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 Niņo-region
indices decreased for the fifth consecutive month (Table
T2, Fig. T5),
with the Niņo 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.
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
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).