For the first time this year, all of the computer models used by the US Climate Prediction Center to forecast the El Niño-Southern Oscillation are predicting the current El Niño to end by the peak of the Atlantic Basin hurricane season.

El Niño is a warming of the sea surface temperatures in the Pacific Ocean that is known to create stronger upper-level wind shear in the Atlantic Basin.  The higher wind shear inhibits the formation and strengthening of tropical cyclones and has been credited by meteorologists for the quiet 2009 Atlantic hurricane season. Conversely, hurricane seasons when the El Niño-Southern Oscillation is neutral or in a La Niña – or cooler than normal – phase tend to support active hurricane seasons.

The Climate Prediction Center’s weekly El Niño forecast report issued 19 April showed all models in agreement with at least a neutral ENSO condition by summer, with the majority of models predicting negative temperature anomalies, although still in the neutral range.   The neutral range is considered any temperatures within 0.5 degrees Celsius – plus or minus - of normal.

Several models have the tropical Pacific Ocean temperatures dropping low enough to cause a full-fledged La Niña.  One of the models, the European Centre for Medium-Range Weather Forecasts, is particularly good at predicting the El Niño-Southern Oscillation, said Colorado State University scientist Phil Klotzbach at the recent US National Hurricane Conference in Orlando.  That particular model shows ENSO into a full-blown La Niña by the three-month period of July, August and September.

The Climate Prediction Center’s ensemble model, which basically takes the average of all the models, forecasts ENSO to enter a neutral condition during the three month period of April-May-June and then to slide into a negative neutral value, close to a La Niña, during the three month period of July-August-September.

Although years with neutral ENSO conditions have supported very active and destructive hurricane seasons – the record breaking 2005 Atlantic hurricane season took place during a neutral ENSO phase – a La Niña event could bode even worse for the Cayman Islands, especially late in the hurricane season when the Western Caribbean becomes the primary breeding ground for tropical cyclones.

Klotzbach said that the Caribbean feels the effects of ENSO even more strongly than other parts of the Atlantic Basin and that the combination of the current active thermohaline circulation phase along with a La Niña event could have significant impacts. 

In particular, at the end of a hurricane season, when wind shear is often too strong for hurricane formation in other parts of the Atlantic Basin, La Niña can keep the sheer levels down in the Caribbean, where in combination with the extremely warm waters, strong hurricanes can form.