Middle Atlantic Bight meridional wind component effect on bottom water temperatures and spawning distribution of Atlantic croaker

Abstract

Predominantly southerly winds over the Middle Atlantic Bight result in offshore flow in surface waters and corresponding onshore flow in a bottom Ekman layer. Seasonal heating results in maximum surface temperatures in August while the bottom onshore Ekman flow results in cold summer bottom water temperatures. Bottom temperatures do not peak until autumn after the seasonal wind shift resulting in a cessation of the southerly winds and overturning of the surface waters. Strong episodes of this onshore bottom flow of cold water (upwelling) have been previously reported as unusually cold surf zone temperatures. Fifteen-day moving averages of meridional components of winds from Norfolk International Airport, Virginia revealed few of the 15 years examined in which there was an abrupt seasonal shift in wind patterns from summer to winter regimes. Twenty-six collections of bottom water temperature data were reviewed for the Middle Atlantic Bight during the years 1967–1981. The average time of cessation of the strong southerly (summer) meridional wind component during those years was the end of August/beginning of September. A correlation of 0.74 exists between the time of cessation and the extent (km 2) of warm nearshore bottom waters, as indicated by the 16°C isotherm. The transition from a southerly to a northerly wind pattern occurs over a period of time ranging from 10 days to 3 months, while the response of ocean bottom isotherms to the cessation of the southerly component can typically be seen in 10–20 days. Migration and spawning behavior of many fish is cued by water temperatures. The autumn areal distribution of the Atlantic croaker ( Micropogonias undulatus) is strongly correlated ( r 2 = 0.78) with the areal distribution of bottom waters warmer than 16°C on the shelf. Furthermore, there is a correlation ( r 2 = 0.64) between the time of cessation of the summer southerly wind and the areal extent of croaker. Therefore, we propose that if the wind relaxation occurs prior to the autumn migration of croaker out of the estuaries, spawning would occur in northern and middle sections of the Middle Atlantic Bight. Prolonged summer winds would keep the nearshore waters cool, and force croaker to migrate further southward to spawn, potentially shifting the distribution of juvenile recruitment to Pamlico Sound with entrance through Oregon Inlet in the Middle Atlantic Bight or through inlets south of Cape Hatteras.