Hurricane season, which officially started June 1, sure got a slow start in the Atlantic.
Bill, the very first Atlantic hurricane of 2009, did not form until this past Monday. At the same time, there was a flurry of tropical activity, with tropical storm Claudette making landfall on the Florida panhandle, and tropical depression Ana weakening in the Caribbean.
There were no storms in June or July, though on average only one or two storms form during those months. Last year was quite a bit more active, with four storms by the end of July. The recent development of El Niño seems to be playing its part in reducing the number of tropical storms. According to the Climate Prediction Center’s lead seasonal hurricane forecaster, Gerry Bell, “El Niño produces stronger upper-level westerly winds over the Caribbean Sea and tropical Atlantic Ocean, which help to reduce hurricane activity by blowing away the tops of growing thunderstorm clouds that would normally lead to tropical storms.”
The Climate Prediction Center has now revised its seasonal hurricane forecast from May, to lower the amount of storms expected and to add a higher probability of a below-average number of hurricanes.
With the effects of El Niño getting more attention recently, it would make sense to explain how a change in ocean temperatures in the equatorial Pacific Ocean could have such a far reaching influence on weather across the globe, including a less snowy upcoming winter for the valleys across the Inland Northwest.
Both the atmosphere and the ocean are part of a system which transfers energy from one part of the globe to another. Where ocean and atmosphere meet, winds can drive ocean currents, and exchanges of energy occur through evaporation and condensation over the water. The temperature of the water affects the temperature of the air above it, which in turn affects where areas of high and low pressure form in the atmosphere. The location of highs and lows determine the prevailing winds, which drive the ocean currents. See the cycle?
During El Niño, weakening trade winds allow warmer water from the western Pacific Ocean to flow toward the east, increasing the temperature of the water in the eastern Pacific. The change in water temperature affects the vertical ocean circulations (upwelling). Since weather such as thunderstorms and hurricanes are fueled by warm, moist air, the change in ocean temperature causes a shift in storm patterns as well; this has a ripple effect across the globe.
The magnitude of the abnormal warming of the equatorial Pacific waters determines the strength of the El Niño episode, which can vary widely through the years. The general result, however, is flooding across South America, and drought and forest fires across areas of the western Pacific. Warmer temperatures are experienced in Canada, while the southern U.S. sees colder than normal temperatures.