Role of the tropical Atlantic sea surface temperature in the decadal change of the summer North Atlantic Oscillation

Abstract

Recent observational studies have shown that the southern center of the summer North Atlantic Oscillation (SNAO) shifted eastward after the late 1970s. In this study, this phenomenon and its causes are fürther explored. It is found that the decadal spatial shift of the SNAO southern center is related to the decadal variability of the tropical Atlantic sea surface temperature (TASST). In the past half century, the TASST experienced an abrupt change around the late 1970s, with a rapid wanning in the recent 2 decades. Thus in the period after the late 1970s when the TASST is relatively warmer, the TASST released more energy into the atmosphere, then stimulated strong convection over the tropical Atlantic, and fürther excited anomalous wave-like patterns. This strengthened the circulation in the region of the Mediterranean Sea, namely the east part of the SNAO southern center, consequently leading to an eastward shift of the SNAO southern center. Meanwhile, in the period before the late 1970s when the TASST is relatively cooler, the TASST released less energy into the atmosphere, so its impact on the overlying atmosphere was significantly weakened and confined to the lower-level atmosphere of the tropical Atlantic. Thus the TASST possibly did not influence the variability of the SNAO southern center, and the SNAO pattern exhibited a traditional distribution with two centers over the North Atlantic. Copyright 2009 bv the American Geophysical Union.