Monitoring the velocity structure of the Faroe Current

This report is intended to document continuing efforts to optimize the monitoring of volume, heat, and salt transport in the Faroe Current. The present monitoring system is based on a combination of in situ observations and data from satellite altimetry as documented by Hansen et al. (2015). The altimetry data are freely available, whereas in situ observations are costly in equipment, ship-time, and manpower. Efforts have therefore continuously been made to reduce the reliance on in situ observations.

Although the altimetry data have become a very useful tool for analyzing variations in surface velocity, they do have three major problems: (1) Due to uncertainties in the geoid, absolute surface velocities based solely on altimetry are not reliable. (2) A priori, the altimetry data only apply to surface, not subsurface, velocities. (3) Altimetry data have no direct information on the temperature and salinity distributions.

In this report, we address the first two of these problems. By using the complete data set of in situ observations, especially current measurements from moored ADCPs, we are able to calibrate the altimetry data so that they give reliable surface velocities, solving problem (1). These observations furthermore show that the vertical variations in velocity are sufficiently consistent that subsurface velocities and transport values can be derived from surface velocity, and hence altimetry, throughout the layer of Atlantic water on the section on monthly time scales or longer, solving problem (2).

Based on the analysis presented, we therefore conclude that in situ current measurements are no longer necessary to monitor the velocity structure of the Faroe Current, although we recommend that one of the long-term ADCP mooring sites is maintained to guard against potential drastic changes in the system. The new information has been incorporated into the algorithms for deriving transport time series, but the differences between the new time series and previously published time series are minimal.

This leaves the third problem: monitoring the temperature and salinity fields. We have a large historical data set from ship-borne observations and moored temperature sensors, but a central component is missing. This is the data from two moored PIES (Pressure Inverted Echo Sounders), borrowed from the University of Hamburg, which due to German cruise cancelations, bad weather conditions, and instrument malfunction have not yet been recovered. The decision on how to monitor the temperature and density structure, and how to use this information to determine Atlantic water transport, is therefore delayed and will be treated in a second report, planned to be finalized later in 2019.