Monitoring the hydrographic structure of the Faroe Current

This report is intended as a component in the continuing effort to optimize the system monitoring volume-, heat-, and salt- transports of Atlantic water in the Faroe Current. In this system, the 4°C isotherm is used as the deep boundary of the Atlantic water layer over most of the monitoring section and the main focus of this report is how to monitor the depth of this isotherm with sufficient accuracy in a cost-efficient and sustainable way.

In the established monitoring system, determination of this isotherm depth is mainly based on data from satellite altimetry. The theoretical basis for this relationship is based on geostrophic adjustment of the density field and we discuss how efficient this adjustment is and what uncertainties may be expected in isotherm depth determination with this method. Compared against CTD profiles, the Root-Mean-Square error of this method was found to be between 31 m and 76 m at the various standard stations on the monitoring section, which is between 65% and 92% of the standard deviations of the 4°C isotherm depth. For the monthly averages used in transport time series, the error may be less. When the altimetry data have been updated to include the latest deployment periods, this will be investigated in a study on how to process the historical data set optimally.

Whatever the result of that study, it would clearly be beneficial to have an independent and hopefully more accurate method than altimetry to monitor the depth of the 4°C isotherm on the section. For that purpose, two PIES (Pressure Inverted Echo Sounders) were deployed at two of the CTD standard stations in 2017 and recovered in 2019 in a cooperation between Havstovan and the University of Hamburg. The results from this experiment indicate that a PIES is able to continuously monitor the depth of this isotherm above the PIES location with a Root-Mean-Square error around 30 m, again compared to snapshot CTD observations.

In addition to this, analysis of the historical CTD data set indicates that monitoring isotherm depth at every second standard stations should allow coverage of the whole section by interpolation. We therefore recommend that 3 PIES are acquired and more or less continuously deployed at stations N05, N07, and N09.