Hydrographic observations

During the summer of 1981 a large scale sampling effort took place in the shelf waters of the southeastern United States. The goal of this effort was to quantify the effect of the intrusion of deep Gulf Stream water into these shallow, euphotic waters. By making repeated hydrographic measurements over the entire shelf area, the actual volume of the intrusions was determined. Two main intrusion events were observed: one in June and early July and a second in late July and early August. The intruding water entered the shelf in the region south of St Augustine and was transported northward by the mean northward shelf circulation. The subsurface cold water mass was isolated from the Gulf Stream by a ridge of warmer water along the shelf break.The intrusions resulted in increased stability in shelf waters with vertical temperature gradients as high as 10°C m⁻¹. The potential energy in the stratified shelf waters typically ranged from 1000 to 2000 J m⁻² that would require 2 to 4 mW m⁻² to mix. However, since only 0.1 to 1.0mW m⁻² was available from wind and tidal mixing, the water column stayed stratified in most cases.The amount of nitrate transported onto the shelf was determined by two methods: direct observation by synoptic cruises and transport measurements using current meter arrays. Both methods gave similar results with about 18,000 tons nitrogen in the large June/July intrusion. On a seasonal basis, 40,000 tons of nitrogen could be advected into shelf waters which could result in 200,000 to 400,000 tons carbon production.