WHY WAS STORM SURGE ON SAN SALVADOR ISLAND, BAHAMAS, LOWER FOR HURRICANE JOAQUIN (2015) THAN FOR HURRICANE FRANCIS (2004)?

Storm surge—ephemeral elevated sea level associated with low atmospheric pressure and directional, storm-force wind—remodels coastlines during tropical cyclones because surge can entrain and transport sediment of a wide range of grain sizes. Several factors including central pressure, wind speed, wind field size, angle of approach and shoreline characteristics influence the height and erosive potential of storm surge. Hurricanes Joaquin (landfall 4 p.m. EDT 2 October 2015) and Francis (landfall 3 p.m. EDT 2 September 2004) allow us to evaluate these factors because the hurricanes shared similar features at landfall. Both were Category 3 on the Saffir-Simpson Scale, and had maximum sustained winds of 110 kts; Joaquin and Francis had pressures of 941 mb and 948 mb, and wind field diameters of 240 miles and 290 miles respectively. Both hurricanes made landfall during ebbing tides, approximately two hours before low. We measured surge heights at four locations around the island. Maximum surge height was marked by a continuous line of marine debris, the location and elevation of which was measured with GPS and survey techniques. Maximum surge heights for Hurricane Joaquin were compared to those of Dick and Cartright (2011, 13^th Symposium of the Natural History of the Bahamas) for Hurricane Francis. Surge of 5–6 m was reported for Francis on the eastern side of San Salvador; 2.7 m was measured on the west side of the island. In January 2016, our team measured strand lines of approximately 1–3 m for Hurricane Joaquin, with the highest (3.4 m) at Grotto Beach and lowest (0.95 m) at the northern end of East Beach. Differences in surge heights can be explained by storm track and location of landfall. Hurricane Joaquin was moving at a heading of 031° at the time of landfall, skirted the western side of the island and made landfall at North Point. Francis was on a heading of 331° when it made landfall perpendicular to the shoreline on San Salvador’s eastern coast. For Francis, cyclonic rotation over open water piled water and pushed it onshore, generating high surge values to the north of landfall. For Joaquin, winds blew over the island during approach, muting the surge.