Modes of Tropical Circulation, Southern Oscillation, and Sahel Rainfall Anomalies

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Abstract
This empirical investigation is devoted to the parsimonious description and diagnostic understanding of large-scale climate variability in the tropics. The role and importance of the Southern Oscillation (SO) over the Atlantic, eastern Pacific, and Indian means is analyzed and compared with other surface circulation modes that contribute to interannual climate variations. Ship data, condensed into clusters of sea level pressure, surface wind, cloudiness, and sea surface temperature, are studied using orthogonally rotated principal component analysis. All four ocean-atmosphere elements are combined in the analysis. Five rotated principal components (PCs) appear as near optimal solutions in both boreal winter and summer. Most of these circulation modes feature a highly correlated, kindred mode in either the preceding or following extreme season, resulting in a preferred 2-year sequence of modes that starts with the boreal summer SO mode. An overall shift in the climate regime of the tropics is found at the end of the 1960s, heralding the recurrent climate extremes of the 1970s. The SO is confirmed as the most important circulation mode. In boreal summer, the second and third PC, called Atlantic Sahel and global Sahel mode, both correlate well with Subsaharan rainfall. The Atlantic Sahel mode focuses on the Atlantic near-equatorial trough, whose northward shift in conjunction with above-average North Atlantic SST has been associated previously with wet West African monsoon seasons. The global Sahel mode complements the Atlantic mode by emphasizing features in the Indian and South Atlantic Ocean domains, in particular, in the SST field which shows strong negative departures for abundant Sahel rainfall years, often preceded by a positively scoring SO mode in boreal winter (i.e., anomalously high/low pressure over the eastern/western Pacific). While the Atlantic Sahel mode relates better to the year-to-year variability of Subsaharan rainfall, the global Sahel mode features a strong negative trend that corresponds best to the long-term downward trend of this climate feature. Since both Sahel modes correlate strongly with precursor modes in boreal winter, some of the Sahel rainfall variance may be foreshadowed a semester in advance.