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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 rainfall.

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 Atlantic.

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 North Atlantic.

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 Period:

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 hurricane activity.

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 period.

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.