With hurricane Earl sweeping up the East Coast, we might ask if, in recent years, we have experienced an unusual number of hurricanes. According to the meteorology department of the University of Florida, tropical cyclone activity remains at a 30-year low, see graph below.
The Florida University study says, “August ACE for the Northern Hemisphere was 63 which is much less than the climatological average of 115. The Western Pacific again was much below average. It was indeed the Atlantic that produced two long-lived storms, Danielle and Earl that picked up some of the slack. The global and NH ACE remains at/near 30-year lows.”
ACE is the “accumulated cyclone energy” which is a measure used by NOAA to express the activity of a cyclone, and is an approximation of the system’s total energy.
Some researchers correlate hurricane frequency with sea surface temperatures. Holland and Webster, for instance, report: “We find that long-period variations in tropical cyclone and hurricane frequency over the past century in the North Atlantic Ocean have occurred as three relatively stable regimes separated by sharp transitions. Each regime has seen 50% more cyclones and hurricanes than the previous regime and is associated with a distinct range of sea surface temperatures (SSTs) in the eastern Atlantic Ocean.” They were studying the period 1850 to present, see graph below (gray line is SST).
A longer study, however, concludes that El Nino and the West African Monsoon controls hurricanes in the Atlantic. (Donnelly, J.P. and Woodruff, J.D. 2007. Intense hurricane activity over the past 5,000 years controlled by El Niño and the West African Monsoon. Nature 447: 465-468.) These researchers, from Woods Hole Oceanographic Institution, based their study on sea sediment cores and corals. They found three major intervals of intense hurricane strikes: one between 5,400 and 3,600 calendar years before present, one between 2,500 and 1,000 yr BP, and one after 250 yr BP. They also say that “an analysis of Caribbean hurricanes documented in Spanish archives indicates that 1766-1780 was one of the most active intervals in the period between 1500 and 1800, when tree-ring-based reconstructions indicate a negative (cooler) phase of the Atlantic Multidecadal Oscillation.”
So, are there more hurricanes now than in the past? The answer seems to depend on the time period studied. Overall, neither the number of hurricanes nor their frequency is unusual.