Coastal polynyas in the southern Weddell Sea: variability of the surface energy budget

The surface energy budget of coastal polynyas in the southern Weddell Sea has been evaluated for the period 1992 to 1998, using a combination of satellite observations, meteorological data and simple physical models. The study focuses on polynyas that habitually form off the Ronne Ice Shelf. The coastal polynya areal data are derived from an advanced multi-channel polynya detection algorithm applied to passive microwave brightness temperatures. The surface sensible and latent heat fluxes are calculated via a fetch-dependent model of the convective-thermal internal boundary layer. The radiative fluxes are calculated using well-established empirical formulae and an innovative cloud model. Standard meteorological variables that are required for the flux calculations are taken from automatic weather stations and from the NCEP/NCAR reanalyses.

The 7-year surface energy budget shows an overall oceanic warming due to the presence of coastal polynyas. For most of the period, the summertime oceanic warming, due to the absorption of short wave radiation, is approximately in balance with the wintertime oceanic cooling. However the anomalously large summertime polynya of 1997-98 allowed a large oceanic warming of the region. Wintertime freezing seasons are characterised by episodes of high heat fluxes interspersed with more quiescent periods and controlled by coastal polynya dynamics. The high heat fluxes are primarily due to the sensible heat flux component, with smaller complementary latent and radiative flux components. The average freezing season area-integrated energy exchange is 3.48x10¹⁹ J, with contributions of 63%, 22% and 15% from the sensible, latent and radiative components respectively. The average melting season area-integrated energy exchange is -5.31x10¹⁹ J, almost entirely due to the radiative component.

There is considerable interannual variability in the surface energy budget. The standard deviation of the energy exchange during the freezing (melting) season is 28% (95%) of the mean. During the freezing season, positive surface heat fluxes are equated with ice production rates. The average annual coastal polynya ice production is 1.11x10¹¹ m³, with a range from 0.71x10¹¹ (in 1994) to 1.55x10¹¹ m³ (in 1995). This can be compared to the estimated total ice production for the entire Weddell Sea: on average the coastal polynya ice production makes up 6.08% of the total, with a range from 3.65% (in 1994) to 9.11% (in 1995).