CPC and the contributors to the Forecast Forum caution potential users of
this predictive information that they can expect only modest skill.
Alert System Status: La
conditions are expected to develop during May-June 2011 and continue through the
Northern Hemisphere summer 2011.
April 2011, La Niña continued to weaken as indicated by increasing surface and
subsurface temperature anomalies across the equatorial Pacific Ocean (Figs.
T9 and T15).
The Niño indices reflected below-average sea surface temperatures (SSTs)
in the central and east-central Pacific (–0.7°C in Nino-4 and –0.8°C in
Nino-3.4 regions), and near-average to above-average SSTs in the eastern Pacific
(–0.3°C in Nino-3 and +0.2°C in Nino-1+2 regions; Fig. T18
and Table T2).
The subsurface oceanic heat content anomalies (average temperatures in
the upper 300m of the ocean) increased slightly, due to an expanded area of
above-average temperatures at thermocline depth (Fig. T17).
Consistent with other transitions to ENSO-neutral conditions, the
atmospheric circulation anomalies related to La Niña remained considerable over
the tropical and subtropical Pacific. Convection
was enhanced over much of Indonesia and suppressed over the western and central
equatorial Pacific (Fig. T25).
Also, anomalous low-level easterly and upper-level westerly winds have
persisted in this region (Figs. T20
Collectively, these oceanic and atmospheric anomalies reflect a weakening
La Niña, but with ongoing global impacts.
observed trends, along with forecasts from nearly all of the ENSO models,
indicate La Niña will continue to weaken in the coming months, with a return to
ENSO-neutral during May-June-July 2011 (three-month average in the Nino-3.4
index between –0.5°C and +0.5°C; Figs. F1-F13).
Thereafter, the majority of models and all multi-model forecasts predict
ENSO-neutral conditions to continue through the remainder of 2011.
However, the status of ENSO beyond the Northern Hemisphere summer remains
uncertain due to lower model forecast skill at longer lead times.
updates of oceanic and atmospheric conditions are available on the Climate
Niño/La Niña Current Conditions and Expert Discussions).