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CLIMATE CONDITIONS DURING THE 1995 ATLANTIC HURRICANE SEASON
As of October 31, the 1995 Atlantic hurricane season featured 19 tropical storms (Fig. 1 bottom). This is the second largest number
of tropical storms observed in any hurricane season (June-November) since 1871, and the second
largest number of hurricanes observed in one season since 1886, according to the National Hurricane
Center (NCDC, 1981). On average, 9-10 tropical storms are observed over the North Atlantic
between June and November, with 5-6 of these systems typically becoming hurricanes. A comparison
with other active hurricane seasons (Table 1) shows that although 1995 was a very active year, many
of the listed tropical storm parameters were equaled or surpassed in prior years. A notable exception
was the 115 Named Storm Days (NSD) through the end of October, which exceeded all previous
years since 1950.
The active 1995 hurricane season follows four consecutive years (1991-1994) of extremely
low Atlantic tropical storm and hurricane activity (Table 2, Figs. 1 and 2). For example, the 19
tropical storms observed in 1995 is more than twice the total observed during each of the 1991-1994
seasons (6-8 events). Additionally, the 11 Atlantic hurricanes during 1995 far exceeds the 3-4
hurricanes observed each year during 1991-1994.
While the 1995 Atlantic hurricane season was extremely active and was preceded by four
relatively quiet years, tropical activity during each of the past five years was within the range of
observed climate variability. This special climate summary is not intended to address issues such as
hurricane and climate modeling, long-term trends, and global warming. The reader is referred to
previously listed contacts at NOAA's Geophysical Fluid Dynamics Lab (GFDL), and the December
11, 1995 seminar and multimedia presentation on hurricanes planned by the U.S. Global Change
Research Program (USGCRP).
Contributing Factors to the Active 1995 Hurricane Season
There are many factors which affect tropical storm and hurricane activity over the North
Atlantic. Some of these factors show surprisingly strong long-range predictive signals for Atlantic
basin seasonal tropical cyclone activity up to 11 months in advance (Gray et al., 1992; Gray et al.,
1994a). Research by Dr. William Gray and colleagues at Colorado State University has identified
three prominent factors associated with this predictive skill: (1) the stratospheric Quasi-Biennial
Oscillation (QBO); (2) the El Nino-Southern Oscillation (ENSO) cycle; and (3) West African
Dr. Gray and colleagues indicated in November 1994 (Gray et al., 1994b) that the above three
factors would be in a phase favorable for above normal Atlantic tropical activity this year, by
contributing to: (a) suppressed vertical wind shear across the subtropical North Atlantic between
10 N-20 N, and (b) a tendency for stronger easterly low-pressure waves (at approximately 15N)
which typically move across western Africa and propagate westward over the subtropical North
These easterly waves are in many cases the very systems which eventually intensify into
tropical storms. However, the potential for this intensification is controlled by the vertical wind shear;
strong vertical shear acts to inhibit intensification, while weak vertical shear acts to aid intensification.
Under favorable conditions of weak vertical wind shear, the intensification potential of these low
pressure disturbances into tropical storms and eventually hurricanes is further aided by large-scale
patterns of abnormally warm ocean water and below normal surface pressure over the subtropical
During the 1995 Atlantic hurricane season, atmospheric and oceanic conditions were
extremely favorable for enhanced tropical storm and hurricane activity (Figs. 3, 4, 5, & 6). In particular,
minimal vertical wind shear was observed during the season throughout the entire region from
western Africa to the Gulf of Mexico and the Caribbean Sea (Figs. 3 bottom, and 4 top).
Additionally, below normal sea-level pressure (Fig. 5 bottom) and above normal sea-surface
temperatures (Fig. 6) were observed throughout the western and central subtropical North Atlantic
during August and September, particularly in the vicinity of the Caribbean Sea and the Gulf of
Mexico. These conditions, combined with a series of intense easterly waves originating over north-
central Africa, provided the primary ingredients for a highly active hurricane season throughout the
subtropical North Atlantic basin.
These atmospheric conditions during 1995 were in strong contrast to those noted during the
1991-1994 hurricane seasons. For example, during the 1994 hurricane season the subtropical North
Atlantic was dominated by enhanced vertical wind shear (Figs. 3 middle, and 4 bottom) and above
normal sea-level pressure (Fig. 5 middle). Similar conditions were also noted during the 1991, 1992,
and 1993 hurricane seasons (not shown).
Possible Causes of the Increased Atlantic Hurricane Activity during 1995 over the 1991-1994
The most important identifiable phenomenon associated with year-to-year climate variability
is the global oceanic and atmospheric oscillation known as the El Nino/ Southern Oscillation (ENSO).
During the warm phase of this oscillation (commonly referred to as the El Nino), above normal sea
surface temperatures are observed throughout the central and east-central equatorial Pacific. Due to
this increase in ocean temperatures, the normal patterns of tropical cloudiness and precipitation are
disrupted, which in turn affects the atmospheric wind and pressure patterns throughout the Northern
Hemisphere. The resulting wind and pressure patterns often act to substantially inhibit Atlantic
In particular, the mid-1991 through early 1995 period was dominated by one of the most
prolonged Pacific warm (ENSO) episodes on record. Important manifestations of this episode
included extremely persistent patterns of enhanced vertical wind shear and above normal air pressure
over much of the subtropical North Atlantic. These conditions resulted in the marked decrease of
tropical storm and hurricane activity over the North Atlantic previously noted during the 1991-1994
Atmospheric and oceanic conditions throughout the tropical Pacific began a rapid return to
normal in March 1995. This evolution marked an end to the prolonged El Nino conditions which
dominated the past four years. It also signaled the demise of the patterns of enhanced vertical wind
shear and above normal air pressure that dominated the subtropical North Atlantic during the past
four hurricane seasons.
As noted above, there are two other important factors which tend to be correlated with an
active Atlantic hurricane season well in advance, although their dynamical links are not well
understood. The first is the westerly phase of the stratospheric Quasi-Biennial Oscillation. This
westerly phase reached peak amplitude during the past few months and may have contributed to low
vertical wind shear throughout the subtropical North Atlantic this year.
The second is above normal rainfall over the African Gulf of Guinea during August-November
of the previous year. These surplus rains favor enhanced soil moisture, which is thought to contribute
to the development of the next year's spring and summer West African rainy season, and subsequently
to stronger easterly low-pressure waves over West Africa. During August-November 1994, rainfall
was above normal over the African Gulf of Guinea (CAC, 1994), indicating a positive influence on
Atlantic tropical cyclone activity this year. In contrast, the much below normal precipitation observed
over West Africa during many of the last 25 years is thought to have exerted an overall suppressing
influence on Atlantic tropical cyclone activity during the period.
The National Hurricane Center will continue to monitor tropical cyclone activity during the
remainder of the 1995 Atlantic hurricane season. For additional information, refer to the updated
forecast (Gray et al., 1995a) and preliminary forecast verification (Gray et al., 1995b) of the 1995
Atlantic hurricane season by Dr. William Gray and colleagues.
CAC, 1994: Special Climate Summary #94/2, Wettest Rainy Season in 30 Years Across African
- Sahel. United States Department of Commerce, National Oceanic and Atmospheric
- Administration, National Weather Service, National Meteorological Center,
- Climate Analysis
Center, Camp Springs, MD, 6 pp.
Gray, W. M., C. W. Landsea, P. W. Mielke, Jr., and K. J. Berry, 1992: Predicting
- Atlantic Seasonal
Hurricane Activity 6-11 Months in Advance. Wea. Forecasting, 7, 440-455.
Gray, W. M., C. W. Landsea, P. W. Mielke, Jr., and K. J. Berry, 1994a: Predicting Atlantic Basin
- Seasonal Tropical Storm Activity by June 1. Wea. Forecasting, 9, 103-115.
Gray, W. M., C. W. Landsea, P. W. Mielke, Jr., and K. J. Berry, 1994b: Extended Range
- Forecasting of Atlantic Seasonal Hurricane Activity for 1995, as of 30 November 1994.
- Department of Atmospheric Sciences, Colorado State University, Fort Collins, CO, 9 pp.
Gray, W. M., C. W. Landsea, P. W. Mielke, Jr., and K. J. Berry, 1995a: Early August Updated
- Forecast of Atlantic Seasonal Hurricane Activity for 1995, as of 9 August 1995. Department
- of Atmospheric Sciences, Colorado State University, Fort Collins, CO, 26 pp.
Gray, W. M., C. W. Landsea, P. W. Mielke, Jr., and K. J. Berry, 1995b: Early Analysis (as of
- October 19) of the 1995 Atlantic Tropical Cyclone Activity and Verification of Author's
- Seasonal Predictors. Department of Atmospheric Sciences, Colorado State University,
- Fort Collins, CO, 10 pp.
NCDC, 1981: Tropical Cyclones of the North Atlantic Ocean, 1871-1980. United States
- Department of Commerce (DOC), National Oceanic and Atmospheric Administration
- (NOAA), National Weather Service (NWS), Environmental Data and Information Service,
- Environmental Research Laboratories, Asheville, NC, and the DOC/NOAA/NWS/National
- Meteorological Center, National Hurricane Center, Coral Gables, FL, 180 pp.