With the official start of the hurricane season roughly one week away, and with the year having already spawned two named storms (Alberto and Beryl), now is an appropriate time to review the potential for hurricane activity during the upcoming summer.
This outlook is not a forecast per se; we firmly believe that one number (such as the number of named storms), or set of numbers, is a potentially misleading way to view the activity for a hurricane season. Instead, we will examine the current state of the Earth’s climate system which can provide general guidance for the season.
In addition to this outlook, we have recently published an in-depth paper on how climate impacts hurricanes, and other catastrophic weather perils in the United States: Catastrophic Weather Perils in the United States: Climate Drivers. Many of the terms and science used in this outlook are discussed in much greater depth in this paper and can be downloaded from www.tigerrisk.com; alternatively a paper copy can be obtained from Steve Smith.
Our expectation for 2012 is that the hurricane season will likely have average to below average activity, with the majority of the activity occurring in the first half of the season. 2012 will likely see hurricanes form closer to land with an increased potential for a southeastern US or Gulf of Mexico hurricane.
Atlantic Sea Surface Temperature
One of the primary drivers of hurricane activity is the temperature of the water in the Atlantic basin since hurricanes derive their energy from warm sea water. Hurricanes, as a general rule, only form and sustain when the water temperature is 26.5°C or greater.
Currently, the Atlantic is cooler over a larger area than at least any time since 2008. Possibly, the present conditions of the Atlantic are cooler than we have observed since 2005. While we remain in a long term period of above average Atlantic sea surface temperatures (SSTs), the present conditions are closer to average than we have seen in some time. However, the Atlantic does remain warm enough to support hurricane formation over a wide area even though SSTs are suppressed relative to recent years. We note that, unlike the other years since 2005, the Caribbean Sea is currently relatively cool; in previous years even when the open Atlantic was on the cool side, the Caribbean was a relative hot spot.
Given that 2012 Atlantic SSTs are close to the long term average, this implies that the activity is likely to be average for the forthcoming season.
El Niño Southern Oscillation
The El Niño Southern Oscillation (ENSO) is a regular warming and cooling of the equatorial Pacific Ocean. ENSO impacts hurricane activity by changing the level of wind shear over the Atlantic (wind shear is the change in wind speed and direction with height in the atmosphere). During the ENSO warm phase (also known as El Niño), wind shear over the Atlantic is increased and hurricane activity is suppressed (increased wind shear acts to pull hurricanes apart). During ENSO neutral and cool (La Niña) phases, hurricane activity is not suppressed.
The forecasting of ENSO requires the use of computer modeling, modeling which is often subject to errors. However, through the use of ensembles (i.e., running the same model multiple times), an envelope of likely outcomes can be determined. For the upcoming season, most of the models are predicting that an ENSO warm phase (El Niño) will build by the middle of the season (around early September).
Given that the current state of ENSO is neutral, the expectation is the early part of the hurricane season will be relatively average. Once ENSO warm (El NiÃ±o) is established, however, wind shear over the Atlantic will likely increase leading to a suppression of hurricane activity. It is possible, therefore, that the hurricane season will come to an early end with little activity after mid-September.
Hurricane Steering Influences
Hurricanes, as large and powerful as they are, are subject to the influences of larger weather patterns, patterns which change on short time scales (which makes hurricane track forecasting the tricky problem that it is). There are some climate variations, however, which can give some guidance as to the general tendency of hurricane tracks.
The North Atlantic Oscillation (NAO) is a variation of air masses over the Atlantic. While the NAO generally has more influence on winter storm tracks affecting Europe, the NAO can affect hurricane tracks by shifting the position of the Bermuda High which itself acts to block hurricanes from tracking northwards. During NAO positive years, hurricanes have an increased likelihood of either hitting the US East Coast or remaining over the open Atlantic. During NAO negative years, hurricanes have an increased likelihood of hitting the southeastern US or entering the Gulf of Mexico. In recent years the NAO has been predominately negative but the last year has seen a very strong positive NAO (which has been evidenced by the unusual winter weather in the US and Europe). It is difficult to forecast the NAO more than 15 days ahead of time, but recent model forecasts are showing the NAO is becoming negative again. If this forecast holds up for the hurricane season, there is an increased likelihood of a southeastern US or Gulf of Mexico hurricane.