Throughout the summer I have updated the blog on the progress of the hurricane season. Now, and assuming that the season is over, it is time to look at the full picture and summarize the main features. In the figure below the tracks of the 19 storms that developed in the season are shown. The strongest ones, category 4 hurricanes Danielle, Earl, Igor and Julia that developed in late August and in early and mid September, stayed mainly over the ocean.
By and large, the 2010 Atlantic hurricane season was extremely active. In fact it was the most active season since 2005 with only two seasons since 1944 recording more named storms: 1995 (19) and 2005 (28). It ties with the 1995 and 1987 hurricane seasons for the third most named storms (19) and with the 1969 and 1887 seasons for the second most hurricanes (12). In the table below the number of storms, hurricanes and major hurricanes recorded up to 22nd November as well as the forecasts by different centers is given. Most of the forecasts were actually very good in particular the ones from NOAA.
Why was this year’s Altantic hurricane season so active?
As discussed in a previous post the three main reasons are the following:
-La Niña: it acts to reduce the vertical wind shear in the tropical Atlantic and Caribbean Sea which increases the hurricane activity in these regions (Gray, 1984);
-Above average SSTs: the sea surface temperatures in those regions were in general 1-2C above average throughout most of the season;
-Tropical multi-decadal signal: key components in the tropical Atlantic and Caribbean Sea include reduced vertical wind shear, weaker easterly trade winds, and an extensive area of cyclonic shear at 700hPa along the equatorward flank of the African Easterly Jet, factors that are also conductive for hurricane formation.
Some of the ingredients mentioned above can be seen in the plots below where the SST anomalies observed at the beginning of the season (17th June 2010) and near the end of it (22nd November) are shown. The La Niña strengthened throughout the season (cooler SSTs over the Equatorial Pacific Ocean) while the very warm SSTs over the tropical Atlantic and the Caribbean Sea were also present most of the time but started to weaken from mid-September in particular over the north-western part of the Caribbean Sea and the Gulf of Mexico.
Having said that, this year’s hurricane season will be remembered more for the “near misses” to the United States than for making the all-time active list. Twelve hurricanes and no US landfalls? It has never happened before in the observational record going back to the late 1800s. Only one tropical storm landfall (Bonnie) out of 19 storms?
The mean geopotential height at 500hPa averaged from 15th July to 15th October 2010 when most of the storms developed is shown below. The underlying shape of the atmosphere during the heart of the season featured upper-level high pressure systems over the Central Atlantic and over Texas and north-eastern Mexico. The clockwise flow around the core of these systems often led to a high-altitude southerly wind just east of North America and upper-level northwesterly or northerly flow over the Gulf of Mexico as can be seen in the figure below where the anomalous winds averaged over the same period are plotted.
Many of this year’s tropical cyclones that migrated westward across the tropical Atlantic found a weakness in the upper flow over the eastern Atlantic between the two ridges from 75°W to 55°W where the winds aloft were often directed northward. The steering currents over this sector of the Atlantic effectively shoved approaching storms towards the north and back out to sea. Further to the west over the Gulf of Mexico, the frequently observed northerly flow blocked storms that formed in the Caribbean from turning northward in time to cross the U.S. Gulf Coast and as a result this year’s tropical systems avoided the United States (source).
Any comments/suggestions are more than welcome.