11:45 AM | *Stubborn La Nina hangs on in the equatorial Pacific Ocean despite model predictions of a transition to El Nino*
Paul Dorian
Current sea surface temperature anomalies with La Nina conditions in the tropical Pacific Ocean; map courtesy NOAA, tropicaltidbits.com
Overview
To be fair, most of the computer model predictions of a transition from La Nina (colder-than-normal) to El Nino (warmer-than-normal) in the equatorial Pacific Ocean suggest this will take place over the next few months, but the track record of these same models from just one year ago is not all that confidence-building and the latest 7-day change in sea surface temperatures is certainly not yet showing any kind of dramatic turnaround. The overall conditions in the equatorial Pacific Ocean are very important to the Atlantic Basin tropical season which officially begins in just over one month’s time. Computer forecast model predictions of an El Nino last year were simply not accurate and the surprise return of La Nina late last summer contributed to the suddenly very active tropical season of 2017.
Numerous computer models in April 2017 predicted El Nino would form and last throughout the year; Plot courtesy IRI/CPC, NOAA, ECMWF
Details
About one year ago in April of 2017, most computer forecast model predictions of the El Nino Southern Oscillation (ENSO) suggested that the early year La Nina-to-neutral conditions in the tropical Pacific Ocean would transition to a moderate or even potentially strong El Nino by the all-important summer tropical season. Typically, El Nino is an inhibiting factor in the Atlantic Basin tropical season as it tends to increase wind shear in the breeding grounds region of the Atlantic Ocean and this tends to either prevent tropical storm formation or inhibit its intensification.
ENSO index values are shown back to 2010 on a rolling 3-month average where negative represents La Nina (cold) conditions in the equatorial Pacific Ocean and positive indicates El Nino (warm) conditions. La Nina actually re-formed in the July/August/September period of 2017 (boxed value) despite numerous computer forecast model predictions for an El Nino. Data courtesy NOAA/CPC
As the year progressed in 2017, it became more and more apparent that the numerous model predictions of an intensifying El Nino were simply not going to materialize and La Nina actually started to re-form by the July/August/September time frame (see rolling 3-month average of ENSO index values). The quick demise of weak El Nino and subsequent somewhat surprising rise of La Nina during the heart of the 2017 Atlantic Basin tropical season suddenly changed overall atmospheric conditions from unfavorable-to-favorable. Indeed, 2017 turned out be be a hyperactive tropical season in the Atlantic Basin – the most active since 2005 - featuring 17 tropical storms, 10 hurricanes, and 6 majors.
Numerous computer models in April 2018 predict La Nino will transition to El Nino over the next few months and El Nino will then continue through the remainder of the year. Plot courtesy IRI/CPC, NOAA, ECMWF
The latest “monthly-updated” computer model predictions for ENSO in the remainder of 2018 suggest once again that there will be a development of El Nino in the tropical Pacific Ocean - though not quite as strong as last year’s (incorrect) forecasts. As of now, however, La Nina conditions remain over an extensive region in the equatorial Pacific Ocean and there has not yet been any kind of dramatic or noticeable warming in sea surface temperatures.
Sea surface temperature changes over the past 7 days are not showing any kind of important signal of imminent change in the tropical Pacific Ocean; map courtesy NOAA, tropicaltidbits.com
We’ll continue to closely monitor the conditions in the tropical Pacific Ocean over the next few months to see if La Nina actually does transition to El Nino. If this transition does not materialize, there could be important ramifications on our anticipated "near normal" Atlantic Basin tropical season. For more on the Vencore Weather “2018 Tropical Outlook” click here.
Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com