Midlatitude Atmosphere-Ocean Interaction During El Niño. Part II: the Northern Hemisphere Atmosphere

Michael A. Alexander, Dept. of Meteorology, University of Wisconsin, Madison, WI


The influence of midlatitude air-sea interaction on the atmospheric anomalies associated with El Niño is investigated by coupling the Community Climate Model (CCM) to a mixed layer ocean model in the North Pacific. Prescribed El Niño conditions, warm sea surface temperatures (SST) in the tropical Pacific, cause a southward displacement and strengthening of the Aleutian Low. This results in enhanced (reduced) advection of cold Asian air over the west central (northwest) Pacific and northward advection of warm air over the eastern Pacific. Allowing air-sea feedback in the North Pacific slightly modified the El Niño induced near surface wind, air temperature and precipitation anomalies. The anomalous cyclonic circulation over the North Pacific is more concentric and shifted slightly to the east in the coupled simulations. Air-sea feedback also damped the air temperature anomalies over most of the North Pacific and reduced the precipitation above the cold SST anomaly which develops in the central Pacific.

The simulated North Pacific SST anomalies and the resulting Northern Hemisphere atmospheric anomalies are roughly 1/3 as large as those related to the prescribed El Nino conditions in a composite of five cases. The geopotential height anomalies associated with changes in the North Pacific SSTs have an equivalent barotropic structure and range from -65 m to 50 m at the 200 mb level. Including air-sea feedback in the North Pacific tended to damp the atmospheric anomalies caused by the prescribed El Niño conditions in the tropical Pacific. As a result, the zonally elongated geopotential height anomalies over the West Pacific are reduced and shifted to the east. However, the atmospheric changes associated with the North Pacific SST anomalies vary widely among the five cases.