Abstract
The influence of a summer storm event in 2007 on the North Sea and its effects on the ocean stratification are investigated using a regional coupled ocean (Regional Ocean Modeling System, ROMS)-atmosphere (Weather Research & Forecasting model, WRF) modeling system. An analysis of potential energy anomaly (PEA, Φ) and its temporal development reveals that the loss of stratification due to the storm event is dominated by vertical mixing in almost the entire North Sea. For specific regions, however, a considerable contribution of depth-mean straining is observed. Vertical mixing is highly correlated with wind induced surface stresses. However, peak mixing values are observed in combination with incoming flood currents. Depending on the phase between winds and tides, the loss of stratification differs strongly over the North Sea. To study the effects of interactive ocean-atmosphere exchange, a fully coupled simulation is compared with two uncoupled ones for the same vertical mixing parameters to identify the impact of spatial resolution as well as of SST feedback. While the resulting new mixed layer depth after the storm event in the uncoupled simulation with lower spatial and temporal resolution of the surface forcing data can still be located in the euphotic zone, the coupled simulation is capable to mix the entire water column and the vertical mixing in the uncoupled simulation with higher resolution of the surface forcing data is strongly amplified. These differences might have notable implications for ecosystem modeling since it could determine the development of new phytoplankton blooms after the storm and for sediment modeling in terms of sediment mobilization. An investigation of restratification after the extreme event illustrates the persistent effect of this summer storm.
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Acknowledgments
We would like to thank the International Research Training Group INTERCOAST and the Deutsche Forschungsgemeinschaft (DFG) for funding for this study. Atmospheric surface data for evaluation were kindly provided by the German Deutsche Wetterdienst (DWD) and the National Center for Environmental Predictions (NCEP). We acknowledge the public deployment of very valuable information and different tools for pre- and post-processing of ROMS and WRF data by the ROMS/TOMS Group, ROMS AGRIF (ROMSTOOLS), Emanuele Di Lorenzo (ROMS Numerical Toolbox), Hernan G. Arango, John C. Warner and the COAWST team, and MMM/NCAR. Many thanks to Mark Hadfield and colleagues for the support and fruitful discussions and also for the possibility to visit and work at the NIWA Wellington during a part of this study. Further thanks for valuable general discussions regarding the COAWST system, the PEA analysis, and North Sea modeling are going to A. Aretxabaleta, H. Burchard, T. Pohlmann and colleagues, and J. C. Warner and colleagues. We also would like to thank the two anonymous reviewers, who clearly helped with their very constructive, detailed, and careful comments to improve this work.
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Gronholz, A., Gräwe, U., Paul, A. et al. Investigating the effects of a summer storm on the North Sea stratification using a regional coupled ocean-atmosphere model. Ocean Dynamics 67, 211–235 (2017). https://doi.org/10.1007/s10236-016-1023-2
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DOI: https://doi.org/10.1007/s10236-016-1023-2