The Average Distribution of Volume Transport and Potential Vorticity with Temperature at Three Sections across the Gulf Stream

Abstract

Average cross sections of downstream velocity and temperature, obtained using PEGASUS current profiles at three locations along the Gulf Stream, have been partitioned into 2.5°C temperature intervals to examine the distribution of transport increase versus temperature between the two southern sections (27° and 29°N) and off Cape Hatters(73°W). Between 27° and 29°N the total transport of the Florida Current over the sections increased only by about 3 ? 106 m3 s?1 (3 Sv) but the current broadens by about 50%. By Cape Hatteras, the transport has increased nearly three-fold to 93.7 Sv, of which two-thirds of the increase is contained in the 19.5°?17.0°C (?18°?) layer and in water colder than the 7°C ?still? temperature found at 27°N. Cross-stream distributions of layer transport, potential vorticity, and thickness are estimated. At each section, the 10 ? 10?7 m?1 s?1 contour tends to be a boundary (independent of temperature) between the region of relatively uniform layer potential vorticity on the anticyclonic (offshore) side of the current and an area with high lateral potential vorticity gradients on the cyclonic (onshore) side. In the colder (<7°C) waters off Cape Hatteras, layer potential vorticity also tends to be uniform at ?5 ? 10?7 m?1 s?1. Layer potential vorticity in the 18° layer is quite uniform with minimum values ?3.5 ? 10?7 m?1 s?1 at 27° and 29°N and somewhat less off Cape Hatteras, which is close to where 18°C water is formed in the wintertime. At Cape Hatteras this same layer shows a peak in transport/unit width at the point where the layer begins to thin as one moves into the Gulf Stream core from the southeast. A simple model based on conservation of layer potential vorticity is proposed to describe this transport structure.