Precursors of summer heat waves in the Eastern Mediterranean

Reanalysis and observational data are used to identify the precursors of summertime heat waves over the Eastern Mediterranean. After compiling a list of heat waves using objective criteria, we identify robust precursors present 7–10 days before the onset of the heat wave, longer than the typical horizon for trustworthy weather forecasts. If these precursors are present, there is a significant warming over the Eastern Mediterranean over the following 10 days that persists for weeks after. These precursors include a weakened Indian monsoon, a strengthened Sahelian monsoon, warm Western/Central Mediterranean sea-surface temperatures, and a midlatitude low-pressure system from the west. Further, horizontal temperature advection is the proximate cause of the heat wave in the days before the extreme; in particular, a weakening of the Etesian winds that would otherwise advect relatively cool maritime air inland accounts for around half of the warming. There has been a clear tendency for more heat extremes in recent years. These results have implications for forecasting anomalous summer temperatures in the Eastern Mediterranean, and the framework developed here can also be applied in other regions.