It has been said that the current "active cycle" which began in 1995 might bring similar hurricane impacts to what was experienced by the U.S. in the 1940s through the 1960s. We remember storms like Donna, Carla, Audrey, Betsy, Beulah, Celia, Dora, Ione, and Camile (to name a few). But how common were such major hurricanes during the last active cycle that began in 1926 and ended in 1969?
I'm glad you asked! Let's take a look at some graphics that I'm preparing for my 2007 presentations. First, let's define the term "active cycle". This refers to the warm phase of the Atlantic Multidecadal Oscillation (AMO) when the Atlantic SSTs are warmer than normal. Dr. Gray has found that the warm and cool cycles are natural fluctuations driven by the Atlantic Thermohaline Circulation (conveyor belt). The theory is that this broad circulation is salinity-driven. Salty water is heavier than fresh water, so it tends to sink in regions with a good bit of fresh water input (like the northern latitudes in the Atlantic). This sinking water off Greenland draws more warm tropical water northward, increasing the speed of the conveyor belt, leading to a warm phase of the AMO. With time, so much warm water is driven northward that there is more ice melt up north, increasing the amount of fresh water in the northern latitudes and slowing the conveyor belt (cold phase AMO). The cycle appears to repeat every 25-45 years:
Conveyor Belt:
So what does a "warm cycle" or a "cool cycle" look like? Here are a couple of graphics depicting named storms, hurricanes, and major hurricanes during the last warm and cool cycles. I used only the last 25 years of the previous warm cycle (1945-1969) for comparison with the 25 year cool cycle from 1970-1994.
Note that there is virtually no difference in the number of named storms in each cycle. Hurricanes are pretty much the same, too. The main difference is in the number of major hurricanes. More ocean heat content appears to lead to a greater number of hurricanes reaching major hurricane strength. That appears to be one main difference.
Warm Cycle (1945-1969):
Cool Cycle (1970-1994):
So one main difference is in the number of major hurricanes that form in active vs. non-active AMO cycles. But the Pacific Decadal Oscillation (PDO) also comes into play. When the eastern Pacific cools, as it did in the 1940s-1960s, there is a tendency for the Bermuda High to be a little stronger. This can affect the tracks of hurricanes, driving more of them through the Caribbean Sea and/or toward the U.S. Mainland.
The two graphics below represent landfalling U.S. hurricanes and major hurricanes from 1926 through 2006. The blue line represents a 5-year moving average. The black line is a trendline.
Of particular intrest (maybe) is the black trend line pointing to a distinct downward trend in U.S. hurricane landfalls since 1926. But one can manipulate the data to fit one's agenda. I could take the same data and start in the mid 1950s and show an upward trend in hurricane landfalls. Beware scientists with political agendas!
In general, the U.S. can expect to see between 1 and 2 hurricane landfalls per season. It could be argued that Ernesto was possibly a hurricane which hit the east U.S. coast this year. Maybe it'll be upgraded to a U.S. hurricane landfall post-season. In any case, there are a number of years in the study in which no hurricanes impacted the U.S. Mainland. During the current active cycle, we went 3 years without a hurricane landfall from late 1999 to October 2002 (Lili).
The second graphic is the same period but indicates only major hurricane landfalls. As you can see, the 2004-2005 seasons were quite unusual. Typically, a major hurricane impacts the U.S. on average about once every 2 years. Often, the U.S. goes from 2 to 4 years without a major hurricane landfall.
In conclusion, seasons like 2006 are to be expected even in the most active of cycles. In alll probability, the average number of named storms over the next few decades will be from 10-12, with 6 hurricanes and 4 major hurricanes. I say from 10-12 because the NHC is now naming subtropical storms, which wasn't the case back in the last active cycle. Also, the NHC has shown a tendency to err on the side of caution and name some storms that may not have been named in the 1940s-1960s (Grace, for example).
So don't make the mistake of assuming that we'll be seeing 15-30 named storms each year for the next few decades. That's highly unlikely. But do expect more Gulf Coast and southeat U.S. impacts in the coming decades, particularly from Florida to the Carolinas. Not every year will have a major impact, though. Be thankful 2006 was quiet.
U.S. Hurricane Landfalls 1926-2006:
U.S. Major Hurricane Landfalls 1926-2006:
