Definition and classification of Arctic heatwaves

The Arctic is warming approximately two time faster than the entire globe, producing considerable local environmental changes that not only affect the fragile Arctic ecosystem but cause also significant socio-economic impacts on the midlatitudes and influence global climate and ocean circulation. In this context the Arctic warm extremes are receiving a growing attention. Until now they have been studied mostly from a overall point of view, considering their mean characteristics and precursors, or focusing on single particularly intense events. This thesis work aims to classify them on the base of their large scale circulation features. Firstly, a definition of Arctic heatwave is proposed, combining the methodology used by some recent studies for identifying Arctic warm extremes with the structure common to the majority of the heatwave definitions used in scientific literature. Then Arctic heatwaves are classified, separately for each season, on the base of their 500-hP a geopotential height anomaly maps in the extra-tropical Northern Hemisphere. Particular attention is given to winter extremes. On the basis of a clustering method, winter events are grouped in three different circulation patterns. Two of these latter are characterized by a pronounced anticyclonic anomaly in the Eurasian sector of the Arctic and by an extensive cold anomaly over southern Siberia (WACE pattern). The third circulation regime is composed by a limited number of events and is countersigned by the prevalence of cyclonic anomalies in the Arctic region. The temporal series of these three regimes shows decadal variations that suggest possible periodicities or Arctic temperature trend dependencies. In the other seasons the circulation regime with the highest number of heatwaves is characterized by an anticyclonic anomaly over the high Arctic. Finally, both on a seasonal level and for single regimes a strong positive correlation between heatwave duration and intensity is observed.