Differences in teleconnection over the North Pacific and rainfall shift over the USA associated with two types of El Niño during boreal autumn

Abstract

This work investigates the teleconnection patterns over the North Pacific/North America sector and regional rainfall variability over the southwestern USA during boreal autumn, associated with two types of El Niño. These two types, called cold tongue (CT) and warm pool (WP) El Niños, have an opposing impact on atmospheric circulation over the eastern North Pacific. When CT El Niño occurs, a strong cyclonic anomaly tends to appear over the North Pacific, and the associated southwesterly winds bring unusually moist air and thereby enhance rainfall over the southwestern USA. However, during WP El Niño autumns, a tripolar anomaly develops over the North Pacific. The associated northerly and northeasterly winds transport unusually dry air to the southwestern USA causing a reduction in rainfall. In this region, the rainfall response to WP El Niño is similar to that of La Niña, but opposite to that of CT El Niño. Since the early 1990s, the WP El Niño event has occurred more frequently, while the CT El Niño events has become less. The La Niña events remain roughly unchanged in terms of the zonal location. Autumn rainfall deficits over the southwestern USA have also been more frequent after the 1990s. The El Niño regime change thus appears to contribute to a decadal difference in the regional autumn rainfall. © 2012, Meteorological Society of Japan.