Nino SSTA Regions Trend Cooler Last Week

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Last week southern oscillation index returned to weak positive phase and trade winds off the northwest coast of South America also returned. As a result, the increased trade winds up-welled cool waters to cause all Nino SSTA regions to fall to within La Nina thresholds. However, the subsurface at 75+ meters continue to warm and subsurface cool water supply to sustain La Nina is steadily losing amplitude.

Highlight: Last week Nino SSTA regions turned cooler. But subsurface continues to warm.

Executive summary: La Nina 2020-21 is weakening but in a choppy manor. Last week southern oscillation index returned to weak positive phase and trade winds off the northwest coast of South America also returned. As a result, the increased trade winds up-welled cool waters to cause all Nino SSTA regions to fall to within La Nina thresholds. However, the subsurface at 75+ meters continue to warm and subsurface cool water supply to sustain La Nina is steadily losing amplitude.

Discussion: The subsurface equatorial Pacific Ocean continues to warm east of the Dateline (Fig. 1). Most of the warming is beneath 75 meters and extends to 120W (Fig. 2). Previously, warming waters were beneath 100 meters. The cool layer just-beneath the equatorial Pacific surface is more shallow but still sufficiently plentiful to allow a borderline La Nina to hang on. The warming of the subsurface east of the Dateline is in response to a Kelvin Wave enabled by a sudden shift of the southern oscillation index (SOI) from a strong La Nina signature rapidly toward neutral to negative phase in recent weeks (Fig. 3). Last week, the SOI started to return to positive phase which occurred in-part due to increasing trade winds off the northwest coast of South America. The returning trades caused the Nino SSTA to trend cooler last week (Fig. 4). Right now, all dynamic and statistical forecast models indicate neutral Nino34 SSTA for 2021 with the current La Nina episode ending during quarter 2 of 2021 (Fig. 5).

Fig. 1: The sudden warming signature of the subsurface equatorial Pacific Ocean east of the Dateline.

Fig. 2: Last week trade winds off the northwest coast of South America increased and up-welled cooler waters to the surface while below 75 meters warm waters continue to shift east.

Fig. 3: The eastward shift of the Kelvin Wave has been helped, in-part by a sharp trend away from positive southern oscillation index. During -SOI trade winds weaken. However, during the past week or so +SOI has returned to force the return of the cooler SSTA in the Nino regions.

Fig. 4: The 12-week SSTA observations identify warming early-to-mid-March. However, last week ALL Nino SSTA regions shifted back to the La Nina thresholds.

Fig. 5: The latest collection of dynamic and statistical Nino34 SSTA forecasts indicate neutral ENSO is ahead for 2021.