This forms a strong temperature front along the offshore side of the upwelled water. Subsequent evolution of the upwelling front occurs as illustrated in the figure. Each year, these upwelling centers are located off the same inlets and estuaries previously identified as regions of recurrent low dissolved oxygen. Shipboard temperature surveys across the Mullica River upwelling center reveal the same complicated thermal structure below the surface each year. Rather than a single upwelled bulge of cold water in the center of the feature, the coldest upwelled water is split into one bulge along the offshore front and a second along the shore.
Wind forcing following the midsummer upwelling events differ from year to year.
After the 1993 upwelling event and its phytoplankton bloom, the winds reversed, causing a strong thermal cap to form as the warm surface water was transported back toward the shore. Offshore of the Mullica estuary, dissolved oxygen levels below the thermocline plummeted and remained well below normal until Hurricane Emily thoroughly mixed the region and returned oxygen values to their normal levels in September. In August of 1994, however, the midsummer upwelling event was immediately followed by a northeaster. In 1995, the upwelling was followed by a series of hurricanes. In both cases, the storms thoroughly mixed the layers of water beneath the surface, the late-summer warm water cap never formed, and bottom dissolved oxygen levels remained elevated.