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North Atlantic hurricanes constitute a threat to both life and property. The warm seas found in tropical low-latitudes provide a breeding ground for hurricanes, with nearly continuous heat and moisture fluxes into near-surface air. Traditionally, the sun's role in hurricane climate studies is acknowledged as a time-marker for ocean heat content, with calendar date predicting hurricane frequency and intensity. However, a series of investigations into a different type of sun-hurricane relationship has uncovered a link between solar activity and hurricane intensity and frequency. High solar activity at a daily timescale is understood to weaken hurricanes in the southwest Atlantic yet correspond to increased hurricane intensity in the southeast Atlantic. At a seasonal timescale, high solar activity is shown to correspond with fewer U.S.-landfalling hurricanes. A gap in the knowledge exists on how and where solar activity influences seasonal hurricane frequency over and within the North Atlantic basin. This study is quantitative featuring exploratory analysis and inferential modeling, with diagnosis and prediction of the sun-hurricane count relationship over space being the primary contribution to science and society. It is carried out via exploratory data analysis and statistical modeling. Hurricane and climate data are binned in equal-area hexagon regions. Count differences for periods of high solar activity (i.e, high sunspot number) feature fewer hurricanes across the Caribbean, Gulf of Mexico, and along the eastern seaboard of the United States when sunspots are numerous. In contrast, fewer hurricanes are observed in the central North Atlantic when sunspots are few. The sun-hurricane connection is as important as the El Ni\~no Southern Oscillation toward statistically explaining regional hurricane occurrences. Regression results indicate a 30\% reduction in probability of annual hurricane occurrence for southeastern Cuba, the southern Bahama islands, Haiti, and Jamaica when the September sunspot number is 115 sunspots. In contrast, hurricane risk in regions of the southeastern Atlantic is predicted to more than double when the September sunspot number is 160 sunspots. Regions within the southwest Atlantic indicate a negative relationship. A physical explanation for the eastern basin increase in counts and count probability is still unclear. Additional warming of the sea surface in these regions from increased solar activity would lead to increased hurricane frequency. However, the sea-surface temperature response to solar activity appears marginal. Future work will address potential explanations including circulation changes to African weather systems in response to changes in solar activity, and earlier hurricane development leading to more storms leaving the deep tropics and tracking into these regions due to coriolis effects on the storm. The study can be expanded to include storms worldwide.
A Dissertation submitted to the Department of Geography in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
James Elsner, Professor Directing Thesis; Robert Hart, University Representative; Victor Mesev, Committee Member; Tetsuo Kobayashi, Committee Member; Thomas Jagger, Committee Member.
Florida State University
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