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This paper discusses and explains the phenomenon of salinity inversions in the thermocline offshore from an upwelling region during upwelling-favorable winds. Using the non-tidal Central Baltic Sea as an easily accessible natural laboratory, high-resolution transect and station observations in the upper layers are analyzed. The data show local salinity minima in the strongly stratified seasonal thermocline during summer conditions under the influence of upwelling-favorable wind. A simple analytical box model using parameters (including variation by means of a Monte-Carlo method) estimated from a hindcast model for the Baltic Sea is constructed to explain the observations. As a result, upwelled water with high salinity and low temperature is warmed up due to downward surface heat fluxes while it is transported offshore by the Ekman transport. The warming of upwelled surface water allows maintenance of stable stratification despite the destabilizing salinity stratification, such that local salinity minima in the thermocline can be generated. Inspection of published observations from the Benguela, Peruvian and Eastern Tropical North Atlantic upwelling systems show that also there salinity inversions occur in the thermocline, but in these cases thermocline salinity shows local maxima, since upwelled water has a lower salinity than the surface water. It is hypothesized that thermocline salinity inversions should generally occur offshore from upwelling regions whenever winds are steady enough and surface warming is sufficiently strong.

We use an observational data set of tidal gauges in the North Sea to investigate the annual cycle of the M2 and M4 amplitudes and phases. The sea surface elevation amplitude of the M2 can vary by 8–10% and the M4 amplitude by 12–30% over the course of the year, with larger amplitudes in summer. The annual phase variations are in the range of 3–15?. The reason for these variations is the thermal structure of the North Sea: a well-developed thermocline in summer and well-mixed water column during winter. The interaction of the M2 and M4 tides is one of the main drivers of the residual sediment transport. Using an analytical model, the seasonal variability in residual sediment transport is estimated. This transport can vary by 10–50% over the course of the year. These variations are mainly related to the seasonal variability of the M2 and M4 amplitudes.

[1] This study presents, for the first time, micro-structure observations in tidally energetic, weakly stratified regimes obtained in the Wadden Sea, a tidal shallow coastal area in the South-Eastern North Sea characterised by barrier islands separated by tidal gulleys. The tidal currents are typically overlaid by a weak horizontal density gradient due to freshwater run-off from land. The observations in an energetic tidal channel clearly show the expected effects of tidal straining: destratification during flood and increased stratification during ebb. Microstructure observations are consistent with the tidal straining dynamics: during the flood relatively high values of viscous dissipation are observed whilst during ebb the values are substantially smaller. It is also shown that the tidal cycle of stratification and destratification depends on the position in the tidal channel. In parts of the channel, increased stratification occurs during full flood, a phenomenon which can only be explained by advection of stratified water masses formed outside the tidal channel. The observations presented here show the general significance of the tidal straining process for tidally energetic weakly stratified regimes.

[1] Synoptic, high-resolution, measurements of turbulent kinetic energy dissipation, current velocity and water column stratification across a fast (up to 0.7 m s -1 ) oceanic saline gravity current are presented. Our data provide, for the first time, a detailed two-dimensional picture of the turbulence structure inside a gravity current. Strong boundary-layer and interfacial turbulence can be distinguished from a quiet core, and a strong asymmetry of mixing near the outer edges of the gravity current is apparent. This asymmetry is mirrored by the computed entrainment velocities, varying approximately by a factor of 5 across the gravity current. It is argued that the asymmetry is due to rotational effects that can be clearly identified also in the velocity and density fields.