Descent and mixing of the overflow plume from Storfjord in Svalbard: an idealized numerical model study

[摘要] Storfjorden in the Svalbard Archipelago is a sill-fjord that producessignificant volumes of dense, brine-enriched shelf water through iceformation. The dense water produced in the fjord overflows the sill and canreach deep into the Fram Strait. For conditions corresponding to a moderateice production year, the pathway of the overflow, its descent and evolvingwater mass properties due to mixing are investigated for the first timeusing a high resolution 3-D numerical model. An idealized modeling approachforced by a typical annual cycle of buoyancy forcing due to ice productionis chosen in a terrain-following vertical co-ordinate. Comparison withobservational data, including hydrography, fine resolution currentmeasurements and direct turbulence measurements using a microstructureprofiler, gives confidence on the model performance. The model eddy diffusivityprofiles contrasted to those inferred from the turbulence measurements giveconfidence on the skill of the Mellor Yamada scheme in representing sub-gridscale mixing for the Storfjorden overflow, and probably for gravity currentmodeling, in general. The Storfjorden overflow is characterized by lowFroude number dynamics except at the shelf break where the plume narrows,accelerates with speed reaching 0.6 m s⁻¹, yielding local Froudenumber in excess of unity. The volume flux of the plume increases byfive-fold from the sill to downstream of the shelf-break. Rotationalhydraulic control is not applicable for transport estimates at the sillusing upstream basin information. To the leading order, geostrophyestablishes the lateral slope of the plume interface at the sill. Thisallows for a transport estimate that is consistent with the model results byevaluating a weir relation at the sill.

[学科分类] 海洋学与技术