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).