Atmospheric conditions influencing Antarctic ice sheet surface mass balance
Creators: name: Reid, Phillip A.
Publisher: name: University Of Tasmania
Year: 2023

description: An analysis is made on climate variables to establish the role of dynamic and thermodynamic processes involved in the movement of water vapour southward to the high southern latitudes. Atmospheric moisture fluxes over the Southern Hemisphere are analysed and decomposed into covariant components to obtain an understanding of the dynamical processes involved in the southward transport of moisture to the Antarctic continent. A method is used such that space and time covariant components of mixing ratio and wind velocities are calculated enabling an analysis of water vapour fluxes attributed to separate transient and quasi-stationary cyclonic activity and a mean flow component. Particular emphasis is placed on the snow accumulation rate over the Antarctic. Two data sets are used for this study: global assimilated analyses from the Australian Bureau of Meteorology's Global Assimilation and Prediction Scheme (GASP) and output from the Commonwealth Scientific and Industrial Research Organisation's (CSIRO) nine level coupled atmosphere/ocean model. The GASP data have been used successfully in similar studies and are presented here as a climatology with which to compare model results. A transient C02 model run using the CSIRO model at R21 resolution was conducted and run out with increasing greenhouse gases to 3 times the pre-industrial equivalent C02 conditions following the C02 scenario IS92(a) as specified by IPCC (1992). Data are presented for each of the separate covariant components for snow accumulation rates over the Antarctic (diverging vertically integrated moisture fluxes on a grid point basis), vertical profiles of horizontal moisture fluxes across the Antarctic continental edge and global water vapour stream functions and vertically integrated horizontal moisture fluxes. The snow accumulation rate over Antarctica is calculated at 156.7 mm year-1 from the GASP analyses and 178.2 mm year-1 from the initial conditions of the CSIRO model. On a seasonal basis the transient cyclonic activity provides the greatest net inflow of moisture into the Antarctic continent, followed by that of the quasi-stationary and mean flow components respectively. This establishes cyclonic activity as the major contributor to the transport of moisture into the Antarctic. Results from the stream function analysis show that the transient cyclonic activity is responsible for the increased snow accumulation during the winter months. Vertical profiles of horizontal fluxes show that while summer inflow is close to that of winter inflow, outflow of moisture during summer is far greater than during winter. The transient component is positive in both summer and winter with outflow occurring within the mean flow and the stationary component during summer. Water vapour flux movement across 70¬¨‚àû south is at a maximum at about 900 hPa. While the mean flow circulation is the main contributor to low latitude atmospheric moisture circulation, transient cyclonic activity is the major contributor to mid and high latitude moisture circulation. Results for changes over time from increasing greenhouse gases are presented. Annual net snow accumulation over Antarctica increases by 26.5% after 3xC02, although decreases in some areas are apparent during summer months. Colder months show the greatest increase in net snow accumulation. As time progresses the transient component of the moisture fluxes is the major contributor to accumulation increase in most areas. Vertical profiles of horizontal fluxes show significant increases in inflow into the continent over time, particularly during the winter months. A close association is found between the increases in temperature and net snow accumulation from the climate model. Similar increases are found from the observation based atmospheric analyses over the available period. The similarity between the results from the climate model and the observational analyses provides increased confidence in the reliability of the model results.