Ocean‐atmosphere heat fluxes at the Ronne Polynya, Antarctica

The Ronne Polynya is a coastal polynya, a\udregion of thin ice or open water in sea ice,\udcaused by the offshore transport of the ice\udby strong winds from the land (Figure 1). As\udsoon as the ice is transported offshore, new\udice forms on the exposed ocean surface and\udis also advected offshore in a continual process,\udearning this type of polynya the nickname\ud‘ice factory’. These polynyas have an\udimportant impact on the regional meteorology\udand oceanography of the high latitudes\udas well as on the global ocean circulation.\udThe exposed ocean surface is relatively\udwarm compared to the cold polar atmosphere,\udand the large temperature and\udhumidity differences result in large sensible\udand latent heat fluxes from the ocean to the\udatmosphere. This leads to a warming and\udmoistening of the atmospheric boundary\udlayer above and downwind of the polynya\udand, through vigorous convective mixing,\udthe formation of a CIBL (convective internal\udboundary layer) (Figure 1). A decrease in the\udocean-atmosphere temperature and humidity\udgradients is caused by this warming and\udmoistening, which results in a decrease in\udthe surface heat fluxes with fetch from the\udshore, or ice shelf front (Renfrew and King,\ud2000). The depth of the CIBL increases with\udfetch due to the warming and also the\udentrainment of warm air from above the\udCIBL (Garratt, 1992).\udAs well as this turbulent heat transfer,\udpolynyas can also influence the balance of\udradiative heat transfer through the generation\udof ice fog (Smith et al., 1990) and\udconvective clouds or plumes (Pinto and\udCurry, 1995). Polynyas, therefore, have the\udpotential to modify and induce mesoscale\udatmospheric motion, impacting on regional\udclimate (Pinto et al., 1995).\udHigh rates of ice production due to the\udlarge ocean-atmosphere heat fluxes and\udthe continual removal of the newly formed\udice by the wind result in extensive brine\udrejection, whereby sea water rejects salt\udon freezing, leaving the sea ice relatively\udfresh and the modified water column relatively\udsalty and therefore dense. This dense\udwater sinks, as shown in Figure 1, accumulating\udon the continental shelf and forming\uda water mass, which eventually contributes\udto the temperature- and salinity-driven global\udocean circulation, known as the thermohaline\udcirculation (THC). Therefore the\udice-formation mechanism within polynyas\udis important for the ventilation of deep\udand bottom water in both the Southern\udand Arctic Oceans (Morales Maqueda et al.,\ud2004). It follows that, in order to accurately\udmodel the response of both the high latitudes\udand global THC to a changing climate,\udprocesses occurring within polynyas must\udbe investigated.\udI was lucky enough to be given the opportunity\udto participate in a British Antarctic\udSurvey (BAS) fieldwork campaign at the\udend of the Antarctic summer field season\udin February 2007. Using an instrumented\udTwin Otter aircraft belonging to the BAS,\udthree flights were conducted over the\udRonne Polynya between 25 and 28 February\ud2007 to investigate ocean-atmosphere heat\udfluxes. Quantification of these heat fluxes is\uda step towards quantifying the surface heat\udbudget, which, together with the surface\udsalinity budget, will aid understanding of\udthe key processes governing deep-water\udformation within polynyas.\udAircraft observations of th