Quarterdeck Volume 4, Number 2, Summer 1996
Coastal Upwelling off south Texas
Mathew K. Howard
John D. Cochrane
Nan D. Walker
Wind-driven coastal upwelling probably occurs each year along the south Texas coast, yet little has been published on this topic. Using meteorological and ocean-ographic station data collected during the Louisiana-Texas Shelf Physical Oceanography Program (LATEX) and infrared satellite imagery, we found evidence of upwelling along the south Texas coast each year from 1992 to 1994.
Upwelling is the process by which subsurface water is brought to the surface. Upwelled water is usually colder, saltier, and often has higher con-centrations of nutrients and dissolved oxygen than the water it replaces. Nutrients provided by upwelling pro-mote the growth of phytoplankton which form the basis of the marine food chain. Many of the world's major fisheries are located in areas where strong persistent upwelling occurs. Not coincidentally, such regions are frequently located along the coast.
In 1905, V.W. Ekman showed that due to the earth's rotation, the net transport of water by winds is directed 90° to the right of the wind direction (in the northern hemisphere). In the open ocean an unlimited reservoir of water on the left supplies transport to the right. Near a coast, however, the supply can only come from below. On the south Texas coast, winds that favor upwelling are those which transport water offshore; winds with an upcoast component. Along the Texas coast, "upcoast" indicates winds directed from Brownsville, near the border with Mexico, northward toward Galveston.
Winds favorable to upwelling exist when the alongshore wind component is positive (see figure). This occurs as a series of episodic events beginning in April or May then becoming more persistent from June through August each year.
Water temperature is well correlated with the alongshore wind component and decreases following positive alongshore winds as expected. Current changes lag six to ten hours behind winds, but temperature changes lag by as much as a day. The temperature peak in mid-August is probably due to a nearby eddy. Although increased salinity is an expected consequence of upwelling, higher salinities can also result from the flow of water from the south due to the seasonal reversal in the general circulation at this time of the year. Salinity is not a robust indicator of upwelling in this region.
The infrared satellite image provides compelling evidence for upwelling along the Texas coast. It shows a coastal band of cool water extending from 24°N to 28°N along the shore. Contour plots of surface and subsurface temperature taken on the LATEX hydrography cruises in early August 1993 confirmed low water temp-eratures below the surface along the coast. The temperatures ranged from 26-27°C in the south to 29-30°C upcoast.
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Last updated February 5, 1997