Abstract
Historical evidence shows that areas of hypoxia are co-located with upwelling events along the New Jersey coast during summer. Past research provide evidence that these events, which tend to occur near bathymetric highs within 30 km of the coast, are known to be forced by sustained north-northeastward winds that drive the Ekman transport of coastal surface waters seaward. The resulting hypoxia is a condition in which dissolved oxygen levels drop below 5.0 mg/L based on criteria set by New Jersey Water Quality Standards. Since 2009, Slocum glider autonomous underwater vehicles have been used to measure the dissolved oxygen and other water properties along the New Jersey coast during summer. Here we report on our analysis of a dataset from the September 2013 glider RU28 and complimented it with buoy-winds, satellite sea surface temperatures, and high frequency radar-derived surface currents. The sub-surface measurements from the glider, which zig-zagged southward along the New Jersey coast, measured both the lowest temperature and lowest dissolved oxygen values in the northern New Jersey coastal waters. We defined the glider mission into three transects consisting of pairs ofadjacent legs that coincided with three regimes of north-northeastward wind. The first transect, A, took place following an extended 79 hour upwelling favorable northeastward wind event which drove an uplifted thermocline, a surface aggregation of chlorophyll fluorescence, and bottom hypoxia above rises in bathymetry. Sea surface temperature showed the development of cold patches along the coast. The use of surface Coastal Ocean Dynamics Applications Radar data and sub-surface glider depth-averaged velocities were consistent with offshore Ekman transport during transect A. The second transect, B, occurred during both downwelling and upwelling favorable winds, of which the upwelling winds lasted about 19 hours. An uplifted thermocline was in place as the glider approached from offshore during initial downwelling favorable conditions. Chlorophyll fluorescence is concentrated mid water column until the switch to upwelling favorable winds where there was an increase of surface chlorophyll fluorescence. Hypoxia was located in the middle of transect B below the uplifted thermocline and surface chlorophyll. Sea surface temperature indicated a cooler surface patch during the upwelling favorable period. Transect B has a comparatively flat bathymetry. Sub-surface glider depth-averaged velocities indicated coastward movement. As the glider entered Transect C, it measured a top to bottom mixed water column above several bathymetric highs during downwelling favorable wind conditions. Winds changed to a northeastward diurnal oscillation as the thermocline was re-established near bottom while the glider proceeded offshore. Sea surface temperature didn't indicate a cold surface patch along the gliders track. However, there was coastward movement of the sub-surface glider depth-averaged velocities. Water mass analysis from transect A suggests that the near-by Mid Atlantic Bight Cold Pool water mass is just offshore in deeper water and mixing within the area of upwelling and hypoxia. In addition, the Cold Pool does not appear to be contributing to the loss of oxygen in the hypoxic areas, but rather providing oxygenated waters.