ENSO forecast for spring 2007 - winter 2008

Summary: La Niña conditions are expected to develop by summer 2007 and persist through the following winter (CF = 50, Odds = 3:1 that La Niña rather than neutral or El Niño conditions will occur).

The first sign of developing La Niña conditions in the equatorial Pacific was observed in the summer (JJA) of 2006, when the Australian sea level pressure (SLP) index sharply increased to 2.82. For the period since 1949, when the index reached 2.1 or above, there were six La Niña, one neutral, and none El Niño events developed during the following year. 

We are also closely monitoring the dynamics of the Hawaiian High. So far this winter, positive SLP anomalies dominated in the Hawaiian region, which is favorable for development of La Niña conditions. The distribution of winter sea surface temperature (SST), surface air temperature (SAT), and precipitation anomalies over the Pacific Ocean and elsewhere appear to resemble the patterns characteristic La Niña, rather than El Niño event.

Several studies have shown a strong lagged relationship between various indices of the sub-surface temperature structure and subsequent surface temperatures in the eastern equatorial Pacific. The animation from the Climate Prediction Center clearly demonstrates the propagation of the negative temperature anomaly eastward (from about 170E in December 2006 to 130W in February 2007) and toward the surface (from about 170m to 70m for the same period). It is expected that this anomaly may surface sometime in mid- to late spring 2007. 

The latest ENSO outlook issued by the CPC indicates a transition from a weak El Niño to ENSO-neutral conditions by March-May 2007. An ensemble mean forecast produced by the Climate Forecast System (CFS) from NCEP suggests a relatively steep decline in SST in the east-central equatorial Pacific (Nino 3.4 region) reaching -1C below the 1971-2000 mean by the fall 2007. This is qualitatively consistent with the results of the linear inverse modeling in Earth System Research Laboratory (NOAA), although in the latter case the anomaly is weaker. In contrast, the so-called “ENSO Prediction Plume” (a summary of 12 dynamical and 8 statistical models from various organizations) issued by the International Research Institute in February 2007 points to a gradual decline of El Niño conditions throughout 2007. Our confidence in those models, however, is quite low.   

Summarizing the above information and taking into account the rate of decline in SSTs and upper-ocean heat content, it is expected a La Niña event to develop by the summer 2007. We do not have enough information at this point to estimate the magnitude of the event.