This is not that obvious or easy to understand. From Scientific American:
A combination of cooler seas and a quiet West African monsoon season made for a less active Atlantic hurricane season, giving the South and East Coasts of the United States one of their lengthiest reprieves in history from a major hurricane, forecasters said on Monday.
“This is the longest without a major hurricane hitting the U.S. since the Civil War era,” said Jeff Masters, chief meteorologist for Weather Underground.
Wilma in 2005 was the last major hurricane to make U.S. landfall. Sandy was not a hurricane by the time it reached land.
“There’s been a whole sequence of conditions that suppress these storms,” said Gerry Bell, NOAA’s lead seasonal hurricane forecaster.
Among factors that tamped down storm formation were below-average temperatures in the tropical Atlantic and an active Pacific storm season that saw more than 20 named storms in its most active season since 1992.
“It’s a seesaw effect; often when the Atlantic is more active the Pacific will be suppressed,” Bell said.
At the start of the season forecasters had predicted up to 13 tropical storms with winds topping 39 mph (63 kph), and at least one major storm.
They left out Ike, which hit Texas in 2008 and caused $27B in damage, #3 all time, and Irene, which hit the East Coast in 2011 and caused $15B in damage, #7 on the all time list. That’s major, even though they were category 2 and 1, respectively. (source)
Is it the overall global water temperature that is increasing? Are we seeing overall more storm activity globally?
It turns out the measure of global storm activity is called Accumulated Cyclone Energy. There is data back to 1970 and it can be found here. This chart shows the total global accumulated cyclone energy, which is reported monthly, as an annual summation (each data point is the total energy for the previous 12 months):
Here’s the global water temperature data, showing both the top 700m and the top 2000m:
That explains the “cooling oceans.” They are cooling in the top layer, or at least not continuing to warm as quickly. For a longer time scale, here’s dating back to 1960, but only for the deep layer:
These graphs are from RealClimate.
So it would appear that total storm energy has not been increasing along with ocean heat content. At least not yet. But then why did the NOAA credit cooler ocean temperatures for fewer storms?
This climate stuff is just not that simple.