You're not alone in noticing that our coastal ocean temperatures suddenly became much colder over the last couple of weeks! While the average July/August ocean water temperatures for coastal Delaware ranges from 71-76 degrees Farhenheit, recently reported temperatures (July 2003) have been as low as 62 degrees Farhenheit. This change in water temperature is caused by a physical process known as upwelling which is the vertical movement of cold water from the deep ocean to the surface.

Upwelling usually results when winds blowing nearly parallel to a continental coastline transport the light surface water away from the coast. Seawater must rise from the depths to replenish the surface water that is being pushed away from the coast. These cooler, deeper nutrient-rich waters rise from depths of 50-300 meters to the surface of the ocean. Therefore, temperatures of water just along the coast can decrease dramatically in a short period of time.

(Photo courtesy of NOAA)

Surface ocean water along our coast is warmed by the sun, and may typically reach temperatures as high as 78 degrees Fahrenheit. You may have noticed that we have experienced several weeks of strong southwesterly winds blowing along the Delaware coast. These offshore winds work to blow the warm surface water offshore, and the colder bottom water moves upwards to take its place. It's actually not quite as simple as I've described here, and the circulation process involves physics - friction and the Coriolis effect, which combine in a process known as Ekman transport.

When wind blows over water, the surface water does not move directly in front of the wind but moves about 45 degrees toward the right of the wind's motion in the Northern Hemisphere. This process is called Ekman transport and is a result of the Coriolis effect. Basically warm surface ocean water is pulled directly off the coast - leaving room for colder deep water to rise and replace the displaced water.
(Photo courtesy of NOAA)

While the colder water may be uncomfortable to swimmers, regions of upwelling are among the richest biological areas of the world. While surface waters are usually depleted of nutrients such as phosphates and nitrates that are critical to plant growth, deeper waters have high concentrations of these nutrients. Upwelling replenishes the surface layers with the nutritional components necessary for biological productivity.

For example, at the University of Delaware, Dr. Charles Epifanio and Dr. Rich Garvine have documented the importance of upwelling events to the life cycle of the blue crab. Drifter studies and computer simulation models demonstrate that upwelling events are important in retaining blue crab larvae near the parent estuary. Because blue crab larvae remain near the water surface, transport of the larvae in the coastal ocean is primarily wind driven. The supply of crab larvae to juvenile habitat is controlled by the wind patterns and buoyance forces over the adjacent nearshore and continental shelf areas. For more information about blue crab larvae, visit Dr. Epifanio's web site.

For more information on upwelling, visit the following website: http://www.noaanews.noaa.gov/magazine/stories/mag32.htm

NOAA's National Data Buoy Center website includes water temperature information for
Buoy Station 44009, located 26 nautical miles southeast of Cape May, New Jersey