Janine J. Nauw

The origin of low-frequency variability of double-gyre wind-driven flows

Bifurcation analysis on e ows 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 e ow. From these transient e ow computations, the current view is that these regimes, and transitions between them, arise through a complex nonlinear interaction between the mean e ow 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 e ows. The low-frequency modes have distinct spatial patterns and introduce preferential patterns oscillating on interannual to decadal time scales into the e ow. In addition, these lowfrequency modes are shown to be robust to the presence of (idealized) topography; the latter may even have a destabilizing effect.

Regimes of low-frequency variability in a three-layer quasi-geostrophic ocean model

The temporal variability of the midlatitude double-gyre wind-driven ocean circulation is studied in a three-layer quasi-geostrophic model over a broad range in parameter space. Four different types of flow regimes are found, each characterized by a specific time-mean state and spatio-temporal variability. As the lateral friction is decreased, these regimes are encountered in the following order: the viscous antisymmetric regime, the asymmetric regime, the quasi-homoclinic regime and the inertial antisymmetric regime. The variability in the viscous and the inertial antisymmetric regimes (at high and low lateral friction, respectively) is mainly caused by Rossby basin modes. Lowfrequency variability, i.e. on interannual to decadal time-scales, is present in the asymmetric and quasi-homoclinic regime and can be related to relaxation oscillations originating from low-frequency gyre modes. The focus of this paper is on the mechanisms of the transitions between the different regimes. The transition from the viscous antisymmetric regime to the asymmetric regime occurs through a symmetry-breaking pitchfork bifurcation. There are strong indications that the quasihomoclinic regime is introduced through the existence of a homoclinic orbit. The transition to the inertial antisymmetric regime is due to the symmetrization of the time-mean state zonal velocity field through rectification effects.

Residual water transport in the Marsdiep tidal inlet inferred from observations and a numerical model

At tidal inlets, large amounts of water are exchanged with the adjacent sea during the tidal cycle.The residual flows, the net effect of ebb and flood, are generally small compared with the gross flux;they vary in magnitude and sign from one tidal period to the other; and their long-term mean variesfrom year to year. Here, we focus on the temporal variability of the residual flows in the Marsdieptidal inlet, which is the western-most inlet of the Wadden Sea, a tidal lagoon along the coasts of theNetherlands, Germany, and Denmark. We compare the transport from a high-resolution numericalmodel with the transport from velocity profile data collected beneath a ferry that crosses the inletdaily. The comparison works in two ways: for the areas and times covered by the measurements, thedata serve to validate the model, and conversely, the model is employed to assess the consequencesof spatial and temporal gaps in the data. Modeled and observed transports over the region of theflow that is covered by the acoustic Doppler current profiler are in good agreement for gross andresidual quantities. Results indicate that uncertainties due to spatial gaps can be overcome with asimple extrapolation approach applied to the velocity profiles, whereas uncertainties due to temporalgaps are more problematic and leave large discrepancies in the residuals.

Frictionally Induced Asymmetries in Wind-Driven Flows

The effect of the parameterization of lateral friction on the separation of western boundary currents is addressed in an idealized context. The study is motivated by a puzzling issue that arises from the nonlinear theory of the wind-driven double-gyre circulation in shallow-water models. Subtle changes in the representation of the lateral friction in these models have a substantial effect on both steady-state and transient flows. The aim of this paper is to explain how lateral friction introduces a north‐south asymmetry in the steady double-gyre flows and why the degree of this asymmetry depends on the type of frictional parameterization. A more conceptual model of a zonal jet in a channel turns out to be very useful to determine the dynamical processes behind the asymmetries. It is also shown that the north‐south asymmetries have an impact on the low-frequency variability of the timedependent flows. This is caused by changes in stability behavior of the steady-state flows.

Observations of suspended matter along the Dutch coast

Large amounts of suspended matter are transported through the Dutch coastal zone in the southern North Sea. Current estimates, based on budget studies, are in the order of 15 - 20 Mton per year transported in northward direction, which should take place in a small strip of 5 - 10 km wide. For this study we have performed a series of measurements on total suspended matter in an area in the most northerly extent of the Rhine region of fresh water influence. The measurements focused on observations both in the vertical and in the horizontal on the behavior of suspended matter in the nearshore zone up to 7 km from the shoreline. A peak in bottom concentrations is observed close to the coast along the coastal stretch. This hot spot location is found in the cross-shore direction at about 1.5 km from the coastline at a water depth of 15 m. Here, total suspended matter concentrations near the bottom exceed 200 mg/l. These peak concentrations have not been identified before and add to the suggestion that a large part of the northward suspended matter transport occurs very close to the coast.

Numerical modeling of physical processes in the North Sea and Wadden Sea with GETM/GOTM

At the Royal Netherlands Institute for Sea Research (NIOZ), several research projects are carried out on the concentration and transport of Suspended Particulate Matter (SPM) in the Southern North Sea and Wadden Sea. So far the focus has been on field data collection and analysis, but in recent years a numerical modeling capacity has been set up, using the GETM/GOTM model. The underlying purpose is twofold. Firstly, a numerical model can provide insight into the hydrodynamics and SPM transport, complementing field observations, helping to interpret and identify the key physical mechanisms. Secondly, it provides a much-needed tool in ecological studies, forming the basic physical core on which the transport of for instance nutrients and larvae depends; thus, these kinds of numerical models provide a valuable bridge in interdisciplinary studies. First steps in the use of the model are reported here, offering a synopsis of its potential.