Anomalously severe cold nights and warm days in northeastern Spain: their spatial variability, driving forces and future projections

Spatial and temporal variability of the most extreme temperatures during extended winter (NovemberFebruary, NDJF) and summer (MayAugust, MJJA) have been analyzed over northeast Spain. This work employed a high-quality daily temperature dataset of 128 station records spanning the period from 1959 to 2006. The uppermost (99th) percentile of daily maximum temperature distribution was used to define very warm days (hereafter VWD), while the lowermost (1st) percentile of daily minimum temperature distribution was adopted to identify very cold nights (hereafter VCN). The climatological conditions related to the initialization and persistence of these rare events were also examined. Based on daily Sea Level Pressure (SLP) and 200 hPa and 500 hPa geopotential height fields from the NCEP/NCAR re-analyses, major large-scale circulation patterns corresponding to these events over large domain of the North Atlantic and European regions were used in order to explore their interrelations with anomalous temperature in the study domain. Our results demonstrate that the occurrence of VCN was mainly associated to the dominance of cyclonic (anticyclonic) conditions over southern Europe and the Mediterranean (Scandinavia). This situation corresponded to an increase in the meridional circulation over much area of Western Europe, with strong advection of colder air from northern continental Europe and the north Atlantic to the study domain. For VWD, the most likely factors contributing to the frequency of these events were the north-eastward displacement of the Atlantic subtropical high and the increase in the European blockings. Based on data derived from a suite of regional climate models (RCMs) with the best validation results, future projections of the frequency and intensity of VWD and VCN events were also assessed under the A1B greenhouse gas emission scenario. Our results demonstrate that future VWD extremes could become more frequent and severe during the latter decades of the 21st century, while a number of simulations projected a general tendency for fewer VCN extremes. Over the study domain climate models predict even faster warming rates for the continental areas relative to coastal portions, which clearly comes in contrast with the observed climatology.

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