Subsurface East Equatorial Pacific Continues to Warm Weakening La Nina

Fig. 1: The subsurface equatorial Pacific Ocean temperature anomalies identify and east-shifting Kelvin Wave notorious for weakening La Nina.

Discussion: In January, trade winds weakened, the southern oscillation index (SOI) flipped briefly to negative phase and La Nina started to weaken. The set of circumstances described were inspired by an eastward shift of the Madden Julian oscillation (MJO) across the equatorial Pacific Ocean. Additionally, the MJO episode caused already warm subsurface temperatures in the equatorial West Pacific to shift eastward well past the Dateline. Normally, east-shifting warm zones of water at 100-300 meters depth is a sign of a Kelvin Wave notorious for weakening La Nina episodes. Last week the Kelvin Wave pushed farther east to almost 110W longitude and the cool waters remaining continued to shrink in aerial coverage plus amplitude just northwest of tropical South Africa (Fig. 1). The Nino SSTA regions are generally steady although Nino34 SSTA weakened to -0.6C last week indicating weak La Nina is barely hanging on (Fig. 2).

Fig. 2: The Nino SSTA regions and their anomalies for each of the last 12 weeks which indicate La Nina is weakening.