Temperature and salinity variability in heterogeneous oceanic convection

In regions of active oceanic convection, such as the Labrador Sea, small- a nd mesoscale spatial variability is observed in the temperature and salinit y fields (T and S). Often T and S structures are "density-compensated," wit h the density contribution of the S anomaly nearly equal and opposite to th e contribution from the T anomaly; this is manifest as variability in the " spice" field, tau = alpha T + beta S, where alpha and -beta are the local e xpansion coefficients for rand S. Here the mechanisms for generating T and S variability by convection around a preexisting mesoscale eddy, with parti cular attention to tau variability, are investigated. The authors perform s everal numerical experiments with identical density stratification, mesosca le circulation, and surface buoyancy forcing, but with different combinatio ns of T and S in the stratification and surface flux. In all cases with bot h T and S variations present, it is found that spice variability exceeds th at of density. In particular, there are substantial heat and salt fluxes at the base of the convecting region where the density flux vanishes. This ta u variability is well predicted by a simple parcel exchange scaling argumen t, and it depends on preexisting vertical and lateral tau gradients as well as the tau component of the surface forcing. The tau variance is generated both by upright plume convection and by slantwise mixing and lateral stirr ing associated with the convectively induced baroclinic instability of the mesoscale eddy. In regions dominated by convective plumes, tau variance is dissipated more rapidly than in regions where the fluxes primarily take the form of mesoscale interleaving.