The regulation of sea ice thickness by double-diffusive processes in the Ross Gyre

Yana Bebieva, Florida State University, Geophysical Fluid Dynamics Institute, Tallahassee, FL, United States and Kevin Speer, Florida State University, Geophysical Fluid Dynamics Institute(GFDI), Department of Scientific Computing(DSC), Tallahassee, United States
New fine-scale observations from the central Ross Gyre reveal the presence of double-diffusive staircase structures underlying the surface mixed layer. These structures are persistent over seasons, with more developed mixed layers within the double-diffusive staircase in winter months. The sensitivity of the ice formation rate with respect to mixing processes within the main pycnocline (double-diffusive versus purely turbulent mixing) is investigated with the 1D model. A scenario with purely turbulent mixing results in significant underestimates of sea ice thickness. However, a scenario when double-diffusive mixing operates in the presence of weak shear yields plausible ranges for sea ice thickness that agrees well with the observations. The model results and observations suggest a peculiar feedback mechanism that promotes the self-maintenance of double-diffusive staircases. Suppression of the vertical heat fluxes due to the presence of a double-diffusive staircase, compared to purely turbulent case, allows Upper Circumpolar Deep Water to be more exposed to surface buoyancy fluxes. Our results shed light on the process - double diffusion - that might account for estimated rates of winter water mass transformation in the central Ross Gyre.