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Temperature and Salinity Variability in Heterogeneous Oceanic Convection

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  • 1 Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
  • | 2 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California
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Abstract

In regions of active oceanic convection, such as the Labrador Sea, small- and mesoscale spatial variability is observed in the temperature and salinity fields (T and S). Often T and S structures are “density-compensated,” with the density contribution of the S anomaly nearly equal and opposite to the contribution from the T anomaly;this is manifest as variability in the “spice” field, τ = αT + βS, where α and −β are the local expansion coefficients for T and S. Here the mechanisms for generating T and S variability by convection around a preexisting mesoscale eddy, with particular attention to τ variability, are investigated. The authors perform several numerical experiments with identical density stratification, mesoscale circulation, and surface buoyancy forcing, but with different combinations of T and S in the stratification and surface flux. In all cases with both T and S variations present, it is found that spice variability exceeds that of density. In particular, there are substantial heat and salt fluxes at the base of the convecting region where the density flux vanishes. This τ variability is well predicted by a simple parcel exchange scaling argument, and it depends on preexisting vertical and lateral τ gradients as well as the τ component of the surface forcing. The τ variance is generated both by upright plume convection and by slantwise mixing and lateral stirring associated with the convectively induced baroclinic instability of the mesoscale eddy. In regions dominated by convective plumes, τ variance is dissipated more rapidly than in regions where the fluxes primarily take the form of mesoscale interleaving.

Corresponding author address: Dr. Sonya Legg, Woods Hole Oceanographic Institution, Mail Stop 21, Woods Hole, MA 02543.

Email: slegg@whoi.edu

Abstract

In regions of active oceanic convection, such as the Labrador Sea, small- and mesoscale spatial variability is observed in the temperature and salinity fields (T and S). Often T and S structures are “density-compensated,” with the density contribution of the S anomaly nearly equal and opposite to the contribution from the T anomaly;this is manifest as variability in the “spice” field, τ = αT + βS, where α and −β are the local expansion coefficients for T and S. Here the mechanisms for generating T and S variability by convection around a preexisting mesoscale eddy, with particular attention to τ variability, are investigated. The authors perform several numerical experiments with identical density stratification, mesoscale circulation, and surface buoyancy forcing, but with different combinations of T and S in the stratification and surface flux. In all cases with both T and S variations present, it is found that spice variability exceeds that of density. In particular, there are substantial heat and salt fluxes at the base of the convecting region where the density flux vanishes. This τ variability is well predicted by a simple parcel exchange scaling argument, and it depends on preexisting vertical and lateral τ gradients as well as the τ component of the surface forcing. The τ variance is generated both by upright plume convection and by slantwise mixing and lateral stirring associated with the convectively induced baroclinic instability of the mesoscale eddy. In regions dominated by convective plumes, τ variance is dissipated more rapidly than in regions where the fluxes primarily take the form of mesoscale interleaving.

Corresponding author address: Dr. Sonya Legg, Woods Hole Oceanographic Institution, Mail Stop 21, Woods Hole, MA 02543.

Email: slegg@whoi.edu

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