Tropical Highlights - May 2003

The evolution toward a Pacific cold episode (La Nina) continued during May 2003 as sea surface temperatures (SST) anomalies became increasingly negative across the east-central and eastern equatorial Pacific (Table T2). During May SSTs averaged more than 1.0�C below normal in the eastern equatorial Pacific east of 140�W, but remained near normal in the vicinity of the date line (Fig. T18). All four Ni�o-region indices decreased for the fifth consecutive month (Table T2, Fig. T5), with the Ni�o 3.4 index becoming negative for the first time since January 2002 and the Nino-4 index dropping to +0.3 (Fig. T5).

Consistent with this cooling subsurface ocean temperatures were below normal across the central and eastern equatorial Pacific during May, with values more than 4�C below normal recorded at thermocline depth between 100� and 110�W (Fig. T17). This subsurface cooling is associated with decreasing depths of the oceanic thermocline across the eastern half of the equatorial Pacific, and is consistent with the continued evolution toward a Pacific cold episode (Figs. T15, T16).

Tropical convection was near normal across the equatorial Pacific during May (Fig. T25), consistent with ENSO-neutral conditions. The upper-level atmospheric circulation across the tropical Pacific also continued to reflect ENSO-neutral conditions. However, at low-levels (850-hPa) enhanced equatorial easterlies were again observed over the central and eastern Pacific (Fig. T20), which is consistent with the cooling trend in ocean temperatures that typically precedes the onset of La Nina.

The tropical sea-level pressure (SLP) pattern during May featured above normal pressure over the Atlantic and Indian Ocean and below normal pressure over Indonesia and most of the Pacific Ocean (Fig. T19). These anomalies were associated with a small negative value (-0.6) of the SOI (Table T1) and a small positive value (0.4) of the equatorial SOI (Fig. T2).