During the 1990-1994 period above-normal pressure dominated the Atlantic Ocean, Indonesia, and the western Pacific Ocean, and below-normal pressure covered the eastern tropical and subtropical Pacific (Figs. 2.1 and 2.2). In the Pacific, this anomalous pressure pattern reflects a sustained negative phase of the Southern Oscillation (Fig. 2.3), as defined by the Southern Oscillation Index (SOI). Accompanying this pattern, positive sea surface temperature anomalies have been prominent in the central and east-central equatorial Pacific (Fig. 2.4) and weaker than normal low-level easterlies have dominated the equatorial Pacific (Fig. 2.5). Since 1991, mature warm episode conditions have been observed during November 1991-May 1992, January - June 1993, and beginning in November 1994. During these periods, atmospheric convection (as indicated by negative outgoing longwave radiation (OLR) anomalies) was greatly enhanced over the central equatorial Pacific (Fig. 2.6). In addition, the South Pacific Convergence Zone was shifted northeastward, and the Intertropical Convergence Zone in the Northern Hemisphere was shifted equatorward. These conditions were accompanied by abnormally warm ocean waters from the date line to the South American coast and by a substantial reduction in the strength of the equatorial easterlies in the area east of the date line.
The redevelopment of mature warm episode conditions in the tropical Pacific during late 1994 for a third time in four years is unprecedented in the last 50 years. However, there is at least one other period during this century, and possibly a few others in the latter half of the 19th century, that featured similar prolonged warm episode conditions. The period 1911-1915 appears to be most similar to the present, at least as indicated by the Darwin SLP anomalies (Fig. 2.7). During that period, 1911, 1912, and 1914 are considered El Nino years.
Also since 1990, above-normal sea level pressure has dominated the tropical and subtropical latitudes of the Atlantic, along with large portions of both the North and South Atlantic (Figs. 2.1 and 2.2). In the tropics, these positive anomalies have been stronger and more persistent than those observed over Indonesia during the period. These conditions have likely contributed to reduced hurricane activity over the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico throughout the period.
Oceanic and Atmospheric Anomalies Related to ENSO:
The 1994 Pacific Warm Episode
Climate and Global Change Issues: Atmospheric Angular Momentum
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