La Nina Continues to Strengthen...A Weak to Moderate La Nina for the Winter

La Nina conditions continue to build in the Pacific this Autumn 2011 along with an encompassing negative Pacific Decadal Oscillation. La Nina conditions of course refer to cooler than average sea surface temperatures /SST/ extending in a band from the coast of South America, westward across the central Pacific. A negative PDO /-PDO/ refers to cooler than average general Pacfiic weather temperatures west of the U.S. across the central and northern Pacific west of which, will only aid the La Nina. In April 2008, scientists at NASA’s Jet Propulsion Laboratory announced that while the La Niña was weakening, the Pacific Decadal Oscillation—a larger-scale, slower-cycling ocean pattern—had shifted to its cool phase (negative).

The ultimate maximum strength of this La Nina should be interesting as I've noticed a subtle shift in model projections toward a consensus to the lower end of the plot scale (with the mean or average in weak La Nina territory rather than Neutral). Where it finally lands, in part, will affect our weather patterns this winter in Southeast Lower Michigan and the Great Lakes. Of course, the important wild card, as always, will be the dominant phase of the North Atlantic Oscillation.

CPC gave an interesting narration recently on the current La Nina and some of the the individual model projections:

During September 2011, La Niña conditions strengthened as indicated by increasingly negative sea surface temperature (SST) anomalies across the eastern half of the equatorial Pacific Ocean.

Currently, La Niña is not as strong as it was in September 2010. Roughly one- half of the models predict La Niña to strengthen during the Northern Hemisphere fall and winter (Fig. 6). Of these models, the majority predict a weak La Niña (3-month average in the Nino-3.4 region less than -0.9°C). In addition, a weaker second La Niña winter has occurred in three of the five multi-year La Niñas in the historical SST record since 1950. However, the NCEP Climate Forecast System (CFS.v1) predicts a moderate-strength La Niña this winter (between –1.0°C to –1.4°C) and CFS.v2 predicts a strong La Niña (less than –1.5°C), which rivals last year’s peak strength. For CFS forecasts made at this time of year, the average error for December-February is roughly ±0.5°C, so there is uncertainty as to whether this amplitude will be achieved. Thus, at this time, a weak or moderate strength La Niña is most likely during the Northern Hemisphere winter. 

In any event, our new La Nina seems to be chugging along nicely so another moderate La Nina I think is as likely as a weak one (a "borderline" La Nina?;-) 

It will be very important to analyze the latest SST temperature plot and computer projections before I issue the Winter 20011-12 Outlook early November.

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