Changes of Subseasonal Variability Associated with El Niño

Abstract

This paper is concerned with assessing the impact of the El Niño?Southern Oscillation (ENSO) on atmospheric variability on synoptic, intraseasonal, monthly, and seasonal timescales. Global reanalysis data as well as atmospheric general circulation model (AGCM) simulations are used for this purpose. For the observational analysis, 53 yr of NCEP reanalyses are stratified into El Niño, La Niña, and neutral winters [Jan?Feb?Mar (JFM)]. The AGCM analysis is based on three sets of 180 seasonal integrations made with prescribed global sea surface temperatures corresponding to an observed El Niño event (JFM 1987), an observed La Niña event (JFM 1989), and climatological mean JFM conditions. These ensembles are large enough to estimate the ENSO-induced changes of variability even in regions not usually associated with an ENSO effect. The focus is on the anomalous variability of precipitation and 500-mb heights. The most important result from this analysis is that the patterns of the anomalous extratropical height variability change sharply from the synoptic to the intraseasonal to monthly timescales, but are similar thereafter. In contrast, the patterns of the anomalous tropical rainfall variability are nearly identical across these timescales. On the synoptic and monthly scales, the anomalous extratropical height variability is generally opposite for El Niño and La Niña, and is also roughly symmetric about the equator. On the intraseasonal scale, however, the anomalous height variability is of the same sign for El Niño and La Niña in the Atlantic sector, and is antisymmetric about the equator in the Pacific sector. In the North Pacific, these intraseasonal variance anomalies (which are consistent with a decrease of blocking activity during El Niño and an increase during La Niña) are of opposite sign to the monthly and seasonal variance anomalies. The sharp differences across timescales in the ENSO-induced changes of extratropical variability suggest that different dynamical mechanisms dominate on different timescales. They also have implications for the predictability of extreme events on those timescales. Finally, there is evidence here that these impacts on extratropical variability may differ substantially from ENSO event to event, especially in the northern Atlantic and over Europe.