North Atlantic Tropical Cyclone - Climatology

Climatology

Climatology does serve to characterize the general properties of an average season and can be used as one of many other tools for making forecasts. Most storms form in warm waters several hundred miles north of the equator near the Intertropical convergence zone from tropical waves. The Coriolis force is usually too weak to initiate sufficient rotation near the equator. Storms frequently form in the warm waters of the Gulf of Mexico, the Caribbean Sea, and the tropical Atlantic Ocean as far east as the Cape Verde Islands, the origin of strong and long-lasting Cape Verde-type hurricanes. Systems may also strengthen over the Gulf Stream off the coast of the eastern United States, wherever water temperatures exceed 26.5 °C (79.7 °F).

Although most storms are found within tropical latitudes, occasionally storms will form further north and east from disturbances other than tropical waves such as cold fronts and upper-level lows. There is a strong correlation between Atlantic hurricane activity in the tropics and the presence of an El Niño or La Niña in the Pacific Ocean. El Niño events increase the wind shear over the Atlantic, producing a less-favorable environment for formation and decreasing tropical activity in the Atlantic basin. Conversely, La Niña causes an increase in activity due to a decrease in wind shear.

According to the Azores High hypothesis by Kam-biu Liu, an anti-phase pattern is expected to exist between the Gulf of Mexico coast and the North American Atlantic coast. During the quiescent periods (3000–1400 BC, and 1000 AD to present), a more northeasterly position of the Azores High would result in more hurricanes being steered towards the Atlantic coast. During the hyperactive period (1400 BC to 1000 AD), more hurricanes were steered towards the Gulf coast as the Azores High was shifted to a more southwesterly position near the Caribbean. Such a displacement of the Azores High is consistent with paleoclimatic evidence that shows an abrupt onset of a drier climate in Haiti around 3200 14C years BP, and a change towards more humid conditions in the Great Plains during the late-Holocene as more moisture was pumped up the Mississippi Valley through the Gulf coast. Preliminary data from the northern Atlantic coast seem to support the Azores High hypothesis. A 3000-year proxy record from a coastal lake in Cape Cod suggests that hurricane activity has increased significantly during the past 500–1000 years, just as the Gulf coast was amid a quiescent period of the last millennium.