Indian summer monsoon versus mid-latitude drivers of boreal summer tropospheric circulation and heat extremes in the eastern Mediterranean

The lower tropospheric circulation over the eastern Mediterranean during boreal summer is markedly characterized by northerly winds known as the Etesians (Tyrlis et al., 2013). These winds are accompanied by large scale subsidence and clear skies, and can mitigate the emergence of heat waves by bringing colder air from the Eurasian landmass. Here, we employ Causal Effect Networks, obtained by applying the Peter and Clark Momentary Conditional Independence (PCMCI) causal discovery algorithm (Runge, 2018), to identify causal precursors of the Etesians both in mid-latitude circulation fields and tropical convective activity at two different intraseasonal time scales (3 and 7-day average). We identify wave train activity over the North Atlantic and North American region and convective activity over the Arabian Sea and western coast of the Indian peninsula to be causal precursors of Etesians winds defined as 850 hPa meridional wind variations over the eastern Mediterranean at a lag of 3-to-6 days. In general, the influence of tropical drivers, i.e. the Indian summer monsoon (ISM) system,  is found to be stronger than that of the mid-latitude wave train, thus corroborating the hypothesis that the ISM affects the circulation over the Mediterranean and Northeast Africa, as suggested by the monsoon-desert mechanisms (Rodwell and Hoskins, 1996). Moreover, at longer time scales (7 to 14-day lag), the main causal influence comes from tropical convective activity over the Indian peninsula, while the effect of the mid-latitude circulation weakens and becomes not significant. We finally employ event coincidence analysis to explore the relationship between Etesians and heat extremes in the eastern Mediterranean and assess the presence of trends in the strength of Etesians outbreaks at intraseasonal variability in the historical period.

References

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Runge, J.: Causal network reconstruction from time series: From theoretical assumptions to practical estimation, 28, 075310, https://doi.org/10.1063/1.5025050, 2018.

Tyrlis, E., Lelieveld, J., and Steil, B.: The summer circulation over the eastern Mediterranean and the Middle East: Influence of the South Asian monsoon, Clim. Dyn., 40, 1103–1123, https://doi.org/10.1007/s00382-012-1528-4, 2013.