Using casual discovery to assess the effect of Indian summer monsoon on summer heat extremes in the eastern Mediterranean

Understanding climate and atmospheric teleconnections and the ability of forecasting models in reproducing these with sufficient accuracy represents a key step to better understand model performance and forecasting skill. In this work, we apply the Peter and Clark causal discovery algorithm (PCMCI) to analyse the relationship between the Indian summer monsoon (ISM) and heat extremes in the eastern Mediterranean in ERA5 reanalysis and SEAS5 seasonal forecast data over the 1981-2023 period. The analysis aims to (a) determine the effect of ISM interannual variability on heat extremes, (b) assess the ability of SEAS5 in reproducing the observed causal chain and (c) understand how El Niño affects this teleconnection. Our results show that in ERA5, weak ISM years are connected to an increased probability of heat extremes in Egypt, the Middle East and the Anatolian Peninsula, while in SEAS5 this connection is shown predominantly for Egypt, but it is very weak or absent in the other two regions. Applying PCMCI at sub-seasonal (3-day) time scales shows that SEAS5 can quantitatively well reproduce the causal links connecting the ISM convective activity to the Etesians via the Middle East ridge (ME-ridge). In turn, the Etesians (summer surface northerly winds blowing over the Aegean Sea) affect temperature variability over the region, with weak Etesians leading to higher surface temperature. In contrast, the ability of SEAS5 to reproduce observed causal links diminishes when monthly time scales are analysed. SEAS5 struggles to reproduce the sign of the link from El Niño towards the ISM and from the latter toward eastern Mediterranean geopotential heights. Finally, we assess historical trends and the effect of El Niño on the detected causal links, showing that (a) the effect of the ISM on the ME-ridge has increased since 1981, and that the influence of the ISM on both the ME-ridge and the Etesians is enhanced during La Niña years.

Di Capua et al. “Increased risk of heat extremes in the Eastern Mediterranean during weak Indian summer monsoon years”, in review
Pre-print available at https://www.researchsquare.com/article/rs-6887363/v1