Ocean‐atmosphere heat fluxes at the Ronne Polynya, Antarctica

The Ronne Polynya is a coastal polynya, a region of thin ice or open water in sea ice, caused by the offshore transport of the ice by strong winds from the land (Figure 1). As soon as the ice is transported offshore, new ice forms on the exposed ocean surface and is also advected offshore in a continual process, earning this type of polynya the nickname ‘ice factory’. These polynyas have an important impact on the regional meteorology and oceanography of the high latitudes as well as on the global ocean circulation. The exposed ocean surface is relatively warm compared to the cold polar atmosphere, and the large temperature and humidity differences result in large sensible and latent heat fluxes from the ocean to the atmosphere. This leads to a warming and moistening of the atmospheric boundary layer above and downwind of the polynya and, through vigorous convective mixing, the formation of a CIBL (convective internal boundary layer) (Figure 1). A decrease in the ocean-atmosphere temperature and humidity gradients is caused by this warming and moistening, which results in a decrease in the surface heat fluxes with fetch from the shore, or ice shelf front (Renfrew and King, 2000). The depth of the CIBL increases with fetch due to the warming and also the entrainment of warm air from above the CIBL (Garratt, 1992). As well as this turbulent heat transfer, polynyas can also influence the balance of radiative heat transfer through the generation of ice fog (Smith et al., 1990) and convective clouds or plumes (Pinto and Curry, 1995). Polynyas, therefore, have the potential to modify and induce mesoscale atmospheric motion, impacting on regional climate (Pinto et al., 1995). High rates of ice production due to the large ocean-atmosphere heat fluxes and the continual removal of the newly formed ice by the wind result in extensive brine rejection, whereby sea water rejects salt on freezing, leaving the sea ice relatively fresh and the modified water column relatively salty and therefore dense. This dense water sinks, as shown in Figure 1, accumulating on the continental shelf and forming a water mass, which eventually contributes to the temperature- and salinity-driven global ocean circulation, known as the thermohaline circulation (THC). Therefore the ice-formation mechanism within polynyas is important for the ventilation of deep and bottom water in both the Southern and Arctic Oceans (Morales Maqueda et al., 2004). It follows that, in order to accurately model the response of both the high latitudes and global THC to a changing climate, processes occurring within polynyas must be investigated. I was lucky enough to be given the opportunity to participate in a British Antarctic Survey (BAS) fieldwork campaign at the end of the Antarctic summer field season in February 2007. Using an instrumented Twin Otter aircraft belonging to the BAS, three flights were conducted over the Ronne Polynya between 25 and 28 February 2007 to investigate ocean-atmosphere heat fluxes. Quantification of these heat fluxes is a step towards quantifying the surface heat budget, which, together with the surface salinity budget, will aid understanding of the key processes governing deep-water formation within polynyas. Aircraft observations of the