Wind and tidal forcing on the meso-scale circulation in Storfjorden, Svalbard

Abstract

The Bergen Ocean Model (BOM), a three-dimensional sigma-layered numerical ocean model, is used to simulate the circulation in Storfjorden with the sounds Heleysundet and Freemansundet opened. Two simulations forced by four tidal constituents are carried out; one where the wind forcing is from the high resolution, non-hydrostatic atmospheric model MM5, and one where the wind forcing is from the coarse resolution NCEP reanalyzed fields. Through comparison of the two wind fields, it becomes clear that a high resolution wind is necessary to resolve the topographic influence on the wind field over the Svalbard Archipelago. The stronger and more variable fine-scale MM5 wind includes more events with high wind speeds than the NCEP reanalyzed field and is required to reproduce realistic currents in Storfjorden. Due to the surface Ekman transport, easterly winds seem to favor a cyclonic surface current, whereas southerly and westerly winds tend to stop it. The mean easterly wind flow over the Svalbard Archipelago is represented in both wind fields, but the mean cyclonic circulation in Storfjorden is only reproduced with fine-scale wind forcing. The residual currents through Heleysundet and Freemansundet compensate for the Ekman transport and the accompanying surface elevation difference along the sounds during different wind directions. The modelled tidal currents up to 1.5 and 1.7 ms - 1 in Heleysundet and Freemansundet, respectively, are controlled by the 200 ∘ – 220 ∘ phase shift of the tidal wave in Storfjorden and northwestern Barents Sea, which exists independent of the opening or closing of the sounds. The enhanced mixing due to the fine-scale wind forcing favors a more homogenized modelled water column at the onset of freezing. Wind governs the surface water distribution, which indicates that fine-scale wind forcing in addition to fine-scale thermal forcing, are necessary to improve the representation of the source water of brine-enriched shelf water (BSW) at the onset of freezing and hence, BSW itself.