Precursors of summer heatwaves in the Eastern Mediterranean

Reanalysis and observational data are used to identify the precursors of summertime
heatwaves over the Eastern Mediterranean over the historical period. After compiling
a list of heatwaves using objective criteria, we identify robust precursors present 7 to 10
days before the onset of the heatwave, 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 then persists for weeks
after. These precursors include a weakened Indian monsoon, warm West/Central
Mediterranean Sea surface temperatures, and a low disturbance from the west. Further,
horizontal temperature advection is the proximate cause of the heatwave in the days
before the extreme, and 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 is a clear tendency for more heat extremes in recent years. These
results have implications for the forecasting of summer heatwaves in the Eastern
Mediterranean, and the framework developed here can be applied in other regions as
well.