El Niño ending

Increases chances for an active hurricane season

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.