Decelerating La Nina Historical Climate for MAR/APR/MAY

Comments on the 2020 North Atlantic Tropical Cyclone Season
12/09/2020, 3:07 pm EST
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Fig. 1: The consensus Nino34 SSTA forecast for the first half of 2021 indicating decelerating La Nina and analog years (since 2000).

Introduction:  La Nina 2020-21 has likely reached the mature stage. All leading dynamic model Nino34 SSTA forecasts indicate La Nina weakens to neutral phase by April. Decelerating ENSO events can be as influential on global climate as the mature stage of established ENSO regimes. A look at the MAR/APR/MAY season decelerating La Nina climate is reviewed.

Data: The analog years are taken from the past 20 years. During the past two decades the polar ice cap has constricted and global oceans warmed and unique to the period of record of ENSO data. Since 2000 a total of 6 decelerating La Nina years have occurred. Each analog year began with moderate-to-strong La Nina followed by steady weakening to neutral phase during MAR/APR/MAY (Fig. 1). Temperature and precipitation anomaly data from NCDC/PSD is used to identify climate bias for MAR/APR/MAY.

Texas/Oklahoma/Kansas dryness and warmth: The MAR/APR/MAY decelerating La Nina climatology indicates high risk of anomalous warmth for New Mexico/Texas to the Midwest U.S. (Fig. 2). The western half of this warm anomaly zone is also drier than normal (Fig. 3). Drought conditions are anticipated in this region as meteorological summer approaches.

The maximum temperature anomalies are exceptional across northwest Texas and vicinity (Fig. 4). Implied is high risk of over-achieving heat for late spring (accelerating dry soils). The minimum temperature warm risk is less extreme but covers the entire eastern half of the U.S. (Fig. 5).

Cool/wet bias Northwest U.S.: Historically, during decelerating La Nina the meteorological springtime climate is cooler than normal in the Far Northwest States slowing snow melt while precipitation is moderately above normal for the West Coast from northern California to the Oregon/Washington Cascades and westward. If the “warm blob” (an area of anomalous warm SSTA in the northeast Pacific Ocean) is present the cool/wet anomaly is likely confined to Oregon/Washington (missing California) and less intense. Northern California desperately needs the precipitation!

The Appalachian States to the Great Lakes region are warmer than normal but also wetter than usual during MAR/APR/MAY when La Nina is weakening. The Southeast States are included in the wet anomaly zone. The coastal Northeast Corridor is typically drier than normal.

U.S. Conclusion: Leading dynamic forecast models indicate La Nina will weaken the first half of 2021. Decelerating ENSO phase influences climate patterns. Based on 6 analog years (since 2000) the effect of decelerating La Nina on U.S. climate during meteorological spring is dry and eventually hotter than normal across Texas to the central Great Plains and vicinity. Northern California has a chance at much needed above normal spring precipitation. The Appalachian States and Southeast U.S. are wetter than normal.

Fig. 2-3: Decelerating La Nina years (since 2000) during MAR/APR/MAY and typical temperature and rainfall anomalies across the U.S.

Fig. 4-5: Decelerating La Nina years (since 2000) during MAR/APR/MAY and typical maximum/minimum temperature anomalies across the U.S.

Global anomalies: ENSO influence is significant across South America and Australia. While ENSO affects North America climate due to the proximity to the Pacific Ocean, the effects on Eurasia are not consistent. A look at decelerating La Nina for South America, Australia and Southeast Asia is reviewed.

During MAR/APR/MAY decelerating La Nina has limited influence on the South America temperature regime (Fig. 6). However, a wet climate has a tendency to dominate the northern continent while harsh dryness strikes northeast Argentina/Southeast Brazil (Fig. 7).

The climate bias across Australia during decelerating La Nina during MAR/APR/MAY is cooler and wetter than normal for central and northeast sectors of the continent (Fig. 8-9).

Fig. 6-7: Decelerating La Nina years (since 2000) during MAR/APR/MAY and typical temperature and rainfall anomalies across South America.

Fig. 8-9: Decelerating La Nina years (since 2000) during MAR/APR/MAY and typical temperature and rainfall anomalies across Australia.