Drivers behind the summer 2010 wave train leading to Russian heatwave and Pakistan flooding

The summer of 2010 saw devastating extreme weather events: a record heatwave in western Russia and catastrophic flooding in Pakistan. These events, causing significant societal impacts including thousands of deaths and widespread damage, were linked by a strongly meandering jet stream. The Russian heatwave was characterized by a persistent blocking anticyclone, leading to subsidence, adiabatic warming, and drought conditions. Simultaneously, Pakistan experienced heavy rainfall, exceeding three times the climatological average, resulting from a combination of a mid-tropospheric trough, high Arabian Sea temperatures, and La Niña conditions. The unusually wavy jet stream connecting these events highlights the potential impact of atmospheric wave trains on weather extremes. While thermodynamic atmospheric warming contributes to increased heatwaves and rainfall extremes, the dynamic effects of global warming on wave trains and blocking are less understood. Previous studies suggested increased frequency of the mid-latitude wave pattern responsible for the 2010 events, with sea surface temperatures (SSTs) and high-latitude land warming playing significant roles. The influence of soil moisture anomalies as a source of Rossby waves and their impact on large-scale dynamics also needed further investigation. This study aimed to quantify the contributions of SST patterns (including global warming), high-latitude land warming, and early-summer soil moisture conditions to the 2010 Russian and Pakistan extremes using a very large ensemble of general circulation model simulations.

Existing literature highlighted the individual characteristics of the 2010 Russian heatwave and Pakistan floods, noting the role of blocking anticyclones, SST anomalies, and La Niña conditions. Studies indicated a connection between these extremes through an unusually wavy jet stream and emphasized the impact of atmospheric wave trains on mid-latitude weather. The role of thermodynamic warming was acknowledged, but uncertainties surrounding the dynamic effects of global warming on wave patterns and blocking remained. Previous work suggested links between Arctic amplification, mid-latitude circulation changes, and the increased frequency of certain wave patterns. Studies also investigated the influence of SST, particularly in the tropics, on shaping atmospheric waves and the potential of tropical convection as a source of circumglobal wave trains. The importance of high-latitude land warming in affecting atmospheric wave patterns was also highlighted, along with the role of soil moisture anomalies as a source of Rossby waves and their potential for inducing or exacerbating heatwaves.

The study employed a very large ensemble of general circulation model simulations from the weather@home/climateprediction.com project. The simulations used the HadAM3P global climate model and a nested 50km regional climate model (HadRM3P) covering South Asia. The models were forced with observed SST anomalies and radiative forcings for the period 1987-2015 (climatology) and for 2010. Indices were defined for western Russia surface air temperature (SAT) and Pakistan rainfall, using spatial averages over specific regions and temporal averages from July 24th to August 8th, 2010. These indices were standardized using the 1987-2015 climatology. Sub-selections of the 2010 ensemble were created based on specific conditions: High-latitude land warming (T65N) was identified using a correlation between zonal SAT profiles and a climatological QRA fingerprint, selecting ensemble members above the 90th percentile. Drier-than-usual early-season (June) soil moisture conditions (soilM) were identified by selecting ensemble members with soil moisture values in western Russia below the 10th percentile. The relative contributions of SST patterns (combined with global warming), high-latitude land warming, and early-summer soil moisture conditions to the probability of the 2010 extremes were then quantified.

The model reproduced the 2010 circumglobal wave train connecting the Russian heatwave and Pakistan floods. Even in the model's climatology, the probability of simultaneous extremes in both regions (both exceeding the 90th percentile) was twice as high as expected for independent events. The 2010 SST anomalies, combined with radiative forcings, significantly increased the probabilities of both the western Russia heatwave (2-fold increase) and Pakistan rainfall extremes (4-fold increase). Conditioning on high-latitude land warming further enhanced these probabilities, by a factor of ~2.5 for western Russia SAT and ~5 for Pakistan rainfall. Similarly, drier-than-usual soil moisture in western Russia in June also increased the probabilities, suggesting not only an impact on local heat extremes but also on remote rainfall extremes via the connecting wave train. The probability of concurrent extremes increased fourfold in the 2010 ensemble compared to climatology and even further when high-latitude warming or dry soils were considered. Analysis of La Niña and El Niño years in the model climatology showed that high-latitude land warming and enhanced Pakistan rainfall were associated with increased SAT anomalies in western Russia during La Niña years. The model simulations also reproduced the atmospheric circulation pattern linking the two extreme events.

The findings demonstrate the model's ability to reproduce the atmospheric circulation pattern linking the 2010 Russian heatwave and Pakistan floods and highlight the synergistic interactions between different drivers. The study shows that the concurrent nature of these events is not simply coincidental but rather favored by a combination of factors. The model's climatological run suggests the existence of a recurrent wave train connecting these extremes even in the absence of the specific 2010 conditions. The significant increase in the probability of concurrent extremes in the 2010 simulation underscores the importance of the 2010 La Niña-like SST anomaly. The additional influence of high-latitude land warming and low soil moisture indicates complex, potentially nonlinear interactions within the climate system. Although the study doesn't allow strict separation of GHG and SST effects, it clearly demonstrates the crucial role of these drivers in exacerbating the concurrent extreme events.

The weather@home model successfully reproduced the atmospheric wave train linking the 2010 Russian heatwave and Pakistan floods, demonstrating the interconnectedness of these events. The study quantified the significant contributions of 2010 SST anomalies, high-latitude land warming, and early-summer soil moisture conditions in increasing the probability of both single and concurrent extremes. Future research should focus on disentangling the complex causal chains between these drivers and the persistence of atmospheric wave trains and on improving the understanding and modelling of these interactions under future climate scenarios, which could result in increasingly frequent and intense extreme weather events, posing significant risks to vulnerable societies. Improved understanding of the non-linear interactions will be critical to accurately predict the changing risks of these high-impact events.

The model's ability to completely reproduce the observed extremes was limited. For instance, the observed SAT anomalies in western Russia were about four times stronger than those in the ensemble mean, and the phase of the wave pattern was slightly different. This highlights the need for additional research to explore the causes of this mismatch. The study does not allow for a complete separation between GHG-induced and SST-driven changes. The choice of the target period for the analysis (24 July-8 August 2010) might slightly influence the results. The lead time of one month between soil moisture conditions and their impact on Pakistan rainfall could be refined. Finally, the exact causal chain between high-latitude land warming and extreme events was not fully elucidated.