Tropical Highlights - April 2004
Sea surface temperature (SST)
anomalies remained near-average across the entire equatorial Pacific during
April 2004 (Fig. T18, Table T2), while atmospheric indices continued
to exhibit month-to-month variability associated with intraseasonal
(Madden-Julian Oscillations - MJO) activity (Figs. T11,
T12, T13, Table
T1). The pattern of
Tropical SST anomalies during April featured small positive values over the
western and central Pacific and small negative values over the eastern
Pacific (Fig. T18). The
values in the four Niño regions were all within 0.3°C
of zero (Table T2).
The oceanic thermocline,
measured by the depth of the 20°C
isotherm remained deeper than average in the western and west-central
Pacific and shallower than average in the eastern Pacific (Figs. T15,
Consistent with these conditions, oceanic temperatures at thermocline
depth were 2-3°C
above average in the western and west-central equatorial Pacific and 3-4°C
below average in the east-central and eastern Pacific (Fig. T17).
The monthly low-level
(850-hPa) and upper-level (200-hPa) equatorial zonal wind indices, the
Tahiti-Darwin SOI and the outgoing longwave radiation (OLR) index have all
exhibited considerable variability since November 2003 in association with
intraseasonal activity (Table T1). During April, stronger-than-average low-level easterly winds were
again observed over the central Pacific (Fig. T20).
Despite the intraseasonal activity, stronger-than-normal easterlies
have been dominant over this region since November 2003 (Fig. T13).
The global Tropics featured
small OLR anomalies during April. Enhanced
cloudiness and precipitation (negative OLR anomalies) were observed over the
west-central Pacific, while suppressed cloudiness and precipitation
(positive OLR anomalies) were observed over Indonesia, the western Pacific
and northern Australia (Fig. T25).
Enhanced convection has been more persistent between 160°E
and the date line for the past two months (Fig. T11).
The Tahiti - Darwin SOI was
-1.3 during April (Table T1, Fig. T1), while the equatorial SOI
increased to 0.8 (Fig. T2). The
Tahiti - Darwin SOI has exhibited large month-to-month variability since
November 2003, in response to MJO activity (Fig. T10).
The equatorial SOI has exhibited much less variability during the
past year, remaining between 0 and 0.9 since March 2003 (Fig. T2).