The weekly SSTA observations from NOAA indicate weak La Nina continues (Fig. 1). But significant changes may be on the way. In the subsurface the large supply of cooler than normal water in the eastern equatorial Pacific Ocean providing fuel for La Nina 2017-18 is weakening (Fig. 2). A recent surge of warmer water into the subsurface East Pacific known as a Kelvin Wave has eroded the cool water supply.
The influence on the atmosphere by the Kelvin Wave coupled with Madden Julian oscillation caused southern oscillation index to shift into a very El Nino-like signature for February (Fig. 3). There’s little doubt that La Nina is eroding.
Meanwhile near the Dateline the next Kelvin Wave is evolving and appears quite powerful according to NOAA analysis. When this Kelvin Wave shifts into the East Pacific La Nina will dissipate rapidly.
Normally, an east-traveling MJO across the Pacific is the catalyst to push a Kelvin Wave eastward. The 30-day MJO forecast from ECMWF indicates the powerful February (MJO) episode weakens and does not regenerate (Fig. 4). If so, the Kelvin Wave either remains quasi-stationary or shifts east and weakens.
HOWEVER, a look at persistence (Fig. 5) yields another more likely result. A MJO pass across the Pacific as observed in February previously occurred in December albeit much weaker. Persistence suggests the next MJO pass across Pacific is in April. In April La Nina should crash given the strength of Central Pacific subsurface warmth which needs a push to surge into the remaining subsurface cool waters of the East Pacific.
Keep in mind, an MJO pass through the central and eastern equatorial Pacific coupled with a Kelvin Wave not only erodes La Nina but amplifies the jet stream pattern into a strong trough/ridge signature causing weather extremes including severe weather and possible preceding record warmth and following record cold.
Fig. 1: 12-week Nino SSTA observations indicate a steady weak La Nina.
Fig. 2: Subsurface equatorial Pacific Ocean temperature anomalies identify a recent weak Kelvin Wave shifting across the Pacific eroding the cool fuel in the East Pacific to sustain La Nina while another more potent Kelvin Wave is emerging near the dateline.
Fig. 3: Southern oscillation index is very El Nino-like in February due to the influence on sea level pressure patterns across the tropical Pacific caused by the Madden Julian oscillation coupled with a subsurface Kelvin Wave.
Fig. 4: The ECMWF 30-day MJO forecast indicates weakening. Needed is a resurgent MJO shifting across the Pacific – much like we’ve seen in February – to push the Kelvin Wave eastward and finish La Nina.
Fig. 5: The 90-day MJO observations indicate a DECEMBER and FEBRUARY pass across the Pacific which – when coupled with a subsurface Kelvin Wave – causes warming of the eastern equatorial Pacific. Based on persistence the next likelihood is APRIL and if so La Nina will certainly dissipate.