La Nina Is Intensifying. Peaks As A Strong La Nina in December.

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La Nina is strengthening and a robust episode is forecast to peak in December. The first half of 2021 La Nina slowly fades. Primary climate impacts will be developing dryness to drought across the Southern U.S. and a very wet regime in Eastern Australia.

Highlight: La Nina is intensifying. Peaks as a strong La Nina in December. La Nina fades first half of 2021.

Discussion: Entering the 4th quarter of 2020 the Nino34 SSTA has cooled to -1.2C and farther east the Nino3 SSTA is a robust -1.5C (Fig. 1). La Nina is strengthening and likely to approach a “strong” intensity peaking in December according to the Australian Bureau of Meteorology (Fig. 2). The southern oscillation index continues robustly positive (Fig. 3) indicating the atmosphere is responding to the equatorial East Pacific cooling and producing a global La Nina climate. The equatorial Pacific upper ocean heat has spiked cool again the past 2-3 months after a short-lived spike earlier this year (Fig. 4). The best La Nina analogs are 1995-96, 2007-08 and 2010-11 (Fig. 5).

La Nina typically occurs when the cool phase Pacific decadal oscillation (-PDO) is present. However, during the past month a weak -PDO appears to have ended as a “warm blob” of ocean water in the Gulf of Alaska shifted east to the coastal North America region (Fig. 6). Rare is evolution of a strong La Nina without a -PDO episode but global SSTA forecasts indicate that regime for the December 2020 mature (peak) phase of La Nina 2020-21 (Fig. 7).

The 1995-96 La Nina occurred without a -PDO pattern. In fact, PDO favored the positive phase during that event. The 2007-08 and 2010-11 La Nina analogs were accompanied by a strong -PDO regime. The 1995-96 La Nina lasted from October to March (similar to 2020-21 expectations). During the 6-month La Nina episode in 1995-96 the precipitation pattern was very dry across the southern Great Plains and Texas, dry across central Brazil and northeast Argentina, and very dry across northern Australia while eastern areas were wetter than normal.

Fig. 1: The weekly Nino SSTA regions trend cooler as La Nina intensifies.

Fig. 2: The Australia Bureau of Meteorology trends stronger with an already robust La Nina.

Fig. 3: The southern oscillation index shifts to a robust La Nina signal the past 2-3 months.

Fig. 4: The equatorial Pacific Ocean upper ocean heat anomalies for the past one year. Another cool spike has generated to support La Nina evolution.

Fig. 5: The Climate Impact Company constructed analog ENSO phase forecast to August 2021.

Fig. 6: Shift into positive phase of the Pacific decadal oscillation in recent weeks is very unlike most La Nina regimes when this zone is typically cooler than normal.