Geostrophic Velocity Measurement Techniques for the Meridional Overturning Circulation and Meridional Heat Transport in the South Atlantic

Abstract

Two ocean general circulation models are used to test the ability of geostrophic velocity measurement systems to observe the meridional overturning circulation (MOC) and meridional heat transport (MHT) in the South Atlantic. Model sampling experiments are conducted at five latitudes (between 15° and 34.5°S) spanning the range of extratropical current regimes in the South Atlantic. Two methods of estimating geopotential height anomalies and geostrophic velocities are tested, simulating dynamic height moorings (T–S array) and current and pressure recording inverted echo sounders (CPIES) deployed within the models. The T–S array accurately reproduces the MOC variability with a slight preference for higher latitudes, while the CPIES array has skill only at higher latitudes resulting from the increased geopotential height anomaly signal. Whether direct model velocities or geostrophic velocities are used, MHT and the MOC are strongly correlated, and successful reconstruction of MHT only occurs when there is skill in the MOC reconstructions. The geopotential height anomaly signal is concentrated near the boundaries along 34.5°S, suggesting that this is an advantageous latitude for deployment of an in situ array. Four reduced arrays that build upon the sites from two existing pilot arrays along 34.5°S were examined. For these realistically sized arrays, the MOC and MHT reconstructions from the T–S and CPIES arrays have comparable skill, and an array of approximately 20 instruments can be effectively used to reproduce the temporal evolution and vertical structure of the MOC and MHT.