|ABSTRACT||Atmospheric blocking episodes are an important component of the intra-seasonal and inter-annual variability at mid-latitudes. Their impacts in the European continent, and in particular, in the Mediterranean basin, depend on: i) blocking location; ii) spatial characteristics; iii) temporal length. A characterization of Euro-Atlantic blocking occurrence within different sectors (Atlantic, European and Russian) was performed, focusing on the impacts in temperature and precipitation regimes. High-latitude blocks were distinguished from sub-tropical ridges using a novel ridge detection scheme. Ridges do not require wave-breaking occurrence as blockings do, although they are frequent precursors of wave-breaking and subsequent high-latitude blocking episodes. The distinct seasonal and regional impacts associated with different blocking/ridge locations were analyzed, as well as the dynamical mechanisms driving the temperature and precipitation responses associated to each pattern, namely: the role of cyclonic activity; moisture transport; large-scale atmospheric instability; balances between horizontal advection, subsidence and radiation budgets.
Our analysis clarifies that increasing extreme heat episodes in southern Europe and Mediterranean areas should not be attributed to blockings, but rather to sub-tropical ridges. In northern Europe, both regimes are responsible for warm conditions in summer, due to enhanced radiative heating and increased subsidence. Opposite temperature responses are found for winter blocking/ridges. While blockings reinforce cold northerly advection and foster cold spells, ridge patterns are related with mild Atlantic flows associated. An opposite north-south dipole is also found regarding precipitation anomalies. While blocks force a split of the Jetstream and storm-track, ridges are characterized by a stronger zonal flow at higher latitudes. Accordingly, negative (positive) precipitation anomalies during blocks occur at higher (lower) latitudes. We also demonstrate how enhanced atmospheric instability and cyclonic activity occurs south of blocking centers, highlighting the importance of torrential regimes in Mediterranean areas. On the other hand, the presence of sub-tropical ridge enhances a northward deflection of moisture corridors, leading to very dry conditions in southern Europe, and significantly increasing the chances of drought occurrence in Mediterranean areas.
Finally, we discuss how sub-tropical ridge frequency has been increasing, in the context of a warming atmosphere and an expanding Hadley Cell, and how the expected further expansion of the tropics poses a severe threat for water availability in the Mediterranean area.
This work was supported by project IMDROFLOOD – Improving Drought and Flood Early Warning, Forecasting and Mitigation using real-time hydroclimatic indicators (WaterJPI/0004/2014) funded by FCT. Alexandre M. Ramos was also supported by a FCT postdoctoral grant (FCT/DFRH/ SFRH/BPD/84328/2012).|