BOSTON — Nevermind long-term global warming. There is rapid climate change under way right now in the Atlantic Ocean. The North Atlantic sea surface is getting warmer. The South Atlantic is cooling. This strongly suggests that we're in for a spate of intense Atlantic hurricane activity like we had last year.
That's the lesson hurricane forecaster William Gray draws from the rocky experience of trying to predict last year's hurricane season, which ended Nov. 30. A meteorologist at Colorado State University in Fort Collins, Dr. Gray and his colleagues have a stellar track record in forecasting the general activity level of the Atlantic hurricane season. But last year they blew it.
Even with a midseason correction, they called for only average or slightly above average action. Instead, we had nine hurricanes, six of them intense. That compares with an average of about six hurricanes (two of them intense) for the 1950-1990 period. What's more, Gray's statistical measure of the season's tropical storm activity turned out to be nearly twice the long-term average.
Gray doesn't expect his statistical forecasting technique to hit the mark all the time. But the size of this failure suggests something new has entered the weather picture. That something, he now says, is a dramatic change in Atlantic sea-surface temperature patterns that began two years ago. For decades, the June-to-November hurricane-season temperature patterns featured a relatively cool North Atlantic and a warmer South Atlantic. Those patterns, in turn, favored a diminished tendency for intense hurricanes to form. Now the temperature patterns appear to be shifting back to what they were in the 1930s and 1940s, when hurricane activity was more intense.
In a postseason analysis, Gray's team says it is convinced that this temperature change is the culprit. Their forecast scheme takes account of a number of so-called predictors including stratospheric wind patterns and surface temperatures of the equatorial Pacific Ocean. The El Nino effect that periodically warms equatorial Pacific waters is especially important. An El Nino affects global weather patterns in ways that tend to suppress Atlantic hurricane activity. When there's no El Nino, Atlantic hurricanes tend to bloom. But Atlantic sea-surface temperature patterns had been much the same from one hurricane season to another until 1994. So Gray's scheme ignored them. It will do so no longer.
The team says it is convinced that what's happening in the Atlantic is a significant climate change. The summertime North Atlantic is at most less that 2 degrees C warmer than the 40-year average. The South Atlantic is less than 2 degrees cooler. Those small changes are enough to affect hurricane activity. The team suggests that this reflects a speedup of the so-called Atlantic conveyor-belt circulation, in which cold, salty water sinks to great depth in the Arctic and flows south while warm water flows north at shallower depths. This circulation, which had been sluggish for decades, now may be starting to pick up.
That begs the question of what is causing this apparent rapid climate change. It probably is not human-induced global warming. Even if that were going on, it would occur too slowly to account for what's happening in the Atlantic. Gray probably is right in saying this is a natural climatic shift that has happened many times in the past.
Whatever the cause, Gray's analysis is an important "heads up" for land-use planners and disaster-management officials. There seems to be little doubt that Atlantic summer sea-surface temperatures are reverting to a pattern that, in the past, has favored intense hurricane formation. Communities vulnerable to hurricanes should take this threat seriously and prepare to deal with it realistically. In particular, there is little rational excuse now to continue residential and commercial development on barrier islands and vulnerable coastlines. Gray may have blown last year's forecast. But his failure provides new scientific insight that is important for public safety.