Subsurface Moderately Cold in Equatorial East Pacific as La Nina Likely to Linger well into NH Spring

Fig. 1: Peak month of subsurface cooling in the equatorial Pacific Ocean east of the Dateline during La Nina events since 1979.  

Discussion: Since 1979 the subsurface equatorial Pacific Ocean to the east of the Dateline has produced peak subsurface cool signatures during La Nina, most dramatically during late last century with more frequent but less intense peak subsurface cool signatures during La Nina in this century (Fig. 1). The JAN-25 subsurface cool anomaly was -1.32 and the coolest monthly signature from the 2024-25 cold ENSO episode. Compared to modern-day La Nina episodes of the current century, the 2024-25 La Nina ranks 4^th strongest of 9 episodes as defined by the strength of the supporting subsurface temperature anomaly. The Nino34 SSTA for JAN-25 is projected near -0.9C and multivariate ENSO index (MEI) at -1.1, both supportive of a moderate El Nino event. During the past 2 weeks, the southern oscillation index (SOI) has shifted into strong positive phase induced by heavy convection associated with the Madden Julian oscillation (MJO) over Maritime Continent. Forecast models indicate MJO shifts eastward, possibly reaching the Dateline during the next 2 weeks, then fading moving across the cooler La Nina waters east of the Dateline. However, this process should weaken +SOI (and attendant trade winds) at least slightly, as La Nina intensity eases mid-month. While all of this is happening, the subsurface west of the Dateline is heating (Fig. 2). Eastward shifting MJO events across the equatorial Pacific try to create an environment that allows the West Pacific subsurface warmth to shift east to weaken La Nina. The next MJO event to move across the Pacific is in March followed by an early May event. Each will try to weaken La Nina. Oceanic and atmospheric La Nina should last until the May MJO episode.

Fig. 2: Subsurface temperature anomalies in the global equatorial ocean regions.