2013 Atlantic Hurricane Season: Much Ado About Very Little

Despite near unanimous forecasts for another above average season across nearly all major forecasting organizations, the 2013 Atlantic Hurricane season was the least active in the last 30 years.

Did you know?

• Of the 13 named storms that formed in 2013, only two have reached hurricane strength (Humberto and Ingrid), and none became major hurricanes (Category 3+). In comparison, on average (1950-2012), the Atlantic Basin produces 11-12 named storms during a season, six-seven of which go on to become hurricanes, including two-three that reach major hurricane status.
• The last time a season produced this few hurricanes was 1982.
• It is also the first season since 1994 not to have produced a major hurricane.
• 2013 was the first season in 11 years without a recorded hurricane by the end of August, and only the second season since 1944 where a hurricane had not formed by the climatological peak of hurricane season (September 10).

From an intensity perspective, the statistics are even more surprising. Hurricane forecasters measure the overall damage potential of individual tropical cyclones and tropical cyclone seasons using a metric called Accumulated Cyclone Energy, or ACE. This hurricane season’s ACE total is just over 30, which is only 30% of the long-term ACE average. Since 1950, only four other Atlantic hurricane seasons have yielded lower ACE totals: 1983, 1982, 1977, and 1972.

But why was the season so inactive?

With much of the scientific community still debating this question, a consensus has yet to be reached. Complicating matters even further is the fact that the large-scale atmospheric signals, such as the absence of El-Niño conditions and warmer-than-average sea-surface temperatures (SSTs) across most of the tropical Atlantic, indicated an average to above average season. Nevertheless, we can get a first glimpse at the most likely suppression factors.

• Drier-than-normal air settling into the eastern Atlantic in August-September, likely a result of dry Saharan air pushing sand and dust into the atmosphere off the coast of Africa. These conditions made it extremely difficult for tropical waves moving off the West African coast to develop and intensify.
• Atmospheric instability during the season’s peak months was reduced, making conditions less conducive for thunderstorm development, a key driver of hurricane growth and intensification
• Intra-seasonal variability of the Atlantic Multi-Decadal Oscillation (AMO) /Thermohaline Circulation (THC), large-scale patterns in the Atlantic Ocean that are driven by fluctuations in SST. Both weakened abruptly during spring and early summer as a result of cooler-than-normal SSTs across most of the Atlantic, which may have had a negative downstream impact on hurricane formation and development during the rest of the season.

So what does this season’s inactivity mean?

• Is global warming starting to impact the atmospheric conditions that drive the Atlantic hurricane season?
• Is the Atlantic Ocean finally starting to show signs of shifting from an active phase of the AMO to an inactive phase?
• Or is this season just an outlier in the longer period of above normal hurricane activity?

The jury is still out at this point, but it’s safe to say that confidence levels are low, especially if conclusions are being drawn from this season alone.

When analyzing the physical drivers of the climate, particularly for hurricane activity, it’s important not to discern long-term trends from short-term signals due to the high degree of variability associated with them. Rather, it benefits scientists and organizations to limit random, naturally-occurring variability by studying robust datasets or conducting experiments that encompass a long period of time.

For instance, the 2013 RMS Medium-Term Rates (MTR) forecast, which was released earlier this year as part of the Version 13.0 North Atlantic Hurricane Model suite, incorporates updates informed by an original study that involved simulating over 20 million years of hurricane activity to better understand the likelihood of hurricane landfalls along the U.S. coastline. The high number of simulations helped establish a higher degree of confidence in results, which has led to an increase in market agreement of the new MTR outlook.

Although the 2013 Atlantic hurricane season was a far quieter than previous years, it does provide the scientific community with plenty to consider as we look ahead to next season, which begins in less than six months.