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

Abstract

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