Heavy rainfall episodes in Ecuador during El Niño events and associated regional atmospheric circulation and SST patterns

Abstract. To date very little is known about the relation between regional circulation patterns and sea surface temperature development in the Niño 1,2 region and the occurrence of heavy precipitation in Ecuador and northern Peru. The current study uses a comprehensive data set of 2544 Meteosat-3 imagery to investigate the dynamics of heavy precipitation during El Niño in 1991/92. Rainfall maps are retrieved by means of an adjusted version of the Convective Stratiform Technique (CST) and Cloud Motion Winds (CMW) are extracted from image sequences by using a special cross-correlation approach. A spatial factor analysis is applied to extract specific weather situations with heavy precipitation during El Niño events. The factor analysis yielded 16 factors. It has been proven that the factor patterns with the highest variance explanation also occur during the rainy season of non-El Niño years. However, 6 El Niño-specific situations could be derived which cause heavy rainfall, especially in coastal Ecuador and northern Peru. Multi-channel Sea Surface Temperatures (MCSST) and cloud motion winds are used to describe atmospheric and oceanic dynamics for these specific weather situations. The analysis shows that high SSTs in combination with strong SST gradients off the coast and warm SST bubbles lead to regional differences in moist instability and heavy rainfall. Both large scale circulation (reversal of the Walker cell) and regional dynamics (extended land-sea-breeze system) have been proven to contribute to El Niño rainfall.