Highlight: The La Nina forecast is revised stronger!
Increasing risk of cold U.S. winter!
Fig. 1: Propelled by the sharp thermal gradient between a cool East Pacific and warm West Pacific/east Indian Ocean accelerated trade winds up-well cool subsurface waters to sustain and intensify La Nina. Pictured is the ECMWF November 2020 global SSTA forecast.
Fig. 2: The Climate Impact Company ENSO forecast identifies the La Nina analog years…2010-11, 2007-08 and 1995-96.
Fig. 3: The NOAA/NCEP Nino34 SSTA forecast is revised toward a stronger La Nina for late 2020/early 2021.
Fig. 4: The Australian Bureau of Meteorology revises their La Nina forecast more intense for late 2020.
Fig. 5: Subsurface equatorial Pacific temperature anomaly analysis identifies plenty of cool water to sustain La Nina east of the Dateline.
Fig. 6: The NOAA/CPC La Nina climate anomalies for DEC/JAN/FEB with added La Nina analog year (anomalies). Note the cold risk in much of the northern hemisphere with wet risk for northern Brazil, South Africa and north Australia.
Fig. 7: Temperature probability forecasts from the International Research Institute for Climate and Society valid DEC/JAN/FEB 2020-21 indicate potential cold risk in similar locations to the La Nina analog years.
Discussion: There is a significant change in the ENSO outlook: Stronger La Nina for late 2020 into early 2021. The catalyst to this stronger La Nina forecast is the previously proposed influence (on ENSO) by an evolving negative Indian Ocean Dipole (-IOD). The -IOD pattern warms the (already very warm) eastern Indian Ocean and western Pacific Ocean tropics while cooler-than-normal waters dominate the equatorial Pacific east of the Dateline. The sharp thermal gradient between the two warm and cool SSTA patterns drives stronger trade winds which up-well cool waters in the central and east tropical Pacific Ocean to sustain and strengthen the La Nina episode (Fig. 1).
The ENSO forecast by Climate Impact Company identifies the La Nina analog years: 2010-11, 2007-08 and 1995-96 (Fig. 2). The analog forecast is similar to the updated Nino34 SSTA forecasts by NOAA/NCEP CFS V2 (Fig. 3) and POAMA (Fig. 4) each indicating a revised stronger La Nina peaking in late 2020.
Latest subsurface temperature analysis across the equatorial Pacific Ocean identifies a resurgent vast supply of cool water east of the Dateline (Fig. 5). Strengthening trade winds up-well the cool subsurface waters to strengthen the La Nina episode.
The NOAA/CPC La Nina climate anomalies for DEC/JAN/FEB with CIC La Nina analog years added identify interesting northern hemisphere cold risk and southern hemisphere wet risk (Fig. 6). The cold risk stretches from Central Asia to Eastern Europe and Western Canada into the U.S. In the southern hemisphere there is abundant wet risk including South Africa, north and east Australia and northern Brazil plus Uruguay. The temperature probability forecast for DEC/JAN/FEB 2020-21 (which are usually quite warm) offer cold risk in the La Nina cold regions of western Canada, the central U.S. and central Asia (Fig. 7).
Summary: There is an ENSO forecast change: Stronger La Nina. Forecast confidence is increasing. Of note is the analog year implication of increased cold risk for the northern hemisphere. For summer 2020-21 in the southern hemisphere there is limited drought risk but widespread wet risk (for Brazil, South Africa and Australia).