Elevated East Coast Sea Levels Anomaly: July – June 2009

Sweet, William; Zervas, Chris; Gill, Stephen

NOAA sea level stations managed by the Center for Operational Oceanographic Products and Services (CO-OPS) recorded higher than normal sea levels (SL) along the U.S. East Coast in June and July 2009. Near-peak levels in the latter half of June coincided with a perigean-spring tide, an extreme predicted tide when the moon is closest to the Earth during a spring tide. This tidal event added to the observed SL anomaly, produced minor coastal flooding, and caught the attention of many coastal communities because of the lack of coastal storms that normally cause such anomalies. In terms of SL heights, the event was not very abnormal as many locations have higher levels in the late-summer. June – July 2009 SL heights are anomalous because of their unexpected timing and geographic scope. The SL event is anomalous in terms of its ‘residual’ values unaccounted for within the predictions of the earth-moon-sun tides and normal seasonal cycles of the wind sand atmospheric pressure, ocean currents, and heating/cooling of coastal waters. The mean residual SL values for June 2009 were significant (> 0.2 m) from North Carolina to New Jersey,substantial (> 0.1 m) from Florida to Maine, and the most extreme to occur simultaneously over the entire East Coast during a spring/summer period as far back as 1980. There are two probable mechanisms responsible for the June – July 2009 high SL residuals. The first is northeasterly (NE) wind forcing. In June 2009, winds over the entire geographic area from Cape Hatteras, NC to the Gulf of Maine had a moderate NE wind component, whose transport caused coastal SL to rise. South of Cape Hatteras, winds were primarily southwesterly (SW). The other mechanism is the changing transport of the Florida Current, which is measure din the Florida Straits before it supplies the Gulf Steam off of Cape Hatteras, NC. When the Florida Current / Gulf Stream transport is low, the eastward-rising cross-current slope relaxes and raises coastal SL. In June 2009, the SL residual rise was concurrent with a noted decrease in transport of the Florida Current. The June – July 2009 SL event decays in mid-July 2009 as the SL residuals diminish. During this period, transport of the Florida Current sharply increases and the winds oscillate between SW and NE along the East Coast. The June – July 2009 SL anomaly is unique in that the NE winds were not at a multi-year high or the Florida Current transport at its low. But the coupled effect of the two forces created SL residuals that were at highest levels all along the East Coast. Highest SL residuals between North Carolina and New Jersey highlight the region of greatest overlap of the two forces.