Our interest here is a network of hurricanes affecting the United States. First we take an exploratory look at the data that will be used in creating the network. A chronological list of all hurricanes that have affected the continental United States in the period 1851-2005, updated from Jarrell et al. (1992) is available from the U. S. National Oceanic and Atmospheric Administration (NOAA) at http://www.aoml. noaa.gov/hrd/hurdat/ushurrlist.htm. We use the May 2006 version of the data.
A hurricane is a tropical cyclone with maximum sustained (one-minute) 10 m winds of 65 kt (33 m/s) or greater. Hurricane landfall occurs when all or part of the storm's eye wall passes directly over the coast or adjacent barrier islands. Since the eye wall extends outward a radial distance of 50 km or more from the hurricane center, landfall may occur even in the case where the exact center of lowest pressure remains offshore. We also include hurricanes that do not make direct landfall, but produce hurricane-force winds at the coast. A hurricane can affect more than one region as hurricanes Andrew (1992) and Katrina (2005) did in striking southeast Florida and Louisiana.
Here it is assumed that the data on hurricanes affecting the United States are complete back to 1899, but less so in the interval 1851-1898. Since we are interested in multiple strikes rather than trends over time the fact that a few hurricanes may have been missed or that a few multiple hit storms are counted only as single hits will not materially influence the network.
The record contains 275 hurricanes affecting the United States in the period 1851-2005. Regions are divided along state lines from Texas to Maine, but Texas is further divided into south, central, and north Texas and Florida is further divided into four regions including northwest, southwest, southeast, and northeast Florida.
This gives a total of 23 non-overlapping regions. The state two-letter abbreviation is used. South, central, and north Texas are denoted ATX, BTX, and CTX, respectively. Northwest, southwest, southeast, and northeast Florida are denoted AFL, BFL, CFL, and DFL, respectively.
Figure 2 shows the frequency of hurricanes by region. The Gulf and Southeast coasts from Texas to North Carolina are affected most often by hurricanes. Within this high frequency zone, Louisiana, northwest Florida, and North Carolina have the most frequent hurricanes. Within the low frequency zone, the region from New York to Massachusetts has the largest frequency. We note that within Florida, the northeast coast has the fewest hurricanes and the northwest coast has most. It should be kept in mind that the regions used in this study do not have the same area or the same coastal exposure to hurricanes so it is not advisable to make anything more than broad generalizations of hurricane frequency. The frequency of major hurricanes (category three or higher) shows similar results (not shown) with most activity occurring in the region from Texas to North Carolina.
It is also interesting to consider the time variation in hurricane frequency. Figure 3 shows the cumulative sum of hurricanes by year for selected regions. Hurricane rates and how they fluctuate over time can be inferred directly by examining changes in slope on the cumulative sums. We see that the rate of hurricanes affecting Louisiana is rather constant over time as indicated by a nearly straight line cumulative sum, whereas the rate of hurricanes affecting southeast Florida is variable with activity appearing in clusters.
More relevant to the present work is the occurrence of years in which two different regions are affected by hurricanes. For example, Fig. 4 shows the cumulative sum of years in which both southeastern Florida and Louisiana were affect by hurricanes. Note that here the requirement is the both regions were affected in the same year, not necessarily by the same hurricane.
Again we see variations depending on regions. The overall rate of multiple hit years for Louisiana and southeast Florida is relative steady, whereas for northwest Florida and North Carolina the period from about 1875 through 1910 was quite active. Next we consider the relationship between regions affected by the same hurricane using the methods of network analysis.
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