Abstract
Strong offshore wind events (SOWEs) occur frequently near the Antarctic coast during austral winter. These wind events are typically associated with passage of synoptic- or meso-scale cyclones, which interact with the katabatic wind field and affect sea ice and oceanic processes in coastal polynyas. Based on numerical simulations from the coupled Finite Element Sea-ice Ocean Model (FESOM) driven by the CORE-II forcing, two coastal polynyas along the East Antarctica coast––the Prydz Bay Polynya and the Shackleton Polynya are selected to examine the response of sea ice and oceanic properties to SOWEs. In these polynyas, the southern or western flanks of cyclones play a crucial role in increasing the offshore winds depending on the local topography. Case studies for both polynyas show that during SOWEs, when the wind speed is 2–3 times higher than normal values, the offshore component of sea ice velocity can increase by 3–4 times. Sea ice concentration can decrease by 20–40%, and sea ice production can increase up to two to four folds. SOWEs increase surface salinity variability and mixed layer depth, and such effects may persist for 5–10 days. Formation of high salinity shelf water (HSSW) is detected in the coastal regions from surface to 800 m after 10–15 days of the SOWEs, while the HSSW features in deep layers exhibit weak response on the synoptic time scale. HSSW formation averaged over winter is notably greater in years with longer duration of SOWEs.
Highlights
Around the Antarctic continent, areas with no or little sea ice cover along the coastlines can be observed even during winter
The new sea ice production within coastal polynyas and the associated brine rejection result in the formation of high salinity shelf water (HSSW), which is a critical source water mass for Antarctic Bottom Water (AABW), a key component of the lower cell of the meridional overturning circulation (Comiso and Gordon 1998; Ohshima et al 2013; Whitworth et al 2013).Antarctic coastal polynyas are regarded as important atmospheric C O2 sink areas (Arrigo et al 2008; Hoppema and Anderson 2007; Tortell et al 2012) resulting from deep convection and massive phytoplankton accumulation compared with adjacent waters (Tremblay and Smith 2007), and play a noticeable role in the global climate system
After these two strong offshore wind events (SOWEs), the center of the cyclone moved eastward out of the polynya region or was succeeded by a weaker cyclone (Fig. 3h–i). These wind patterns support the findings of Turner et al (2009) and Weber et al (2016) that extreme offshore wind speed over the coastal areas of East Antarctica is usually caused by the interaction of local topography and synoptic-scale cyclones. While these studies addressed the role of the western flank of cyclones in enhancing northward wind component that act as offshore wind for most of the East Antarctica coast, we found that the southern flank of cyclones are more important for increasing offshore winds over polynyas that lie on the leeside of protruding topography
Summary
Around the Antarctic continent, areas with no or little sea ice cover along the coastlines can be observed even during winter. The coastal winds of Antarctica are characterized by high-frequency wind events that are often associated with the passage of synoptic- and mesoscale cyclones (Turner et al 2009; Weber et al 2016), and the offshore winds could either be enhanced or weakened during these events depending on the relative location of the coast and the cyclones. We argue that these wind events significantly impact the sea-ice formation and oceanic properties of Antarctic coastal polynyas
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