(i) Overview
The North Atlantic hurricane season runs from June through November, with peak activity between August to October primarily linked to systems developing from African easterly wave disturbances. Overall, 9-10 tropical storms are observed over the North Atlantic in an average season, with 5-6 becoming hurricanes and 2-3 reaching intense hurricane status [measured by a category 3, 4, or 5 on the Saffir-Simpson scale (Simpson 1974)]. The suppressed 1997 hurricane season featured 7 named storms (Fig. 38), with 3 of these systems becoming hurricanes and 1 reaching intense hurricane status. This latter system, Erica, developed in September and remained intense for only 2 days. Interestingly, hurricane Erica was the only tropical cyclone to form over the North Atlantic basin during August-September, a record low number for the period since the beginning of the aircraft reconnaissance era in 1944.
In contrast, the 1995 and 1996 Atlantic hurricane seasons were very active (Fig. 38), with a high percentage of tropical storms becoming hurricanes in each year (Halpert and Bell 1997, Halpert et al. 1996). During 1995, 11 of 19 tropical systems became hurricanes, with 5 reaching intense hurricane status. During 1996, 9 of 13 tropical storms became hurricanes, with 6 reaching intense hurricane status. A significant majority of these tropical cyclones and hurricanes in both years (16 of 19 systems in 1995 and 9 of 13 systems in 1996) developed from African easterly wave disturbances during August-October.
Over the eastern North Pacific, the 1997 hurricane season featured 17 named storms (normal is 16), 9 of which became hurricanes (normal is 9) with 7 becoming major hurricanes (normal is 5). The season also featured an expanded area of tropical cyclone activity compared to normal, with four systems moving well west of 135°W and two major hurricanes affecting southwestern North America. In contrast, the 1995 and 1996 seasons featured well below-normal tropical storm and hurricane activity across the eastern North Pacific.
(ii) Vertical wind shear
Tropical storm and hurricane activity over the North Atlantic and eastern North Pacific ocean basins is strongly affected by the vertical wind shear between the upper (200-hPa) and lower (850hPa) levels of the atmosphere. Strong vertical shear inhibits tropical cyclogenesis while weak vertical shear (less than approximately 8 m s-1) favors tropical cyclone development. Climatologically, strong vertical wind shear during the hurricane season is observed throughout the Caribbean, large portions of the subtropical North Atlantic and the northern Gulf of Mexico. In contrast, weak vertical shear is normally observed over the tropical eastern North Atlantic and over a large area of the eastern North Pacific between 10°-17.5°N (Fig. 39a). Thus, tropical cyclone formation is normally favored over the eastern tropical Atlantic and North Pacific basins, while comparatively less activity is favored in the Caribbean region.
The El Niño/Southern Oscillation (ENSO) can substantially influence the year-to-year variability of vertical wind shear over both the North Atlantic and eastern North Pacific ocean basins, and thus the interannual variability of hurricane activity in these regions. Gray (1984) has shown that Pacific warm episodes (El Niño) often favor suppressed tropical storm activity and a reduction in intense hurricane activity over the North Atlantic by helping to maintain or enhance the normally high vertical wind shear. In contrast, he notes that Pacific cold episodes (La Niña) often favor enhanced tropical storm activity and increased intense hurricane activity by helping to reduce the vertical wind shear across most of the tropical North Atlantic. Extreme phases of ENSO often have an opposite impact on tropical storm and hurricane activity over the eastern North Pacific, with El Niño favoring an expanded area of tropical cyclone activity by reducing the vertical wind shear in that region, and La Niña favoring suppressed tropical cyclone activity by enhancing the vertical wind shear.
This ENSO influence on tropical storm and hurricane activity has been particularly prominent during the 1990s. The prolonged ENSO-like conditions during 1991-February 1995 were accompanied by extremely low Atlantic tropical storm and hurricane activity. In contrast, the cold-episode years of 1995 and 1996 featured an increase in tropical storm activity over the North Atlantic and substantially reduced activity across the eastern North Pacific. Subsequently, the transition to very strong warm episode conditions during 1997 brought a return to below-normal activity over the North Atlantic and with an increase tropical storm activity over the eastern North Pacific.
During August-October 1997 large vertical wind shear covered most of the western and central North
Atlantic, the Caribbean Sea and the Gulf of Mexico, with favorable shear conditions (under 8 m
s-1) confined to the
eastern tropics generally south of 10° latitude (Fig. 39a). Enhanced vertical wind shear was observed
primarily over the Caribbean Sea and off the west coast of Africa between 10°-15°N (Fig. 39b), with near average
shear observed over the central subtropical North Atlantic. However, these normal shear values remained too large
to support tropical cyclone development.
A vertical profile of the atmospheric winds over the Caribbean Sea region (Fig. 40), where no tropical storms developed during the 1997 hurricane season, indicates enhanced vertical shear resulting primarily from an ENSO-related increase in the upper-level westerlies. In contrast, the near absence of vertical wind shear over the region during August-September 1995 (Fig. 40) resulted from weak easterly winds throughout the depth of the troposphere in association with La Niña conditions.
The entire eastern North Pacific featured low vertical wind shear during August-October 1997 (Fig. 39a), with generally reduced wind shear throughout the primary region of tropical storm formation between 10°-17°N and 105°-125°W (Fig. 39b). This reduced shear resulted primarily from an ENSO-related collapse of the normal easterly winds in the upper atmosphere (Fig. 41). These conditions contrast with the enhanced easterly winds and stronger-than-normal vertical shear observed throughout the region during the suppressed 1995 season.
(iii) The African easterly jet and African wave disturbances
Over the North Atlantic, another notable distinction between the inactive 1997 hurricane season and the active 1995 season was a marked difference in the percentage of tropical storms and intense hurricanes that developed from African easterly waves. These disturbances typically move across western Africa between 10o-15°N, and then propagate westward across the subtropical North Atlantic. During the peak of the hurricane season in August-September these easterly waves are in many cases the very systems which eventually intensify into tropical storms. However, the potential for this intensification is heavily influenced by two factors: the vertical wind shear (discussed in the previous section) and the structure/location of the low-level African easterly jet within which the disturbances move and evolve (Reed et al. 1977).
The easterly jet normally extends westward from western Africa to the central subtropical North Atlantic (Figs. 42a, b ) and reaches peak strength between the 600-700-hPa levels. This jet provides the "steering flow" for the easterly waves and is an important initial energy source for these disturbances as they propagate through the cyclonic shear zone (denoted by the region of red shading) along the southern flank of the jet (Reed et al. 1977). This cyclonic-shear zone is normally well-defined over the eastern tropical North Atlantic and western Africa between 8°-15°N, and overlaps the area of low vertical wind shear (Figs. 43a, b). The overlap is normally most extensive in September (Fig. 43b) during the climatological peak in the Atlantic tropical cyclone activity.
During August and September 1997, the African easterly jet was centered 2°-3° south of normal near 11°N. The jet was also broader than normal, with an abnormally weak meridional gradient in wind speed evident along its cyclonic-shear side (Figs. 42c, d). As a result, the primary region of cyclonic vorticity was displaced to south of 10°N in both months, a region generally considered too far south to favor efficient tropical cyclogenesis. Also during August 1997, the jet was weaker than normal and quite diffuse over the eastern tropical North Atlantic, with a relatively small region of cyclonic vorticity present. This area of weak cyclonic vorticity was displaced well south of the region of low vertical wind shear (Fig. 43c), with almost no overlap of the two features present. In September the easterly jet and accompanying cyclonic vorticity structure were better defined and extended farther west than normal (Fig. 42d). However, the overlap region of cyclonic relative vorticity and low vertical wind shear generally remained south of 10°N (Fig. 43d). Also during September, the vertical wind shear was much larger than normal across the central and western North Atlantic, further precluding any significant tropical development.
In contrast, during the active August and September 1995 period the African easterly jet was
well-defined and centered north of normal (approximately 1°-3° latitude) to between
15°-18°N (Figs. 42e, f
). Also, there was a strong meridional gradient in zonal wind in the region immediately south of the jet core in both
months, resulting in large areas of cyclonic relative vorticity covering the entire eastern North Atlantic between
10°-15°N. These conditions contrast with the near-absence of cyclonic vorticity at these latitudes during 1997.
Also, August and September 1995 featured an extensive overlap of the regions of large cyclonic relative vorticity
and low vertical wind shear between 10°-15°N across the central and eastern North
Atlantic (Figs. 43e, f
), compared with no overlap of these two features in this latitude band during 1997. This favorable location and
hori
zontal structure of the African easterly jet during August-September 1995, combined with its proximity to
the extended region of low vertical wind shear, contributed to recurring tropical cyclogenesis and intense
hurricane development from easterly waves throughout the period.
