Stability and slow overturning of the water column in the Weddell Sea under a sea ice cover

Abstract

A crucial controlling factor on the stability of the two-layered water column in the Weddell Sea is its sea ice cover, which acts through a negative feedback mechanism (NFM) due to the thermal barrier at the mixed layer base. The present study revisits the problem of how the NFM affects the stability and overturning of the water column using a fine-resolution nonhydrostatic model to run experiments employing various background mixing coefficients (viscosity $\nu$ and diffusivity $\kappa$) and Earth rotation (Coriolis parameter f) with and without the sea ice cover. In the ice-free case without the NFM, destabilization and overturning of the water column are accelerated by lower $\nu$ ($\kappa$) and $f=0$ , since convection is enhanced in the mixed layer. Vigorous convection increases the upward salt flux from the deep layer and densifies the mixed layer against the concurrent upward heat flux. In the ice-covered case with the NFM, to the contrary, the destabilization and overturning are decelerated by lower $\nu$ ($\kappa$) and $f=0$. The upward heat flux due to mixed layer convection delays destabilization by reducing ice formation through the NFM. On the other hand, lower $\kappa$ decelerates the overturning, suppressing cabbeling instability at the mixed layer base. As a result, the entire process of destabilization and overturning needs more than two months at the maximum. Additional experiments, together with observational data, suggest that such a slow process may have occurred in the Weddell Sea.