moderate-strength La Niña
continued during December 2007. This is reflected by a continuation since
October of below-average equatorial sea surface temperatures (SSTs) from west of
the Date Line to the South American coast (Figs. T9,
T18). The equatorial SSTs were more than 1.5°C
below average east of 170oW during December (Figs. T9,
T18), which is consistent with the negative values
of the latest monthly Niño-3.4 index (-1.5°C) and the monthly Niño-1+2
index (-2.0°C) (Table T2).
The sub-surface temperature departures remained negative across the
eastern equatorial Pacific, where
temperatures at thermocline depth ranged from -2°C to -4°C below average (Fig.
During December 2007,
strong low-level easterly anomalies (more than 3.0 m s-1)
spanned the western and central equatorial Pacific – in fact, the 850
mb wind index in the western Pacific (+3.7) is the largest value observed in the
historical record back to 1979 (Fig.
T20, Table T1). This
pattern is consistent with a continued shallower-than-average thermocline in the
central and eastern equatorial Pacific (Figs.
These conditions were associated with enhanced convection (above-average
rainfall amounts) across the eastern tropical
and a continuation of suppressed
convection (below-average rainfall amounts) across the central and eastern
equatorial Pacific (Figs. T25, T26, E3).
Consistent with these anomalies, the
– Darwin SOI was +1.8 during December (Table
T1, Fig. T1).
For the latest status of the ENSO cycle see the ENSO Diagnostic Discussion at: