Storms

We now examine some attempts to link storm numbers and storm track locations to solar activity. The number of both tropical cyclones and thunderstorms has increased and decreased with time and location as a function of solar activity. In fact, an early correlation between the number of Indian cyclones and solar activity proved so startling it caused an explosion of related research. In the previous century, tropical cyclones were called hurricanes or typhoons. Today tropical cyclones refer only to the weaker tropical storms with sustained winds above 31 miles per hour. Here, tropical cyclones refer to the stronger storms like those in the previous century. Anywhere from 1 to about 30 hurricane-strength storms can form each year. Among other factors, formation of these storms requires oceanic water temperatures above 26 °C (79 °F). William Gray at Colorado State University has successfully predicted the number of Atlantic Ocean hurricanes each year. This number is a function of the equatorial wind direction, the sea-level air pressure in the Caribbean, the strength of the westerly winds near the top of the lower troposphere, the presence or absence of an El Niño current, and, particularly, the amount of rainfall in the Sahel in Africa. Earlier we noted that increased solar activity produces a corresponding increase in rainfall in some regions. Figure 6.4 indicates that increased rainfall in the Sahel is expected, so based on this expectation and Gray’s theory, hurricanes should increase in number. Higher solar activity and a higher solar irradiance can also be expected to increase the tropical ocean temperatures by a few tenths of a degree. These increased water temperatures tend to increase both the number of tropical cyclones and their intensity. Figure 7.1 illustrates the number of Atlantic Ocean hurricanes observed between 1962 and 1994 as a function of the sea-surface temperatures (SST). A sharp gradient exists in the number of storms produced between 23 and 25 °C. In some regions, even a small increase in SST can lead to sharp increases in the number of tropical cyclones. Changes in solar brightness on the 11-year time scale could be expected to cause a corresponding cycle in the number and strength of tropical cyclones.