Tropical Highlights - June 2003

Current atmospheric and oceanic conditions do not support the development of La Niña in the next few months. Strong low-level westerly wind anomalies over the western and central equatorial Pacific during late May-early June initiated an eastward-propagating oceanic Kelvin wave, possible linked to recent MJO activity, that led to a reversal in the oceanic cooling trend observed during the first 5 months of 2003. As a result, an upward trend was observed in all Niño indices during June 2003 (Table T2), positive equatorial SST anomalies strengthened and expanded in the central and western Pacific, and negative anomalies weakened in the eastern Pacific. The average SST anomaly pattern for June (Fig. T18) shows weak positive anomalies west of 150°W and near zero or negative anomalies east of 150°W.

Oceanic thermocline depths, as measured by the depth of the 20°C isotherm, and subsurface temperature anomalies increased throughout the western and central equatorial Pacific during June in response to the eastward-propagating Kelvin wave. At the same time, the equatorial easterly winds were near or slightly weaker than normal between 180°W and the South American coast (Table T1, Fig. T20), and the Tahiti-Darwin SOI remained negative with a value of -1.1 (Table T1).

Abnormally dry conditions affected Indonesia and Malaysia, and portions of Southeast Asia (Figs. T25, E3). These areas have experienced considerable month-to-month variability since March, probably been related to MJO activity (e.g., see Figs. T11, T12).