The La Nina climate phenomenon is strengthening, increasing the likelihood an active hurricane season could get even busier.
The update from the National Oceanic and Atmospheric Administration on Thursday comes as residents of Texas and Mexico are cleaning up from the deluge of Tropical Storm Hermine, and Tropical Storm Igor is drifting in the Atlantic.
La Nina is marked by a cooling of the tropical Pacific Ocean and was reported to be developing a month ago. It strengthened throughout August and appears likely to last at least through early 2011, NOAA's Climate Prediction Service said.
"La Nina can contribute to increased Atlantic hurricane activity by decreasing the vertical wind shear over the Caribbean Sea and tropical Atlantic Ocean," the center noted.
Wind shear is a sharp difference in wind speed at different levels in the atmosphere. A strong wind shear reduces hurricanes by breaking up their ability to rise into the air, while less shear means they can climb and strengthen.
NOAA has been calling for an above-normal tropical storm. The forecast issued in August anticipates 14 to 20 named tropical storms. The hurricane season started June 1 and ends Nov. 30, but the peak period runs from August through October.
La Nina's cooling of the tropical Pacific is the opposite phase of the El Nino event, which is marked by unusually warm tropical water in that region. Each can take place every few years, usually with neutral conditions in between.
Both can impact climate worldwide by changing the direction and strength of winds and altering air pressure and rainfall patterns.
In addition to hurricanes in the Atlantic and Gulf of Mexico, the impact of La Nina can include above-average rain or snowfall in the Pacific Northwest and below-average precipitation in the Southwest.
In other regions, La Nina tends to suppress hurricane activity across the central and eastern tropical North Pacific and increases rainfall in Indonesia.
NOAA said its computer climate models disagree on how strong this La Nina will be, but all concur it will last at least through early 2011.