By Philip J. Klotzbach, Ph.D.
The 2017 Atlantic hurricane season, which officially ends on November 30, has been extraordinary by any standard, with a total of 17 named storms, including 10 hurricanes—six of which were classified as major, storms, measuring Category 3-5 on the Saffir-Simpson Wind Scale. Historically, 2017 ranked as a top-ten year in most widely recognized tropical cyclone (TC) metrics (Figure 1).
Figure 1: 2017 Atlantic tropical cyclone statistics compared with the 1981-2010 average as well as its rank compared with all historical Atlantic hurricane seasons since 1851.
What really distinguished the 2017 Atlantic hurricane season, however was the month of September (Figure 2). All other months of 2017’s hurricane season had near-normal activity, while September broke the Atlantic calendar month record for named storm days, hurricane days, major hurricane days and Accumulated Cyclone Energy (ACE). (ACE is an integrated metric that takes into account the frequency, intensity and duration of storms.)
Figure 2: Atlantic ACE by month in 2017 compared with the 1981-2010 average.
Atmospheric and oceanic conditions were conducive for an active season and this was especially true during September. During the period from late August to late September when the season was most active, the tropical Atlantic had very low vertical wind shear (Figure 3). Vertical wind shear is the change in wind direction with height in the atmosphere. Strong vertical wind shear tilts the hurricane vortex, disrupting the circulation and preventing the pressure fall necessary to sustain a strong hurricane. Sea surface temperatures were also much warmer than normal, providing more fuel for developing tropical cyclones (Figure 4).
Figure 3: Vertical wind shear anomalies from late August to late September. Blue colors indicate reduced vertical wind shear. Reduced vertical wind shear dominated most of the tropical Atlantic into the eastern and central Caribbean.
Figure 4: Sea surface temperature anomalies across the tropical Pacific and Atlantic near the peak of the Atlantic hurricane season. (The tropical Atlantic and Caribbean were much warmer than normal in 2017.)
Three of the 17 storms that formed in 2017 accounted for the lion’s share of damage. Hurricane Harvey made landfall in central Texas as a Category 4 hurricane, then stalled near Houston, inundating the metropolitan area with record-setting rainfall. Hurricane Irma cut a path of destruction across the Caribbean and tropical Atlantic, devastating several islands before becoming the first Category 5 hurricane to make landfall in Cuba since 1924 (Figure 5). Irma then made landfall on the Florida Keys as a Category 4 hurricane. Hurricane Maria became the first Category 5 hurricane on record to make landfall in Dominica, then it buffeted the US Virgin Islands before slamming into Puerto Rico as a Category 4 hurricane. Maria was the strongest hurricane to make landfall in Puerto Rico since 1928. While the final damage estimates are still being tallied, this season will certainly go down as one of the most devastating Atlantic hurricane seasons of all time.
Figure 5: Satellite imagery of Hurricane Irma as it pummeled the northern coast of Cuba.
Philip J. Klotzbach, Ph.D. is Research Scientist, Department of Atmospheric Science, Colorado State University and an I.I.I. Nonresident Scholar. You can follow him on Twitter at @PhilKlotzbach