The piercing of the Atlantic Layer by an Arctic shelf water cascade in an idealised study inspired by the Storfjorden overflow in Svalbard

Abstract

A plume of dense brine-enriched water, resulting from sea ice production in the Storfjorden polynya (Svalbard), cascades into Fram Strait and encounters a layer of warm, saline Atlantic Water. In some years the plume continues to sink into the deep Fram Strait while in other years it remains at Atlantic Layer depths. It has been unclear what parameters control whether the plume pierces the Atlantic Layer or not.We use a high-resolution 3-D numerical ocean model (NEMO-SHELF) to simulate an idealised scenario where a cascade descends a conical slope into an ambient 3-layer stratification. The model uses 1km horizontal resolution and a blend of s- and z coordinates with 42 layers in the vertical arranged to resolve the plume at the bottom. We vary the salinity ‘S’ and the flow rate ‘Q’ of the simulated Storfjorden overflow to investigate both strong and weak cascading conditions. In agreement with observations the model reproduces three regimes: (i) the plume is arrested within or just below the Atlantic Layer, (ii) the plume pierces the Atlantic Layer and continues to the bottom of the slope and an intermediate regime (iii) where a portion of the plume detaches from the bottom, intrudes into the Atlantic Layer while the remainder continues its downslope propagation. For our idealised case the cascading regime can be predicted from the initial values of S and Q. In those model experiments where the initial density of the overflow water is considerably greater than of the deepest ambient water mass we find that a cascade with high initial S does not necessarily reach the bottom if Q is low. Conversely, cascades with an initial density just slightly higher than the deepest ambient layer may flow to the bottom if the flow rate Q is high. A functional relationship between S/Q and the final depth level of plume waters is explained by the flux of potential energy (arising from the introduction of dense water at shallow depth) which, in our idealised setting, represents the only energy source for downslope descent and mixing.