Thermobaric cabbeling over Maud Rise: Theory and large eddy simulation

A Large Eddy Simulation (LES) of the wintertime upper ocean below seasonal Antarctic ice cover over Maud Rise was carried out using observed time-dependent surface forcing from 1994 Antarctic Zone Flux Experiment (ANZFLUX) observations. Surface ice formation increases the density of the cold, fresher Surface Mixed Layer (SML), that overlies warmer, saltier Weddell Deep Water (WDW). This reduces the stability of the thermocline until it reaches a critical point for instabilities arising from the nonlinear equation of state (NES) for seawater density ρ. This simulation was intended to model the thermobaric detrainment of SML fluid, a NES instability predicted to result from the dependence of seawater density on the product θP of temperature and pressure. Instead, model results demonstrate a different instability arising from the combination of thermobaricity with cabbeling, the NES effect due primarily to the dependence of ρ on θ2. This combined thermobaric cabbeling instability drives turbulent convection in a deep interior mixed layer (IML) that may grow hundreds of meters thick below the thermocline, largely decoupled from SML dynamics. In the LES, thermobaric cabbeling and IML convection shoals the SML through entrainment from below until ice motion increases in the observationally-based model forcing. Increased upper ocean model heat flux due to higher ice speed melts surface ice, increasing thermocline stratification and eventually bringing the simulated instability to a halt. In an auxiliary simulation the lull preceding strong ice motion in field observations is artificially extended by temporarily holding model surface forcing constant until the SML shoals entirely, bringing the modified WDW of the IML, 2 °C above freezing, directly to the surface. Subsequently, reverting to the observed surface forcing and its attendant strong ice motion melts the ice cover entirely, demonstrating a possible mechanism for open ocean Antarctic polynya formation. The same process, as halted prematurely in the LES using the forcing observed, may also be responsible for thick, deep internal layers and localized 'chimney' structures observed in the Weddell Sea.