4. Regional climate highlights
1) The 1998 North Atlantic and eastern North Pacific hurricane season
i) Overview
The North Atlantic hurricane season runs from June through November and exhibits a peak in activity from AugustOctober, primarily in response to systems developing from African easterly wave disturbances. The 1998 hurricane season was extremely active and one of the deadliest in history with 11,629 lives claimed. The season also featured the strongest October hurricane (Mitch) on record, which claimed more than 11,000 lives in Central America. The last time that a single Atlantic hurricane caused so many deaths was 1780.
Overall, 910 named tropical storms are observed over the North Atlantic in an average season, with 5-6 becoming hurricanes and 2 reaching intense hurricane status [measured by a category 3, 4, or 5 on the Saffir-Simpson scale (Simpson 1974)]. The 1998 hurricane season featured 14 named storms (Fig. 32), with 9 of these systems becoming hurricanes and 3 reaching intense hurricane status. These conditions are in marked contrast to the suppressed 1997 hurricane season, which featured 7 named storms (Fig. 32), with 3 of these systems becoming hurricanes and only 1 reaching intense hurricane status.
Most of the storms during the 1998 season developed during the 35-day period of 19 August23 September when ten tropical storms formed (7 of which became hurricanes). During the peak of activity on 25 September, there were four Atlantic hurricanes in progress at the same time. This is the first time since 1893 that such an event has occurred. In contrast, August-September 1997 featured the formation of only one hurricane (Erica) over the entire North Atlantic basin, which is a record low for the period since the beginning of the aircraft reconnaissance era in 1944.
Other aspects of the 1998 Atlantic hurricane season included an exceptionally late start to the tropical storm activity, with the first tropical storm (Alex) developing on 27 July. The season also featured a continuation of activity well into November, with the final system of the season (Nicole) becoming a tropical storm on 24 November and a hurricane on 29 November.
Over the eastern North Pacific the 1998 hurricane season featured slightly below-normal activity, with 13 named storms (normal is 16), 9 of which became hurricanes (normal is 9). The season also featured a reduced area of tropical cyclone activity compared to normal, particularly along the west coast of North America where only one system (Hurricane Isis) made landfall in Mexico. Normally, 3-4 tropical storms impact this region during the eastern Pacific hurricane season.
Tropical storm and hurricane activity over both the North Atlantic and eastern North Pacific ocean basins is strongly affected by the vertical wind shear between the upper (200 hPa) and lower (850 hPa) levels of the atmosphere. Strong vertical shear inhibits tropical cyclogenesis while weak vertical shear (less than approximately 8 m s-1) favors tropical cyclogenesis and possible hurricane development.
During June-July 1998, the lack of tropical cyclone activity over the North Atlantic and Caribbean Sea was linked to very strong vertical wind shear throughout the region (Fig. 33a), with larger-than-normal values dominating the region off the southeastern coast of the United States, the central subtropical Atlantic, and central Caribbean Sea (Fig. 33c ). The high wind shear off the eastern seaboard of the U. S. was linked to enhanced upper-level westerly winds along the eastern flank of a very strong subtropical ridge that dominated the southern tier of the United States from April through mid-July [see section 4a(2), Fig. 40b]. This amplified ridge was directly linked to intense convection over the eastern equatorial Pacific in association with strong El Niņo conditions (see section 3). Farther east, the high wind shear over the central subtropical Atlantic was linked to enhanced upper-level southwesterly flow in association with a stronger-than-normal tropical upper-tropospheric trough (TUTT) situated immediately downstream of the subtropical ridge. These conditions contrasted with the low shear observed across the western Atlantic during June-July 1997 (Figs. 33b, d) in association with a reduced strength of the TUTT. Three tropical depressions formed in this region during that period.
Farther south during June-July 1998 lower-than-average shear values in the vicinity of
the subtropical ridge axis covered Mexico, the southern Gulf of Mexico, and the northern
Caribbean. Nonetheless, the actual shear values in these regions, which far exceeded 8 m s-1,
combined with strong upper-level convergence and sinking motion (not shown), inhibited
tropical cyclone formation during the period.
The inactivity during June-July 1998 was followed by an extremely active August and September, with 10 tropical storms (7 of which became hurricanes) forming during the period. This transition to well above-normal activity during the climatologically active part of the season contrasts to the previous year, when only one Atlantic basin hurricane formed during August-September 1997.
Climatologically, tropical storms and hurricanes often develop from African easterly waves during August-September. These wave disturbances move across western Africa between 10o-15°N and then propagate westward across the subtropical North Atlantic. However, the potential for tropical storm development within the easterly waves is heavily influenced by two factors: the vertical wind shear (Gray 1984) and the structure/ location of the low-level African easterly jet within which the disturbances move and evolve (Reed et al. 1977). The coherent interannual variations in these two fields is discussed by Bell and Halpert (1998) and Bell and Chelliah (1999).
Climatologically, the region of low vertical wind shear during August-September extends from Africa to northwestern South America (Fig. 34a), with the lowest shear values centered near 10°S over the central North Atlantic. High vertical wind shear typically dominates much of the subtropical North Atlantic and the Caribbean sector, making these regions unfavorable for tropical cyclogenesis. In contrast, active hurricane years such as 1998 (Fig. 34c ) feature low vertical wind shear across the entire North Atlantic between 10°-15°N and throughout the Caribbean sector (see also Landsea et al. 1998; Bell ad Halpert 1998; Bell and Chelliah 1999). A vertical profile of the atmospheric winds over the Caribbean Sea region during August and September 1998 (blue curves in Figs. 35a, b) shows that the reduced vertical wind shear resulted from the development of easterly winds at upper levels and from anomalous westerly winds at lower levels. Similarly, the extremely active August-October 1995 season was associated with very low vertical wind shear across the Caribbean throughout the three-month period (green curves).
In contrast, both the inactive August-October 1997 (Fig. 34e) and October 1998 periods featured an expanded region of high wind shear across the western North Atlantic and Caribbean, and a confinement of low shear values to the central and eastern Atlantic. A vertical profile of the atmospheric winds over the Caribbean Sea region during both periods (Figs. 35ac, and 35c, respectively) indicates that the enhanced vertical shear resulted primarily from strong upper-level westerly winds across the region, which is a reversal in wind direction at these levels from that observed during the active periods.
There are also considerable changes in the location and structure of the African easterly jet between active and inactive hurricane years (Bell and Halpert 1998; Bell and Chelliah 1999). This easterly jet normally extends from western Africa to the central subtropical North Atlantic (Fig. 34a), with the jet core located near 15°N. The jet reaches peak strength between the 600-700-hPa levels and provides the "steering flow" for the easterly waves. It is also an important initial energy source for the easterly waves, which propagate through the cyclonic shear zone (Fig. 34b) 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 (Fig. 34b ). This overlap is normally most extensive in September during the climatological peak in the Atlantic tropical cyclone activity.
During the active August-September 1998 period (Fig. 34c) the African easterly jet was well-defined and shifted approximately 2.5° north of its normal position. It was also stronger than normal across virtually the entire subtropical North Atlantic, with an increased meridional gradient in zonal wind in the region immediately south of the jet core. These conditions resulted in large areas of cyclonic relative vorticity covering the entire central and eastern North Atlantic between 10°-15°N (Fig. 34d ). Also, there was a substantially increased 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, particularly when compared to that associated with the climatological mean and the inactive 1997 period (Fig. 34f). This favorable location and horizontal structure of the African easterly jet during August-September 1998, 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.
In contrast, the African easterly jet during the inactive August-September 1997 period was centered near 12.5°N over the eastern Atlantic (Fig. 34e), which is 2°-3° south of normal and approximately 5° south of its mean 1998 position. The jet core was also weaker and more diffuse than normal, with a weaker meridional gradient in wind speed evident along its cyclonic-shear side. As a result, the primary region of cyclonic vorticity was displaced to south of 10°N (Fig. 34f), which is generally considered too far south to favor efficient tropical cyclogenesis. This area of weak cyclonic vorticity was also displaced well south of the region of low vertical wind shear, with comparatively little overlap of the two features present. This combination of both an unfavorable structure and location of the African easterly jet, along with high vertical wind shear across the western Atlantic and Caribbean, resulted in suppressed development of the African easterly waves and a dearth of tropical cyclogenesis during the period.
2) April-June 1998 U. S. drought in the South; Flooding in the Midwest and Northeast
Extreme dryness during April-June 1998 covered much of the south-central and southeastern United States (Fig. 36), with record low statewide rainfall totals dating back to 1895 observed in New Mexico, Texas, Louisiana, and Florida. Many locations across Florida and Louisiana received under 150 mm of rain during the period, which is less than half the long-term average. Broad sections of Texas and New Mexico received 25-100 mm of rain, which is less than 25% of the long-term average. This dryness was a dramatic change from the surplus rainfall observed in much of these areas from late 1997 through March 1998.
The dryness was accompanied during May-June by record heat in Texas, Louisiana, Arkansas, and Florida (Fig. 37), with temperatures averaging 2°-4°C above normal across most of the region. Temperatures across much of Texas and parts of Florida, Georgia, and Alabama reached or exceeded 35°C on more than 50% of the days during May-June (Fig. 38). These conditions resulted in widespread drought from New Mexico to Florida, with the most severe drought occurring in Texas and Florida.
In Florida, one consequence of these extreme conditions was the development of widespread wildfires during June-early July 1998 in all 67 counties. By 5 July, 483,000 acres and 356 structures had been consumed by fires, resulting in an estimated $276 million in damages according to the Florida State Emergency Operations Center (personal communication). Over 100,000 people had been ordered to leave their dwellings during this period, including the entire population of Flagler County. Increased rainfall and humidity over the 4 July weekend allowed firefighters to make progress toward controlling several large wildfires. Additional control of the fires, particularly those in Flagler County, came on 6-7 July as widespread rainfall covered central Florida. Wildfires also occurred in Texas during May-June and burned an estimated 143,000 acres, according to the Texas Emergency Response Division (personal communication).
In contrast, above-normal rainfall covered much of the western, midwestern, and northeastern portions of the country during April-June (Fig. 36), with totals exceeding 400 mm in most of the primary cities in the Ohio Valley, Tennessee Valley, central Appalachians, and lower Northeast (Fig. 39). Record high statewide rainfall totals (dating back to 1895) were observed in Rhode Island and Massachusetts, while the third and fourth highest totals in the record were observed in Tennessee and Iowa, respectively. The rains during May and June were often associated with strong thunderstorms, including tornadoes, hail, damaging winds, and flash floods. According to data from the National Severe Storm Prediction Center (personal communication) 372 tornadoes were recorded during June over the nation as a whole, which is nearly 200 more than average. The rainfall was particularly intense in some states during June, which was the second wettest June in 104 years in Massachusetts, New Hampshire, Rhode Island, and Vermont, and the third wettest in Iowa and Maine.
The anomalous temperature and rainfall patterns observed across North America during April-June were essentially manifestations of an exceptionally persistent, large-scale atmospheric circulation pattern (Fig. 40). Primary features of this pattern included: 1) a pronounced amplification of the subtropical ridge across the eastern North Pacific, Mexico, and the south-central United States (Fig. 40a ); 2) increased jet stream winds [1.5-2 times normal] (Fig. 40b) and increased storminess across the central and northern United States; 3) an amplified upper-level trough over the western United States, which acted as a continuous source region for the storms; and 4) above-normal heights and weaker-than-normal jet stream winds across Canada, which helped to concentrate the jet stream and storminess over the central United States.
All of these features were linked to the 1997-98 El Niņo and many were prominent aspects of the atmospheric circulation since January. In fact, one difference in the atmospheric circulation features between January-March (see Figs. 29c, d ) and April-May was simply an overall poleward shift of the anomaly centers in the Northern Hemisphere in association with the normal progression of the seasons. Another difference was the establishment of the amplified ridge directly over Mexico and the southern United States, which resulted from a collocation of El Niņo-related positive height anomalies with the development of the climatological mean ridge over Mexico. Accompanying these conditions was a corresponding northward movement in the regions of hot and dry conditions from Central America and southern Mexico [see section 4a(3)] to northern Mexico and the Gulf Coast States, and in the regions of strong storminess, increased rainfall, and severe weather from the southern states to the central and northern sections of the U. S..
The links between the above circulation features and the 1997/98 El Niņo are further highlighted in Figs. 40, 41 , and 42. The enhanced subtropical ridge during April-June 1998 was associated with a large-scale pattern of above-normal heights that spanned the entire Tropics and subtropics of both hemispheres (Fig. 40a). These areas of above-normal heights are consistent with the strong convection that prevailed across the eastern half of the tropical Pacific for most of the period (Figs. 27a, b). Moreover, a remarkably coherent and symmetric pattern of height, temperature, and wind anomalies was evident over the eastern Pacific in the subtropics and extratropics of both hemispheres during the period (Fig. 41). Thus, the amplified subtropical ridge reflected only one of many components of the large-scale atmospheric circulation which remained linked to the patterns of anomalous tropical convection during the period. During June, the continuation of an enhanced subtropical ridge across Mexico and the southern United States was also consistent with a continuation of enhanced tropical rainfall (Fig. 42a) and abnormally warm sea surface temperatures (Fig. 42b) over the extreme eastern tropical and subtropical North Pacific. Thus, while SSTs had cooled significantly in the central equatorial Pacific by June, residual warm waters in the extreme eastern Pacific helped to maintain the amplified subtropical ridge over the southern United States and Mexico.
3) The 1997/98 Mexican Drought
In Mexico the July 1997 through June 1998 period was the driest in the historical record dating back to 1945 (Fig. 43), with below-normal rainfall observed in every month except November 1997. During March-June 1998 this dryness, in combination with prolonged periods of extreme heat, led to an intensification of drought conditions which culminated in widespread forest fires. The onset, intensity, and duration of the Mexican drought were also linked to the strong 1997-98 El Niņo.
The onset of the drier-than-normal conditions occurred during JJA 1997 (Fig. 44a ), in association with the establishment of strong El Niņo conditions. During this period rainfall totals averaged 20%-60% of normal over much of the country, with the largest deficits observed in the southern and eastern sectors. This dryness is attributed to a substantially weakened monsoon trough and a southward displacement of the Inter-tropical Convergence Zone (ITCZ). In the north, the generally drier-than-normal conditions were linked to a persistent upper-level ridge that was embedded within an anomalous large-scale circulation pattern across the United States and Mexico.
During SON 1997 drier-than-normal conditions persisted across large sections of interior Mexico (Fig. 44b), in response to an early retreat of the summer monsoon. These conditions were associated with an El Niņo-related strengthening of the subtropical ridge from the central North Pacific to the Caribbean Sea (see Bell and Halpert 1998, their Fig. 27d). In contrast to this dryness, the Baja Peninsula recorded above-normal precipitation during the season, in response to rains associated with hurricane Nora in September. Portions of southern Mexico also recorded above-normal precipitation during the season, in response to three tropical systems during October and November.
An unusual aspect of the Mexican drought was the continuation of substantially below-normal rainfall during DJF 1997-98 across most of the country (Fig. 44c), despite the continuation of strong El Niņo conditions. Historically, above-normal winter rainfall is observed across central and northern Mexico during these episodes (Ropelewski and Halpert 1986), which helps to alleviate the rainfall deficits that typically develop during the previous summer and fall.
This continuation of drought conditions during DJF 1997-98 was related to three distinct changes in the large-scale flow and vertical motion field. First, much of Mexico was situated slightly upstream of the mean upper-level trough axis in a region of descending motion (see Fig. 29c). This feature represented an extreme eastward shift of the mid-Pacific trough to the Americas, and was consistent with previously described El Niņo-related changes in the upper-level height field across the subtropical North Pacific (see section 3c, Figs. 29, 30). This positioning of Mexico with respect to the upper-level trough differs from the climatological mean, in which the country is situated downstream of the mid-Pacific trough in a region of broad southwesterly flow and ascending motion.
Second, Mexico also experiences ascending motion in the climatological mean due to its location in the right entrance region of the subtropical jet. However during DJF 1997-98, this jet entrance region was exceptionally ill-defined (see section 3c), and did not favor ascending motion over much of the country. Third, dryness during the period also resulted from reduced storminess across Mexico, in response to a very intense wintertime jet stream that kept the main storm track over the southern tier of the United States.
Although the MAM season is normally dry in Mexico, precipitation totals during MAM 1998 were less than 20% of normal throughout most of the country (Fig. 44d). This dryness was directly related to strong subsidence beneath the highly amplified subtropical ridge (Figs. 27b , 40). The dryness during May and June reflected a 4-6 week delay in the onset of monsoon rains and culminated in widespread forest fires which peaked in May and early June 1998. Accompanying these conditions, extreme drought also developed across the Gulf Coast states of the United States during May-June 1998 [see section 4a(2)]. The Mexican drought eased considerably during July and August, as the ITCZ shifted northward and the monsoonal rains became reestablished in association with the development of La Niņa conditions.
