This map shows the tracks of each tropical storm this year.
There are two types of weather that are generally considered the worst: tornadoes and hurricanes. This year, the United States has experienced very low levels of both tornado and hurricane activity. This is great news, of course! But it still begs the question... why? In this 2-part series, I'll break down the numbers, put them in historical context, and present a theory or two for the lack of bad weather. Today's topic is the hurricane season of 2013, which has practically been a non-factor.
Hurricane season is almost over (it officially ends on November 30th), and we've only had twelve named storms this year. This is lower than the seasonal prediction that NOAA issued back in May (13-20 named storms), but it's about the average number of named storms since the satellite era began in 1961 (11.42 per year). The more dramatic change is the number of hurricanes, especially major hurricanes, in 2013. We only had two hurricanes all year, and both were Category 1 storms, which is the lowest on the Saffir-Simpson scale! NOAA had predicted a season with 7-11 hurricanes, and they expected that between 3-6 of them would be major hurricanes, with winds of 111mph or stronger. Obviously, the number of hurricanes is way lower than expected, and we didn't have a single major hurricane all year.
This article from National Geographic online theorizes that the weak tropical season came from an infusion of dry Saharan air, due to an unusually long-lasting Bermuda-Azores high in the Atlantic Ocean. In most summers, the Bermuda high is a prevalent, but not a permanent feature in our weather pattern. Also, the center of the high is usually closer to the USA coastline, whereas this summer, it was further west. (As a side note, I also think this unusual placement is partly responsible for our lack of 90- and 100-degree temperatures this summer in DC!) The picture below shows color contours denoting higher pressure areas in red and orange. It clearly shows the large area of high pressure that dominated the Atlantic this past summer.
|Courtesy of the National Centers for Environmental Prediction (NCEP)
The dry air infusion is likely the most important factor that kept our tropical storm numbers so low. It's one of the three main factors that can prevent tropical storms from developing or strengthening. The other two are wind shear and colder water. Wind shear, which usually comes in the form of a strong jet stream, was not heavily mentioned as a factor in the tropical weather outlooks issued by the National Hurricane Center. Tropical wind shear is usually a bigger factor during El Nino periods, and we have been ENSO-neutral (neither El Nino nor La Nina conditions present) for more than a year now. So really, nothing is pointing to wind shear as a big factor in this year's low hurricane number. The last factor I mentioned, cold water, doesn't appear to be a factor this year either. At the beginning of the season, there was a weak cooling in the waters north of Hispaniola, but that anomaly eroded away as the peak of hurricane season neared (see pictures below, and please forgive my lack of skill with MS Paint!).
So, why was simple dry air so destructive for developing tropical storms? In order to understand why, you have to first understand that a hurricane is very different from other storms. It is a very consistent column of warm, moist air from sea level all the way to the top of the storm. There is very little variance in temperature, wind speed, or moisture level within this tropical column. So, when dry air enters a tropical system, it slices into it like a knife.
Want to learn more about this weak hurricane season? Check out this article I posted last month.