Estimating Currents From Argo Trajectories in the Bothnian Sea, Baltic Sea

Abstract

Argo floats have been used in the environmental monitoring of the very shallow Bothnian Sea, a sub-basin of the Baltic Sea, for 5 years as part of the Finnish Euro-Argo programme. The Bothnian Sea is so far considered to be an environmentally healthy part of the Baltic Sea because the deep waters of the basin are well-ventilated by inflowing oxygen-rich saltier and heavier surface layer waters of the Baltic Sea proper. Thus the deep water flow is of interest in the Bothnian Sea. In this study, we used Argo float data from six different long-term missions, from 111 to 512 days, to analyze the deep-water flow in the Bothnian Sea where no continuous monitoring of currents exist. We estimated mainly the flow below the expected halocline from the paths of the floats. We analyzed the movements statistically and estimated the error caused by the surface drift of the floats during their stay at the surface by using 3D hydrodynamic model results as reference data. Our results show a northward flowing resultant current in the deep trench of the Bothnian Sea. There seemed to be very little exchange between coastal zone and open-sea waters in deeper layers. The drifting speed of the floats in the deep layers of Bothnian Sea generally was around 2 cm/s but instantaneous speeds of up to 30 cm/s in the middle-layer (50 dbars) were observed. In the Bothnian Sea deep, the deep trench on the Finnish side of the Bothnian Sea, the vast majority of the observations showed deep currents from south to north, with the same average speed of around 2 cm/s but the instantaneous maximum was smaller at 13 cm/s. Our study indicates that the routine Argo float observations can be used to get information on the deep currents in the basin in addition to hydrographic observations.

Applying area-locked, shallow water Argo floats in Baltic Sea monitoring

“…In case of MBIs, the spreading of the incoming water is of interest, and then spatial variability is important. With enough Argo floats for longer time periods, the deep currents of the areas can also be researched (Roiha et al 2018). Especially in the northern parts of the Baltic Sea, with proper ice avoidance, the possibility to measure under ice can provide valuable information that is hard to obtain in other ways.…”

“…Other basins with similar characteristics should be considered for further float deployments. Another larger area is operated in the Gulf of Bothnia (Haavisto et al 2018;Roiha et al 2018). Other interesting areas for deployment in the Baltic Sea include Bay of Bothnia, Gulf of Gdansk, Bornholm Basin (IOPAN 2018) and Western Gotland Basin.…”

A Synthesis of Marine Monitoring Methods With the Potential to Enhance the Status Assessment of the Baltic Sea

“…It frequently surfaces by changing buoyancy due to an oil filled bladder. In general, an Argo Float profiles at 10-day intervals, but intervals can also be programmed to generate, for instance, multiple profiles a day (Roiha et al, 2018;Siiriä et al, 2019). In marine monitoring Argo Floats can be deployed to autonomously obtain high-resolution vertical profiles of the water column to assess eutrophication and hydrographical conditions.…”

Baltic Sea Operational Oceanography—A Stimulant for Regional Earth System Research

“…In recent years, the shallow water Argo profiler has been demonstrated as an efficient tool for monitoring hydrographic conditions in operational mode (Haavisto et al, 2018). There is also a potential to derive the currents from the shallow water Argo profilers (Roiha et al, 2018). A recent survey on the observational infrastructure to BOOS members showed that the member institutes (not including Russian) own or have access to 211 tide gauges, 7 shallow water Argo floats, 29 buoys, 22 Research Vessels (R/Vs), 6 Remotely Operated Vehicles (ROVs, e.g., gliders), 22 Acoustic Doppler Current Profilers (ADCPs), 25 other fixed stations and 23 FerryBoxes.…”