IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis North Atlantic

The North Atlantic hurricane record begins in 1851 and is the longest among cyclone series. Values are considered fairly reliable after about 1950 when measurements from reconnaissance aircraft began. Methods of estimating wind speed from aircraft have evolved over time and, unfortunately, changes were not always well documented. The record is most reliable after the early 1970s (Landsea, 2005). The North Atlantic record shows a fairly active period from the 1930s to the 1960s followed by a less active period in the 1970s and 1980s, similar to the fluctuations of the AMO (Figure 3.33).

Beginning with 1995, all but two Atlantic hurricane seasons have been above normal (relative to the 1981 to 2000 base period). The exceptions are the two El Niño years of 1997 and 2002. As noted in Section 3.8.3, El Niño acts to reduce activity and La Niña acts to increase activity in the North Atlantic. The increased activity after 1995 contrasts sharply with the generally below-normal seasons observed during the previous 25-year period (1970–1994). These multi-decadal fluctuations in hurricane activity result nearly entirely from differences in the number of hurricanes and major hurricanes forming from tropical storms first named in the tropical Atlantic and Caribbean Sea. The change from the negative phase of the AMO in the 1970s and 1980s (see Section 3.6.6) to the post-1995 period has been a contributing factor to the increased hurricane activity (Goldenberg et al., 2001) and is well depicted in Atlantic SSTs (Figure 3.33), including those in the tropics. Nevertheless, it appears likely that most of the warming since the 1970s can be associated with global SST increases rather than the AMO (Trenberth and Shea, 2006; see Section 3.6.6).

During 1995 to 2004, hurricane seasons averaged 13.6 tropical storms, 7.8 hurricanes and 3.8 major hurricanes, and have an average ACE index of 159% of the median. The record-breaking 2005 season is documented in more detail in Section 3.8.4, Box 3.6. In contrast, during the preceding 1970 to 1994 period, hurricane seasons averaged 8.6 tropical storms, 5 hurricanes and 1.5 major hurricanes, and had an average ACE index of only 70% of the median. NOAA classifies 12 (almost one-half) of these 25 seasons as being below normal, and only three as being above normal (1980, 1988, 1989), with the remainder as normal. The positive phase of the AMO was also present during the above-normal hurricane decades of the 1950s and 1960s, as indicated by comparing Atlantic SSTs (Figure 3.33) and seasonal ACE values (Figure 3.40). In 2004, there were 15 named storms, of which 9 were hurricanes, and an unprecedented 4 hit Florida, causing extensive damage (Levinson, 2005). In 2005, record-high SSTs (Figure 3.33) and favourable atmospheric conditions enabled the most active season on record (by many measures), but this was not fully reflected in the ACE index (see also Section 3.8.4, Box 3.6). In 2005, the North Atlantic ACE was the third highest since 1948, while the PDI was the highest on record, exceeding the previous high reached in 2004.

Key factors in the recent increase in Atlantic activity (Chelliah and Bell, 2004) include: (1) warmer SSTs across the tropical Atlantic; (2) an amplified subtropical ridge at upper levels across the central and eastern North Atlantic; (3) reduced vertical wind shear in the deep tropics over the central North Atlantic, which results from an expanded area of easterly winds in the upper atmosphere and weaker easterly trade winds in the lower atmosphere; and (4) a configuration of the African easterly jet that favours hurricane development from tropical disturbances moving westward from the African coast. The vertical shear in the main development region where most Atlantic hurricanes form (Aiyyer and Thorncroft, 2006) fluctuates interannually with ENSO, and with a multi-decadal variation that is correlated with Sahel precipitation. The latter switched sign around 1970 and remained in that phase until the early 1990s, consistent with the AMO variability. It has been argued that the QBO is also a factor in interannual variability (Gray, 1984). The most recent decade has the highest SSTs on record in the tropical North Atlantic (Figure 3.33), apparently as part of global warming and a favourable phase of the AMO. In the Atlantic generally, the changing environmental conditions (Box 3.5) have been more favourable in the past decade for tropical storms to develop.