Atmospheric large‐scale dynamics during the 2004/2005 winter drought in portugal

Abstract

Abstract: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.

A process-based typology of hydrological drought

“…Santos et al, 2007;Trigo et al, 2010;Li et al, 2010). A more generally applicable typology of hydrological drought is needed, both for process understanding of drought propagation and for improvement of drought forecasting and management.…”

The role of large‐scale eddies in the occurrence of winter precipitation deficits in Portugal

“…Although a descriptive analysis of several dynamical aspects prevailing during extremely dry winters in Portugal has already been presented in a previous study (Santos et al, 2007a), the underlying physical processes will be investigated here in greater detail, giving much more emphasis to the forcing mechanisms. The present study thus aims at getting a deeper insight into the dynamics.…”

“…Particular attention has been devoted to the recent 2004/05 drought, since it was among the severest according to a number of drought indices (e.g. Palmer Drought Severity Index, Standard Precipitation Index) and it had significant socio-economic impacts across almost all sectors of the Portuguese economy (Santos et al, 2007a).…”

“…The precipitation regime in Portugal is deeply influenced by the seasonal migration of the mid-latitude westerlies and associated weather systems, where the location, extent and intensity of the North Atlantic ridge, manifested at the surface as the Azores High, play a controlling role in establishing or blocking rain-generating mechanisms over Portugal (Santos et al, 1980;Ferreira and Ferreira, 1983;Santos et al, 2007a). The total rainfall amounts in Portugal are thus highly controlled by large-scale anomalies in the atmospheric circulation over the North Atlantic (Santos et al, 2007b).…”

Detection of heat and cold waves in Montevergine time series (1884–2015)

“…According to the results achieved by the studies of Cassou et al (2005) and Carril et al (2008), the anticyclonic circulation over the eastern North Atlantic plays a relevant role in the triggering of heat waves in Europe. Furthermore, linkages between the occurrence of extreme events and the North Atlantic Oscillation have been reported by Santos et al (2007) and by Scaife et al (2008). In a recent study, Andrade et al (2012) have discovered a relevant connection between the winter variability of the cold nights and warm days in Europe and the dipolar pattern associated to the North Atlantic Oscillation.…”