ENSO Forecast: La Nina Peak DEC/JAN; Ending MAR/APR

Highlight: La Nina peaks DEC/JAN and dissipates by MAR/APR

Executive summary: The monthly Climate Impact Company ENSO Outlook indicates ongoing La Nina will strengthen, peak in DEC/JAN and dissipate by MAR/APR of next year. Typical of La Nina, a cool phase Pacific decadal oscillation coincides and enhances the La Nina climate. Modern-day La Nina climate is different from older episodes due to the anomalous warmth of the middle latitude oceans. La Nina 2021-22 will produce a wet climate in the Northwest (but not California), dryness across the southern states which could lead to drought and expansive Ohio valley rain and snow. The South and East U.S. are biased warmer than normal for the 2021-22 cold season.

Fig. 1: Climate Impact Company constructed analog Nino34 SSTA forecast regressed 2 years and forward forecast for 2 years implies lack of intensity.

Discussion: La Nina 2021-22 onset was in September and is the follow-up event to the 2020-21 episode. Many La Nina episodes have multiple years. The multivariate ENSO index (MEI) which measures the atmospheric response to the ENSO regime indicates La Nina 2020-21 never ended. Daily global SSTA reveals the developing La Nina in the equatorial East Pacific Ocean (Fig. 2). Typical of La Nina presence, the cool phase of the Pacific decadal oscillation (-PDO) is also present – note the cool SSTA in the eastern oceans of both the North and South Pacific. There are some unique signatures to the global SSTA pattern – unusual warm zones including the Pacific “warm blob” located near the Dateline and an unusually warm basin-wide North Atlantic.

Fig. 2: The daily global SSTA analysis and key SSTA areas related to ENSO.

La Nina is forecast to intensify and peak either in December or January. The strengthening forecast is based in-part due to the plentiful cooler than normal upper ocean of the equatorial East Pacific (Fig. 3). In fact, the upper ocean cool anomalies for the new La Nina is stronger than the previous which implies potential for a stronger than forecast (La Nina) episode. Most forecast models agree on peak intensity for La Nina 2021-22 in December, most intensely by NOAA (Fig. 4). Similar to the Climate Impact Company constructed analog, dissipation of La Nina 2021-22 is likely by next March according to most dynamic models (Fig. 5).

Fig. 3: Depleted upper ocean heat is indicated east of the Dateline in the equatorial Pacific Ocean. The heat depletion is greater than the previous La Nina.

Fig. 4-5: A collection of dynamic models forecasts by the Australia Bureau of Meteorology for ENSO phase in December (peak La Nina) and next March (La Nina dissipates).

Historically, the cold season (NOV-MAR) climate across the U.S. during La Nina favors cold temperatures across the West/North-central U.S. and a mild winter in the South and into the East (Fig. 6). The Northwest States are stormy during La Nina winter while dryness can lead to a drought across the southern states to the Mid-Atlantic (Fig. 7).

A typical La Nina usually causes a global SSTA pattern to cool with the coolest anomalies across the central and east tropical Pacific basin as observed in October 1988 (Fig. 8). However, modern-day La Nina generates in a much warmer ocean. Note the anomalous warmth of most middle/subtropical latitudes in the October 2021 analysis (Fig. 9).

Fig. 6-7: La Nina temperature and precipitation climatology for November through March across the U.S. taken from the 1950-2000 climatology.

Fig. 8-9: La Nina of October 1988 identifies the global tendency for anomalous cool SSTA while modern-day La Nina (from October 2021) generates while poleward waters are much warmer.

Consequently, modern-day La Nina climatology is a little different. La Nina episodes during 2000-2020 are not as chilly in the Northwest while the Northeast is warmer for NOV-MAR (Fig. 10). The primary difference with modern-day La Nina precipitation patterns during the cold season is much drier in California while the Ohio Valley is wetter and the Northwest is not as wet (Fig. 11).

Fig. 10-11: Modern-day (2000-2020) La Nina U.S. temperature/precipitation anomalies for NOV-MAR.

The PDO regime often parallels ENSO. Currently, a robust cool phase PDO is in-place and certainly supportive of developing La Nina. The Climate Impact Company constructed analog forecast of PDO indicates peak intensity now with slow weakening through the northern hemisphere winter season but weak -PDO lingers well into 2022 (Fig. 12). Presence of -PDO should enhance La Nina climate anomalies the next several months.

Fig. 12: Climate Impact Company constructed analog PDO index forecast.

The Atlantic multi-decadal oscillation (AMO) regime is not necessarily related to the ENSO phase. However, research has shown that AMO and PDO regimes imply drought risk in the U.S. somewhat independently from ENSO. Currently, the AMO regime is unusually warm for late northern hemisphere autumn. The Climate Impact Company constructed analog forecast indicates +AMO fades quickly to neutral phase which lasts well into 2022 (Fig. 13). Given the unusual warmth this late in the year there is the possibility that the constructed analog is too cool. The ECMWF global SSTA forecast for next March (Fig. 14) weakens the +AMO regime only slightly. There is potential for weak -PDO/+AMO to co-exist well into next year which supports above normal drought risk for the Southwest U.S. and the U.S. Corn Belt.

Fig. 13: Climate Impact Company constructed analog AMO index forecast.

Fig. 14: ECMWF global SSTA forecast for next March.