The Role of Conservation of Mass in the Satellite-Derived Poleward Moisture Transport over the Southern Ocean

Poleward meridional moisture transport across the Southern Ocean during 1988 is investigated by applying conservation of mass to the wind derivation approach of Slonaker and Van Woert. The moisture field is from the Television and Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) Pathfinder A dataset. The wind field is first derived from a combination of the TOVS temperature profiles and a satellite-based surface wind field using the thermal wind relationship. Then a Lagrange multiplier is introduced in a variational procedure to constrain the wind to conserve mass. The introduction of the conservation of mass reduces the estimates of the moisture flux and net precipitation dramatically in comparison with the nonmass-conserved method in Slonaker and Van Woert. For instance, the estimates of the zonally averaged, vertically integrated moisture flux across 50°S are reduced by 56% and the net precipitation between the 50°S and 60°S latitude belt are reduced by 63%. The reason for the difference is that the nonmass-conserved approach leads to unrealistically strong annual-mean winds in the lower troposphere, which results in an exaggerated mean moisture transport. In contrast, the mass-conserved annual-mean wind compares favorably with the radiosonde observations at Macquarie Island and European Centre for Medium-Range Weather Forecasts and National Centers for Environmental Prediction–National Center for Atmospheric Research reanalyses, and it yields a mean moisture flux consistent with historical estimates. In contrast, the satellite-derived eddy moisture flux is underestimated by about 45% when compared with the radiosonde and analysis studies. This underestimation is probably due to the lower spatial and temporal resolutions of the satellite observations and lack of certain types of ageostrophic winds in the wind derivation.