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The suppressed Atlantic hurricane season featured 7 named storms (Figure 1), with a record low of one system forming during August-September and a tied record low of 3 systems forming during August-October. The eastern North Pacific season featured a near-average of 17 named storms, 9 of which became hurricanes and 7 of which became major hurricanes (Figure 2). Some of the prominent large-scale atmospheric conditions contributing to this suppressed 1997 North Atlantic season and to the comparatively active 1997 eastern North Pacific season are described in this Special Climate Summary. These conditions are contrasted with those accompanying the very active 1995 Atlantic hurricane season and the inactive 1995 eastern Pacific hurricane season.
The increase in tropical storm and hurricane activity over the eastern North Pacific during 1997 compared to the previous two years was partly related to an expanded area of low vertical wind shear (the change in winds with height), in response to weaker-than-normal upper-level easterly flow throughout the region. Another contributing factor was abnormally warm ocean waters (1 - 2 °C above normal) across the subtropical eastern North Pacific (Figure 3, bottom ), in association with the occurrence of very strong El Niño conditions [A detailed analysis of the current major El Niño, its impacts and outlooks can be found in the Climate Prediction Center's Special Climate Summary #97/3).
Over the North Atlantic, the suppressed activity during August - October 1997 was related to sustained high vertical wind shear across the Caribbean Sea and the western/ central subtropical North Atlantic, and to the development of only one tropical storm from African easterly waves during August and September. This minimal easterly wave development was partly related to an unfavorable location and structure of the 700-mb African easterly jet stream, and to the aforementioned high vertical wind shear.
The large-scale pattern of reduced vertical wind shear over the eastern North Pacific and normal-to-enhanced vertical wind shear over the western and central North Atlantic was also linked to the development of strong El Niño conditions. In contrast, opposite patterns of vertical wind shear over both basins during the 1995 and 1996 hurricane seasons were related to cold episode conditions in the tropical Pacific (known as La Niña). The direct impact of El Niño on changes in the location and dynamical structure of the African easterly jet during the 1997 Atlantic hurricane season is under investigation.
A brief overview of both the 1997 North Atlantic and eastern North Pacific hurricane seasons is presented in section 2. A general description of the variability in hurricane activity over the North Atlantic and eastern North Pacific during the past several years is presented in section 3 . The patterns of vertical wind shear during both the 1997 and 1995 hurricane seasons are then described in section 4. A diagnosis and comparison of the African easterly jet location and structure between the 1997 and 1995 seasons is presented in section 5 , and a summary follows in section 6 .
2. Overview of 1997 North Atlantic and eastern North Pacific hurricane seasons
According to the National Hurricane Center, the 1997 Atlantic hurricane season featured 7 named storms (normal is 9.3), 3 of which became hurricanes (normal is 5.8) (Figure 1). According to Dr. Chris Landsea, the net tropical cyclone activity was only 52% of normal for the season as a whole. The El Niño impacts on tropical storm activity were most evident during August-October, when only three systems developed. In fact, a record low [since the beginning of the aircraft reconnaissance era in 1944] of only one system formed during August and September, normally the months of peak tropical storm activity. These conditions contrast with the previous two very active Atlantic hurricane seasons, which featured 19 named storms in 1995, 11 of which became hurricanes and 13 named storms in 1996, 9 of which became hurricanes. During these two years the "net tropical cyclone activity" was 229% and 198% of normal, respectively.
The National Hurricane Center also notes that the 1997 eastern North Pacific hurricane season featured 17 named storms, 9 of which became hurricanes and 7 of which became major hurricanes (Figure 2). This compares with an average of 16 named storms, 9 of which typically become hurricanes and 5 of which typically become intense hurricanes. The 1997 season also featured an expanded area of tropical cyclone activity, with four systems moving well west of 135°W and two major hurricanes affecting North America.
3. Recent year-to-year variability in tropical storm and hurricane activity over both the North Atlantic and eastern North Pacific Ocean
There is substantial year-to-year variability in tropical storm and hurricane activity over both the North Atlantic and eastern North Pacific. One prominent atmospheric factor contributing to this variability is the vertical wind shear between the upper (200 mb) and lower (850 mb) levels of the atmosphere: strong vertical shear inhibits tropical storm intensification while weak vertical shear aids tropical storm intensification. A second contributor to the year-to-year variability of Atlantic basin tropical storm and hurricane activity is the location and intensity of the African easterly waves, which typically move across western Africa between 10o-15°N and 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 and the structure/ location of the low-level African easterly jet within which the disturbances move and evolve.
The El Niño/ Southern Oscillation (ENSO) is a known contributor to the year-to-year variability of vertical wind shear, and may influences the structure and location of the African easterly jet in some instances. The ENSO basically refers to an abnormal warming (known as El Niño) or cooling (known as La Niña) of the ocean waters across the central and eastern Tropical Pacific. Past research by Dr. William Gray at the Colorado State University has shown that El Niño often favors suppressed tropical storm activity over the North Atlantic by contributing to enhanced vertical wind shear, while La Niña often favors enhanced Atlantic hurricane activity by helping to reduce the vertical wind shear. Over the eastern North Pacific El Niño often favors an expanded area of tropical cyclone activity by reducing the vertical wind shear in that region, while La Niña favors suppressed tropical cyclone activity through enhanced vertical wind shear.
The very active ENSO cycle during the 1990's has contributed to large year-to-year variations in hurricane activity over both the North Atlantic and the eastern North Pacific. For example, the prolonged Pacific warm episode of 1991-1994 was accompanied by extremely low Atlantic tropical storm and hurricane activity. The cold-episode of 1995-1996 contributed to active 1995 and 1996 Atlantic hurricane seasons and to suppressed activity across the eastern Pacific. Subsequently, the transition to very strong warm episode conditions since April 1997 has contributed to reduced tropical storm activity over the North Atlantic and to a substantial increase in tropical storm and hurricane activity over the eastern North Pacific compared to the last two seasons.
4. Vertical wind shear during August-October 1997 and 1995
One of the most important atmospheric factors influencing tropical cyclone formation is the change in winds with height (termed vertical wind shear) between the upper (200 mb level) and lower (850 mb level) atmosphere. Weak vertical wind shear in this layer (less than approximately 8 ms-1) is required for tropical convective complexes to develop into tropical storms and ultimately into hurricanes, while stronger shear suppresses deep tropical convection and inhibits tropical cyclone formation.
Over the North Atlantic the vertical wind shear pattern during August - 8 November 1997 (Figure 4) reflected the persistence of high shear conditions across the Caribbean Sea, the Gulf of Mexico and most of the western and central subtropical North Atlantic, with favorable shear conditions (under 8 ms-1) confined to the eastern subtropical and central tropical regions. A vertical profile of the atmospheric winds over the Caribbean Sea region (Figure 5) 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 resulted from weak easterly winds throughout the depth of the troposphere in association with La Niña conditions.
The entire eastern subtropical North Pacific was dominated by low vertical wind shear during the 1997 hurricane season (Figure 4 top), with generally reduced wind shear throughout the primary region of tropical storm formation between 10°-17°N and 105°-125°W (Figure 4, bottom). During August - October 1997 this reduced vertical wind shear resulted primarily from an ENSO-induced collapse of the normal easterly winds in the upper atmosphere (Figure 6). These conditions contrast with the enhanced easterly winds and strong vertical shear observed during the suppressed 1995 season.
5. The African easterly jet and African wave disturbances
A second important contributor to Atlantic hurricane activity during August and September is the African easterly wave, which appears as a disturbance propagating westward from the southwestern coast of western Africa at approximately 12N°-15°N. During active hurricane seasons the easterly waves amplify while moving across the eastern and central North Atlantic, and are often the trigger for tropical storm formation. In contrast, inactive hurricane seasons often feature a lack of significant easterly wave development, resulting in reduced tropical cyclone activity.
An important flow characteristic affecting easterly wave development is the location and structure of the African easterly jet. This easterly jet normally extends westward from western Africa to the central subtropical North Atlantic (Figure 7, top) and reaches peak strength between the 600 mb-700 mb level. This jet provides the "steering flow" and is an important initial energy source for the easterly waves, which propagate through the cyclonic shear zone (denoted by the region of red shading in Figure 7) along the southern flank of the jet. This cyclonic-shear zone is normally well-defined over the eastern North Atlantic and western Africa between 8°-15°N, and overlaps the area of low vertical wind shear (Figure 8, top panels). The overlap is normally most extensive in September during the climatological peak in the Atlantic tropical storm and hurricane activity.
During August and September 1997, the African easterly jet was shifted south of normal to an average latitude of 11°-12°N (a 2°-3° latitude southward shift), with the primary area of cyclonic vorticity confined to the region south of 10°N (Figure 7, middle). This latitude is generally considered too far south to favor efficient tropical cyclogenesis. Also during August the north-south variations in wind speed across the easterly jet were much weaker than normal over the eastern North Atlantic, with only a small region of cyclonic vorticity present. This area of weak cyclonic vorticity was located south of the region of low vertical wind shear (Figure 8, middle left ), with no substantial overlap of the two features evident. In September the easterly jet and accompanying cyclonic vorticity structure were better defined and extended farther west than normal. However, the overlap region of cyclonic relative vorticity and low vertical wind shear generally remained equatorward of 10°N (Figure 8, middle right ). Also during September, the vertical wind shear was much higher than normal across the central and western North Atlantic, thus further precluding any significant tropical development.
Thus, it appears that the location of the African easterly jet was unfavorable for substantial easterly wave development in both August and September 1997. Also, the nearly complete separation of the region of weak cyclonic vorticity and low vertical wind shear during August, and the high vertical wind shear across the western and central North Atlantic during September, represented a further degradation of normally favorable conditions for easterly wave development.
In contrast, during both August and September 1995 the African easterly jet was well-defined and shifted northward to 15°-18°N (1°-3° latitude north of normal) (Figure 7, bottom ), approximately 4°-6° latitude north of its 1997 counterpart. This northward shift in the mean jet location allowed for large values of cyclonic vorticity to cover the entire eastern North Atlantic between 10°-15°N during the period, compared to a near-absence of cyclonic vorticity at these latitudes during 1997. Also, both months featured an extensive overlap between the regions of large cyclonic relative vorticity and low vertical wind shear that extended across the central and eastern North Atlantic (Figure 8, bottom ). Thus both the location and horizontal structure of the 1995 easterly jet were favorable for easterly wave amplification.
The El Niño contributed to markedly reduced tropical storm and hurricane activity over the North Atlantic during August - October 1997, and to an enhanced area of favorable conditions for tropical cyclone and hurricane formation over the eastern North Pacific.
Over the eastern North Pacific the large increase in tropical cyclone and hurricane activity during the 1997 season compared to the 1995 and 1996 seasons was partly related to an expanded area of low vertical wind shear in response to anomalous upper-level westerly flow throughout the region, and to abnormally warm sea-surface temperatures (1-2°C above normal) across the region. These conditions contrast with the high vertical wind shear and near-normal sea-surface temperatures over the eastern subtropical North Pacific during the suppressed 1995 and 1996 seasons when La Niña conditions were present.
Over the North Atlantic, the decrease in tropical storm activity during 1997 was most prominent during August - October, when only three systems developed. This suppressed activity was related to high vertical wind shear across the Caribbean Sea and the western/ central subtropical North Atlantic throughout the period, and to a lack of tropical storm development from African easterly waves during August and September. This lack of easterly wave development was related to 1) a southward shift of the 700-mb African easterly jet stream, resulting in less cyclonic vorticity available for easterly waves moving off the African coast between 10°-15°N, and 2) to the high vertical wind shear farther west. In contrast, the active 1995 Atlantic hurricane season featured a high rate of easterly wave development during August and September, in response to 1) very low vertical wind shear across the central and western North Atlantic, and 2) a northward shift of the African easterly jet stream so that its accompanying region of cyclonic vorticity strongly overspread the large area of low vertical wind shear between the critical 10°-15°N region within which the easterly waves typically amplify.
It is generally accepted that the changes in vertical wind shear between the 1997 and 1995 seasons over both the North Atlantic and North Pacific are linked to the ENSO cycle. The relationship (if any) between ENSO and changes in the location and structure of the African easterly jet between the two years is not clear and requires further investigation.