Wintertime rainfall over Portugal is strongly coupled with the large‐scale atmospheric flow in the Euro‐Atlantic sector. A K‐means cluster analysis, on the space spanned by a subset of the empirical orthogonal functions of the daily mean sea‐level pressure fields, is performed aiming to isolate the weather regimes responsible for the interannual variability of the winter precipitation. Each daily circulation pattern is keyed to a set of five weather regimes (C, W, NAO−, NAO+ and E). The dynamical structure of each regime substantiates the statistical properties of the respective rainfall distribution and validates the clustering technique. The C regime is related to low‐pressure systems over the North Atlantic that induce southwesterly and westerly moist winds over the country. The W regime is characterized by westerly disturbed weather associated with low‐pressure systems mainly located over northern Europe. The NAO− regime is manifested by weak low‐pressure systems near Portugal. The NAO+ regime corresponds to a well‐developed Azores high with generally settled and dry weather conditions. Finally, the E regime is related to anomalous strong easterly winds and rather dry conditions. Although the variability in the frequencies of occurrence of the C and NAO− regimes is largely dominant in the interannual variability of the winter rainfall throughout Portugal, the C regime is particularly meaningful over northern Portugal and the NAO− regime acquires higher relevance over southern Portugal. The inclusion of the W regime improves the description of the variability over northern and central Portugal. Dry weather conditions prevail in both the NAO+ and E regimes, with hardly any exceptions. The occurrence of the NAO+ and the NAO− regimes is also strongly coupled with the North Atlantic oscillation. Copyright © 2005 Royal Meteorological Society

As temperature extremes have a deep impact on environment, hydrology, agriculture, society and economy, the analysis of the mechanisms underlying their occurrence, including their relationships with the large-scale atmospheric circulation, is particularly pertinent and is discussed here for Europe and in the period 1961–2010 (50 yr). For this aim, a canonical correlation analysis, coupled with a principal component analysis (BPCCA), is applied between the monthly mean sea level pressure fields, defined within a large Euro-Atlantic sector, and the monthly occurrences of two temperature extreme indices (TN10p – cold nights and TX90p – warm days) in Europe. Each co-variability mode represents a large-scale forcing on the occurrence of temperature extremes. North Atlantic Oscillation-like patterns and strong anomalies in the atmospheric flow westwards of the British Isles are leading couplings between large-scale atmospheric circulation and winter, spring and autumn occurrences of both cold nights and warm days in Europe. Although summer couplings depict lower coherence between warm and cold events, important atmospheric anomalies are key driving mechanisms. For a better characterization of the extremes, the main features of the statistical distributions of the absolute minima (TNN) and maxima (TXX) are also examined for each season. Furthermore, statistically significant downward (upward) trends are detected in the cold night (warm day) occurrences over the period 1961–2010 throughout Europe, particularly in summer, which is in clear agreement with the overall warming.

The unusually dry conditions during the 2004/2005 winter in Portugal led to the development of an extreme/severe drought episode throughout the country with major socioeconomic impacts. In fact, at some locations, this winter was the driest in at least the last 60 years. A K‐means classification of days into a set of five weather regimes (WRs), relevant for winter precipitation in Portugal, reveals a large prevalence of the two driest weather regimes during the 2004/2005 winter. These two regimes are basically linked to either anticyclonic circulation or easterly winds over Portugal and their prevalence explains the significant precipitation deficit. Winter precipitation variability in Portugal is indeed skillfully represented by linear models where the predictors are the frequencies of occurrence (FO) of these weather regimes. The dominance of the ‘dry phases’ of the main coupled modes between winter precipitation in Portugal and the large‐scale atmospheric circulation also supports the prevalence of the dry regimes and the corresponding lack of precipitation.The predominance of the dry regimes can be explained by a remarkably strong enhancement of the climate‐mean North Atlantic ridge, manifested by dynamically coherent anomalies in the geopotential heights, vorticity and temperature fields over the North Atlantic. The persistence of a warm‐core asymmetrical eddy over the North Atlantic, with a nearly barotropic equivalent structure, is a manifestation of this large‐scale anomaly. The blocking of the westerlies and the consequent northward shift in the axis of maximum moisture transports over the North Atlantic was one of the most striking changes in the large‐scale atmospheric flow. Consequently, the main track of the developing baroclinic disturbances was sufficiently distant from Portugal to hamper the development of rain‐generating conditions. As these dynamical conditions are common to other reportedly dry winters, they effectively constitute a key factor for the occurrence of a precipitation deficit in Portugal. Copyright © 2006 Royal Meteorological Society

Cloud-to-ground lightning in a sector covering the Iberian Peninsula, the Balearic Islands and nearby seas (36–44° N, 10° W–5° E) is analysed in the period from 2003 to 2009 (7 yr). Two Iberian lightning detection networks, composed of 18 sensors over Portugal and Spain, are combined for the first time in the present study. The selected characteristics are cloud-to-ground flashes (CGFs), first stroke peak current, polarity and multiplicity (number of strokes in a given flash). This study examines the temporal (on hourly, monthly and seasonal timescales) and spatial variability of CGFs. The influence of five forcing factors on lightning (elevation, lifted index, convective available potential energy and daily minimum and maximum near-surface air temperatures) over the Iberian sector is also assessed. For regional-scale assessments, six subsectors with different climatic conditions were analysed separately. Despite important regional differences, the strongest lightning activity occurs from late spring to early autumn, and mostly in the afternoon. Furthermore, CGFs are mainly located over high-elevation areas in late spring to summer, while they tend to occur over the sea in autumn. The results suggest that (1) orographically forced thunderstorms over mountainous areas, mostly from May to September, (2) tropospheric buoyancy forcing over western-central and northern regions in summer and over the Mediterranean regions in autumn, and (3) near-surface thermal contrasts from October to February largely control the location of lightning in Iberia. There is no evidence of different forcings by polarity. A clear correspondence between summertime precipitation patterns and CGFs is also found.

The severest winter precipitation deficits in Portugal result from an enhancement in the stationary wave pattern of the atmospheric large‐scale circulation over the Northern Hemisphere, which implies a strengthening of the axially asymmetric eddies, mainly over the North Atlantic and adjacent continental areas. This enhancement leads to a strong warm‐core ridge westward of Iberia, with a nearly equivalent barotropic structure, and a clear separation between the subtropical and polar‐front jet. These atmospheric conditions are clearly unfavourable to the establishment of rain‐generating mechanisms over Portugal, leading to a lack of precipitation and to extremely dry conditions with a high potential to trigger drought episodes. Conversely, the atmospheric flow during extremely wet winters contrasts highly with the above‐described conditions by presenting a significantly weakened North Atlantic ridge and a relatively unclear separation between the two westerly jets. A diagnosis of the main forcing dynamical mechanisms that generate and maintain the anomalous flow is carried out in two ways: by analyzing the Eliassen–Palm fluxes, zonally averaged over a North Atlantic window, and by calculating the third and sixth components of the empirical forcing functions. The former approach enables a quantification of the contributions made by the transient and stationary‐eddy transports of enthalpy and momentum to the establishment of the contrasting dynamical structures. The latter approach explores the role of local transient transports of enthalpy and angular momentum in forcing large‐scale asymmetries. Copyright © 2008 Royal Meteorological Society