Tropical Highlights - April 1999

Mature cold episode (La Niña) conditions continued throughout the tropical Pacific during April 1999, as sea surface temperatures (SSTs) were more than 1°C below normal across the central and parts of the eastern equatorial Pacific (Fig. T18). Since the beginning of the year, SSTs in the central equatorial Pacific have increased (partially related to the normal annual cycle), while the negative SST anomalies have decreased in magnitude to near -1.0°C (Table T2, Fig. T5).

The oceanic thermocline remained shallower than normal throughout the central and eastern Pacific east of 165°W and deeper than normal west of 165°W during the month (Figs. T15, T16). Consistent with this sub-surface structure, equatorial ocean temperatures at thermocline depth remained more than 3°C below normal in the central and eastern Pacific, and more than 3°C above-normal in the western Pacific (Fig. T17).

Tropical convection during April [as inferred from anomalous outgoing longwave radiation (OLR)] remained suppressed across the western and central equatorial Pacific and enhanced across Indonesia (Fig. T25 ). This pattern of convection has persisted since June 1998 and is consistent with a continuation of mature cold episode conditions (Fig. T11). The OLR index has remained above 1.0 since September 1998 (Table T1, Fig. T1), indicating substantially reduced convection in the equatorial central Pacific. Since the beginning of the year, tropical instraseasonal (Madden-Julian Oscillations) activity (with a period near 60 days) has increased, and has contributed to enhanced convection over Indonesia during January and March and to decreased convection during February and late April (Fig. T11).

Low-level (850-hPa) equatorial easterly winds were observed across the entire Pacific during April (Fig. T20 ). The largest easterly wind anomalies were again observed over the western equatorial Pacific, where winds averaged 6-9 ms^-1 above normal. These anomalies resulted in a zonal wind index of 2.9 over the western Pacific, which is the third largest value observed in the historical record back to 1979, exceeded only during January and March of this year (Fig. T4 ). In contrast, the winds remained weaker than normal (westerly anomalies) over the eastern Pacific for the third consecutive month (Table T1, Fig. T20). Westerly wind anomalies were also observed over the eastern Indian Ocean, which contributed to low-level convergence and enhanced convection over Indonesia (Fig. T25).

At upper levels of both hemispheres, well-defined mid-Pacific troughs were observed during the month, and the low-latitude sub-tropical ridges were confined to the Indonesian sector (Fig. T22). This circulation contributed to a westward retraction of the East Asian jetstream toward Asia and to a very strong jet exit region over the central North Pacific (Figs. T21). It was also associated with enhanced low latitude westerlies in the Northern Hemisphere east of the date line (Fig. T21 ).

The Southern Oscillation Index (SOI) was 1.4 during April (Table T1, Fig. T1) while the equatorial SOI was 2.4 (Fig. T2). The equatorial SOI has averaged 2.4 for the past 5 months and continues to better depict the continued strength of the cold episode than the conventional SOI. These index values reflect above-normal sea level pressure (SLP) over the central Pacific and below-normal SLP over Indonesia and the eastern Indian Ocean (Fig. T19).