Highlights - June 2000
Negative sea surface temperature (SST) anomalies
continued to weaken across the tropical central Pacific during June, as the magnitude of
SST anomalies was less than 1°C throughout the region (Fig. T18).
As a result, the Niņo 3.4 and Niņo 4 region indices were only 0.4°C and 0.5°C,
respectively, during June (Table T2, Fig.
T5). These were the smallest negative values for these indices since May 1998 and
July 1998, respectively (Fig. T5). Across the eastern
Pacific negative SST anomalies returned, after becoming briefly positive in April and near
zero in May (Table T2).
The oceanic thermocline remained deeper than normal in the equatorial west-central and
western Pacific during the month (Fig. T15), with
temperatures again averaging more than 4.0°C above normal at thermocline depth (Fig. T17). Over the eastern Pacific, the negative thermocline
depth anomalies continued to weaken (Fig. T15), resulting
in a lessening of negative temperature anomalies in the upper 100 m of the ocean (Fig. T17). As a result subsurface temperatures more than
1.0°C below normal were confined to the region between 95°W and 130°W. The recent
evolution of the oceanic thermocline and subsurface temperature anomalies is similar to
that observed during March-June 1999, with the major differences being an increase in the
magnitude of the positive subsurface temperature anomalies in the western Pacific and a
decrease in the magnitude of the negative anomalies in the eastern Pacific. This basic
east-west dipole of subsurface temperature anomalies, which is typical of the mature phase
of La Niņa episodes, has been highly persistent since late 1998.
The pattern of tropical convection during June [as inferred from anomalous outgoing
longwave radiation (OLR)] remained consistent with cold episode conditions, with
suppressed convection over the western and central equatorial Pacific and enhanced
convection over Indonesia (Fig. T25). Elsewhere,
convection was enhanced over southeast Asia and India during the month (Fig. T25), implying a continued strong start to the
Indian/Southeast Asian summer monsoon system. Convection was also enhanced over
Mexico/Central America and portions of the southwestern United States, in association with
an amplified monsoon circulation in that region. Strong monsoonal circulations in each of
these regions are consistent with ongoing La Niņa conditions.
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). However, consistent with the decrease in magnitude
of the negative SST anomalies across the central equatorial Pacific in recent months, the
low-level (850 hPa) easterly wind anomalies across the central and western tropical
Pacific have also decreased (Fig. T7). At upper-levels
(200-hPa), well-developed mid-Pacific troughs were again observed over the low-latitudes
of both hemispheres, with anticyclonic circulation anomalies dominating the subtropics and
lower mid-latitudes of both hemispheres (Figs. T21, T22). This anomaly pattern has also persisted since mid-1998, in
association with ongoing La Niņa conditions.
The sea level pressure (SLP) pattern across the Tropics during June featured weak
positive anomalies across the tropical central and eastern Pacific and weak negative
anomalies over Indonesia/ Malaysia (Fig. T19). This
pattern is consistent with ongoing cold episodes, and was associated with a positive value
of the equatorial Southern Oscillation Index (SOI) (1.2) (Fig. T2).
The June value of the Tahiti-Darwin SOI was 0.6 (Table T1, Fig. T2), as SLP at Darwin was above normal in association with
an anomalously strong high pressure system that dominated all of Australia (Fig. T19).