Jeff Masters

August 30, 2013

The end of August is upon us, and with just one day remaining in the month, the remarkably quiet Atlantic hurricane season of 2013 is highly likely to be just the 6th season since the Hurricane Hunters began flying in 1944 without a hurricane forming by the end of August (the other years: 2002, 2001, 1988, 1984, and 1967.) Although there have been two tropical storms in August (Erin and Fernand), these storms were weak and short-lived, and August 2013 had one of the lowest Accumulated Cyclone Energy (ACE) totals on record for an August in the Atlantic. ACE is calculated as the square of the wind speed every 6 hours for every named storm with at least 40 mph sustained winds (scaled by a factor of 10,000 for usability.) Since the damage potential of a hurricane is proportional to the square or cube of the maximum wind speed, ACE is not only a measure of tropical cyclone activity, but also a measure of the damage potential. During the 20-year period 1981 - 2010, the Atlantic averaged 104 ACE units, and the 20-year average ACE by the end of August was 30. So far in 2013, we've managed just 9 ACE units, with only 1.9 of those occurring in August. Since the current active hurricane period we are in began in 1995, only 2002 had a lower ACE by this point in the year (4.9), and only 1997 had a lower August ACE (zero.) Both were El Niņo years, when we expect hurricane activity to be low due to high wind shear. If we go back to the beginning of reliable Atlantic ACE statistics in 1966, when good satellite data first became available, only five other years had August ACE values comparable to 2013's. Three of those years were El Niņo years, and the other two had ocean temperatures in the hurricane Main Development Region (MDR, from 10 - 20°N, 20 - 70°W) that were more than 0.5°C (0.9°F) cooler than in 2013. This year's combination of no El Niņo, warm MDR SSTs, and an exceptionally low August ACE is an event unparalleled in the historical record, going back to 1966.

Figure 1. August Accumulated Cyclone Energy (ACE) totals in August 2013 were among the lowest on record for an August in the Atlantic. Other Augusts with low ACE in the Atlantic all occurred during El Niņo years, or when sea surface temperatures (SSTs) were cool.

Why the quiet season?
The main reason for the quiet August has been the large amount of dry, stable air over the Atlantic. This dry air has two sources: the Sahara desert of Africa, and sinking air from aloft, which warms and dries as it sinks. Even so, I find it highly perplexing that activity has been so low when all of the other factors--lack of an El Niņo, low wind shear, an active African Monsoon spitting out plenty of tropical waves, and above average ocean temperatures--have favored development. Instability has increased over the tropical Atlantic over the past few days, thanks to the influence of the MJO and a Convectively Coupled Kelvin Wave (CCKW) bringing rising air to the Atlantic. However, there do not appear to be any tropical waves positioned to take advantage of this increased instability and intensify to hurricane strength over the coming week (though 96L could become a tropical storm.) The next tropical wave with a shot at becoming a hurricane will not exit the coast of Africa until September 7. If we make it all the way to September 11 without a hurricane in the Atlantic, it will beat the record set by Hurricane Gustav of 2002 for latest date of formation of the season's first hurricane, going back to when the Hurricane Hunters first began flying in 1944. Even if we do get a first-half of September hurricane, the steering pattern features a strong trough of low pressure over the U.S. East Coast, which would have high odds of recurving any hurricane that manages to form out to sea, without affecting any land areas.

Figure 2. Vertical instability over the tropical Atlantic in 2013 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Instability has been much lower than average during most of August, due to outbreaks of dry air from Saharan Air Layer (SAL) , and an unusual amount of dry, sinking air in the tropical Atlantic. Low instability reduces the potential for tropical storm formation. During the last few days of August, instability has increased, which should raise the odds of tropical storm formation. Image credit: NOAA/NESDIS/CIRA.