Ocean Dynamics DOI 10.1007/s10236-008-0159-0 Temporal and spatial circulation patterns in the East Frisian Wadden Sea Emil V. Stanev · Sebastian Grayek · Joanna Staneva Received: 14 May 2008 / Accepted: 18 October 2008 © Springer-Verlag 2008 Abstract This work deals with the analysis of simula- plications of the results in hindcasting and forecasting tions carried out with a primitive equation numerical of hydrodynamics and sediment dynamics in the coastal model for the region of the East Frisian Wadden Sea. zone are considered. The model, with 200-m resolution, is forced by wind, air–sea heat, and water fluxes and river runoff and is Keywords Tidal basins · Statistical characteristics · nested in a German Bight 1-km-resolution numerical Predictability of circulation model, the latter providing tidal forcing for the fine resolution model. The analysis of numerical simula- tions is focused both on responses due to moderate 1 Introduction conditions, as well as to extreme events, such as the storm surge Britta, for which the model demonstrates A considerable body of work concerning the response very good skills. The question addressed in this paper of the coastal ocean to tides based on local observa- is how well the model output can be compressed with tions, tidal gauges data, etc., already exists. However, the help of empirical orthogonal function analysis. It spatial patterns are much less clear because direct ob- is demonstrated that, for the short-time periods of the servations over large areas with high spatial resolution order of a spring–neap cycle, only a few modes are are limited. Remote sensing data also do not help necessary to almost fully represent the circulation. This much: altimeter data are not precise enough in the is just an illustration that the circulation in this region near coastal zone and ocean color or AVHRR data is subject to the dominating tidal forcing, creating clear are affected by cloud conditions and do not provide and relatively simple response patterns. However, for complete information with sufficient temporal resolu- longer periods of about several months, wind forcing is tion. In the East Frisian Wadden Sea, which is the area also very important, and correspondingly, the circula- of our research interest, the temporal and spatial vari- tion patterns become much more complex. Possible ap- ability derived from statistical analyses of outputs from numerical models has not been sufficiently explored, although some initial steps have already been made (Stanev et al. 2007b). Responsible Editor: Jörg-Olaf Wolff The work presented here has been carried out within the framework of the research programme “BioGeo- B · E. V. Stanev ( ) J. Staneva Chemistry of Tidal Flats,” which combined various Institute for Coastal Research, GKSS Research Centre, Max-Planck-Strasse 1, 21502 Geesthacht, Germany research activities aimed to improve the basic under- e-mail: [email protected] standing of functioning of these areas, including cir- culation and sediment transport (Staneva et al. 2008). · E. V. Stanev S. Grayek Our major aim here is to make an initial effort towards Institute for Chemistry and Biology of the Sea, University of Oldenburg, Carl-von-Ossietzky-Strasse 9-11, developing techniques that would enable the best use 26111 Oldenburg, Germany of data for state estimates and forecasting purposes. Ocean Dynamics One necessary step, before going deeper into this issue Wadden Sea water periodically mixes with North Sea and before addressing data assimilation, is to under- waters (during flooding) and is partially transported stand the dominating statistical properties of coastal into the North Sea (during ebb). In this way, the dynamics. Wadden Sea acts as a buffer mixing-zone between The German Bight (Fig. 1a), which is bordered by ocean and land. The exchange of water and properties the coasts of the Netherlands, Germany, and Denmark, between the Wadden Sea and the German Bight are is situated in the south-eastern corner of the North Sea. known from numerical simulations (Stanev et al. 2003, It is well established that the wind in this area results b, 2007b) and numerous observations (see this special in a cyclonic residual circulation, i.e., a circulation in issue). the direction of propagation of the tidal wave (from It is well-known that one fundamental characteristic west to east along the southern boundary and from of the East-Frisian Wadden Sea is the vigorous south to north along the western coasts of Germany and suspended matter dynamics (Santamarina Cuneo and Denmark). Flemming 2000;Burchardetal.2008) triggered by the The East-Frisian Wadden Sea is one of the shallow- turbulence due to velocity shear (Stanev et al. 2007a) est areas of the German Bight, being characterized by and wind waves (Gayer et al. 2006;Stanevetal.2006). a series of barrier islands, each 5–10 km long in the Understanding sediment dynamics is, thus, crucially de- east–west direction and 2–3 km wide (Fig. 1b). The tidal pendent upon the knowledge of the turbulence regime. range varies from ∼ 2.5 m (Isles of Borkum and Sylt) to Therefore, specific attention in this paper is also paid ∼ 3.5 m (the Elbe Estuary), i.e., the region is exposed to the question of how complex the turbulence patterns to upper meso-tidal conditions. are in the region of the East Frisian Wadden Sea. Detailed validation of numerical models against ob- servations at fixed locations (Stanev et al. 2003, 2007b), a) and over larger areas (Staneva et al. 2008) demon- strates a good quality of numerical simulations and motivates us to address the question: what are the dominating temporal and spatial patterns? Provided that the latter show some robustness, we could hope that it would not be too difficult to extend this research towards the issue of predictability, which has never been addressed for this region. The paper is structured as follows: Section 2 de- scribes the numerical model and setup, Section 3 provides a description of the simulated spatial and temporal patterns, and Section 4 presents the statisti- cal reconstruction of simulations. The paper ends with brief conclusions. b) 2 Numerical model The model system used here consists of two models with different horizontal resolution: a German Bight model (GBM) and a Wadden Sea model. Both models are based on the 3-D General Estuarine Transport Model (Burchard and Bolding 2002), in which the equa- tions for the three velocity components u,v,andw; sea- level elevation (SLE) ζ; temperature T; and salinity S, as well as the equations for turbulent kinetic energy (TKE) and the eddy dissipation rate due to viscosity, Fig. 1 Topography of the German Bight (a) and East Frisian are solved. The models use terrain-following equidis- Wadden Sea (b). Dashed line in b contours regions, which are σ subject to drying. The position of continuously operating data tant vertical coordinates ( coordinates) and are capa- stations are given with red squares ble of simulating drying and flooding of tidal flats. The Ocean Dynamics vertical column is discretized into ten nonintersecting layers. The application of the models to the area of our study is described by Staneva et al. (2007b, 2008), and we refer to these papers for more details. The Wadden Sea model with a resolution of 200 m is nested in a GBM with a resolution of about 1 km (Staneva et al. 2008). Both models use spherical hori- zontal grids. The atmospheric forcing for both models is computed through bulk aerodynamic formulas using 6- hourly European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses data. Below, we briefly formulate the differences in the forcing of the two models. a) For the outer mode (GBM), the river run-off for the rivers Elbe, Ems, and Weser is taken from the 1-hourly data provided by the Bundesamt für Seeschifffahrt und Hydrographie (BSH). The lateral boundary conditions of sea surface elevations, salinity, and temperature are taken from the operational BSH model (Dick and Sötje 1990; Dick et al. 2001). For the inner (Wadden Sea) model, the forcing at the open boundaries is taken from the simulations with the GBM (Staneva et al. 2008) and interpolated in time and space onto the grid points along the boundaries of the Wadden Sea regional model. The nesting is one- way, i.e., the nested model receives boundary values from the coarser model but does not influence the coarser resolution model. The fresh-water fluxes from the main tributaries in the region are taken from the observations compiled by Reinke et al. (2000). Initial conditions are set to zero for sea level and velocity. Temperature and salinity are taken from climatology. b) The experience with the coupled GBM and Wadden Sea model showed that the circulation reaches a qua- siperiodic state in only a few tidal cycles. The present study analyzes numerical simulations carried out for the period September to December 2006. This 4-month period is enough to resolve the dominant temporal scales associated with tidal (flood- ebb, spring–neap) and atmospheric (synoptic) forcing. Compared to earlier modelling studies for the same re- gion based on this model, the present one also demon- strates the response to extreme events. One such event was the storm surge Britta on 1 November 2006. This event extended over the entire North Sea (Fig. 2a, b), atmospheric pressure was below 984 hPa, and extreme c) values of sea level (Fig. 2c) were reached in many con- Fig. 2 Surface pressure (a), satellite image (b), and visual obser- tinental locations (e. g. Emden 3.59 m above the mean vation (c) during storm surge Britta. Courtesy: European Centre high water), as well as at barrier islands (Langeoog for Medium-Range Weather Forecasts (ECMWF), National und Wangerooge).