Monitoring the in/out flow through the Danish straits using satellite synthetic aperture radar

The aim of this study is to investigate the potential of spaceborne synthetic aperture radar (SAR) to monitor the Baltic Sea inflow/outflow circulation through the Danish straits. The flow in the Danish straits is mainly driven by changes in atmospheric forcing and is dominated by irregular inflow and outflow events. SAR provides high spatial resolution observations of the sea surface, which are particularly relevant in coastal areas and shelf seas. During the last decade, a new application of SAR measurements based on the analysis of the Doppler shift has emerged. The SAR Doppler shift is directly related to the surface circulation, thus direct measurements of surface currents are possible. It is however a challenging problem in practice due to the wave contribution to the observed Doppler shift.

The main limitation of spaceborne SAR for monitoring fast evolving ocean processes is the long revisit time. In order to overcome this limitation, data from three satellites are combined in this study, namely Sentinel-1A, Sentinel-1B and TanDEM-X. Sentinel-1 is a conventional single-antenna SAR, while TanDEM-X is an along-track interferometric SAR. In addition, the two systems differ in the operating frequency and in the imaging mode. In this study, two months of opportunistic data (June and July 2020) covering the Danish strait (Fehmarn Belt) are used. This time period is constrained by the availability of coincident (Sentinel-1 and TanDEM-X) data covering the area of interest. Since TanDEM-X is not an ocean-dedicated mission, acquisitions suitable for ocean current retrieval are sporadic.

Comparison of the derived radial velocities shows a good agreement between Sentinel-1 and TanDEM-X, provided both datasets are calibrated over land and the time delay between acquisitions is below ~20 min. The residual difference is probably due to the wave-induced Doppler shift. The SAR derived velocities are compared to the Copernicus analysis product (BALTICSEA\_ANALYSIS\_FORECAST\_PHY\_003\_006) and in-situ measurements. A reasonable agreement is found, provided that the wave-induced Doppler shift is taken into account. The study also investigates the relationship between the surface current along the Fehmarn Belt, the sea surface wind and the sea level, as an attempt to understand the main drivers of the surface flow. First, a high variability in the duration of inflow/outflow is observed. The shortest and the longest durations are one day and 10 days, respectively. Second, it is found that the surface current is predominantly in the east-to-west direction (outflow). Third, the relationship between the local wind and the surface current is stronger in the outflow situation, whereas the relationship between the surface current and the sea level gradient is stronger in the inflow situation. Though these observations agree with previous studies, it is however difficult to draw firm conclusions on the driving force from these limited dataset, hence additional data are required to verify these results. However, the study clearly demonstrates the potential of SAR for monitoring sea surface flows.