The cold transit of Southern Ocean upwelling

The upwelling of deep waters in the Southern Ocean is a critical component of the climate system. The time- and zonal-mean dynamics of this circulation describe the upwelling of Circumpolar Deep Water and the downwelling of Antarctic Intermediate Water. The thermodynamic drivers of the circulation and their seasonal cycle play a potentially key regulatory role. Here, an observationally-constrained ocean model and an observation-based seasonal climatology are analysed from a thermodynamic perspective, to assess the diabatic processes controlling overturning in the Southern Ocean. This reveals a seasonal two-stage cold transit in the formation of intermediate water from upwelled deep water. First, relatively warm and saline deep water is transformed into colder and fresher near-surface Winter Water via wintertime mixing. Second, Winter Water warms to form Intermediate Water through summertime surface heat fluxes. The mixing-driven pathway from deep water to Winter Water follows mixing lines in thermohaline coordinates indicative of nonlinear processes. This mixing-driven pathway is corroborated by winter-time density profiles from an ocean model, Argo-based and ship-based observations. These data show that where Winter Water overlies Deep Water the stratification is always statically stable. Instead, profiles projected into temperature and salinity space lie along a tangent to the isopycnal at the temperature, salinity and pressure of Deep Water. This suggests mixing may occur due to cabbeling, which is linked to non-linearities in the equation of state (EOS) for sea water. The effect of EOS non-linearities is further investigated using a 1-D model to represent the configuration of Winter Water and Deep Water.