Multi-sensor in situ observations to resolve the sub-mesoscale features in the stratified Gulf of Finland, Baltic Sea

Abstract

Abstract. High-resolution numerical modeling, remote sensing, and in situ data have revealed significant role of submesoscale features in shaping the distribution pattern of tracers in the ocean's upper layer. However, in situ measurements are difficult to conduct with the required resolution and coverage in time and space to resolve the sub-mesoscale, especially in such relatively shallow basins as the Gulf of Finland, where the typical baroclinic Rossby radius is 2-5 km. To map the multi-scale spatiotemporal variability in the gulf, we initiated continuous measurements with autonomous devices, including a moored profiler and Ferrybox system, which were complemented by dedicated research-vesselbased surveys. The analysis of collected high-resolution data in the summers of 2009-2012 revealed pronounced variability at the sub-mesoscale in the presence of mesoscale upwelling/downwelling, fronts, and eddies. The horizontal wavenumber spectra of temperature variance in the surface layer had slopes close to −2 between the lateral scales from 10 to 0.5 km. Similar tendency towards the −2 slopes of horizontal wavenumber spectra of temperature variance was found in the seasonal thermocline between the lateral scales from 10 to 1 km. It suggests that the ageostrophic submesoscale processes could contribute considerably to the energy cascade in such a stratified sea basin. We showed that the intrusions of water with different salinity, which indicate the occurrence of a layered flow structure, could appear in the process of upwelling/downwelling development and relaxation in response to variable wind forcing. We suggest that the sub-mesoscale processes play a major role in feeding surface blooms in the conditions of coupled coastal upwelling and downwelling events in the Gulf of Finland.

Wind-driven stratification patterns and dissolved oxygen depletion in the area off the Changjiang (Yangtze) Estuary

“…Those 410 studies are difficult to arrange with conventional research vessel surveys only. Autonomous measurement platforms, such are profiling moorings (Lips et al, 2016;Sun et al, 2016), moored sensor chains (Bailey et al, 2019;Venkatesan et al, 2016), which allow capturing the variability in necessary vertical-temporal resolution can be used. Underwater gliders (Liblik et al, 2016;Rudnick, 2016) might be complicated to use due to strong tidal velocities and heavy ship traffic, but are worthy to consider as well.…”

Salinity inversions in the thermocline under upwelling favorable winds

“…As such, it does not matter much dynamically, whether or not salinity of upwelled water is higher or lower than the surface waters. When it is higher than in the surface waters such as in the Baltic Sea, salinity minima occur when saltier water, warmed due to surface heat fluxes, is transported over the older surface waters [ Lass et al , ; Lips et al , , this study]. In contrast, as, for example, in the eastern tropical North Atlantic [ Brandt et al , ], the Benguela upwelling system [ Armstrong et al , ], and the Peruvian upwelling system [ Thomsen et al , ], upwelled water is less salty than surface waters such that the same process of offshore Ekman transport and surface warming leads to salinity maxima in the thermocline.…”

“…Observations by Lass et al [] show distinct salinity minima inside the thermocline below the surface mixed layer under upwelling favorable winds, see their Figures 5 and 15. Recently published data from the Gulf of Finland of the Baltic Sea include thermocline salinity minima as well for various summer days in 2010 and 2012; see Figure 7 by Lips et al []. Those observed salinity minima are, however, not further discussed by Lass et al [] and Lips et al [].…”

“…Recently published data from the Gulf of Finland of the Baltic Sea include thermocline salinity minima as well for various summer days in 2010 and 2012; see Figure 7 by Lips et al []. Those observed salinity minima are, however, not further discussed by Lass et al [] and Lips et al [].…”

Propagation of Impact of the Recent Major Baltic Inflows From the Eastern Gotland Basin to the Gulf of Finland

“…Available data from research vessel cruises and moored vertical profilers (e.g., Lips et al, 2016) were used. In addition, HELCOM (Helsinki Commission) data (http://ocean.ices.dk/helcom, 31 January 2018) and historical data collected in the Department of Marine Systems were used.…”