A classification of the atmospheric circulation patterns producing significant daily rainfall in the Spanish Mediterranean area

This study investigates the synoptic atmospheric circulations associated wi th 11 typical spatial patterns for significant rainfall days for Mediterran ean Spain, which have been identified in an earlier paper. Using cluster an alysis on the most relevant T-mode principal components, a classification i nto 19 fundamental circulations emerged, based only on geopotential fields at 925 and 500 hPa, using ECMWF gridded data for the period 1984-1993. The derived atmospheric patterns comprise a wide variety of flows over the Iberian Peninsula, with a clear distinction between Atlantic and western Me diterranean disturbances. Distinct seasonal distributions are also observed . Despite the comparatively higher resolution of the rainfall patterns, and the uncertainty derived from working with a daily time scale, a clear asso ciation emerged between each of the circulation types and a small number of the characteristic rainfall patterns. These associations can be physically interpreted in terms of the position of the 500 hPa trough, disturbances a t the 925 hPa level, and the interaction of the surface rain bearing flows with the complex topography of the region. The final part of the analysis concentrated on establishing links between i dentified circulation patterns and notable (torrential) precipitation event s recorded during the 1984-1993 decade. Some of the identified circulations that are important for the occurrence of significant rainfalls, produce fe w or no torrential rainfall episodes. Most torrential rainfall events in Me diterranean Spain are associated with disturbances located near or over the south of the Iberian Peninsula. Copyright (C) 1999 Royal Meteorological So ciety.