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 0.5°C of normal during
the month (Fig. T18). However, positive SST anomalies
increased in the central equatorial Pacific, with the Niño 4
index reaching +0.5C for the first time since the 1997-98 El Niño
episode (Table T2).
The depth of the equatorial oceanic thermocline (as represented by the depth of the 20°C 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 15°N 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.
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).
Editors 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 Niño
indices in Table T2, the time series in Fig.
T5, and the anomalies in Figs. T9, T18,