Cold episode conditions continued in the tropical Pacific during March, as sea surface temperatures (SSTs) remained well below normal across the central and east-central equatorial Pacific (Fig. T18 and Table T2). Negative subsurface temperature anomalies (Fig. T17) and a shallower than normal oceanic thermocline (Fig. T16) continue to dominate the equatorial Pacific east of 140° W, although the magnitude of the anomalies has decreased from those observed earlier in the year (Fig. T15). Accompanying these conditions, the mean low-level equatorial easterly winds remained much stronger than normal over the west-central and central tropical Pacific and slightly weaker than normal over the eastern Pacific (Fig. T20). Enhanced low-level westerlies were observed over the eastern Indian Ocean and Indonesia. This pattern of low-level winds contributed to enhanced low-level convergence and precipitation, as inferred from the anomalous Outgoing Longwave Radiation (OLR), over Indonesia, the Philippines and northwestern Australia and suppressed precipitation in the equatorial central Pacific (Fig. T25). These patterns of anomalous convection and low-level winds have been highly persistent since the summer of 1998 (Figs. T7 and T8 ), consistent with ongoing cold episode conditions.

Since December 1999 there has been a slow eastward expansion of the positive subsurface anomalies into the central Pacific, which is consistent with a slow evolution of the subsurface thermal structure toward a pre-warm episode state. Historically, this "build-up" phase of the ENSO cycle has lasted between one and two years before the subsequent development of warm episode conditions.