Ocean Heat and Seasonal Sea Ice Thickness in the Southern Ocean

Abstract

Typical values of the winter air-sea ice heat flux in the seasonal sea ice region of the Weddell Sea imply that the observed thermodynamic sea ice thickness in the region is much thinner than that expected in the absence of ice divergence or ocean sensible heat flux. Therefore, the ice must be dynamically thinned by ice divergence and/or by the presence of an ocean sensible heat flux which reduces the latent heat loss component of the air-sea heat flux and thus reduces the amount of ice growth. For an average air-sea heat flux of 35 W/m² over ~5 months of the winter ice covered cooling period, the amount of ice thinned must be ~1.6 m.

Measurements of ice divergence near the central gyre region suggest that it is sufficient to account for ~0.9 m of ice thinning. Consideration of representative characteristics of the temperature and salinity profiles from this region suggest that the ability of the water column to resist free convection, relative to the amount of heat released by any such convection, indicates that at the least (i.e., ignoring upwelling and diffusive fluxes) 20% of the air-sea heat flux must be satisfied in the form of ocean sensible heat (~7 W/m² for a 35 W/² air-sea heat flux). If the upwelling and diffusive contributions to the heat flux are included (~10 W/m² assuming a turbulent diffusivity coefficient of 0.3×10⁻⁴ m/s, and ~4 W/m², respectively), the best minimum estimate of the ocean sensible heat flux is ~19 W/m². This amount of ocean sensible heat will prevent ~1.1 m of ice from growing. This estimate is significantly larger than that from recent model studies (e.g., Lemke et al., 1990, Stössel et al., 1990) suggesting ocean sensible heat fluxes ~2 W/m² in this region, and it is ~40% lower than estimates made from observational studies based on overall property budgets (Gordon and Huber, 1984, 1990), and on explicit model analysis (Martinson, 1990), though as stated, the estimate is a lower limit and subject to considerable uncertainty in some of the specific flux components.