Dynamical Controls of Fjord Circulation

Abstract:

The oceanic connections between tidewater glaciers and continental shelf waters are modulated and controlled

by geometrically complex fjords. These fjords exhibit both overturning circulations and horizontal

recirculations, driven by a combination of water mass transformation at the head of the fjord, variability on

the continental shelf, and atmospheric forcing. However, it remains unclear which geometric and forcing

parameters are the most important in exerting control on the overturning and horizontal recirculation. To address

this, idealized numerical simulations are conducted using an isopycnal model of a fjord connected to a

continental shelf, which is representative of regions in Greenland and the West Antarctic Peninsula. A range

of sensitivity experiments demonstrate that sill height, wind direction/strength, subglacial discharge strength,

and depth of offshore warm water are of first-order importance to the overturning circulation, while fjord

width is also of leading importance to the horizontal recirculation. Dynamical predictions are developed and

tested for the overturning circulation of the entire shelf-to-glacier-face domain, subdivided into three regions:

the continental shelf extending from the open ocean to the fjord mouth, the sill-overflow at the fjord mouth,

and the plume-driven water mass transformation at the fjord head. A vorticity budget is also developed to

predict the strength of the horizontal recirculation, which provides a scaling in terms of the overturning and

bottom friction. Based on these theories, we may predict glacial melt rates that take into account overturning

and recirculation, which may be used to refine estimates of ocean-driven melting of the Greenland and

Antarctic ice sheets.