Nutrient transport and mixing in the Gulf Stream

Abstract

The distribution ofnutrient flux (geostrophic velocity times concentration) in five sections across the Gulf Stream-North Atlantic Curren! (from thc Florida Straits to 35ºW) is charactcrized by an intense core, centered at the depth of the 26.8 u, iso􀀖ycnal surface (lypically 500 m). This '"nutrient stream" carries nutrient transports of the order of lO kmol s -l of nitrate and proportional amounts of other nutrients. Bctwcen the Florida Straits and thc Mid-Atlantic Bight, water transport doubks. but nutrient transport !rebles, because along-isopycnal inl1ow from the subtropical gyre is concentrated in the layers of the upper thermocline, which are rich in nutrients. Beyond the Mid-Atlantic Bight, both water and nutrient transports decline slowly. Water mass and nutrient balances of nine isopycnal layers revea! signilicant upward entrainment and mixing of thermocline waters in the sector of the stream between the Florida Straits and the Mid-Atlantic Bight. A two-box model of the nutrient­depleted surface layers (u1 < 26.8) and the nutrient-rich thermocline layers (26.8 < u1 < 27.5) shows an upward cntrainmcnt rate of ahout 1.6 m 2 s -I per unit length of the stream, or a diapycnal velocity of 2 x 10-5 m s -I over the 80-km width of the stream. In addítion, thcre is two-way diapycnal mass cxchangc al approximatcly 1 thc same rate. The rate of inflow from the surface 1 laycrs of thc Sargasso Sea is about 12 x 106 m s - l, from lhe thermocline laycrs 15 x 106 m 1 s