Data

The hurricane numbers used in our analysis (see Fig. 1) come from the 2006 version of the HURDAT data-set (Jarvinen et al., 1984 and Jarrell et al., 1992). We use sea surface temperatures from HADISST (Rayner et al., 2002). A couple of our models also use Northern Hemisphere temperature anomalies relative to the base period

1900 1940 1980 1900 1940 1980

Fig. 1 Atlantic Hurricane Numbers since 1900. (a) Numbers of Category 1-5 Hurricanes in the Atlantic Basin. (b) Numbers of those hurricanes which hit the U.S. coastline. (c) Number of Category 3-5 Hurricanes in the Atlantic Basin. (d) Number of those hurricanes which hit the U.S. coastline

1900 1940 1980 1900 1940 1980

Fig. 1 Atlantic Hurricane Numbers since 1900. (a) Numbers of Category 1-5 Hurricanes in the Atlantic Basin. (b) Numbers of those hurricanes which hit the U.S. coastline. (c) Number of Category 3-5 Hurricanes in the Atlantic Basin. (d) Number of those hurricanes which hit the U.S. coastline

1951-1980, which can be found at http://www.giss.nasa.gov/gistemp (Hansen and Lebedeff, 1987).

The HURDAT data is considered to be reasonably accurate since 1950. Prior to 1950, the numbers of hurricanes in the basin may be less accurate, however, as there were fewer observational opportunities over the ocean at this time. Aircraft reconnaissance, for example, did not begin until 1944. Numbers of hurricanes hitting the US coastline are generally considered accurate since 1900, although the intensities of such landfalling storms may be poorly estimated due to the distances between proper meteorological measurements and the probability of missing measurements of the most intense windspeeds. Prior to 1900 even the landfall numbers are suspect because of the sparsity of population along the US coast line. Based on this, we never use data before 1900 and only use data from 1950 to build relationships between SSTs, hurricane numbers within the Atlantic basin and hurricane numbers at landfall. Any conclusions drawn from our analyses need to bear in mind these

Table 1 Estimates for Hurricane Numbers in 2007

Historical Ratio 2007 season up to 15 Oct. Likely 2007 season

Landfall Cat 1-5 0.035 1 1

Landfall Cat 3-5 0.029 0 0

issues of data quality, since our forecasts can only ever be as good as the data on which they are based.

Atlantic SSTs are used to help predict hurricane numbers in some of our models, and the strength of the relationship between SSTs and hurricane frequency is dependent on the region of the north Atlantic being considered (Shapiro and Goldenberg, 1989; Raper, 1993; Goldenberg et al., 2001). The SSTs which correlate most highly with hurricane activity are the SSTs within the main development region (MDR), 10-20N, 15-70W. MDR SSTs are also highly correlated with each other in space (see figure 10 in Meagher and Jewson, 2006). It therefore makes sense to construct a single index that captures the MDR SSTvariability. We tested various combinations and came up with an index based on July-September SST variability as having the highest correlation with hurricane activity.

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