in the middle–upper troposphere, with two trains of cyclonic–anticyclonic–cyclonic anomalous circulations aligned meridionally over the Atlantic Ocean. The sea surface temperature (SST) signature of the ASP, which is composed of a distinct SST dipole, is the leading mode of the interannual SST of the Southwest Atlantic Ocean. Experiments with the linear baroclinic model shows that the interhemispheric wave trains of the ASP can be excited as a Gill-type response to convection in the South American monsoon system and the South Atlantic convergence zone. Further studies are warranted to elucidate other aspects of the ASP, including teleconnection in the Northern Hemisphere and interactions with other climatic modes.
How to cite: Tseng, W.-L., Wang, Y.-C., Lee, Y., Hsu, H.-H., and Keenlyside, N.: An Atlantic interhemispheric teleconnection established by South American summer monsoon, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-16755, https://doi.org/10.5194/egusphere-egu23-16755, 2023.
Canonical understanding based on general circulation models (GCMs) is that the large-scale circulation responds only weakly to extratropical sea-surface temperature (SST) anomalies, compared to the larger influence of tropical SST anomalies. However, the horizontal resolution of modern GCMs, which ranges from roughly 200 km to 25 km, is too coarse to fully resolve mesoscale atmospheric processes such as weather fronts. Here, we investigate the large-scale atmospheric circulation response to idealized Gulf Stream SST anomalies in a variable resolution version of the Community Atmospheric Model (CAM6), with regional grid refinement of 14 km over the North Atlantic, and compare it to versions with 28-km regional grid refinement and global 111-km resolution. The high-resolution simulations show a large positive response of the wintertime North Atlantic Oscillation (NAO) to positive SST anomalies in the Gulf Stream, a 1-standard-deviation NAO anomaly for 2°C SST anomalies. The lower-resolution simulations show a much weaker response, and in some cases, a different spatial structure of the response. The enhanced large-scale circulation response at high resolution results from an increase in resolved vertical motions, which enables SST forcing to have a larger influence on transient-eddy heat and momentum fluxes. In response to positive SST anomalies, these processes contribute to a stronger North Atlantic jet that varies less in latitude, as is characteristic of the positive phase of the NAO. Our results suggest that the atmospheric circulation response to extratropical SST anomalies is fundamentally different at higher resolution. Regional refinement in key regions offers a potential pathway towards improving simulation of the atmospheric response to extratropical SST anomalies and thereby improving multi-year regional climate predictions.
How to cite: Jnglin Wills, R., Herrington, A., Simpson, I., and Battisti, D.: Resolving weather fronts increases the large-scale circulation response to Gulf Stream SST anomalies, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2125, https://doi.org/10.5194/egusphere-egu23-2125, 2023.
In this communication, we will present results from an analysis of the variability of the vertically averaged (i.e., barotropic) atmospheric circulation simulated by climate models, which integrate the current Coupled Model Intercomparison Project (CMIP6). The variabilities in two ensembles of Atmospheric Model Intercomparison Project (AMIP) simulations were compared with the variabilities in two ensembles of fully coupled simulation counterparts of the current CMIP6 (Castanheira and Marques, 2022).
The atmospheric models simulate less variability of the barotropic atmospheric circulation over the Northern Atlantic and more variability over the North Pacific when compared with the corresponding variabilities in the ERA5 reanalysis (“observations”), at intraseasonal and interannual scales. When integrated over the whole globe, the variability in the coupled climate simulations is smaller than the variability in the corresponding AMIP simulations. The smaller global variability of the coupled simulations results in no mean overestimation of the subtropical jet variability in the North Pacific, but further underestimation of the jet stream variability in the Northern Atlantic. The results suggest that the reduction of the biases in the barotropic atmospheric variability over the North Pacific, in the coupled climate simulations, is achieved through compensating biases in the mean Sea Surface Temperatures (SSTs). Moreover, the reduction of the positive biases in the North Pacific seems to be associated with a reduction of the excitation of the most unstable barotropic mode of the atmospheric circulation, which contributes also to a reduction of the barotropic atmospheric variability in the North Atlantic region.
Acknowledgements: The CESAM is supported by FCT/MCTES through the project UIDP/50017/2020+ UIDB/ 50017/20201+LA/P/0094/2020.
References
Castanheira, J. M., Marques, C. A. F. (2022). Biases of the Barotropic Atmospheric Circulation Variability in CMIP6 Models. Journal of Climate, Vol. 35, 5071–5085, DOI: 10.1175/JCLI-D-21-0581.1.
How to cite: Castanheira, J. M. and Marques, C. A. F.: How can the most unstable barotropic mode of atmospheric models contribute for the explanation of atmospheric variability biases of climate models in the North Atlantic?, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13716, https://doi.org/10.5194/egusphere-egu23-13716, 2023.
Previous studies have shown that the response of the Atlantic Meridional Overturning Circulation (AMOC) to increasing greenhouse gas forcing is a key driver of inter-model uncertainties. While all models project an AMOC decline, the inter-model spread in the decline rate drives very different climate change impacts, including temperature, precipitation, and large-scale atmospheric circulation patterns. Here we investigate the impacts of a weakened AMOC by performing idealized climate model experiments using EC-Earth3, a state-of-the-art GCM participating in CMIP6. We compare results from a control experiment run under preindustrial forcing, with an experiment in which we force a weakened AMOC by applying a virtual salinity flux in the North Atlantic/Arctic basin. Here we analyze previously unexplored aspects of the climate response to a weakened AMOC, focusing on impacts on wintertime daily timescales in the Euro-Atlantic region.
We find that a weakened AMOC forces an overall drier climate over most of Europe; however, some regions especially in northwestern Europe experience an increase in the number of very wet days. We investigate drivers of precipitation changes by performing a moisture budget and analyzing the association with changes in weather regimes at daily timescales. We find that an increase in the occurrence of the NAO+ days (going from a frequency of ~26% of occurrence to above 42%) together with an enhanced and more central jet, favors drier conditions over southern Europe and wetter conditions over northwestern Europe. Further, enhanced but drier storms cause dryness over Europe while thermodynamic processes per se, namely the Clausius-Clapeyron constraint on temperature, play a second role. Finally, we explore these relationships in additional experiments in which we keep the AMOC constant in a forced 4xCO2 experiment by applying a reversed virtual salinity flux, which allows us to separate the effects of 4xCO2 forcing from the weakened AMOC on climate change impacts. Our results have broader implications for understanding the role of the AMOC response on future climate change, allowing us to separate the impacts of the AMOC from those of the CO2 increase.
How to cite: Bellomo, K., Meccia, V., D'Agostino, R., Fabiano, F., Larson, S., von Hardenberg, J., and Corti, S.: Impacts of a weakened AMOC on the European climate, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5775, https://doi.org/10.5194/egusphere-egu23-5775, 2023.
A potential future slowdown or acceleration of the Atlantic Meridional Overturning Circulation (AMOC) would have profound impacts on global and regional climate. Recent studies have shown that AMOC responds, among many other processes, to anthropogenic changes in tropical Indian ocean (TIO) temperature. However, internal unforced co-variations between these two basins are largely unexplored as of yet. Here, we use the ERSST v5, HadISST v1, and COBE v2 gridded observational products for the period 1870-2014, as well as dedicated simulations with coupled climate models, and show that internal changes in sea surface temperature gradients between the Indian and Atlantic Ocean (SSTgrad) can drive teleconnections that influence internal variations of North Atlantic climate and AMOC.
We separate the unforced observed component (i.e., internal signal) from the forced signal following the residuals method presented by Smith et al. (2019). In the absence of direct AMOC observation we estimate AMOC variability from an SST index (SSTAMOC; Caesar et al., 2018). We find a robust observed relationship between the unforced tropical SSTgrad and SSTAMOC when TIO leads by ~25 years. This time-lag is in line with a recently described mechanism of anomalous tropical Atlantic rainfall patterns that originate from TIO warming and cause anomalously saline tropical Atlantic surface water which slowly propagate northward into the subpolar North Atlantic, ultimately altering oceanic deep convection and AMOC (Hu and Fedorov, 2019; Ferster et al. 2021). Our study now suggests that it is the tropical SSTgrad that drives those AMOC changes, with a limited role for the western tropical Pacific. Pre-industrial control simulations with the IPSL-CM6A-LR model confirm this relationship, indicating a time lag of ~25 years between SSTgrad and SSTAMOC variations. These simulations also confirm that the SSTAMOC is representative of unforced AMOC variations when SSTAMOC leads by 5 years. This work therefore indicates that an unforced pathway between tropical ocean temperature and AMOC exists with a ~20 year lag, which opens the potential for using SSTgrad as precursor to predict future AMOC changes.
Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G., & Saba, V. (2018). Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556(7700), 191-196.
Ferster, B. S., Fedorov, A. V., Mignot, J., & Guilyardi, E. (2021). Sensitivity of the Atlantic meridional overturning circulation and climate to tropical Indian Ocean warming. Climate Dynamics, 1-19.
Hu, S., & Fedorov, A. V. (2019). Indian Ocean warming can strengthen the Atlantic meridional overturning circulation. Nature climate change, 9(10), 747-751.
Smith, D. M., Eade, R., Scaife, A. A., Caron, L. P., Danabasoglu, G., DelSole, T. M., ... & Yang, X. (2019). Robust skill of decadal climate predictions. Npj Climate and Atmospheric Science, 2(1), 1-10.
How to cite: Ferster, B., Borchert, L., Mignot, J., and Fedorov, A.: AMOC variations modulated by Tropical Indio-Atlantic SST Gradient, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7011, https://doi.org/10.5194/egusphere-egu23-7011, 2023.
Regime-oriented causal model evaluation of Atlantic-Pacific teleconnections in CMIP6
Abstract:
The Pacific Decadal Variability (PDV) and the Atlantic Multidecadal Variability (AMV) are two important modes of long-term internal variability that significantly impact the climate system and its spatio-temporal changes. In this study, we use a regime-oriented causal discovery method (Karmouche et al, 2022) to examine the changing interactions between the PDV and AMV. The results of this analysis are used to evaluate the ability of models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) to represent the observed changing interactions between the PDV, AMV, and their extra-tropical teleconnections.
Applying the regime-oriented causal discovery method to reanalysis time series revealed that the interactions between AMV and PDV differ from one regime to the other. The results also show that there are both direct and indirect connections between the Atlantic and Pacific oceans, which are established through various teleconnection patterns.
In order to evaluate the ability of climate models to represent these observed interactions, we applied the same regime-oriented causal discovery method to the CMIP6 Large Ensemble historical simulations. We show that several models performed well in simulating the observed causal patterns when AMV and PDV are "out-of-phase", and that the two models with the largest number of members generally outperformed other models in simulating observed causal patterns during longer regimes. This work shows how causal discovery on LEs complements the available diagnostics and statistics metrics of climate variability to provide a powerful tool for climate model evaluation.
Karmouche, S., Galytska, E., Runge, J., Meehl, G. A., Phillips, A. S., Weigel, K., and Eyring, V.: Regime-oriented causal model evaluation of Atlantic-Pacific teleconnections in CMIP6, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2022-1013, 2022.
How to cite: Karmouche, S., Galytska, E., Runge, J., Meehl, G., Phillips, A., Weigel, K., and Eyring, V.: Regime-oriented causal model evaluation of Atlantic-Pacific teleconnections in CMIP6, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13294, https://doi.org/10.5194/egusphere-egu23-13294, 2023.
Arctic amplification of global warming is accompanied by a dramatic decline in sea ice. This, in turn, has been linked to cooling over the Eurasian subcontinent over recent decades, most dramatically during the period 1998-2012. Such a coherent and pronounced cooling is a counterintuitive impact under global warming. Some studies have proposed a causal teleconnection from Arctic sea ice retreat to Eurasian wintertime cooling; others argue that Eurasian cooling is mainly driven by internal variability. Overall, there is an impression of strong disagreement between those holding the “ice-driven” versus “internal variability” viewpoints. We offer an alternative framing that shows that the sea ice and internal variability views can be compatible. Key to this is viewing Eurasian cooling through the dual lens of dynamics (linked primarily to internal variability with a small contribution from sea ice; cools Eurasia) and thermodynamics (linked to sea ice retreat; warms Eurasia). This framing, combined with recognition that there is uncertainty in the hypothesized mechanisms themselves, allows both viewpoints (and others) to co-exist and contribute to our understanding of Eurasian cooling. A simple autoregressive model shows that strong Eurasian cooling is consistent with internal variability, with some periods being more susceptible to strong cooling than others. Rather than posit a “yes-or-no” causal relationship between sea ice and Eurasian cooling, a more constructive way forward is to consider whether the cooling trend was more likely given the observed sea ice loss, as well as other sources of low-frequency variability. Taken in this way both sea ice and internal variability are factors that affect the likelihood of strong regional cooling in the presence of ongoing global warming. Improving our understanding of the underlying mechanisms is critical for quantifying regional responses and impacts as well as producing reliable near-term climate predictions.
How to cite: Sobolowski, S., Outten, S., and Li, C.: Reconciling conflicting evidence for the cause of the observed early 21st century Eurasian Cooling, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5279, https://doi.org/10.5194/egusphere-egu23-5279, 2023.
The Hadley circulation (hereafter HC) is one of the most prominent meridional overturning circulations in the climate system. In addition to maintaining energy balance and momentum exchange in tropics and extratropics, it can also shape the Intertropical Convergence Zone (ITCZ) and subtropical dry arid zones by regulating the hydrological cycle in tropical and extratropical regions. Weakening and expanding HC and narrowing of the ITCZ are projected with human greenhouse gas emissions. However, no consensus has been achieved regarding the relative importance of direct CO2 radiative effect and indirect effects via SST changes in shaping the future HC changes. This limits our deep understanding of the climate impacts imposed by changes in the HC. Here we analyze a broad range of CMIP5 experiments and show that future changes in SST patterns play the leading role in the determining the future changes in HC and ITCZ. In addition, a series of individual basin perturbation experiments were conducted at 1.5°C, 2°C, and 3°C temperature thresholds to identify key basins that determine HC strength, edges, and ITCZ locations. Our work highlights the overwhelming role of future tropical Indian Ocean warming on the HC and ITCZ changes.
How to cite: Sun, Y., Ramstein, G., Fedorov, A. V., Ding, L., and Liu, B.: Impacts of oceanic warming patterns versus CO2 radiative forcing on the Hadley Circulation, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-10582, https://doi.org/10.5194/egusphere-egu23-10582, 2023.
During the last decades, CMIP5 models simulate a warming trend in the tropical eastern Pacific that has not been present in observations (Seager et al., 2019). Associated with this, the Walker circulation has experienced a westward migration while CMIP5 models simulate an eastward migration. This mismatch is still present in CMIP6 models and might affect climate projections worldwide. In the Caribbean region, CMIP6 models project a strong drying at the end of the 21st century. El Niño-like changes in the Walker circulation are the dominant teleconnections driving the Caribbean drying. The models that project a strong Caribbean drying also simulate generally a strong equatorial eastern Pacific warming trend over the recent decades. Thus, the mismatch between observed and simulated warming trends over the equatorial eastern Pacific questions the reliability of the Caribbean precipitation projections. The warming bias might also have implications for tropical cyclones’ projections in the Atlantic and Pacific through the effect of vertical wind shear, which is related to shifts in the Walker circulation. In addition, the double Intertropical Convergence Zone (ITCZ) bias might be influenced by the mismatching trends. The strong influence of El Niño-Southern Oscillation (ENSO) dynamics on the world’s climate demands more in-depth studies addressing the drivers of the Walker circulation and the equatorial Pacific warming bias.
How to cite: Brotons Blanes, M., Haarsma, R., and Bloemendaal, N.: Impact of tropical eastern Pacific warming bias on Caribbean climate, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5034, https://doi.org/10.5194/egusphere-egu23-5034, 2023.
It is well established that the positive phase of El Niño Southern Oscillation (ENSO) tends to weaken the Northern Hemisphere stratospheric polar vortex (SPV), promoting a negative North Atlantic Oscillation (NAO). Pacific Decadal Variability (PDV) is characterised by a pattern of sea surface temperatures similar to ENSO, but its impacts are more uncertain: some studies suggest similar impacts of ENSO and PDV on the SPV and NAO, while others find the opposite. We use climate model experiments and reanalysis to find further evidence supporting opposite interannual and decadal impacts of Pacific variability on the extratropics. We propose that the decadal strengthening of the SPV in response to positive PDV is caused by a build-up of stratospheric water vapour leading to enhanced cooling at the poles, an increased meridional temperature gradient and a strengthened extratropical jet. Our results are important for understanding decadal variability, seasonal to decadal forecasts and climate projections.
How to cite: Seabrook, M., Smith, D., Dunstone, N., Eade, R., Hermanson, L., Scaife, A., and Hardiman, S.: Opposite Impacts of Interannual and Decadal Pacific Variability in the Extratropics, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6966, https://doi.org/10.5194/egusphere-egu23-6966, 2023.
Decadal Climate Variability (DCV) modes perturb regional climatic parameters across the globe at multi-year timescales. Precipitation is one such climatic parameter of socio-economic importance.
Our study examines the ability of Coupled Model Intercomparison Project Phase 6 (CMIP6) models in representing the observed teleconnection of DCV modes; Pacific Decadal Oscillation (PDO) and Tropical Atlantic SST Gradient with the global precipitation. We chose the subset of CMIP6 models that participate in both historical and hindcast experiments.
In this study we examine the relationship between the model's ability to simulate the long-term DCV pattern and its ability to simulate the teleconnection between DCV mode and global precipitation.
HadGEM3-GC31-MM and MPI-ESM-1-2-HR, which simulate the observed global SST anomaly pattern in the warm phase of PDO considerably well, also simulate observed global precipitation patterns during the warm phase of PDO quite well in regions like central India, Europe, North- and South-America, Eastern Africa, Eastern Australia etc. However, BCC-CSM2-MR and NorCPM1 fail to effectively simulate observed precipitation patterns in the warm phase of PDO in regions like, North- and South-America, Africa etc.
Hence, we found that models that are able to simulate the PDO pattern of SST are also able to represent the teleconnection between PDO modes and precipitation across the globe. We also examined the regression pattern of wind circulation, and the regression pattern of converging and diverging parts of the wind with PDO index. Models that better represent the observed warm phase of PDO pattern, also well represent the observed circulation pattern in respective phases of PDO. Similar analysis is also performed for TAG.
Keywords: Decadal Climate Variability (DCV), CMIP6, historical experiments, teleconnection, precipitation.
How to cite: Dixit, J., Mehta, V. M., and AchutaRao, K. M.: Representation of relationship between PDO and global precipitation in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15027, https://doi.org/10.5194/egusphere-egu23-15027, 2023.
The central role of tropical sea surface temperature (SST) variability in modulating Northern Hemisphere (NH) extratropical climate has long been known. However, the prevailing pathways of teleconnections in observations and the ability of climate models to replicate these observed linkages remain elusive. Here, we apply maximum covariance analysis between atmospheric circulation and tropical SST to reveal two co-existing tropical-extratropical teleconnections albeit with distinctive spatiotemporal characteristics. The first mode, resembling the Pacific-North American (PNA) pattern, favors a Tropical-Arctic in-phase (warm-Pacific-warm-Arctic) teleconnection in boreal spring and winter. The second mode, predominant in summer and autumn, is manifested as an elongated Rossby-wave train emanating from the tropical eastern Pacific that features an out-of-phase relationship (cold-Pacific-warm-Arctic) between tropical Pacific SST and temperature variability over the Arctic. This Pacific-Arctic teleconnection (PARC) mode partially explains the observed summertime warming around the Arctic. The reliability of climate models to replicate these leading teleconnections is of primary interest in this study to improve decadal prediction on regional climate. While climate models participating in CMIP6 appear to successfully simulate the PNA mode and its temporal characteristics, the majority of models’ skill in reproducing the PARC mode is obstructed by apparent biases in simulating low-frequency SST and rainfall variability over the tropical eastern Pacific and the summer climatological mean flow over the North Pacific. Considering the contribution of the PARC mode in shaping low frequency climate variations over the recent decades from the tropics to the Arctic, improving models’ capability to capture the PARC mode is essential to reduce uncertainties associated with decadal prediction and climate change projection over the NH.
How to cite: Feng, X.: Possible causes of model biases in simulating Tropical-Arctic teleconnections in CMIP6, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6017, https://doi.org/10.5194/egusphere-egu23-6017, 2023.
The amplitude and spatial distribution of low-frequency natural variability is determinant for regional climate projections, but it is still poorly understood.
In a previous study, pollen-based temperature reconstructions were used to quantify spatial patterns of millennial temperature variability. This showed an inverse relationship across timescales with sub-decadal variability from instrumental data in extra-tropical regions over land (Hébert et al., 2022, under review). We concluded that due to varying marine influence, regions characterized by stable oceanic climate at sub-decadal timescales experience stronger long-term variability while continental regions with higher sub-decadal variability show weaker long-term variability. Indications of this relationship could also be inferred from instrumental data alone as regions of low sub-decadal variability were more likely to exhibit a steeper increase of variability over multi-decadal timescales and vice versa.
In the current work, the relationship found in the instrumental data was further investigated using different instrumental products. In addition, a large multi-model ensemble of CMIP6 models, as well as single-model ensembles, were considered for analysis and it was found that they do not systematically reproduce the relationship found in the instrumental data. This indicates a fundamental deficiency in the model simulations with regard to the mechanism driving the emergence of low-frequency climate variability. This characteristic being related to multi-decadal variability thus has important significance for multi-decadal regional climate projections and might be used as an emergent constraint in model evaluation and inter-comparison.
How to cite: Hébert, R. and Laepple, T.: Emergence of Low-Frequency Temperature Variability in Instrumental Data and Model Simulations, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-16668, https://doi.org/10.5194/egusphere-egu23-16668, 2023.
A better understanding of the rainfall variability and extremes in tropical regions is crucial for the development of improved statistical and numerical approaches used for climate research and weather prediction. In this study, we present a novel fuzzy rule-based method for classifying atmospheric circulation patterns relevant to heavy rainfall in the Sudan-Sahel region over West Africa. In the first step, we determine large-scale atmospheric patterns to describe important seasonal features of the West African Monsoon like the movement of Saharan Heat Low over the African continent. In the second step, meso-scale monsoon patterns are classified to better describe rainfall variability and extremes during the monsoon period. In addition to a comprehensive predictor screening using more than 30 variables at different atmospheric levels, a detailed sensitivity analysis is performed, which aims to improve the transferability of the classification approach to an independent dataset. Furthermore, crucial aspects of the methodological development of fully automatic classification approaches are addressed. Using mean sea level pressure and stream function fields (700hPa) as final predictor variables, we identified 23 circulation patterns as robust solution to represent key atmospheric processes and rainfall variability in the study region. The two wettest patterns are distinguished by an enhanced Saharan Heat Low and cyclonic rotation near the study region, suggesting the presence of a tropical wave trough and triggering about 50% of the rainfall extremes on 6.5% of the days. The identified atmospheric circulation patterns are currently used to develop a variety of improved statistical approaches for this challenging region, such as pattern-dependent bias correction, geostatistical interpolation, and simulation.
How to cite: Rauch, M., Bliefernicht, J., Laux, P., and Kunstmann, H.: Classification of Atmospheric Circulation Patterns That Trigger Rainfall Extremes in the Sudan-Sahel Region , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12678, https://doi.org/10.5194/egusphere-egu23-12678, 2023.
The Intertropical Convergence Zone (ITCZ), with its twice-annual passage over central Africa, is considered as the main driver of the rainfall seasonality. But recently, this paradigm was challenged. To find out what are the main drivers of the annual cycle of rainfall over central Africa, we present a simple comprehensive paradigm with both local forcings and regional-scale processes playing crucial role. Due to the local evaporative cooling effect, the foot of the ascending branch of Hadley cells occurs where the temperature is the warmest, indicating a thermal low. This distorts the southern Hadley cell by developing its bottom-heavy structure. As result, both shallow and deep Hadley cells coexist over central Africa year–round. The deep mode is associated with poleward branches at upper levels that transport the atmospheric energy. The shallow mode is characterized by a meridional return flow in the mid-troposphere that transports the water vapour instead of lower branches as widely reported. This favours the building-up of the mid-tropospheric moisture flux convergence with a limited contribution of the midlevel easterly jet, conducive to deep convection. Embedded in this strong rising branch of Hadley cells at midlevels, the intense convective rainfall, and with it the rainfall maximum position, is seasonally controlled by the dynamics of the midlevel shallow meridional return flow. This highlights the interhemispheric rainfall contrast over central Africa and outlines its unimodal seasonality. On the other hand, forced by the Congo basin cell, the precipitable water regulates the deep convection from the vegetated surface of Congo basin, acting as a continental sea. This nonlinear mechanism separates the rainfall into three distinct regimes – (i) the moisture-convergence-controlled regime, with convective rainfall exclusively occurring in the rainy season and (ii) the local evaporation-controlled regime with drizzle and (iii) the precipitable-water-controlled regime, with exponential increase of rainfall that both occur during the dry season.
How to cite: Longandjo, G.-N. T., Monyela, B. M., and Rouault, M.: Drivers of the Annual Cycle of Rainfall over Central Africa: The Role of Water Vapor and the Mid-Tropospheric Meridional Circulation, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13299, https://doi.org/10.5194/egusphere-egu23-13299, 2023.
Indian Ocean Dipole (IOD) is an air-sea coupled variability in the Tropical Indian Ocean (TIO), which strongly impacts climate variability over the Indian Ocean rim countries. Though many positive IODs co-occurred with El Niño Southern Oscillation (ENSO), IODs do evolve independently, suggesting the possible role of internal dynamics of the Indian Ocean. In this study, the subtropical IOD (SIOD) is reported as one of the triggers for non-ENSO IODs. The study highlights the existence of cyclic feedback between IOD and SIOD through tropical subtropical interaction, a possible mechanism for the biennial tendency of both IOD and SIOD modes. The positive SIOD induce warming in the southwest of the Subtropical South Indian Ocean (SSIO) during April-May months and creates a meridional cell with subsidence over the southwestern TIO region (10oS). The subsidence expands the existing anticyclonic circulation over SSIO towards the equator and develops easterlies along the equator, warming the western TIO region. A zonal-vertical cell with convection over the western TIO and subsidence over the eastern TIO originates during June-July, which subsequently generates positive IOD in the following months. The positive IOD triggers negative SIOD by developing a stationary Rossby wave train in the midlatitudes. The southeastern anticyclonic circulation develops during the IOD peak season as Gill’s response initiates warm SST anomalies in the northeastern subtropics. As a result of the warming, the evolution of upper-level divergence and high absolute vorticity gradient over the subtropics generate an equivalent barotropic Rossby wave number 3 pattern in the extratropics. The cyclonic circulation over the southwest SSIO related to this Rossby wave pattern creates cold SST anomalies there. The cooling in the southwest and the warming in the northeast SSIO persisted from the IOD peak season, which strengthened the cyclonic circulation over SSIO, reinforcing the existing negative and positive SST anomalies through a positive feedback mechanism and generating negative SIOD, which peaks in the following January-March months.
How to cite: Sebastian, A. and Gnanaseelan, C.: Coupled feedback between the tropics and subtropics of the Indian Ocean with emphasis on the coupled interaction between IOD and SIOD, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-373, https://doi.org/10.5194/egusphere-egu23-373, 2023.
Posters on site: Wed, 26 Apr, 14:00–15:45 | Hall X5
The variability of Indian Ocean shallow meridional overturning circulation (SMOC) is studied using the century-long ocean reanalysis simple ocean data assimilation (SODA) data. Though SMOC exhibits stronger southward transport during boreal summer, it displays stronger variability during boreal winter. The spectrum analysis of the winter SMOC index reveals the presence of the highest amplitude between 5 to 7 years at 95% confidence level, suggesting the dominance of intra-decadal SMOC variability. The robustness of intra-decadal SMOC variability is also confirmed in different ocean reanalysis data sets. Composite analysis of filtered upper Ocean Heat Content, sea level, thermocline depth, and Sea Surface Temperature anomalies for strong (weak) SMOC years show negative (positive) anomalies over north and East of Madagascar. Correlation analysis, of filtered SMOC index and sea level pressure (zonal winds) over the Indian Ocean, found a significant negative (positive) correlation coefficient north of 40 °S (around 10 °S) and a significantly positive (negative) correlation coefficient over the 45 °S to 70 °S (20 °S to 50 °S and north of 5 °S). This meridional pattern of the correlation coefficient for sea level pressure, manifesting the out-of-phase relationship between sub-tropics and high latitude mean sea level pressure, resembles Southern Annular Mode (SAM). We conclude that the intra-decadal variability of mean sea level pressure leads to zonal wind variation around 10 °S modulating SMOC, which in turn affects the upper ocean thermal properties in the east and north of Madagascar. This study for the first time brought out coherent intra-decadal evolution of SAM and SMOC during boreal winter.
How to cite: Pai, R., Parekh, A., Chowdary, J. S., and Chellappan, G.: Intra-decadal variability of the Indian Ocean shallow meridional overturning circulation during boreal winter, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-595, https://doi.org/10.5194/egusphere-egu23-595, 2023.
Tropical variability has long been identified as having an important influence on climate variability and change in the Southern Hemisphere (SH). In all three ocean basins, heating from tropical convection can generate stationary Rossby waves that propagate polewards and eastwards towards Antarctica, influencing temperature and rainfall patterns along the way. Recent studies have also highlighted the reverse – an influence of the polar regions on changes further north, for example, the stratospheric weakening of the SH polar vortex that contributed to the prolonged drought and extreme fire weather in Australia in the spring and summer of 2019. On longer timescales, the climate of the Southern Hemisphere has undergone significant changes over the past 30-50 years. The extratropical atmosphere has seen a shift to a more positive phase of the Southern Annular Mode and a stronger and more poleward eddy-driven jet, particularly in austral summer. While the influence of anthropogenic forcing such as ozone depletion and increasing greenhouse gases on these changes is well-established, the importance of tropical to extratropical interactions in shaping some recent events is becoming more evident. Examples include the deepening of the Amundsen Sea Low which has been associated with tropical Pacific decadal variability and the rapid decline in Antarctic sea ice in 2016 which was linked to a record positive Indian Ocean Dipole event. Recent insights into tropical to extratropical interactions, including the mechanisms through which they operate and links to observed changes on interannual to interdecadal timescales will be discussed.
How to cite: Arblaster, J.: Tropical to extratropical interactions in the Southern Hemisphere, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-16955, https://doi.org/10.5194/egusphere-egu23-16955, 2023.
The teleconnection studies regarding Indian summer monsoon (ISM) clouds are not focused on detail from both observational and modeling aspects. This is despite the fact that clouds play a seminal role in governing rainfall variability through the modulation of heating and induced circulation. Therefore, we find it essential to explore whether the inter-annual variability of ISM clouds is also remotely influenced by the slowly varying predictable component e.g. Sea Surface Temperature (SST).
The findings reveal the linkage of observed TCF (and rainfall) over the ISM region with slowly varying forcing (e.g., global SST). The observed/reanalysis teleconnection pattern of TCF-SST is almost similar to that of rainfall-SST.In the long-term period, TCF and SST show a strong and positive correlation with Extra-Tropics (R ~ 0.41), NAO (R ~ 0.51), and AMO (R ~ 0.41) SST regions, in addition to canonical ENSO teleconnection (R ~ −0.39). This is better captured in CMIP6-MME than in CMIP5-MME. The representation of the global teleconnection pattern has been significantly improved in participating models from CMIP5 to CMIP6. The teleconnection with extra-tropics and north Atlantic mode of variability is markedly enhanced in CMIP6-MME compared to CMIP5-MME. The present study has also shown the lag correlations in the teleconnection analysis, i.e., the correlation of June–September (JJAS) mean of rainfall/TCF with October–December (OND) SST from observation/reanalysis, CMIP5-MME, and CMIP6-MME. The CMIP6-MME performs better than CMIP5-MME as compared to observation/reanalysis.
Thus, the improved understanding of the teleconnection of cloud variables with ENSO and other predictors (ET, NAO, and AMO) will help researchers take up the challenges of improving the ISMR skill far ahead using the new generation coupled climate models. This may facilitate reliable seasonal ISM forecasting.
Keywords: Indian Summer Monsoon, Clouds, Teleconnection, CMIP5, CMIP6
How to cite: Dutta, U., Hazra, A., Chaudhari, H. S., Saha, S. K., Pokhrel, S., and Verma, U.: The Teleconnection of Indian Summer Monsoon Clouds with Global Predictors: An Unexplored Measure for Coupled Model development, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-864, https://doi.org/10.5194/egusphere-egu23-864, 2023.
The South American monsoon system is one key component of the regional climate of South America, and its interannual as well as intraseasonal variability is of great relevance for water availability over vast parts of the continent. To further develop advanced prediction systems for hydro-meteorological conditions, a better understanding of the underlying atmospheric as well as coupled ocean-atmosphere and land-atmosphere processes governing the intraseasonal variations of rainfall is of paramount importance.
In this work, we focus on rainfall variability over the central Amazon basin (CAB) as a particularly vulnerable region during the peak season of the monsoon (December to February). In order to identify causal precursors of CAB rainfall variability and their mutual causal interdependence structure, we employ a causal discovery tool called Peter and Clark Momentary Conditional Information (PCMCI) algorithm to monthly average sea surface temperature (SST), mean sea level pressure (MSLP) and precipitation fields from reanalysis data sets for two different time periods, 1950-2020 and 1979-2020. As a first step, anomaly maps and correlation maps are used to identify potential candidate drivers of the precipitation variability in the CAB at lead times of up to three months. The causal effect networks resulting from the subsequent application of the PCMCI algorithm unveil the causal dependencies of different climate phenomena with CAB rainfall variability during austral summer, confirming previous results based on standard correlation analyses and allowing for a quantitative assessment of the different effects.
Among others, we find that SST changes in the tropical Pacific Nino1+2 region close to the South American west coast have a causal effect on CAB precipitation, with lower SSTs promoting more rainfall with a lag of one to two months. Notably, we do not find any similar statistically significant causal impact of SST variations in the Nino3.4 region in the central tropical Pacific, which is commonly most closely associated with the El Niño Southern Oscillation. Additionally, the obtained causal effect networks demonstrate that the Southern Annular Mode (SAM) causally influences the Amundsen Sea Low (ASL), which in turn causally affects the CAB rainfall. Both links are negative, i.e. a positive SAM mode leads to a deeper ASL with a lag of one month, and a deeper ASL supports higher precipitation in central Amazonia with a lag of three months. Finally, SST variability in the tropical North Atlantic as well the Madden-Julian Oscillation do not show a significant causal relationship with CAB rainfall. Our obtained findings are qualitatively consistent among the two different time periods. However, when analyzing data starting only after 1979, some links increase in strength while generally less causal links show up in the networks.
How to cite: Henningsen, E., Di Capua, G., and Donner, R. V.: Causal drivers of central Amazon precipitation variability during austral summer, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13726, https://doi.org/10.5194/egusphere-egu23-13726, 2023.
Tropical cyclone (TC) activity varies substantially yearly, and tropical cyclone-related damage also changes. Longer-term prediction of tropical cyclones plays an important role in reducing the wear and human loss caused by TCs. In this study, we have used a Causal-network-based algorithm to find the main development regions and precursors responsible for TC genesis and intensification. However, all the extreme events are interconnected through various global links. Therefore, analysis of the teleconnection and correlation of Tropical Cyclones with El Nino Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and North Atlantic Oscillation (NAO) during the satellite era (1980-2020) over the North Indian Ocean (NIO) basins using this Causal Effect Network (CEN) based algorithms is checked. The most appropriate metric for cyclone energy is Accumulated Cyclone Energy (ACE); its correlation with the various factors are investigated. We examined the variation in TCs activity during all three phases (positive, negative, and neutral phases).
The results show an increasing trend in ACE over the NIO region during that specific period. The duration of most intense cyclones is increased, but their frequency decreases in this period. A shift in ACE starts after 1997 and still rises significantly. Analysis of Sea Surface Temperature (SST), Vertical Wind Shear (VWS) between 850 and 250 hPa, mid-tropospheric (800 hPa) Relative Humidity (RH), low level (850 hPa) Relative Vorticity (RV), and Tropical Cyclone Heat Potential (TCHP) is done, and it shows positive changes and variability of ACE. These results may help get better knowledge about the atmospheric or oceanic teleconnections between the events, and improved tropical cyclone prediction can help reduce the loss caused by the TCs.
How to cite: kumar sagar, A.: identification of robust predictors of tropical cyclones using causal effect network over the north indian ocean, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-502, https://doi.org/10.5194/egusphere-egu23-502, 2023.
The evaluation of new generations of global climate models (GCMs) with respect to their large-scale circulation features is crucial for model development and has recently been brought into focus by the downscaling community, interested in the suitability of GCMs for downscaling purposes. In such evaluation experiments, additional uncertainties emerge from differences among the reference datasets used for evaluation, typically reanalyses. In this context, weather typing techniques are a useful tool for the classification of the full diversity of data into a few recurrent patterns that can serve as objective characterizations of either global or regional atmospheric circulation. A well-known weather typing classification algorithm is the Jenkinson-Collison Weather Type (JC-WT, Jenkinson and Collison 1977) approach. Although the methodology was originally developed for the British Isles (Lamb, 1972), the JC-WT approach can in principle be applied to any mid-to-high latitude region (Jones et al, 2013). Fernandez-Granja et al (2023) extended the limits of applicability from 23.5º to 80º latitude on both hemispheres, but the suitability of the method is questionable for certain seasons over some areas of the globe, such as the Mediterranean region in summer.
In this study, we first explore the applicability of the JC classification over the Mediterranean by linking the JC-WTs with main northern hemisphere teleconnection indices and blocking conditions. Further, the diversity of JC-WTs and occurrence of the unclassified type are used to examine the suitability of the method. Results show that the application of the JC-WT classification is physically meaningful in large parts of the domain. Secondly, fundamental characteristics of the JC-WTs such as transition probabilities between consecutive types and persistence of the dominant JC-WTs (number of time-steps staying in the same type) obtained for five different reanalyses are compared. Important differences among reanalyses are found, especially in summer, which may bring additional uncertainties when the method is used in model evaluation experiments.
References:
Fernández-Granja, J. A., Brands, S., Bedia, J., et al (2023) Exploring the limits of the Jenkinson–Collison weather types classification scheme: a global assessment based on various reanalyses. Climate Dynamics. DOI: 10.1007/s00382-022-06658-7
Jenkinson A., Collison F. (1977) An initial climatology of gales over the north sea. synoptic climatology branch memorandum. Meteorological Office, 62
Jones P.D., Harpham C., Briffa K.R. (2013) Lamb weather types derived from reanalysis products. International Journal of Climatology 33(5):1129–1139. DOI: 10.1002/joc.3498
Lamb H. (1972) British isles weather types and a register of daily sequence of circulation patterns 1861-1971. Meteorological Office, Geophysical Memoir 116:1–85
How to cite: Fernández-Granja, J. A., Casanueva, A., Bedia, J., Brands, S., and Fernández, J.: Characterization of Mediterranean large-scale atmospheric circulation based on Jenkinson-Collison Weather Type classification., EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15646, https://doi.org/10.5194/egusphere-egu23-15646, 2023.
Heat wave is a period of prolonged abnormally high surface temperatures relative to those normally expected. Heat waves may form when high pressure system strengthens and remains over a region from several days up to several weeks. Severe and exceptional heat waves, such as those that occurred over the Balkans (2007), France (2003), or Russia (2010), are associated with increased mortality, health hazards, reduced personal work productivity and have significant economic impacts by compromising agricultural harvest. Extremely high air temperature values in the Balkan Region are associated with anticyclones formed at the Azores maximum or high-pressure ridges and advections of hot air from the south and southwest.
In our study we perform statistical analysis of the occurrence, durability and intensity of the heat waves over the Balkan Peninsula for the period 1980-2020 based on historical satellite and reanalysis datasets. We analyse correlation between heat waves occurrence and North Atlantic Oscillation Index and certain historical meteorological data for Atlantic and Mediterranean regions aiming to figure out possible causes and physical drivers of this phenomena. One of the mid-term goals of the project is to develop a CNN based predictive system for short and long-time forecasting of extreme weather conditions over the Balkans.
How to cite: Popov, H. and Stepanyuk, O.: Heat Waves over Eastern Balkans: A statistical analysis, possible causes and physical drivers., EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15694, https://doi.org/10.5194/egusphere-egu23-15694, 2023.
Weather type classification is a well-established and thoroughly researched field of study in atmospheric sciences. One of its applications is the analysis of occurrence of and transitions between large scale synoptic types. This is typically done by calculating the moving average of, or estimating linear or polynomial fits to relative frequencies. The presented work points out the theoretical inconsistencies implied by such approaches and, instead, employs binomial and multinomial logistic regression for consistent estimation of long-term trends in occurrence and transition probabilities between synoptic types, while assuming first-order Markovian behaviour throughout. The methodological framework's functioning is demonstrated using two prominent examples of weather type classification schemes with regional focus on Germany and central Europe. Temporal refinement to seasonal and monthly level and aggregation into combined groups of classes allows for tracing of observed trends, providing a more comprehensive understanding of the systems investigated. The results, by and large, fit in well with expectations about circulatory changes suggested by research about global warming induced climate change and can be verified by existing research in some cases. Inspection of transition probability changes allows for differentiation between changes in occurrence probability caused by changes in the mean vs. changes in circulatory dynamics. Limitations and favourable implementational details of the approach are determined and the Wald Null test is recommended for assessing statistical significance.
How to cite: Schoeller, H.: Occurrence and Transition Probabilities for two Weather Classification Systems over Germany, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2004, https://doi.org/10.5194/egusphere-egu23-2004, 2023.
A detailed assessment of climate variability of the Baltic Sea area for the period 1958-2009 (Lehmann et al. 2011) revealed that recent changes in the warming trend since the mid-1980s, were associated with changes in the large-scale atmospheric circulation over the North Atlantic. The analysis of winter sea level pressure (SLP) data highlighted considerable changes in intensification and location of storm tracks, in parallel with the eastward shift of the North Atlantic Oscillation (NAO) centres of action. Additionally, a seasonal shift of strong wind events from autumn to winter and early spring existed for the Baltic area. Lehmann et al. (2002) showed that different atmospheric circulation regimes force different circulation patterns in the Baltic Sea. Furthermore, as atmospheric circulation, to a large extent, controls patterns of water circulation and biophysical aspects relevant for biological production, such as the vertical distribution of temperature and salinity, alterations in weather regimes may severely impact the trophic structure and functioning of marine food webs (Hinrichsen et al. 2007). To understand the processes linking changes in the marine environment and climate variability, it is essential to investigate all components of the climate system which of course include also the large-scale atmospheric circulation. Here we focus on the link between changes/shifts in the large scale atmospheric conditions and their impact on the regional scale variability over the Baltic Sea area for the period 1950-2021. This work is mostly an extension of previous studies which focused on the response of the Baltic Sea circulation to climate variability for the period 1958-2008 (Lehmann et al. 2011, Lehmann et al. 2014). Now extended time series ECMWF ERA 5 reanalysis for 7 decades are available, highlighting recent changes in atmospheric conditions over the Baltic Sea. The main focus of this work is to identify predominant large scale atmospheric circulation patterns (climate regimes) on a monthly/seasonal time scale influencing the regional atmospheric circulation over the Baltic Sea area. Furthermore, long-term changes on the annual to decadal time scale will also be investigated.
How to cite: Lehmann, A., Post, P., and Myrberg, K.: Changing impact of the large-scale atmospheric circulation on the regional climate variability of the Baltic Sea for the period 1950-2022, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8461, https://doi.org/10.5194/egusphere-egu23-8461, 2023.
In the South Pacific region, the precipitation patterns are mostly driven by a number of processes operating at spatial and temporal scales. One of the most important features is the South Pacific Convergence Zone (SPCZ).
Five Daily Weather Types (WT) of precipitation are presented, based on Principal Component Analysis (PCA) and k-means clustering using ERA5 precipitation and atmospheric circulation variables such as mean sea-level pressure (SLP), day-to-day difference of mean daily SLP or northward and eastward 10-m wind component fields, able to capture distinct precipitation spatio-temporal patterns, interpretable in terms of salient regional climate features such as the SPCZ state and tropical cyclone activity. We then undertake a weather-type conditioned calibration of the TRMM (Tropical Rainfall Measuring Mission) product using in-situ rain gauge records from the PACRAIN database as reference. “Conditioning” is here based on applying separate statistical corrections for each of the generated WTs, since biases might be dependent on specific atmospheric situations that can be partially captured by the clustering procedure, thus adapting the correction factors to specific synoptic conditions.
Our results indicate that the WT-conditioned calibration provides an overall marginal added value over the unconditioned approach, although it makes a significant difference for a better correction of extreme rainfall events, critical in many impact studies. The approach can be extended to compound extreme events, in which several variables are involved (e.g. precipitation, sea level, wind, etc.), in order to better preserve multi-variable consistency.
How to cite: Mirones, O., Bedia, J., Fernández-Granja, J. A., Herrera, S., Van Vloten, S. O., Pozo, A., Cagigal, L., and Méndez, F. J.: Precipitation weather typing over the South Pacific: application to the TRMM satellite product calibration, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-11826, https://doi.org/10.5194/egusphere-egu23-11826, 2023.
Posters virtual: Wed, 26 Apr, 14:00–15:45 | vHall CL
The Mediterranean basin is located between the subtropical high-pressure belt and the mid-latitude westerlies and is characterized by complex topography. Its orography, the relatively warm Mediterranean Sea, which is a source of energy and moisture, and the land-sea interactions result in significant cyclonic behavior in the synoptic and sub-synoptic scales. Due to its high sensitivity to climate change forcings, the Mediterranean region is considered a climate change hot spot, with impacts, such as the decline in the projected precipitation, leading to increasing aridification in an already water-stressed area. The above highlight the importance of examining the cyclonic development in the area and assessing the respective changes under different climate change scenarios. In this research, unsupervised machine learning algorithms are used in order to objectively identify cyclonic development in the Mediterranean basin using CMIP6 data for a subset of the different shared socio-economic pathways (SSP) that explore a wide range of possible future outcomes. In more detail, Sea Level Pressure from selected CMIP6 models is used as an input in a Self-Organizing Map (SOM) which is trained to identify the cyclone activity in the Mediterranean Basin for the 1981-2010 reference period. The ability of the network in terms of identifying effectively cyclogenesis regions and the transition probabilities is evaluated. The trained SOM is used to classify CMIP6 mid-century (2031-2060) projections and changes in the frequencies of occurrence of cyclonic development. These are evaluated in terms of physical drivers and regionally specific mechanisms. Examining the responses of cyclonic development to different forcing scenarios will not only shed light on the physical and dynamical processes that govern these circulations but will also allow identifying high–risk regions with potential socio-economic impacts.
How to cite: Blougouras, G., Philippopoulos, K., Tzanis, C. G., and Cartalis, C.: Cyclonic development in the Mediterranean Basin in CMIP6 models using a neural network approach, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14053, https://doi.org/10.5194/egusphere-egu23-14053, 2023.
The climate of the state of Alaska is influenced not only by regional anthropogenic climate change, but also by the effects of large-scale ocean atmosphere systems like the Pacific Decadal Oscillation (PDO). Whereas positive anomalies in the PDO index coincide with warmer (sea surface) temperatures in the Gulf of Alaska and across the state, negative anomalies have the opposite effect. After analyzing the strength and direction of the correlation between the PDO index and the average temperatures in each of the 13 climate divisions of Alaska – both annually, as well as seasonally (DJF, MAM, JJA, and SON) – it becomes apparent, that the PDO affects the southern coastal and Panhandle regions much stronger than the Interior and North Slope. Over the course of a year, the correlations are strongest during the winter months, decrease during the spring and summer, only to increase again in the fall. Since the effects of large-scale circulations such as the PDO are changing under the influence of natural and anthropogenic climate change, reliable predictions on the future of the Alaskan climate are extremely complicated. In the future, further analysis is needed to support policy makers in their efforts to help adept the state’s ecosystems and economies to the changing climate.
How to cite: Heuer, J., Stuefer, M., and Hartl, L.: The Large-Scale Climate of Alaska - The Effects of the Pacific Decadal Oscillation on the Climate of Alaska, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-11931, https://doi.org/10.5194/egusphere-egu23-11931, 2023.
Climate change affects the hydrological cycle and induces extreme weather events, such as storms, floods and droughts. Adaptation to climate change needs to be based on assessments of future impacts. The new generation of Coupled Model Inter-comparison Project Phase 6 (CMIP6) is widely used in future flood prediction and drought risk assessment. However, many studies have found that CMIP6 global climate models for simulating land surface water and energy fluxes have significant biases, which poses a problem for using CMIP6 as input data for hydrological impact studies. Therefore, the output of CMIP6 cannot be directly used in hydrological models to project the impacts of future climate change. To overcome this problem, the correction of model output towards observations for its subsequent application in climate change impact studies has now become a standard procedure. And hydrological simulations generally use bias corrected output. But bias correction methods cannot really correct bias. The commonly used bias correction approaches only force the model outputs to match observations, and does not consider the mechanisms within the model and the interaction between variables. This study systematically evaluates water and energy fluxes of CMIP6 model over the Tibetan Plateau. Results show that the inter-model variability is substantial in temperature simulations. Snow that the largest component of the cryosphere responds significantly to changes in temperature. In the study, we study snow depth simulations corresponding to temperature simulations of different models over the Tibetan Plateau. Based on the water balance formula, analysis of how water balance fluxes respond to temperature changes in CMIP6, and determine the sources of error and ultimately lead to improved predictions.
How to cite: Liu, S., Liu, Z., and Duan, Q.: Evaluation of CMIP6 models for water and energy fluxes and analysis of source of errors over the Tibetan Plateau, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5066, https://doi.org/10.5194/egusphere-egu23-5066, 2023.
