Increased risk of heat extremes in the Eastern Mediterranean during weak Indian summer monsoon years

Heat extremes in the eastern Mediterranean have increased 3-fold in the last decade, posing a severe threat to human health, socio-economic activities and ecosystems. During summer, northerly winds, called Etesians, can contribute to mitigating or exacerbating heat extremes. Here, we apply the Peter and Clark momentary conditional independence (PCMCI) algorithm to reanalysis data (ERA5) and seasonal forecasts (SEAS5.1), to assess the impact of the Indian summer monsoon (ISM) – Etesians teleconnection on the probability of heat extremes occurrence over the eastern Mediterranean. We find that weak Etesians, associated with suppressed ISM activity and warmer central tropical Pacific sea surface temperatures (El Niño phase), can increase the probability of heat extremes in the region up to 5-fold at 3-day timescales, and up to 2-fold at seasonal timescales. Moreover, over the 1981-2023 period, the effect of the mid-latitude circulation has weakened. In general, SEAS5.1 forecasts struggle to reproduce the dependency of the identified causal pathways originating from the ISM and El Niño intraseasonal variability, while it overestimates the effect of mid-latitude wave trains. These findings highlight that future changes in the ISM system could significantly affect the probability of heat extremes occurrence in the Mediterranean, while climate models might underestimate this link.