Abstract. The Dutch Wadden Sea is a region of intertidal flats located between the chain of Wadden Islands and the Dutch mainland. We present numerical model results on the tidal prisms and residual flows through the tidal inlets and across one of the main watersheds. The model also provides insight into the pathways of fresh water originating from the two sluices at the Afsluitdijk (enclosure dike) through the use of passive tracers. All these results are obtained from threedimensional numerical simulations carried out with the General Estuarine Transport Model (GETM), at a horizontal resolution of 200 m and with terrain-following vertical coordinates with 30 layers. We concentrate on the years 2009-2010, for which we impose meteorological forcing, freshwater discharge, and boundary conditions for tidal forcing and storm surges. Results from the model show an excellent agreement with various observational data sets for sea surface height, temperature, salinity and transport through the Texel Inlet. The simulations show that although tides are responsible for a characteristic pattern of residual transport through the inlets, the wind imposes a large variability on its magnitude and can even invert its direction during strong southwesterly winds. Even though these events are sporadic, they play an important role in the flushing of the Dutch Wadden Sea, as they strongly diminish the flushing time of fresh water. In addition, wind can force a residual transport across the Terschelling watershed of the same order, if not larger, than through any of the main tidal inlets, despite the fact that its tidal prism is much smaller than any of those of the inlets. For the pathways of fresh water, the Terschelling watershed turns out to be more important than was previously thought, while the opposite holds for the Vlie Inlet.

[1] In 2001, two relatively saline intrathermocline eddies (ITEs) were observed southeast of Madagascar at 200 m depth. They are characterized by a subsurface salinity maximum of over 35.8 at potential temperatures between 18°and 22°C. The oxygen concentrations within the high salinity cores are slightly elevated compared with those of the surrounding water. Their horizontal extent is about 180 km, several times the Rossby deformation radius, while their thickness is about 150 m. The observed circulation around the ITEs is anticyclonic and maximum velocities of 20 to 30 cm/s are observed at 200 m depth. In these cores the potential density anomaly (25.0 < g < 25.9 kg/m 3 ) has a relatively low vertical gradient and therefore a low planetary potential vorticity. The hydrographic properties of these ITEs are distinctly different from those of the surrounding thermocline water, and especially from the much fresher water mass in the East Madagascar Current. Strong evidence has been found that the distant formation area of the water mass in the ITEs is the subtropical Southern Indian Ocean east of 90°E and south of 25°S, where Subtropical Underwater (STUW) is formed with similar characteristics. Similar high-salinity cores as the ITEs are also found in the thermocline around 200 m depth along an almost zonal section between Madagascar and 100°E. Differences between the hydrographic properties of these cores and the ITEs near Madagascar may partly be explained by interannual variations in the temperature and salinity of the surface mixed layer water in the possible formation area.

[1] In April 2001 four hydrographic sections perpendicular to the southern East Madagascar Current were surveyed as part of the Agulhas Current Sources Experiment. Observations with a vessel mounted and a lowered ADCP produced information on the current field while temperature, salinity, oxygen and nutrient data obtained with a CTDRosette system, gave information on the water mass structure of the currents southeast of Madagascar. The peak velocity in the pole-ward East Madagascar Current through these four sections had a typical magnitude of $110 cm/s, while the width of this current was of the order of 120 km. The mean pole-ward volume transport rate of this current during the survey above the 5°C isotherm was estimated to be 37 ± 10 Sv. On all four sections an undercurrent was observed at intermediate depths below the East Madagascar Current. Its equator-ward transport rate amounted to 2.8 ± 1.4 Sv. Offshore of the East Madagascar Current the shallow South Indian Ocean Countercurrent was observed. This eastward frontal jet coincided with the barotropic and thermohaline front that separates the saline Subtropical Surface Water from the fresher Tropical Surface Water in the East Madagascar Current. The near-surface geostrophic flow of the East Madagascar Current, derived from satellite altimetry data from 1992 to 2005, suggests a strong variability of this transport due to eddy variability and interannual changes. The long-term pole-ward mean transport of the East Madagascar Current, roughly estimated from those altimetry data amounts to 32 Sv. The upper-ocean water mass of the East Madagascar Current was very saline in 2001, compared to WOCE surveys from 1995. Comparison of our undercurrent data with those of the WOCE surveys in 1995 confirms that the undercurrent is a recurrent feature. Its water mass properties are relatively saline, due to the presence of water originating from the Red Sea outflow at intermediate levels. The saline water was advected from the Mozambique Channel to the eastern slope of Madagascar.

The objective of the study described in this paper is to localize the transport path of suspended particulate matter (SPM) in the Dutch coastal zone in the southern North Sea. It is known that a large mass of SPM is transported northward from the Strait of Dover, which is however mostly hidden from satellite and other surface measurements. The study area is located at 80 km north of the Rhine-Meuse estuary mouth in the far-field plume of the region of freshwater influence (ROFI). We investigate the occurrence and persistence of a turbidity maximum zone (TMZ) in an area closer to the coast than studied in previous observational programs. Shipboard measurements of vertical profiles of SPM concentrations, density and current velocities with a high cross-shore spatial resolution are presented. A turbidity maximum zone is found at a distance between 0.5 and 3 km from the coast along 30 km of the coastline. Observed concentrations are shown to vary strongly within a tidal cycle, and also between contrasting meteorological conditions in terms of the springneap tidal cycle, the significant wave height and the wind force. Temporary stratification is observed during spring tides, and occurs on the ebb phase of the tidal cycle. Cross-shore transports at a transect perpendicular to the coast show an accumulation of SPM in the TMZ within one tidal cycle.Possible mechanisms for this accumulation close to the coast are discussed.2

Long-term measurements with a hull mounted acoustic Doppler current profiler (ADCP) under the ferry, crossing the Marsdiep inlet between the mainland and the island of Texel (the Netherlands), were used to determine the volume flux and the flux of suspended particulate matter (SPM) through this inlet for the period [2003][2004][2005]. Profiles of the SPM concentration were estimated from profiles of the acoustic backscatter intensity in which the shift between the low and high turbulent regime is taken into account. Calibration constants and tuning parameters were estimated by using data collected during 7 different 13 hour anchor stations. The residual (water) volume flux through the inlet appears to vary strongly on a variety of time scales from daily to inter-annual. A regression analysis indicates that the daily residual volume transports correlate well with the daily mean wind component from the south; the latter likely drives the residual flow along the coast of Holland. The observed residual SPM transport of 7 to 11 Mton/yr is dominated by the correlation between tidal velocity and SPM concentration variations. This leads to an import as currents and SPM concentrations during flood were higher than those during ebb, a process generally known as tidal asymmetry. Our analysis has shown that regular observations with a ferry mounted ADCP is an effective method to monitor the volume and SPM transport processes in an estuary.

The North Sea hydrodynamics are key to the redistribution of methane released at site 22/4b, located at (57 o 55'N, 1 o 38'E) in the UK Central North Sea, 200 km east of the Scottish mainland. This review summarizes the current state of knowledge on the North Sea circulation, stratification and variability therein and briefly discusses the potential consequences for the distribution and fate of methane released from site 22/4b or other seabed sources. Astronomical tidal waves follow an anti-clockwise path and tide-topography interaction generates a residual circulation in the same direction. Wind stress forcing can enhance, reduce or even reverse this circulation. Variations in the strength of the Fair Isle Current (FIC) are important. The FIC enters the North Sea between The Orkneys and Shetland, follows approximately the 100-m isobath, passes along site 22/4b, and ends up in the Norwegian Trench. The North Atlantic Oscillation (NAO) also causes variability. A positive (negative) NAO index is associated with stronger (weaker) than normal westerly winds. NAO+ situations strengthen the circulation in the North Sea, whereas it weakens during NAOconditions and is directed northeastward. High positive correlations exist between the SST at site 22/4b and the NAO index. Climate change can have a long-term effect on the hydrodynamics of the North Sea. Seasonal stratification has potentially the most important imprint on methane derived from well site 22/4b. Summertime heating stratifies the northern part of the North Sea. In autumn, loss of heat to the atmosphere causes the stratification to break down until tides and storms mix the entire water column. During the period of stratification, the bulk of (dissolved) methane released from site 22/4b gets trapped below the thermocline. The loss of methane to the atmosphere thus becomes a function of the relative time scales of transport and horizontal and vertical mixing processes versus the time scale of microbial degradation (oxidation) in the water column.

No abstract

Bifurcation analysis on flows in a two-layer shallow-water model is used to clarify the dynamical origin of low-frequency variability of the double-gyre wind-driven ocean circulation. In many previous model studies, generic low-frequency variations appear to be associated with distinct regimes, characterized by the level of kinetic energy of the mean flow. From these transient flow computations, the current view is that these regimes, and transitions between them, arise through a complex nonlinear interaction between the mean flow and its high-frequency instabilities (the eddies). On the contrary, we demonstrate here, for a particular (but relevant) case, that the origin of these high-and low-energy states is related to the existence of low-frequency instabilities of steady-state flows. The low-frequency modes have distinct spatial patterns and introduce preferential patterns oscillating on interannual to decadal time scales into the flow. In addition, these lowfrequency modes are shown to be robust to the presence of (idealized) topography; the latter may even have a destabilizing effect.