Differences in Teleconnection over the North Pacific and Rainfall Shift over the USA Associated with Two Types of El Nino during Boreal Autumn

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 Nino. These two types, called cold tongue (CT) and warm pool (WP) El Ninos, have an opposing impact on atmospheric circulation over the eastern North Pacific. When CT El Nino 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 Nino 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 Nino is similar to that of La Nina, but opposite to that of CT El Nino. Since the early 1990s, the WP El Nino event has occurred more frequently, while the CT El Nino events has become less. The La Nina 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 Nino regime change thus appears to contribute to a decadal difference in the regional autumn rainfall.