Estimating Mooring Motion when the Pressure Sensors Fail: A Method Employing Inverted Echo Sounders

Abstract

In the presence of a strong current, such as the Gulf Stream or the North Atlantic Current, current meter moorings are known to ?blow over? due to drag from the moving water. This dipping of the current meters, which has been documented to exceed 500 m in some cases, can significantly affect estimates of fluxes on level surfaces. Pressure measurements made by sensors collocated along the mooring near each current meter are commonly used to correct for this mooring motion. Data from a current meter mooring near 42°N, 45°W are used to demonstrate that, in cases where there is a failure of the pressure sensors, measurements from an inverted echo sounder near the current meter mooring can be combined with the mooring temperature records and historical hydrography to produce ?synthetic? pressure records for current meters within the main thermocline depth range. Pressures at other current meters on the mooring can then be determined using mooring design parameters. This technique allows corrections for mooring motion when they would otherwise be impossible due to the loss of the directly measured pressure records. Comparison to directly measured pressures in the main thermocline from a mooring near the North Atlantic Current demonstrates that this technique can determine synthetic pressure records to within a root-mean-square difference of about 46 dbar for an instrument with observed mooring motion related pressure dips of 200?500 dbar. The technique is also applied to a number of other current meters in the North Atlantic Current region as well as instruments that were moored in the Subantarctic Front near 143°E to demonstrate where the technique will and will not work.