Cold episode (La Niña) conditions continued in the tropical Pacific during September. Sea surface temperatures (SSTs) averaged more than 1°C below normal across the tropical Pacific between 150°W and the South American coast, with weak negative anomalies extending westward to 160°E (Figs. T9, T18). Consistent with this, tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] remained suppressed across the western and central equatorial Pacific during the month (Fig. T25) and the low-level equatorial winds remained stronger than normal across the same region (Fig. T20).

The pattern of subsurface temperature anomalies has been highly persistent, with positive anomalies in the western equatorial Pacific and negative anomalies farther east (Fig. T17). Consistent with this pattern, the oceanic thermocline remained deeper (shallower) than normal in the western (eastern) equatorial Pacific (Fig. T16). Over the last couple of months temperatures at thermocline depth have decreased to more than 4°C below normal across the east-central and eastern tropical Pacific. Also, the positive sub-surface temperature anomalies in the west-central equatorial Pacific continue to show little evidence of an eastward shift, indicating that cold episode conditions are likely to persist in the tropical Pacific through the Northern Hemisphere winter 1999/2000. This assessment is supported by the most recent NCEP coupled model forecast (Figs. F3 and F4) and by other available coupled model and statistical model predictions that indicate cold episode conditions persisting through the Northern Hemisphere spring of 2000.