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Keywords:

  • extreme temperatures;
  • air-trajectories;
  • East Mediterranean;
  • evolution scenarios;
  • Asian Monsoon;
  • K-Means clustering;
  • Persian trough

ABSTRACT

This research analyses the scenarios that break the monotonic summer regime over the East Mediterranean (EM). The study compares the five upper, median and lower percentiles of the 850-hPa temperature for July–August 1975–2005. The temperature differences were found to be confined to the lower 4 km. They are controlled by the intensity of the negative temperature advection, but not by the prevailing subsidence. Air trajectory analysis shows that the regional prevailing northwest winds are the strongest prior to cool events and the weakest, or nonexistent, prior to hot events. An index representing the location of the air source is proposed as an effective predictor for the temperature. The hot and the cool days were subdivided according to their evolution scenarios. All the cool days were associated with deepening of upper troughs over the EM. Different scenarios were found to generate hot days; ‘tropical’, associated with intrusion of tropical-like air, ‘subtropical’, resulting from an expansion of the subtropical high from North Africa towards the EM, and ‘baroclinic’, associated with a pronounced upper-level ridge, part of a Rossby wave over the Mediterranean Basin. The ‘tropical’ scenario was well identified through the mid-level relative humidity. An alternative classification approach, the objective K-Means clustering method, was applied to the air back-trajectories reaching the EM. They isolated successfully only the ‘tropical’ type. Back-tracking of temperature anomalies indicate two different evolutions for the hot days; one is associated with propagation from the west sector and other is quasi-stationary, associated with intrusion of tropical air. The cool events are all propagating from northwest. This indicates that extreme summer events over the EM are mostly regulated by mid-latitudes disturbances travelling eastward rather than by the Asian Monsoon.