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 Americas to Australasia,
and is strongly linked to the La Niņa-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
These global atmospheric circulation anomalies have resulted from a combination of
ongoing La Niņa 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.
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 Niņo
and La Niņa 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 Niņa-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 Niņa conditions
at least through July. Thereafter, the forecasts tend to diverge, but a majority indicate
either near-normal or weak La Niņa conditions continuing to the end of the year. However,
we expect that even if La Niņa 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.
The evolution of La Niņa, along with the accompanying tropical rainfall and
atmospheric circulation patterns, will be monitored very closely during the next several
months for evidence of significant changes. Our expectation of above-average activity
during 2000 is not based on global warming associated with elevated carbon dioxide levels.