Abstract
During katabatic wind events in the Terra Nova Bay and Ross Sea polynyas, wind speeds exceeded 20 m s-1, air temperatures were below -25 ℃, and the mixed layer extended as deep as 600 meters. Yet, temperature and salinity profiles were not perfectly vertical, as one would expect with vigorous convective heat loss. Instead, the profiles revealed bulges of warm and salty water starting at the ocean surface and extending to the top tens of meters. Considering both the colder air above and colder water below, we surmise that the increase in temperature and salinity reflects latent heat and salt release during unconsolidated frazil ice production throughout the upper water column. We use a simplified salt budget to analyze these anomalies to estimate in-situ frazil ice content 5.8 and 0.13 kg within the top 50 m of the water column. Estimates of vertical mixing by turbulent kinetic energy dissipation reveals rapid convection in these unstable density profiles, and mixing lifetimes from 2 to 30 minutes. The corresponding ice production rates yield an average ice thickness of 52 cm day-1, which compares well with previous empirical and model estimates. However, our individual estimates of production up to 358 cm day-1 reveal the intensity of short-term ice production in the windiest sections of the Terra Nova Bay Polynya.
Highlights
This research focuses on results from two coastal latent heat polynyas in the Ross Sea, which will be referred to as polynyas moving forward
For the purposes of this study, we focus on the 13 stations (CTD 23-35) that occurred within the Terra Nova Bay polynya (TNBP) and 4 stations (CTD 37-40) within the Ross Sea polynya (RSP) during katabatic wind events
3.2 Evaluating the fidelity of the CTD measurements To evaluate the uncertainty associated with the temperature and salinity anomalies at each of the polynya stations, we compared each anomaly to the initial accuracy of the Seabird 911 CTD (SBE 911): ± 0.001 °C and ± 0.0003 S m-1 or 0.00170 g kg-1 when converted to absolute salinity
Summary
Latent heat polynyas form in areas where prevailing winds or oceanic currents create divergence in the ice cover, leading to openings surrounded by extensive pack ice (Armstrong, 1972; Park et al, 2018). Two criteria for ice production in polynyas from supercooled water are large net heat loss from the water and transport of the frazil ice away from the formation region; both criteria are achieved in the polynya by katabatic winds and cold air temperatures (Coachman, 1966). These conditions generate sea ice as fine disc-shaped or dendritic crystals called frazil ice. These conditions generate sea ice as fine disc-shaped or dendritic crystals called frazil ice These frazil ice crystals depicted, measure about 1-4 millimeters in diameter and 1-100 micrometers in thickness (Ushio and Wakatsuchi, 1993). TNBP produces especially dense HSSW, driven by its higher salinity, and despite being smaller than RSP, it produces approximately 1-1.23 Sv annually (Buffoni et al, 2002; Fusco et al, 2009; Orsi & Wiederwohl, 2009; Kurtz & Bromwich 1985, Van Woert 1999b)
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