Nearshore response to cold air outbreaks in the Gulf of Mexico

Abstract

The nearshore response to synoptic meteorological patterns called Cold Air Outbreaks (CAOs) was studied with a cross-shelf mooring array located 7 km west of an estuarine outflow in the Northern Gulf of Mexico. The array consisted of 6 stations that spanned the nearshore measuring waves, currents, temperature, and salinity for a two-week period in December 2013. CAOs are prevalent during winter and are characterized by the passage of atmospheric cold fronts with offshore winds known to cool coastal waters. The prefrontal phases of CAOs coincided with downwelling favorable winds and westward flow that introduced a river plume into the nearshore. Offshore winds during the high pressure phase formed a two-layer circulation with offshore flow in the surface layer and onshore flow beneath, which was moderated by a river plume. The downwind advection of the river plume tended to enhance surface transport until the plume was flushed out of the nearshore, which formed adverse density gradients that reduced surface transport through vertical mixing. When the plume was not fully flushed out of the inner shelf, surface transport remained enhanced during the high pressure phase due to stratification that limited the formation of adverse density gradients. When the plume was initially shore detached at the onset of the high pressure phase, offshore flow was not enhanced by the downwind advection of the plume due to the existence of initial adverse density gradients. These results highlight the effects of nearshore river plumes in regulating inner shelf circulation during cold fronts.