Cool Support for La Nina East Pacific Subsurface Remains BUT West Pacific Is Warming!

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The MJO has been mostly dormant in the vicinity of Indonesia or westward since October. However, a slow eastward shift with intensification is underway the past 1-2 weeks. MJO has shifted toward the Pacific Dateline and a further eastward drift is indicated through the middle third of December. Upper ocean heat in the equatorial West Pacific is responding as warmth increases and shifts to the Dateline.

Highlight: Moderate La Nina continues. MJO is active and West Pacific subsurface is active…signs that La Nina may weaken in January.

Fig. 1: The 12-week Nino region SSTA observations.

Discussion: Last week, the Nino SSTA regions edges slightly cooler as moderate-strength La Nina persists (Fig. 1). The atmospheric signature is stronger as multivariate ENSO index (MEI) for OCT/NOV 2021 was -1.5. Some important developments are occurring in the subsurface and are likely related to the Madden Julian oscillation (MJO). The MJO has been mostly dormant in the vicinity of Indonesia or westward since October. However, a slow eastward shift with intensification is underway the past 1-2 weeks (Fig. 2). MJO has shifted toward the Pacific dateline and further eastward drift is indicated through the middle third of December. Upper ocean heat in the equatorial West Pacific is responding as warmth increases and shifts to the Dateline (Fig. 3). The equatorial East Pacific remains cooler than normal and certainly capable of sustaining moderate La Nina. However, an east-shifting MJO event across the equatorial Pacific will likely weaken the cool East Pacific signature and West Pacific warmth could continue to expand eastward. Another (similar) MJO episode would further weaken La Nina. The next potential MJO episode is likely 6-7 weeks away.

Fig. 2: Madden Julian oscillation has strengthened and is shifting eastward during the past 7-10 days.

Fig. 3: Equatorial Pacific Ocean upper heat indicates the East Pacific is sustaining La Nina while West Pacific is turning warmer.