The oceanic thermocline remained much deeper than normal in the west-central and western Pacific during April and also deepened in the eastern Pacific (Fig. T15). In the west oceanic temperatures were more than 5.0°C above normal during April at thermocline depth (Fig. T17). Across the east-central and eastern Pacific, the deepening thermocline has resulted in a lessening of negative temperature anomalies at thermocline depth, with temperatures more than 2.0°C below normal observed only between 110°W and 140°W during the month (Fig. T17).

The pattern of tropical convection during April [as inferred from anomalous outgoing longwave radiation (OLR)] remained consistent with ongoing cold episode conditions, and featured suppressed convection over the western and central equatorial Pacific and enhanced convection over Indonesia and the Indian Ocean (Fig. T25). This pattern has persisted since the onset of cold episode conditions in late May 1998 (Fig. T8). Elsewhere, the Atlantic ITCZ was enhanced from northeastern Brazil to southwestern Africa during the month (Fig. T25).

The pattern of tropical convection over the central and western Pacific was again accompanied by an enhanced Walker circulation across the central equatorial Pacific (Fig. T29), with low-level (850 hPa) easterly wind anomalies across the central and western tropical Pacific greater than 6 m s^-1 between 140°E and 170°W (Fig. T20). At upper-levels (200-hPa), troughs were observed over the low-latitudes of the central Pacific in both hemispheres, with amplified subtropical ridges dominating most of the remainder of both hemispheres (Fig. T22).