Katabatic winds and polynya activity in Terra Nova Bay during winter seasons in the period 1995-2022

Abstract

Terra Nova Bay (TNB), in the Ross Sea, is characterized by the presence of persistent ice-free areas during the austral winter, known as polynyas. TNB polynya markedly influences air-sea dynamics and physical oceanography in that region as a major sea ice and salt “factories”, maintained by strong persistent katabatic winds. The role of TNB polynya is important in the formation of High Salinity Shelf Water (HSSW), the densest water mass of the Southern Ocean, occurring during sea ice production that increases the salinity of the subsurface water. The aim of this study is to investigate the processes that occur in the TNB coastal polynya and the role of the air-sea interactions in the determination of its opening and activity. First of all, we analysed the role of the katabatic winds, that are widely recognized to be one of the most relevant meteorological features of Antarctica continental margins. The genesis of these winds can be ascribed to the cooling of the near-surface air on the Antarctic Plateau, which determines a strong pressure gradient force downslope. The topographic forcing provides a strong direction consistency to such winds and can cause strengthening in wind speed near steep-sloped coasts. The impact of katabatic winds has been evaluated analyzing the in-situ meteorological data collected in Terra Nova Bay in the framework of the MeteoClimatological Observatory of Italian National Research Antarctic Program (PNRA). We studied the polynya response to the wind forcing and the katabatic regime, establishing subjective criteria to distinguish these strong and persistent winds from a normal wind’s intensification, and find the moments of distribution of katabatic events over the years from 1995 to 2022 examined during the austral winter season (April to October). The characterization of the detected event has been performed through the following indicators, the duration (DUR), the Severity (SEV) and Intensity (INT). Subsequently, we have computed the frequency (FRE, which is the number of events) and the average DUR, SEV and INT of the events belonging to every single year of the 1995-2022 period. In the second step the open water fractions, detected by the Ice Surface Temperature (IST) imagery derived from the MODIS data, were used to estimate the opening and the activity of the polynya during the winter seasons. Then, we estimated the surface heat budget between ocean and atmosphere, which can be assumed to result directly in ice production considering that ocean column is at its freezing point. Assuming that ice production rate depends on the net heat flux and on the polynya extension, it is possible to calculate the total production of salt released during sea ice formation and HSSW volume.