2013: The year of low Atlantic hurricane activity

Despite the doom and gloom, of increasing global tropical cyclone frequency and intensity as the climate warms, predicted by scientists like Kerry Emanuel (as mentioned in the previous post), I recently came across a few other pieces of information that would add a new dimension to the current debate.

2013 Atlantic Hurricane Season

2013 Atlantic hurricane season forecast. Credits: AccuWeather.com

At the start of the year, experts from NOAA predicted that 2013 would likely be an active or extremely active year. They stated that there are 3 climate factors that favour the development of an active Atlantic hurricane season this year:

1. Continuation of the atmospheric climate pattern since 1995 that includes a strong west African monsoon
2. Warm sea surface temperatures (SSTs) over the tropical Atlantic Ocean and Caribbean Sea
3. Absence of El Nino that would otherwise suppress hurricane formation

Yet, thus far, there have only been a total of 12 named storms, of which only 2 are hurricanes (both are short-lived Cat 1 hurricanes i.e. no major hurricanes this season so far). The first hurricane of the season, Hurricane Humberto, came only after the mid-season point on September 11. The Accumulated Cyclone Energy (ACE) developed by NOAA is calculated as the square of the wind speed every 6 hours for every named storm with at least 40mph sustained winds. It is a measure of both tropical cyclone activity and of potential damage. So far, the ACE is 28.55, which is way below the 1981-2010 average of 104 units.

So what happened to the Atlantic hurricane season this year? As highlighted in a post on ClimateCentral.org, researchers have posited that there are a few factors that have led to the low hurricane activity despite the warmer-than-average SSTs over the main developing region and lack of El Nino. Firstly, Brian McNoldy from the University of Miami suggested that the air has been anomalously dry at the lower and higher altitudes and is coupled with large-scale subsidence. Both factors suppress the growth of thunderstorms, which is needed for hurricane development. Secondly, frequent plumes of dry, dusty air coming from the Sahara Desert has also contributed to the dry and sinking air over the Atlantic. Thirdly, Landsea and Klotzbach suggested that there has been a higher-than-average wind shear over the Atlantic that is hindering the intensification of the storms. These factors were missed out from the initial calculations during hurricane forecasting.

Nonetheless, given that there are still a few more days to the end of the Atlantic hurricane season, anything could still happen.

Bigger Picture

The low hurricane activity this year is not restricted to the Atlantic only. In fact, all the ocean basins, other than the North Indian basin, are recording below average ACE values this year. This year also marks the 8^th year since the last major hurricane of Cat 3 strength or greater made landfall in the USA. Although Sandy peaked at Cat 3, it was only a Cat 1 hurricane by the time it made landfall. Moreover, a recent report stated that based on results from climate model simulations, there is a decreasing probability of a steering flow oriented towards the US eastern coast in future under the RCP8.5 pathway (Barnes et al. 2013). In addition, other than the Atlantic Ocean that have been experiencing enhanced hurricane activity associated with the warm phase of the Atlantic Multidecadal Oscillation (AMO) since 1995, the other basins have actually recorded decreasing ACE since 2006 due to the evolution of tropical and North Pacific inter annual and interdedacal climate modes such as ENSO and the Pacific Decadal Oscillation (PDO) (Maue 2011). 

Nonetheless, there is a worry that the lack of major hurricane activity might lead to a sense of complacency among those living in Hurricane Alley. Since Hurricane Wilma in 2005, Florida has not ben hit by a major hurricane. Consequently, the sunshine state has seen a massive influx of people in recent years, many of whom have not had a first-hand encounter with hurricanes. This actually increases the vulnerability of the state to hurricanes and it only takes one hurricane that makes landfall to bring about major devastation to its people.

What all these pieces of information suggest is that the debate may not be as clear-cut as it seems after all. There are still so many factors that can contribute to hurricane and cyclone activity, many of which are still not well understood by scientists.