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 Niña Advisory
Niña conditions have returned and are expected to gradually strengthen and
continue into the Northern Hemisphere winter 2011-12.
Niña conditions returned in August 2011 due to the strengthening of negative
sea surface temperature (SST) anomalies across the eastern half of the
equatorial Pacific Ocean (Fig. T18). With the exception of the
far easternmost Niño-1+2 region, all of the monthly Niño index values were
near –0.5°C or less (Table T2).
Also supporting the return of La Niña conditions was the strengthening
of the below-average subsurface oceanic heat content anomaly (average
temperature anomalies in the upper 300m of the ocean), in response to increased
upwelling and further shoaling of the thermocline across the eastern Pacific
Ocean (Fig. T17). The
atmospheric circulation over the tropical Pacific continued to exhibit La Niña
characteristics, but remained weaker and less canonical than the wintertime
atmospheric patterns. For example,
convection continued to be suppressed near the Date Line, but remained south of
the equator, while convection was only weakly enhanced near Papua New Guinea (Fig.
In addition, anomalous low-level easterly and upper-level westerly winds
persisted over the central tropical Pacific (Figs. T20,
Collectively, these oceanic and atmospheric patterns reflect the return
of La Niña conditions.
the last several months many models have predicted increasingly negative SST
anomalies in the Nino-3.4 region during the upcoming Northern Hemisphere fall
and winter. However, the majority of models continue to predict ENSO-neutral
conditions for this period (Figs. F1-F13). The NCEP Climate Forecast System
(CFS) has performed quite well over the past several months capturing the recent
decrease in SST anomalies. The
better model performance, combined with the historical tendency for significant
La Nina episodes (as in 2010-11) to be followed by relatively weaker La Niña
episodes, leads to increased confidence that La Niña will persist into the
winter. While it is not yet
clear what the ultimate strength of this La Niña will be, La Niña conditions
have returned and are expected to gradually strengthen and continue into the
Northern Hemisphere winter 2011-12.
updates of oceanic and atmospheric conditions are available on the Climate
Niño/La Niña Current Conditions and Expert Discussions).