Dynamics and physical processes involving extreme temperatures in the iberian peninsula and iraq

Resumen

The occurrence of heat waves and cold spells has received special attention in recent years due to their impact on human health, ecosystems and economy. In the context of climate change, there is evidence that extreme temperature episodes, and not only the mean temperature, are changing in response to anthropogenic radiative forcing. This thesis focuses in the study of the dynamics and the physical processes leading to temperature extremes in two regions located at the west and the east sides of the Mediterranean Sea: the Iberian Peninsula (IP) and Iraq. The 12–hourly 2–m height maximum/minimum temperatures obtained from the 1º¿1º ERA–Interim database in the period 1994–2013 are used. At each grid point, hot/cold extremes are defined as those events in which T2MAX/T2MIN was above/under its 99.9th/0.1th percentile. The results in both study areas show that there is a positive trend in the occurrence and persistence of both hot and cold events. By using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, the 10-day back-trajectories of the air masses producing the extremes are analysed. Rather than meridional advection, the primary cause behind the occurrence of hot extremes seems to be progressive diabatic warming, which becomes accentuated the days before the episode. The extremes are caused by air masses with long residence times and by recirculation processes during summer days of weak baric gradient. The air masses producing the extreme cold events have faster trajectories. Cold events are mainly associated with advection of air-masses from northeast and north Europe for IP and from Siberia and East Europe for Iraq. In the IP, Principal Component Analysis of the pressure and geopotential fields has also been carried out to identify the associated synoptic scale configurations, showing that advection during cold episodes is induced by the presence of a relative low in the central Mediterranean and a blocking high in the North Atlantic and/or in northeast Europe. We analyse the correlations between temperature extremes and the modes of climatic variability affecting the Mediterranean basin: North Atlantic Oscillation (NAO), Western Mediterranean Oscillation (WeMO), Arctic Oscillation (AO) and East Atlantic Oscillation (EA). In the IP, a significant positive correlation (p < 0.01) is found between the annual average of the extreme hot temperatures and annual WeMO. Significant negative correlation (p < 0.01) is found between the number of cold days and the annual index for NAO. In Iraq, significant positive correlation (p < 0.01) between the annual average of the extreme hot temperatures and annual EA is found. The mechanisms underlying the occurrence of hot extreme temperatures for specific events occurred in IP have been analysed by using both HYSPLIT and the Weather Research and Forecast (WRF) mesoscale model. For the event that occurred on 10 August 2012, that affected the highest number of grid points of the series, the synoptic situation shows an omega blocking system and local recirculation of warm air masses. The analysis of the only hot extreme event recorded just at one single grid point, on 15 August 2012, shows that not synoptic or mesoscale conditions substantially different from those of the neighbours grid point were given.