Global warming and hurricanes part 1: The natural cycle
Before we can discuss the possible influence of global warming on hurricanes, we need to set the stage by talking about this natural cycle of hurricane activity we hear so much about. The Atlantic Multidecadal Oscillation (AMO) is a cyclic variation in the large-scale atmospheric flow and ocean currents in the North Atlantic Ocean that combine to alternately increase and decrease Atlantic sea surface temperatures (SSTs). As its name indicates, the AMO is "multidecadal"--meaning it operates on a time scale of multiple decades. The cool and warm phases last for 25-45 years at a time, with a difference of about 1�F (0.6�C) between extremes. These changes are natural. Analysis of tree rings, fossil coral, and sediments has shown that the AMO has been around at least 300 years, and probably much longer.
What has the AMO done in recent years?
As seen in Figure 1, the AMO has been though about two complete cycles since detailed measurements of the Atlantic began in the mid-1800s. A cool phase lasted 25 years from 1901-1925, a 44-year long warm phase from 1926-1969, and a 25-year long cool phase from 1970-1994. A new warm phase began in 1995, and the AMO index values since 2001 have been the highest on record. This has resulted in sea surface temperatures over the prime hurricane breeding grounds of the tropical North Atlantic being the highest on record, as well. The AMO index in 2004 was about the same as in 2003, but 2005 has seen about a 10% drop from 2004's level.
Figure 1. The AMO index, 1871 to 2003. The index was computed by averaging sea surface temperatures north of the Equator over the Atlantic Ocean, between 75�W and 7.5�W and south of 60�N. The red regions show warmer than average SSTs over the North Atlantic, and the blue regions, cooler than average. The "trend" has been removed, so the mean and long-term increase in SSTs are not visible. Image credit: Rowan, T.S., and Daniel Hodson, "Atlantic Ocean Forcing of North American and European Summer Climate", Science 309, 115-118, 2005. Reprinted with permission from SCIENCE, � 2005 AAAS. Permission from AAAS is required for all other uses.
How do hurricanes vary during warm and cool AMO cycles?
The AMO is thought to strongly influence the incidence of intense hurricanes, which more than double during the years when the warm phase of the AMO. For example, an average of 1.5 intense (category 3, 4, and 5) hurricanes per year formed in the Atlantic during the last cool phase of the AMO, 1970-1994, while 4.1 intense hurricanes/year formed during the current warm phase (1995 - 2005). The number of tropical storms and weak hurricanes do not change much between cool and warm AMO periods.
What causes the AMO, and can we predict it?
The causes of the AMO are not well-understood, but some "coupled" computer models that simulate the behavior of both the atmosphere and the ocean are beginning to shed light on this. One of the leading theories is that changes in the ocean's salt content causes a speed up or slow down of the Gulf Stream, due to the fact that density differences between lighter fresh water and heavier salty water drive weaker and stronger ocean currents, respectively. This circulation (called the "thermohaline circulation") is thought to cause the warm phase of the AMO and warmer Atlantic SSTs when it speeds up, and cooler SSTs and a cool AMO phase when it slows down. It is also possible, but less likely, that changes in atmospheric circulation cause the AMO. We are a long way from being able to predict when a particular phase of the AMO will begin or end. The last warm phase lasted for 44 years. The current cycle began in 1995, so it may not be until 2040 that the current active period of hurricane activity dies down.
What influence does global warming have on the AMO?
Since the instrument record extends back for only two cycles of the AMO, it is difficult to say if the record warmth in the current AMO cycle is unusual. It has been observed that the tropical North Atlantic SSTs have warmed approximately 0.3�C over the past 100 years due to global warming. However, the current thinking of climate experts is that global warming should act to dump increased fresh water into the North Atlantic and reduce the speed of the Atlantic thermohaline circulation. This would lead to a decrease in Atlantic SSTs, and a reduction of intense hurricanes. Either global warming hasn't acted to dump enough fresh water into the North Atlantic to affect the thermohaline circulation, or else the theory is wrong!