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Tropical Highlights - April 2002

Sea surface temperatures (SSTs) averaged above normal across most of the equatorial Pacific during April 2002, with the largest anomalies exceeding 1C west of the date line and in the extreme eastern Pacific (Fig. T18). This overall pattern of SST anomalies is very similar to that observed last month, with all four Nio index regions indicating positive anomalies in both March and April (Table T2). The atmospheric circulation across the Pacific continued to be influenced by tropical intra-seasonal oscillations associated with the Madden-Julian Oscillation (MJO) with a period of approximately 30–40 days.

The oceanic thermocline remained deeper than normal across most of the equatorial Pacific during April (Fig. T15) with the largest depth anomalies located in the central Pacific between 170E and 155W and in the extreme eastern Pacific (Figs. T15, T16). Ocean temperature anomalies at thermocline depth were also largest (+2 to +4C) in the central and extreme eastern equatorial Pacific, and near normal to slightly below normal between 140W and 105W (Fig. T17). The thermocline has shoaled during the past few months between 170W-110W (Fig. T15), resulting in the disappearance of the large positive temperature anomalies (+4C) observed in this region during February 2002 (Fig. T17).

Enhanced tropical convection during April was observed over the eastern equatorial Pacific, coastal Peru, and Ecuador, and across the equatorial Atlantic and equatorial western Africa (Fig. T25). Tropical convection was near-normal across the western Pacific, and slightly below normal over both the central Pacific (Fig. T8, top), and northern Australia (Fig. T25). The above-average convection over the equatorial Atlantic and western equatorial Africa was associated partly with the MJO (Fig. T11). The enhanced convection that was observed between 160E and the date line (180) during December 2001 – March 2002 was absent during April (Figs. T8, T11).

Both the low-level (850-hPa) and upper level (200-hPa) winds were near normal across the equatorial Pacific during April (Figs. T20, T21). For most of the past year the low-level equatorial winds have generally been near normal over the Pacific basin (Table T1, Fig. T4), while the upper-level winds have fluctuated between near-normal and stronger-than-normal westerlies. As such, no El Nio signal is yet evident in these wind patterns.

The tropical sea level pressures (SLP) during April featured negative anomalies over the entire Tropics except for over the eastern Pacific and South America (Fig. T19). This pattern was associated with a a positive value of the equatorial Southern Oscillation Index (SOI) (0.7), while the Tahiti - Darwin SOI was near zero (–0.3) (Table T1). After remaining strongly positive for the 3 year period ending November 2001, the equatorial SOI has oscillated around zero since December 2001 (Fig. T2).

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