Interannual, Decadal–Interdecadal, and Global Warming Signals in Sea Surface Temperature during 1955–97

Abstract

Sea surface temperature (SST) data from the NOAA analysis for the period of 1955?97 are used to identify dominant spatial and temporal patterns associated with interannual, decadal?interdecadal variabilities, and global warming. For the global annual mean SST variation, a linear trend plus pronounced decadal?interdecadal and interannual variabilities are found. The linear trend reflects a slow warming of about 0.1°C in the past four decades. Superimposed on the linear trend is a relatively fast warming in the past two decades of approximately 0.2°?0.3°C, which is associated with the advent of the warm phase of a decadal?interdecadal oscillation. Also noted is the more frequent occurrence of warm events and the dearth of cold events on interannual timescales in the early and mid-1990s. For the last four decades, the tropical oceans have been warming at the faster rate while the extratropical North Pacific and the northwestern North Atlantic has been cooling at the slower rate. The exceptional intensity of the 1997 warm event stems from the superposition of the warm phases of a moderate interannual signal, a decadal?interdecadal oscillation, and a steady warming trend since 1955. The first annual empirical orthogonal function (EOF) mode shows an El Niño signal, modified by a warming trend. In this mode, the tropical eastern Pacific and Indian Oceans are warmed relative to the western Pacific, leading to a relaxation of the climatological east?west SST gradient. The second annual EOF mode shows that there is a secondary cooling trend in the equatorial eastern Pacific, centered near the Niño-3 (5°S?5°N, 150°?90°W) area. This cooling is most pronounced in the boreal fall and summer. It was strongest in the 1980s, but considerably weakened in the 1990s. The cooling is well correlated with the observed surface wind divergence in the equatorial eastern Pacific cold tongue region and with anomalous surface easterly wind in the equatorial central Pacific, signaling an enhancement of the Walker circulation. The net result of the warming and cooling tendencies in the tropical eastern Pacific produces slightly less warming in the Niño-3 area compared with the equatorial central and eastern Pacific, but with little change in large-scale SST gradient across the Pacific. The implications of the present results on the mechanisms and detection of global warming are discussed.