EL NINO MEETS LA NINA - ANOMALOUS RAINFALL PATTERNS IN THE 'TRADITIONAL' EL NINO REGION OF SOUTHERN ECUADOR

摘要

In this paper, the central Pacific cold event of 2008 and its exceptionally warm conditions in the eastern tropical Pacific are analyzed by using rainfall data of south Ecuadorian meteorological stations, sea surface temperatures in the El Nifio3 and1+2 regions, and simulations with the Weather Research and Forecasting (WRF) model. It can be shown that El Nino-like rainfall conditions with severe inundations occur particularly in the coastal plains of southern Ecuador while a central Pacific cold event prevails. In contrary to previous situations, positive rainfall anomalies as a result of El Nino-like conditions in the El Nihol +2 region during the 2008 La Nina event occurred in both regions, the coastal plains and the highlands, for the first time. A detailed analysis of the ocean-atmosphere system during episodes of heavy rainfall reveals typical El Nino circulation and rainfall patterns as observed during previous El Nino events for the coastal area and La Nina-like conditions for the highlands. The spreading of Pacific instability in the Nino 1+2 region to the eastern escarpment of the Andes could be the result of a temporary eastward shift of the Walker circulation. The unusual combination of El Nino-like conditions in the eastern tropicalPacific during a La Nina state in the central Pacific is the newest indicator for an impact mode shift regarding severe rainfall anomalies during El Nino/La Nina events in the traditional El Nino area of southern Ecuador since the end of the last century. Since 2000, El Nino events unexpectedly provide below average rainfall while central Pacific La Nina conditions generate exceptional severe flooding in the normally drier coastal plains. The novel sea surface temperature (SST) anomaly dipole structurebetween the eastern and central/western tropical Pacific and the weakening of El Nino events since 2000 could be due to natural decadal oscillations in the El Nino background state, the Pacific Decadal Oscillation (PDO). However, the observed atmosphericpatterns and the recent increase of the SST anomaly difference between the central and the eastern tropical Pacific resemble structures that also result from climate change simulations.