Hurricane season outlook ramps up after tame 2015
COLORADO - With the 2016 Atlantic hurricane season officially due to get underway on June 1, the respected climatology team at Colorado State University (CSU) has released its outlook for the upcoming season.
The initial forecast is for a total of 12 named storms, 5 hurricanes and 2 major hurricanes, which is very close to the long-term average of 12 named storms, 6 hurricanes and 2 major hurricanes.
This year’s forecast nevertheless calls for more activity than the 2015 Atlantic hurricane season, which was slightly below normal as hostile winds from El Niño depressed tropical activity.
According to Phil Klotzbach, lead author of the CSU hurricane outlook, El Niño’s demise is one of several factors that CSU considered in developing its outlook for a more active season this year.
El Niño, characterized by warmer-than-normal waters in the eastern and central part of the tropical Pacific, is an important predictor for hurricane activity because it generates stronger upper-level westerly winds that can tear storms apart in the tropical Atlantic and Caribbean.
This year’s El Niño event, which reached historic levels this winter, is nevertheless now in decline.
There are several models that predict the future state of El Niño, most of which are calling for either neutral or La Niña (the climatic opposite of El Nino) in the tropical Pacific for the peak of the Atlantic hurricane season from August-October. Should this prove correct, upper-level winds should be much more conducive for hurricane formation and intensification.
According to Professor Klotzbach, two models which have reasonably good predictive skill for El Niño are the European Centre for Medium-Range Weather Forecasts (ECMWF) model as well as the Climate Forecast System (CFS) model.
Both of these models use an ensemble approach, where the model is run with slightly different initial conditions and run forward in time. The average of these ensembles call for cool neutral to La Niña conditions for the peak of the Atlantic hurricane season.
Currently, the far North Atlantic is much colder than normal. The remainder of the Atlantic is relatively warm, with the exception of some colder than normal water emerging off of the west coast of Africa. It is these two areas of cold water that are one of the reasons why CSU is not predicting an above-average hurricane season.
Typically, colder-than-normal water in the far North Atlantic is indicative of a negative phase of the Atlantic Multi-decadal Oscillation (AMO). When the AMO is negative, it tends to portend quieter periods for Atlantic hurricanes.
The cold waters in the far North Atlantic tend to force higher pressure in the tropics and subtropics, as has been the case this winter. Associated with this higher pressure are stronger low-level trade winds in the tropical Atlantic which then force cooler SSTs in the tropical Atlantic. In addition, higher pressure in the tropics means more dry air and sinking motion which tends to suppress deep thunderstorm activity, the building blocks for hurricanes.
It nevertheless remains to be seen whether the cooling in the tropical Atlantic and higher-than-normal pressures will continue.
At present, the thinking behind the CSU forecast is that the unfavourable Atlantic conditions will counteract the more favourable La Niña conditions and lead to a near-average hurricane season.