| Summary: | Ocean wave climate exerts a fundamental control on geomorphic evolution of coasts, by providing a characterization of the deep-water wave heights, periods, and directions responsible for supplying energy to transport nearshore sediment within the coastal zone. Three decades of western North Atlantic significant wave heights (SWHs) are examined for a latitudinal relationship with the North Atlantic Oscillation (NAO) using a 33-year (1980-2012) numerical hindcast. Analysis of hourly-computed SWHs, from 49 U.S. Atlantic coast (Florida to Gulf of Maine) outer-shelf virtual stations, reveals that 50th percentile SWHs have increased at a rate of approximately 4 cm/decade since 1980. South of 37 degrees North (Chesapeake Bay Inlet), extreme SWHs (90th, 95th percentiles) have increased at lower rates (0-4 cm/decade) than 50th percentile rates, whereas North of 37 degrees North extreme SWHs have increased at higher rates (5-10 cm/decade) than 50th percentile rates. SWH observations subdivided by NAO index category (NAO+, NAO-, NAO null) show that north of 39 degrees North (Delaware Bay), all three subpopulations trend positively for 50th 90th and 95th percentiles, whereas south of 35 degrees North (Cape Hatteras), NAO- extreme SWHs trend negatively, while NAO+ and NAO null extremes trend positively. North of 32 degrees North (Charleston, SC), NAO null extreme SWH rates (10-29 cm/decade) deviate positively from the range of rates exhibited by NAO+ and NAO. Together, a two-tailed one-sample T-test and bootstrap percentile analysis demonstrate that mean SWHs of NAO+, NAO-, and NAO null are statistically distinct. The largest difference among the NAO subpopulations is visible south of Cape Hatteras (from Charleston, SC to central Florida), where NAO+ mean SWHs range from 10-17 cm (~9-15%) higher than NAO- and NAO null means. The results support a hypothesis that wave climate is largely controlled by the North Atlantic 'intergyre' gyre, an atmospheric phenomenon that has recently been linked to sea level rise in the western North Atlantic.
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