Cabbeling due to Isopycnal Mixing in Isopycnic Coordinate Models

Abstract

The cabbeling that arises as a consequence of isopycnal mixing in a North Atlantic model based on MICOM (the Miami Isopycnic Coordinate Model) is quantified. Annually averaged over the model Atlantic, the diapycnal volume flux associated with cabbeling reaches 1.5 Sv, with an associated net density flux of 2 ? 106 kg s?1 (equivalent to an annual-basin mean cooling of 0.6 W m?2). Over the range of densities that incorporate the major water masses of the model Atlantic, cabbeling effectively weakens the density flux due to parameterized diapycnal turbulent mixing by ?25%. The strength of cabbeling varies in proportion to the isopycnal mixing of heat and salt, the local buoyancy frequency, and a ?cabbeling parameter? (which is inversely proportional to temperature). As a consequence of these dependencies, cabbeling is highly localized and seasonal. In the model, strongest cabbeling occurs during summer at the subpolar front in the northwest Atlantic. Model cabbeling arises both physically (due to the independent mixing of heat and salt in isopycnic layers) and, to a lesser extent, nonphysically (due to the advection of heat and salt). Fields of layer thickness changes due to model cabbeling compare reasonably well with changes predicted by ?physical? cabbeling. Physical cabbeling is therefore predicted for a global model (QGIM) based on a more recent version of MICOM, which features salinity-only advection and mixing (and hence no cabbeling). In the circumpolar Southern Ocean of QGIM, intermediate water would be transformed (by cabbeling) to higher density at rates of up to 7 Sv, primarily due to end-of-winter freshwater forcing around Antarctica. This suggests that the cabbeling associated with isopycnal mixing, although neglected in later versions of MICOM, may play a significant role in water mass transformation around the Southern Ocean. However, the layer temperature, salinity, and thickness fields used to initialize MICOM lead to unrealistically strong cabbeling around the Mediterranean outflow during the early stages of spinup, a problem which further highlights the unsuitability of σ0 as a layer variable for water masses below ?1000 m.