Sea ice extent, concentration, and seasonality [in “State of the Climate in 2019']

Antarctic sea ice plays a pivotal role in the global climate system. Forming a highly reflective, dynamic, and insulative blanket that varies seasonally in its areal coverage from ~3 × 106 km² to ~19–20 × 106 km², sea ice and its snow cover strongly modifies ocean–atmosphere fluxes and interaction processes (Bourassa et al. 2013). Moreover, brine rejection into the underlying ocean during sea ice formation on some continental shelf areas leads to the formation of Antarctic Bottom Water that contributes to the global ocean overturning circulation (Johnson 2008). Antarctic sea ice also acts as a protective buffer for ice shelves against destructive ocean swells (Massom et al. 2018) and modulates the interaction of warm ocean waters with ice shelf basal cavities to affect basal melt there (Timmermann and Hellmer 2013). Finally, it also forms a key habitat for a myriad of biota—ranging from microorganisms to whales (Thomas 2017)—that are strongly affected by changes in the presence and seasonal rhythms of the sea ice cover (e.g., Massom and Stammerjohn 2010).