Katabatic wind forcing of the Terra Nova Bay polynya

Abstract

The Terra Nova Bay polynya is a perennial winter feature in the western Ross Sea, Antarctica, which occupies roughly 1000 km2, it is formed and maintained by the combined influence of persistent katabatic winds, which advect newly formed bay ice eastward, and the Drygalski Ice Tongue, which prevents northward drifting pack ice from entering Terra Nova Bay. Existence of anomalously strong katabatic drainage along this coast is predicted by Parish's (1982) simulation of wintertime airflow which reveals a pronounced confluence of surface winds upslope from the Reeves Glacier where the winds are further focused by local topography. The simulation is strongly supported by regional sastrugi orientations. Average wintertime atmospheric conditions and ice sheet topography which control surface air drainage are stable on a climatic time scale; therefore, persistent wintertime katabatic winds should be an annual phenomenon. Further evidence comes from multi‐year Landsat images which consistently show windswept, snowfree areas on the Reeves Glacier. In marked contrast to typical Antarctic katabatic winds, strong persistent winter winds are observed at sea level ∼25 km beyond the coastal slope break. Air probably descends as bora‐type winds and is likely to be significantly denser than the air at sea level; conditions are not favorable for hydraulic jumps apparently typical of other katabatic regimes. The horizontal density difference is maintained during airflow across the Nansen Ice Sheet because relatively little air mass modification occurs there in contrast to situations where air moves over an ice‐laden ocean. Observations thus suggest that katabatic winds maintain their identity for some distance seaward of the coast; qualitative trajectory calculations indicate that for representative geostrophic conditions this distance is on the order of the observed polynya width. Estimated ice freezing rates are ∼20 cm day−1, but wind‐generated waves and currents prevent ice from consolidating. The blocking effect of the Drygalski Ice Tongue is a consequence of its orientation with regard to western Ross Sea ice drift patterns; ice tongue length controls polynya width. Absence of such blocking along other coasts experiencing strong katabatic outflow partly explains why similar polynyas do not form there.