Motivated by the observed behaviour of a salt wedge estuary, I attempted to assess the hydraulic state of a stratified shear flow, (i.e., either sub- or supercritical) by a calculation of the long internal wave speeds.  This turned out to be a spectacular failure, which I now believe to be caused by the commonly used method of calculating wave speeds -- performing a normal mode analysis.  When calculating internal or surface wave speeds, one must specify the ocean currents.  If the currents are represented by a smooth profile U(z), then wave speeds obtained by normal modes always exist outside the range of the U(z) profile (with one exception that I will not go into...).  This bizarre result naively suggests that no current is strong enough to advect a gravity wave downstream, and flies in the face of common sense.  Indeed, hydraulic theory relies on this process of arrested gravity waves in order to define supercritical flows.  I will explore the strange behaviour of gravity waves in strongly sheared flows and show that gravity waves can be advected downstream by currents, but only if we relax our definition of a gravity wave. For this purpose I am going to propose something called a quasi-wave.

Coastal systems with multiple basins and inlets, like the Dutch Wadden Sea, exhibit complex transport dynamics shaped by tides, atmospheric forcing, and basin geometry with strong interannual variability. To elucidate the long-term variability of the Lagrangian transport, it is important to perform a multi‑decadal study. We focus on transport time scales (TTS), residual displacement, and dispersion using high-resolution simulations and particle tracking. First, depth-averaged currents were used to advect the Lagrangian tracers to get a general idea of the long-term variability, and later, the 3D fields were used to understand the importance of 3D processes.

When using the depth averaged currents, most spatial-temporal variability in TTS and residual displacement is driven by winds from the dominant southwesterly sector, aligned with the system’s topography. On the one hand, TTS show marked seasonal variability because strong southwesterly winds are common in autumn and winter and generally absent in spring-summer. The wind variability is linked to large scale atmospheric patterns. On the other hand, large residual displacements are forced by shorter wind events (i.e. storms), and although these events are more common in autum-winter, a seasonal signal is not clearly observed. Finally, dispersion is mainly controlled by the tides.

When using the three-dimensional currents, we observe two main changes in the statistics of the Lagrangian transport: 1) The TTS is reduced in the spring-summer, also reducing the seasonal variations and 2) dispersion increases by a factor of three. These changes highlight the importance of three-dimensional processes due, for example, to stratification, secondary flows, and the estuarine circulation.

A gradient force, together with the Coriolis-Stokes and vortex force are residual surface wave-driven forces showing up in the equations of motion averaged over the wave cycle. They are part of the so-called Craik-Leibovich (CL) equations, which are often used to understand surface wave-driven (Langmuir) turbulence, and to simulate the wave effects in coastal numerical ocean models. Since observational or modelling evidence for the surface-wave driven forces is only partly available,
we validate in a systematic manner the CL equations using a two-phase model of air and water, and explore the limits of the assumptions of the CL equations. It turns out that the Coriolis-Stokes and vortex force are very good approximations within the limits of the assumptions, and that they remain useful even beyond those limits. The gradient force, however, differs from versions put forward in the literature, which has consequences for the treatment of wave-driven (Langmuir) turbulence in mixed layer closures. Examples using LES models for this flaw of the closures will be shown. An energetically consistent way to couple the CL equations to a spectral surface wave model is also discussed and a recent suggestion by Vanneste and Young is expanded by the treatment of the stress by the waves on the ocean. That stress, part of the so-called wind stress on the ocean, turns out to be an additional new wave-driven force with finite extent into the water column.

The Northern California Current (NCC) System, which is the eastern boundary upwelling system that lies along the U.S. West Coast, supports major fisheries that respond strongly to seasonal and interannual variability in water properties — such as temperature, salinity, and dissolved oxygen — that directly influence organisms. The NCC coastal ocean exhibits strong seasonal changes in temperature and salinity associated with coastal upwelling and downwelling circulations. These circulations are affected by the vertical density stratification, which itself affects the strength of the circulation. The density stratification in the NCC has contributions from both temperature and salinity, as does the density stratification farther offshore in the northeast Pacific. The observational literature does not detail the relative contributions of temperature and salinity to the density stratification in the NCC nor the seasonal changes in those contributions. Since 1997, shipboard hydrographic transects have been carried out biweekly to monthly on the inner portion of the Newport Hydrographic (NH) Line. These sections cover the continental shelf and slope off Oregon State, USA at NHL stations NH01–NH25, 1-25 nautical miles (to 46 km) from the coast. Here, we use our gridded version of the >550 cross-shelf CTD sections available in this historic data set to examine the vertical density stratification over the continental shelf and slope. We find (1) the stratification is controlled by salinity except in a shallow seasonal thermocline present July–October, and (2) in March–October, there is substantial salinity stratification in the mid-depth and lower water column. Next, we examine the origin of that mid-depth stratification by using glider sections from 2005-present extending to ~450 km offshore. The mid-water column salinity stratification over the shelf and slope off Oregon in summer and fall connects directly to the permanent halocline of the northeast Pacific. In April–June, the double pycnocline structure present offshore of the continental shelf — the seasonal pycnocline above 25 m depth and the permanent halocline at ~100 m depth — merges over the continental shelf to form a single pycnocline. Consistent with the seasonal evolution of stratification on the Washington shelf observed in 2005 by McCabe et al. (2015), the density stratification over the continental shelf off Oregon decreases from May to September. This decrease in stratification is the opposite of the pattern that would explain the shoaling onshore flow observed as the upwelling season progresses.

Turbulence dynamics within a tidally varying fluid-mud layer were investigated in the hyper-turbid Ems Estuary using horizontally deployed air-foil shear probes and a 1D electromagnetic current sensor at fixed depth over two consecutive tidal cycles. These observations were complemented by ADCP data and vertical OBS and CTD casts. The analysis is focused on the relationship between turbulent kinetic energy (k) and turbulent dissipation (eps) in response to tidally-varying SSC and sediment-induced stratification.

We identify four characteristic phases: flood entrainment, late-flood restratification, fluid mud, and ebb shear dispersion. During flood entrainment, strong near-bed shear coincides with elevated k and eps, consistent with an actively-turbulent, near-equilibrium state. In contrast, the fluid-mud phase exhibits elevated suspended sediment concentration (SSC), persistently weak epsilon, and systematic departures from the canonical scaling k~eps^2/3, indicating a partial decoupling between velocity variance and turbulent dissipation despite comparatively weak stratification in the fluid-mud layer itself. Late-flood restratification and ebb shear dispersion act as transitional states.

Across the tidal cycle, the gradient Richardson number does not consistently predict turbulence activity, particularly when wavelet analysis reveals energetic motions below the buoyancy frequency indicative of internal wave activity. Overall, the results suggest that fluid mud can modify the structure and efficiency of the turbulence cascade through interaction of turbulence, sediment-induced stratification, and internal wave motions. Our findings help to improve the understanding of sediment transport in hyper-turbid systems, which is particularly important for improving parametrizations of turbulence in numerical transport models.

In this short presentation I will sketch how it was for me to pursue a PhD with Hans. The main research question was quite simple: “How does mixing relate to the estuarine exchange flow for an inverse estuary?” I will briefly summarize the pathway to answering this question for the Persian Gulf, which can be thought of as an inverse Baltic Sea.

In my talk I will interweave the scientific part with my experiences working with Hans. This will also include my case of his famous words: “I leave the rest (of what I do not have time for) to you to finish”.

This study investigates the impact of sub-mesoscale structures (SMS) on large-scale circulation in the Baltic Sea. The results of a high-resolution simulation using the General Estuarine Transport Model (GETM), covering a period of ~9 years with realistic atmospheric and hydrological forcing, were analysed and compared with lower-resolution simulations that were otherwise similar. In the high-resolution simulation, SMS are resolved at a horizontal scale of 250 metres, and both Eulerian tracers and Lagrangian particles were employed to analyse the statistical properties of the flow and tracer fields, both with and without considering SMS. The results demonstrate that SMS significantly impact the circulation and the dispersion of substances such as nutrients and pollutants. In regions where there is an increased average occurrence of eddies, and thus SMS, the transit times of particles originating in the coastal region differ considerably from those in coarser-resolution simulations. On average, including SMS hinders the spreading of tracers originating in coastal waters towards basin centres in the surface mixed layer but enhances tracer transport towards the subsurface layer in offshore areas. However, surface layer currents are generally more persistent in low-resolution simulations, moving tracers more rapidly away from their sources than in the high-resolution simulation. Consequently, the influence of SMS on the dispersion of tracers depends on the direction of the mean flow. At high resolution, tracer transport is slowed down by the mean flow; however, it is accelerated perpendicular to the mean flow in regions where eddies are generated by baroclinic instability (e.g. the north-western Gotland Basin and the Gulf of Finland). However, the calculated current patterns are complex. In coastal currents, horizontal velocities are higher at a higher spatial resolution, while in SMS, vertical velocities are higher. The latter results in higher diffusivity of tracers and smaller horizontal transport in the surface layer.

Submarine canyons (trenches) have a major impact on transport across continental shelves, impacting the upwelling and downwelling of deeper water masses and exchange across the shelf. Possibly one of the most well studied shelf seas in the world is the North Sea, yet observations of cross-shelf transport through the Norwegian Trench (NT) are rare due to its remote location and excessive trawling. Here we preset unique observations from the successful deployment and recovery of two moored ADCPs from June 2023–May 2024, highlighting periods of upwelling and downwelling. We show that deep flows are driven by local winds and remotely forced coastal trapped waves (CTWs). Strong AW inflows occur during along-slope winds, while strong westerly winds intensify the transport of matter. A seasonal signal is evident that is related to the NAO and intense storms. We also present the first observations of episodic upwelling of deep Arctic Intermediate Water during south-westerly winds.

Based on the PhD thesis of Anna Enge (in prep).

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Seasonally stratifying shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer. Predictions suggest global warming will accelerate this deficiency. In the first part of my talk we will integrate a novel turbulence timeseries with vertical profiles of water column properties from the seasonally stratifying Celtic Sea to estimate diapycnal oxygen and biogeochemical fluxes. The profiles reveal a significant subsurface chlorophyll maximum and associated mid-water oxygen maximum. We show that the oxygen maximum supports both upward and downwards O2 fluxes. The upward flux is into the surface mixed layer, whilst the downward flux into the deep water will partially off-set the seasonal O2 deficit. The results indicate the fluxes are sensitive to both the water column structure and mixing rates implying the development of the seasonal O2 deficit is mediated by diapcynal mixing. Analysis of current shear indicate that the downward flux is supported by tidal mixing, whilst the upwards flux is dominated by wind driven near-inertial shear.

In the second part of my talk we will present novel measurements of mid-water turbulence in a wake generated by a moderate tidal flow past a floating wind turbine foundation. The measurements reveal that the mid-water turbulence is enhanced by an order of magnitude when compared to the non-wake situation. These new results demonstrate the potential of the tidal flow past the turbine foundations to disrupt stratification and hence enhance diapcynal fluxes of nutrients and oxygen. These results imply that in assessing the environmental impact of proposed new wind farms it is important to consider the impact of mixing associated with the wake generated by the tidal flow past the turbine foundation.

Mixing is one of the most notable physical processes in estuaries, and estuaries are among the best places to study mixing at geophysical scales, owing to its persistence and intensity in these energetic environments in which fresh and salt water meet.   But when we note how the word “mixing” has been used over the years, its meaning has evolved.  From the 1950’s to the 1970’s, estuarine mixing was typically represented by a horizontal dispersion coefficient, which embodies the sum of all of the processes at a vast range of temporal and spatial scales that do the mixing.  As oceanographic instruments got more sophisticated from the 1980’s to early 2000’s, the vertical structure of estuaries could be resolved, and “estuarine mixing” switched to the vertical coordinate.  As the measurements of turbulence advanced, so did the parameterization of vertical mixing via 2-equation turbulence closure formulations.   Ideas like turbulent kinetic energy dissipation and buoyancy flux became the watchwords of the estuarine mixing community.  Then in 2009, Hans Burchard woke us all up to the real meaning of estuarine mixing—the destruction of salinity variance—which was the underlying process in all the previous “mixing” studies but was not explicitly quantified.  In the last 10 years, Hans and other mixing afficionados have linked the destruction of salinity variance, which happens at scales of millimeters, to the estuarine scale salt transport.  Between the molecular scale at which mixing actually happens and the mouth of the estuary where the mixed water joins the ocean resides a remarkable suite of processes that belie the elegant simplicity of Hans’ Universal Law of Estuarine Mixing!

In this summary, I will show how General Ocean Turbulence Model (GOTM) captures complex physical and biogeochemical processes across varying temporal scales when applied to distinct oceanographic sections in the North Atlantic.  Operating as a high-resolution one-dimensional water column framework, GOTM was implemented in both open-shelf and coastal environments to resolve critical upper-ocean dynamics.

In the Santander standard oceanographic section (southern Bay of Biscay), a purely physical configuration of GOTM was utilized to analyze mixed layer depth (MLD) variability over a 60-year reconstruction. Validated against long-term hydrographic time series, the model successfully replicated decadal trends and extreme winter deepening events. It proved that 1D vertical mixing physics can accurately simulate counter-intuitive climate responses, such as the winter MLD shallowing observed during the 1970s and 1980s concurrent with generalized upper-water warming trends.

In the shallow L4 coastal oceanographic station (off Plymouth, English Channel), GOTM was coupled with the European Sea Regional Ecosystem Model (ERSEM) to evaluate the sensitivity of air–sea $C{O}^2$ gas exchange. Validated against extensive in-situ observations, the coupled framework effectively captured the seasonality of nutrients, biomass, and carbonate chemistry, identifying the site as a weak net carbon source. Sensitivity experiments at L4 revealed that resolving fine-scale near-surface vertical temperature gradients and implementing satellite data assimilation drastically alter modeled biogeochemical fluxes.

Together, these applications highlight that despite its one-dimensional nature, GOTM serves as a computationally efficient and highly precise tool for diagnosing vertical transport, validating multi-decadal physical reconstructions, and reducing uncertainties in marine carbon cycle projections across diverse oceanographic sections.

Despite significantly reduced nutrient loading during the last decades, the Baltic Sea has not yet recovered to Good Environmental Status (GES). Using a 69-year (1954–2022) coupled biogeochemical dataset, we isolate mechanisms decoupling external forcing from internal ecosystem responses. Structural analysis reveals highly asynchronous regime shifts: five external loading shifts triggered only three biological shifts, demonstrating initial ecological buffering and subsequent lock-in. Deep-basin biogeochemistry lags external load reductions by 14–19 years. While shallow margins respond reversibly to external loading, phase-space trajectories and Information Flow Rate analyses prove deep basins (≥50 m) are trapped in an alternative stable state. Here, the legacy dissolved inorganic phosphorus reservoir supersedes external forcing as the primary causal hub controlling phytoplankton biomass and primary production. Consequently, systemic recovery requires reducing nutrient forcing significantly below historical thresholds. Achieving basin-wide GES remains bottlenecked by deep-water legacy effects, necessitating spatially differentiated environmental targets and adjusted regulatory timelines.