Impact of Clouds and Large-Scale Climate Forcing on the Surface Energy Balance and Melting of West Antarctica
West Antarctica is experiencing rapid ice loss and complex regional climate change. This dissertation investigates how cloud properties and the large-scale atmospheric circulation influence surface heat exchange and melting on West Antarctic ice shelves and ice sheet margins using field measurements, satellite observations, and atmospheric reanalysis data.
Surface-based shortwave spectral irradiance measurements and satellite data reveal strong orographic controls on West Antarctic cloud formation and ice-phase microphysics. Orographically-forced updrafts and waves favor rapid conversion of supercooled liquid water into ice, which efficiently attenuates incoming solar near-infrared energy. Frequent intrusions of marine air from the Amundsen Sea anchor a semipermanent cloud band over the West Antarctic Ice Sheet (WAIS) that continues downstream along the Transantarctic Mountain range. Cloud systems sampled downstream at Ross Island tend to be optically thin and radiatively dominated by ice water. In contrast, direct onshore flows of marine air from the Southern Ocean bring low clouds with enhanced liquid-phase spectral signatures. Radiative transfer calculations using vertically-resolved cloud data indicate that, owing to a dominance of longwave effects, clouds radiatively warm the surface of the WAIS in every month of the year. On annual average, cloud cover is estimated to warm the grounded ice-sheet by 34 Watts per square-meter. Thin low-level liquid-bearing clouds, which favor strong radiative heat input to the snow surface, are common during the summer melt season.
Summer atmospheric warming in West Antarctica is favored by blocking activity over the Amundsen Sea and a negative phase of the Southern Annular Mode, which both correlate with El Niño conditions in the tropical Pacific Ocean. Extensive melt events on the Ross and Pacific-sector coastal ice shelves are linked to persistent, intense Amundsen Sea anticyclones, which force intrusions of marine air over the ice-sheet. Surface melting is driven by enhanced downwelling longwave radiation from clouds and a warm, moist atmosphere and by downward turbulent mixing of sensible heat by föhn winds. Since the late 1990s, concurrent with accelerating ocean-driven WAIS mass loss, summer surface melt occurrence has increased from the Pine Island and Thwaites Glacier systems to the eastern Ross Ice Shelf, linked to increasing anticyclonic marine influence and regional sea-ice loss.