The Southern Oscillation in Surface Circulation and Climate over the Tropical Atlantic, Eastern Pacific, and Indian Oceans as Captured by Cluster Analysis

Abstract

Clusters of sea level pressure (SLP), surface wind, cloudiness, and sea surface temperature (SST) in the domain of the tropical Atlantic, eastern Pacific, and Indian Oceans are introduced and discussed in terms of general circulation and climate. They appear to capture well the large-scale degrees of freedom of the seasonal fields. In the Atlantic and, to a lesser extent, in the eastern Pacific, most analyzed fields group into zonally oriented ?trade wind? clusters. These are separated distinctly by the near-equatorial trough axis. By contrast the Indian Ocean features strong interhemispheric connections associated with the monsoon systems of boreal summer and, to a lesser degree, of boreal winter. The usefulness of clusters thus established is elucidated with respect to the Southern Oscilation (SO). General circulation changes associated with this planetary pressure seesaw are deduced from correlation maps of surface field clusters for January/February and July/August. During the positive SO phase (i.e., anomalously high pressure over the eastern Pacific and anomalously low pressure over Indonesia), both the Atlantic and eastern Pacific near-equatorial troughs are inferred to be shifted towards the north from July/August SLP, wind, and cloudiness fields. While eastern Pacific trade winds are weakened in both seasons in the positive SO phase, the Atlantic trades appear strengthened at the same time in the winter hemisphere only. Over the Indian Ocean, the monsoon circulation seems to be strengthened during the positive SO phase, with the summer monsoon displaying a more complex picture. Its SLP, cloudiness and SST fields support an enhanced southwest monsoon, while its surface winds appear largely inconclusive. SST is lowered during the positive SO phase in all three tropical oceans. Since all major tropical circulation components over the Atlantic, eastern Pacific, and Indian Ocean participate in the Southern Oscillation, as is evidenced by field significance tests, they become prime movers for the widespread tropical interannual climate variations associated with this global pressure seesaw.