OUTLOOK

The global-scale atmospheric circulation anomalies are presently conducive to an above-average (active) Atlantic hurricane season during 2000, according to a consensus of scientists at the National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center (CPC), National Hurricane Center (NHC) and Hurricane Research Division (HRD). This anomaly pattern features above-normal upper-level heights in the subtropics of both hemispheres extending eastward from the America’s to Australasia, and is strongly linked to the La Niña-related pattern of anomalous tropical convection. It is probable that this anomalous atmospheric circulation will persist through the summer, thereby favoring another season of above-average overall activity in the tropical Atlantic.

These global atmospheric circulation anomalies have resulted from a combination of ongoing La Niña conditions, and a decadal-scale shift in global ocean temperatures during the past several years. This shift includes above-average temperatures across large portions of the North Atlantic since mid-1995. Historically, when the existing atmospheric anomalies were present at this time of the year, 90% of the Atlantic hurricane seasons featured average or above-average activity, with 50% of the seasons being above-average. However, when compared to decades during which global ocean temperatures also favored an active hurricane era (such as the 1950's and 1960's and the period 1995-present), this probability increases to a 75% chance of an above-average hurricane season.

If the current global anomalies persist well into the summer, they will favor an active North Atlantic hurricane season by 1) reducing the vertical wind shear across the tropical Atlantic and Caribbean Sea, and 2) producing a structure and location of the African easterly jet which is thought to be more efficient for providing energy to developing tropical systems as they propagate westward from the African coast. Other expected favorable factors are below-average air pressure and above-average sea-surface temperatures across the tropical Atlantic and Caribbean.

Many of the storms are expected to develop over the tropical Atlantic, and then move westward toward the Caribbean Islands or the United States, thereby putting coastal areas at an increased risk of experiencing a tropical storm or hurricane. In active years the Caribbean Islands and the United States each experience an average of 2-3 hurricane strikes. For the Caribbean Islands, this frequency of hurricane landfall is much larger than that which is observed in inactive years.

DISCUSSION

Tropical rainfall patterns are a primary forcing of the atmospheric circulation throughout the global tropics and subtropics. This global-scale atmospheric circulation which results from these rainfall patterns represents an important climate signal that allows one to predict upcoming conditions over the North Atlantic with some certainty, and therefore to predict overall seasonal hurricane activity.

Ocean temperatures and tropical rainfall patterns impact Atlantic hurricane activity on both yearly and decadal time scales. For example, Pacific Ocean phenomena such as El Niño and La Niña strongly impact the year-to-year variations in hurricane activity, while decadal variations in North Atlantic sea surface temperatures are thought to affect hurricane activity on longer time scales.

The continuation of the current global-scale atmospheric conditions will depend on the persistence of the ongoing La Niña-related tropical rainfall anomalies. This anomaly pattern features enhanced rainfall across Indonesia and the western tropical Pacific, and suppressed rainfall over the central equatorial Pacific. A consensus of the latest numerical and statistical model forecasts indicate a continuation of La Niña conditions at least through July. Thereafter, the forecasts tend to diverge, but a majority indicate either near-normal or weak La Niña conditions continuing to the end of the year. However, we expect that even if La Niña fades by late summer, the existing tropical rainfall anomalies will not be totally destroyed, and therefore will not unduly impact the favorable large-scale atmospheric circulation pattern that currently exists.

Another contribution to conditions favorable for an active season comes from the above-average sea surface temperatures that have persisted since 1995 across large portions of the North Atlantic. Since that time the warmer North Atlantic waters have been associated with an overall dramatic increase in hurricane activity. In contrast, the relatively inactive decades of the 1970's and 1980's were associated with below-average sea-surface temperatures over large portions of the North Atlantic.