Tropical Storm Frequency Research Articles

Downward trends in the frequency of intense at Atlantic Hurricanes during the past five decades

There is concern that the enhanced greenhouse effect may be affecting extreme weather events such as tropical cyclones. The North Atlantic basin offers a reliable, long‐term record of tropical cyclone activity, though it may not be representative of tropical cyclones throughout the rest of the tropics. The most recent years of 1991 through 1994 have experienced the quietest tropical cyclone activity on record in terms of frequency of tropical storms, hurricanes, and intense hurricanes. This was followed by the 1995 hurricane season, one of the busiest in the past 50 years. Despite 1995's activity, a long‐term (five decade) downward trend continues to be evident primarily in the frequency of intense hurricanes. In addition, the mean maximum intensity (i.e., averaged over all cyclones in a season) has decreased, while the maximum intensity attained by the strongest hurricane each year has not shown a significant change.

Historical Developments Leading to Current Forecast Models of Annual Atlantic Hurricane Activity

There is considerable interest in forecasting interannual hurricane activity for the Atlantic basin. Various predictors representing different components of the tropical Atlantic climate have been suggested. The choice of predictors is based on previous research into contemporaneous and lag relationships with seasonal hurricane and tropical storm frequency. Past research is divided into five distinct periods: the search for physical relationships, the use of composite charts, the use of satellite imagery and climatology of easterly waves, the emergence of recent ideas, and the development of prediction models. As an historical summary this paper describes the important research contributions in each period leading to our current understanding of yearly hurricane variability. The paper concludes by describing current methods for forecasting this variability and recommends an area for future investigations.

Mesoscale air‐sea interactions related to tropical and extratropical storms in the Gulf of Mexico

Data indicate that the climatological vorticity of the lower atmosphere of the northwestern Gulf of Mexico is relatively strong, with monthly means exceeding a Rossby number of 3 from October through March, with a maximum of 4.6 in December. It has been hypothesized that wintertime cyclogenesis over the northwestern gulf is enhanced by this lower level atmospheric vorticity field when it occurs simultaneously with appropriate upper level venting. Observations from November 1982 to mid‐February 1983 were studied in which seven significant cyclones were generated in the northwestern gulf. It was found that all seven storms occurred when the vorticity correlate of horizontal air temperature difference was ∼3–5°C above the climatological mean difference. Although these results corroborate the postulate that such lower level vorticity is related to cyclogenesis, the data from mid‐February through March 1983 were less supportive. During those months, only two of the six significant storms developed when the horizontal air temperature difference was above the climatological mean. Surprisingly, the location of the most unstable air within the atmospheric boundary layer is not found in the northwestern Gulf of Mexico but instead along the shelf break. Relatively strong free convection in the atmospheric boundary layer is found over the Loop Current and the deep western gulf as expected. The unstable regions of the deep gulf can enhance the potential for frontogenesis and cyclogenesis, but it is likely that the linear character of the strong shelf break free convection does not enhance cyclogenesis. It is shown that a maximum in the frequency of tropical storms within the Gulf of Mexico exists some 275 km south of the Mississippi delta at 27°N, 90°W. This maximum is a result of only those storms which originate within the gulf. Two plausible effects of the Loop Current and its rings on tropical storms are discussed. One is that these ocean features are large and consolidated heat and moisture sources from which a nearby, slowly moving atmospheric disturbance can extract energy. The second is that of the cyclonic vorticity that can be generated in the lower atmosphere by such oceanographic features.

Global environmental change issues in the Western Indian Ocean region

Mounting evidence from both instrumental and proxy records shows global climate continues to change. Analysis of near-surface temperatures over land and oceans during the past 130 years shows marked warming during the first half of this century with relatively steady temperatures through the mid-1970s followed by a rapid warming during the 1980s. The source of this warming is unclear at present. The warmest decade in the recent record is the 1980s with some of the most pronounced warming occurring in the lower latitudes, including the Western Indian Ocean. In the context of this study, the important consequences of climate warming are: (1) the potential impacts associated with rising sea level due to thermal expansion of the oceans; (2) melting of land-based ice sheets and glaciers; (3) the increased frequency, intensity, and seasonality of tropical storms and the monsoon season; and (4) changes in local land-use practices. Rising sea level, coupled with meteorological changes, creates a potential for increased coastal erosion and loss of coastal habitats such as wetlands, mangroves, and coral reef communities. These potential impacts may affect future land-use and development practices. For example, the curtailment of tourism could alter economic growth and development. Forecasting the potential regional and local impacts of global warming is not a trivial problem. General circulation models presently do not provide the necessary finescale resolution required for ascertaining such smaller-scale effects. In addition, there is little agreement between the forecasts from different models of past, present, or future patterns on such fundamental climate variables as precipitation and air temperature. The absence of quality regional and local tide-gauge data of the appropriate duration necessary for calculating changes in the relative sea level exacerbates the existing theoretical uncertainty. Nevertheless, the potential impacts do represent present-day problems resulting from the alteration and acceleration of naturally occurring processes through man's activities. These types of future problems which typify the impacts forecast as a result of global warming are occurring now, partly because of human activities. Such insular impacts in the Western Indian Ocean region should be used by local governments as case studies for educating the populace, developing remedial activities, analyzing socioeconomic patterns, and formulating long-term management and policy strategies that will minimize unwanted future changes. Finally, a regional strategy is needed to address the following issues: (1) the acquisition and interpretation of tide-gauge and other geodetic data to provide estimates of the local and regional relative sea-level rise ; (2) a plan to identify and suggest possible mitigation for regional and local activities (land use, economic development practices, industrial development etc.) that may directly contribute to enhanced global warming through the release of carbon dioxide and other radiatively active gases; (3) the development of regional and local policies (e.g. land use, coastal zoning, building setbacks, etc.) to minimize the impacts resulting from extant and possible future relative sea-level rise and meteorological changes; and (4) the linkages between potential changes in global (regional) climate and present anthropogenic stress upon the environment—that is, cumulative impact assessment. The challenge is to recognize the need for regional and local action in the absence of site-specific data, or scientific certainty.

The Greenhouse Effect, Climatic Change and Rising Sea Level: Implications for Development

The 'greenhouse effect' is likely to change the global climate substantially, with serious implications for the developing countries. By the year 2030, it is estimated that the global-mean surface temperature may be about 1-20C higher than today. Given the range of uncertainty in future greenhouse gas emissons, the sensitivity of climate and the rate of oceanic heat uptake, the warming could be as little as 0-50C or as high as 2-50C. By 2030, the corresponding rise in global-mean sea level is estimated to range from 5-44 cm, with a best estimate of 17-26 cm higher than today. The developing countries, so far minor contributors to the increase in atmospheric concentrations of greenhouse gases, are quickly becoming major contributors due to growth in both population and energy consumption. This implies that developing countries must be actively involved in any international agreements to reduce greenhouse gas emissions in order to slow global warming. Moreover, in terms of impacts, the developing countries may be among the most vulnerable countries of the world. Although the regional details of climatic change cannot yet be predicted accurately, there is particular concern about possible changes in the frequencies of climatic extremes, in monsoon regimes, and in the magnitude and frequency of severe tropical storms. A rise in global sea level could exacerbate problems of flooding, salinization and erosion already being experienced in many parts of the developing world. There are compelling reasons to initiate actions to reduce the growth rates of greenhouse gas emissions and slow global warming.

Month-to-Month Variability of the Atlantic Tropical Circulation and Its Relationship to Tropical Storm Formation

Abstract Monthly mean winds have been derived from 200 mb and Analysis of the Tropical Oceanic Lower Layer (ATOLL) winds over the southern North Atlantic, Caribbean, Gulf of Mexico and eastern Pacific during the hurricane seasons (June-November) of 1975 through 1985. After removal of the seasonal cycle, the winds are expressed in terms of empirical orthogonal functions. The dominant mode of variability for the combined 200 mb/ATOLL circulation strongly resembles part of a Walker cell confined near the equator. This mode is strongly correlated with El Nin index (Weare, 1986), and is associated with the.El Nin/Southern Oscillation. A positive (El Nin-like) index tends to be associated with more anticyclonic vorticity at the ATOLL level in the tropics and increases in the vertical shear between about 10° and 30°N. This circulation is unfavorable for tropical storm formation. Correlations are derived between the monthly mean winds and monthly tropical storm frequency in the Atlantic basin. Contemporaneous cor...

The effects of a cyclonic storm on coral reef fish assemblages

Visual censusing techniques were used to monitor the fish assemblages on eight shallow water coral patch reefs at Lizard Island (northern Great Barrier Reef) over a 27 month period. Larval settlement was distinctly seasonal with peaks occurring in December or January. Cyclones are most prevalent at this time of year. Three cyclones passed close to Lizard Island during the study period. Fish assemblages were censused before and immediately after one cyclone that struck at the time of peak larval settlement. The cyclone had little effect on adults but caused high juvenile mortality and re-distribution of sub-adult individuals. Because settlement strongly influences the diversity and density of adults, cyclones have marked effects on the fish assemblages as a whole. The high frequency of tropical storms in many coral reef areas and the depths to which their effects penetrate suggest that physical disturbances may be an important determinant of coral reef fish assemblage structure.

Tropical Storm Frequencies Related to Sea Surface Temperatures

A relationship is identified between the area of the North Atlantic with surface temperatures greater than 26.8°C and the frequency of tropical storms and hurricanes. The relationship is direct and near-exponential. When the mean monthly area with sea temperature of 26.8°C is less than about 8.5 × 106 km2, tropical storms and hurricanes do not occur. Estimates of tropical storm frequency based on the above relationship are made for several episodes of the Holocene.

A Preliminary Analysis of Factors Affecting the Frequency of August Southeastern worth Pacific Tropical Storms and Hurricanes Since the Advent of Satellite Observations

Satellite-augmented observations of tropical storm and hurricane frequencies in the southeastern North Pacific during recent Augusts (1966–1974) were compared to various environmental factors. On a regional scale, a relatively strong mean 700 mb ridge from the Gulf of Mexico to Baja California was found to accompany high storm frequency. The linear correlation coefficient between storm frequency and a measure of the strength of this ridge amounted to 0.77. On a larger scale, the 700 mb Subtropical Westerlies Index (20°N to 35°N) for west longitudes from 0° to 180° had a somewhat stronger relation to storm frequency (r=0.86). Average August sea surface temperature in the vicinity of storm formation was poorly correlated with storm frequency. This suggests that the unfailingly warm August water temperatures in this area— always exceeding 27°F—were not a limiting factor in storm development.

Fluctuations in Frequency of Tropical Storms

Fluctuations in Frequency of Tropical Storms