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Tropical Highlights - July 2001

Near-normal conditions prevailed across the tropical Pacific during July 2001. Sea surface temperatures (SSTs) across the tropical Pacific were generally within 0.5C of normal during the month (Fig. T18). However, positive SST anomalies increased in the central equatorial Pacific, with the Nio 4 index reaching +0.5C for the first time since the 1997-98 El Nio episode (Table T2).

The depth of the equatorial oceanic thermocline (as represented by the depth of the 20C isotherm) continued to remain deeper than normal in the western Pacific and near normal in the extreme eastern Pacific during the month (Fig. T15). Consistent with this pattern, oceanic temperatures at thermocline depth were above normal throughout the western and central Pacific and slightly below normal in the extreme eastern Pacific (Fig. T17). Since the beginning of the year, the subsurface ocean temperature anomalies along the equator have slowly shifted eastward and expanded upward (Fig. T17). The evolution of the subsurface temperature anomalies together with the evolution of the SST anomalies are consistent in indicating an incipient warm episode in the tropical Pacific (see the Forecast Forum for a discussion of the most recent SST forecasts).

Tropical convection during July exhibited near normal conditions across the tropical Pacific, with slightly enhanced convective activity over parts of the Sahel, northern Indian subcontinent and along 15N in the western tropical Pacific (Fig. T25). Over the past few months the convective activity across the western and central Pacific has persisted since mid-1998 has returned to near-normal (Fig. T8).

The low-level (850-hPa) winds across the Atlantic, Indian and western and central Pacific Ocean basins were near normal (Fig. T20). At upper levels, near normal winds were again observed during the month over much of the tropical ocean basins with the lowest recorded value of the 200-hPa wind index (0.1) in over two years (Table T1). Upper-level easterly anomalies were observed throughout the global subtropics of both hemispheres (Fig. T21).

The sea level pressure (SLP) pattern during July featured normal conditions across the entire tropics with anomalies less than 1 hPa in all three ocean basins (Fig. T19). This pattern is consistent with the near normal value of the Southern Oscillation Index (SOI) (-0.4) (Table T1).

In the stratosphere, the easterly phase of the QBO, which started around early 2000, continued during July 2001, and has now lasted considerably longer than the climatological average duration (Fig. T14).

Editor’s Note: The climatology used to compute SST anomalies has been changed from the 1961-1990 period to the 1971-2000 base period. This change affects the Nio indices in Table T2, the time series in Fig. T5, and the anomalies in Figs. T9, T18, and E1.

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