Diapycnal Mixing and Mass Transfer in Western Boundary Currents
The distribution of nutrient flux in five sections across the Gulf Stream (from the Florida Straits of 35°W) is characterized by an intense core, centered at the depth of the 26.8 σt isopycnal surface. This 'Nutrient Stream' transports nutrients of O(103 kmol s-1} of nitrate and proportional amounts of other nutrients. Water mass and nutrient balances of nine isopycnal layers reveal significant diapycnal mixing between upper-thermocline and surface waters in the sector of the Stream between the Florida Straits and the Mid-Atlantic Bight. A two-box model of the nutrient-depleted surface layers (σt < 26.8) and the nutrient-rich thermocline layers (26.8< σt < 27.5) shows upward one-way transfer and two-way exchange, both at a a rate at about 1.m2s-1 per unit length of the Stream. We use isopycnic coordinates to analyze a hydrographic section across the Gulf Stream off the Mid-Atlantic Bight. The analysis shows that the Nutrient Stream is characterized by low Richardson numbers, matched by a signature of mixing in the nutrient distribution on isopycnal layers. Analysis of the Jacobian, J = dz/dp (with z the depth and p the potential density), total vorticity and potential vorticity fields, shows that this region has low Jacobians associated with high potential vorticity values, as required theoretically for the case of dominant diapycnal mixing. The diapycnal velocity is calculated from the vertical gradient of the Reynolds density flux, this parameterized as density eddy diffusivity divided by the Jacobian. Regions of large negative diapycnal gradients of the diapycnal velocity coincide with the location of the low Jacobians, suggesting that diapycnal mixing is the consequence of frontogenetically produced anomalies. Fine-scale details of the density distribution in the Nutrient Stream show a 'staircase' structure. We propose a simple model to account for mixed-layer formation in previously stratified regions, once the Richardson number becomes subcritical. Our time scale estimate for the duration of mixing events is of only a few hours. Such events are possibly triggered by enhanced diapycnal gradients of the horizontal velocities in strongly stratified regions, during frontogenesis in Gulf Stream meanders. This process appears responsible for two-way exchange in the Nutrient Stream.
Gulf Stream, Nutrient stream, Ocean currents, Oceanography
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