Preconditioning of the Weddell Sea Polynya by the Ocean Mesoscale: Insights from an Eddying Climate Model
Preconditioning of the Weddell Sea Polynya by the Ocean Mesoscale: Insights from an Eddying Climate Model
Abstract:
Polynyas are large openings in the sea ice cover that are thought to play an important role
in the climate system as they open a window between the atmosphere and deep ocean through
intense deep convection. Open-ocean polynyas, rather than coastal polynyas, are the primary
mechanism for bottom water formation in CMIP5 models, a feature that is generally thought
to be typical of coarse-resolution climate models. In this study, we investigated the occurence
of open-ocean polynyas in a modest-eddying (0.25°) and an actively-eddying (0.1°) ocean of a
climate model run for 200 years under preindustrial forcing. Surprisingly, while the modest-
eddying model does not develop any open-ocean polynya over the course of the simulation, the
actively-eddying model develops two polynyas in the Weddell Sea, similar to the one observed
in the 1970s. The main reason for the absence of a polynya in the modest-eddying model is a
weak strati cation that enhances the vertical mixing between the subsurface and the surface,
hence preventing the formation of a heat reservoir at the subsurface. In contrast, in the
actively-eddying model, a better representation of ocean mesoscale features and topography
enables a more stable strati cation of the water column to be simulated thus allowing the heat
reservoir to build up at the subsurface. The growing deep heat reservoir is eventually strong
enough to destabilize the water column and to maintain a polynya over a decade.
in the climate system as they open a window between the atmosphere and deep ocean through
intense deep convection. Open-ocean polynyas, rather than coastal polynyas, are the primary
mechanism for bottom water formation in CMIP5 models, a feature that is generally thought
to be typical of coarse-resolution climate models. In this study, we investigated the occurence
of open-ocean polynyas in a modest-eddying (0.25°) and an actively-eddying (0.1°) ocean of a
climate model run for 200 years under preindustrial forcing. Surprisingly, while the modest-
eddying model does not develop any open-ocean polynya over the course of the simulation, the
actively-eddying model develops two polynyas in the Weddell Sea, similar to the one observed
in the 1970s. The main reason for the absence of a polynya in the modest-eddying model is a
weak strati cation that enhances the vertical mixing between the subsurface and the surface,
hence preventing the formation of a heat reservoir at the subsurface. In contrast, in the
actively-eddying model, a better representation of ocean mesoscale features and topography
enables a more stable strati cation of the water column to be simulated thus allowing the heat
reservoir to build up at the subsurface. The growing deep heat reservoir is eventually strong
enough to destabilize the water column and to maintain a polynya over a decade.