Abstract
AbstractGeostrophic eddies contribute to the mixing of heat, carbon, and other climatically important tracers. A passive tracer driven by satellite-derived surface velocity fields is used to study the regional and temporal variability of lateral eddy mixing in the North Atlantic. Using a quasi-Lagrangian diffusivity diagnostic, we show that the upstream region (80°–50°W) of the Gulf Stream jet exhibits a significant mixing barrier (with diffusivity of ≈1 × 103 m2 s−1), compared to the downstream region (50°–10°W), which displays no mixing suppression (≈10 × 103 m2 s−1). The interannual variability is 10%–20% of the time mean in both regions. By analyzing linear perturbations of mixing-length diffusivity expression, we show that the across-jet mixing in the upstream region is driven by variations in the mean flow, rather than eddy velocity. In the downstream region, both the mean flow and eddy velocity contribute to the temporal variability. Our results suggest that an eddy parameterization must take into account the along-jet variation of mixing, and within jets such diffusivities may be a simple function of jet strength.
Highlights
Mesoscale eddies, of length scales of 10–100 km, are ubiquitous in the global ocean and dominate the oceanic kinetic energy (Ferrari and Wunsch 2009; Wortham and Wunsch 2014)
We investigated the interannual variability of the lateral mixing of a passive tracer in the North Atlantic, using an observation-driven passive tracer experiment in MITgcm
Using the effective diffusivity of Nakamura (1996), we showed that lateral mixing at the surface exhibits significant along-stream variation in the North Atlantic (Fig. 2)
Summary
Of length scales of 10–100 km, are ubiquitous in the global ocean and dominate the oceanic kinetic energy (Ferrari and Wunsch 2009; Wortham and Wunsch 2014). Upstream exhibits significant mixing suppression (’1000 m2 s21) in the jet core of the Gulf Stream, while downstream, in the presence of the North Atlantic and Azores Currents, exhibits a single large peak (’10 000 m2 s21) in the effective diffusivity. In the upstream region the FN10 expressions reproduces the mixing suppression observed in the effective diffusivity, and we find that the interannual variability is dominated by changes in the magnitude of the mean flow. The North Atlantic Current and the Azores Current do not produce as sharp a front as the Gulf Stream jet, but both the upstream and downstream regions exhibit an energetic mesoscale eddy field that contributes to the lateral mixing of tracers. The KFN10 diffusivity in the downstream region does not exhibit a mixing barrier, consistent with the along-stream variation observed with the effective diffusivity, but has a magnitude approximately 5 times smaller than Keff. It may be the case that the FN10 diffusivity expression does not accurately capture the correct physical processes in this region
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