Turbulent heat transfer as a control of platelet ice growth in supercooled under-ice ocean boundary layers

Miles G Mcphee,Craig L Stevens,Natalie J Robinson,Inga J Smith

doi:10.5194/os-12-507-2016

Abstract

Abstract. Late winter measurements of turbulent quantities in tidally modulated flow under land-fast sea ice near the Erebus Glacier Tongue, McMurdo Sound, Antarctica, identified processes that influence growth at the interface of an ice surface in contact with supercooled seawater. The data show that turbulent heat exchange at the ocean–ice boundary is characterized by the product of friction velocity and (negative) water temperature departure from freezing, analogous to similar results for moderate melting rates in seawater above freezing. Platelet ice growth appears to increase the hydraulic roughness (drag) of fast ice compared with undeformed fast ice without platelets. Platelet growth in supercooled water under thick ice appears to be rate-limited by turbulent heat transfer and that this is a significant factor to be considered in mass transfer at the underside of ice shelves and sea ice in the vicinity of ice shelves.

Highlights

In addition to seaward advection, calving and basal melting, the distribution of mass in ice shelves depends on the socalled ice pump (Lewis and Perkin, 1986)
At Mast A (TICs at 1 and 3 m below the ice undersurface) ice accretion on the instruments limited the duration of acceptable measurements to about two diurnal cycles (∼ 60 h) ending early (UTC) on day of year (DOY) 301
Our data show that turbulence-enhanced transfer of supercooled seawater can be the source of the negative heat flux measured within the ocean boundary layer during the present observations

Summary

Introduction

In addition to seaward advection, calving and basal melting, the distribution of mass in ice shelves depends on the socalled ice pump (Lewis and Perkin, 1986). By this mechanism, water warmer than the in situ freezing point temperature, typically High-Salinity Shelf Water entering the undershelf cavity, encounters glacial ice at, or near, the grounding line. Supercooled water can deposit ice by direct growth of ice crystals attached at the ice underside, or by upward migration of frazil crystals suspended by turbulence in the water (Dieckmann et al, 1986) In this way, fresh glacial ice near the grounding line can be transformed to marine ice (Langhorne, 2008). Evidence from icebergs (Kipfstuhl et al, 1992), borehole (Craven et al, 2005) and radar studies (Engelhardt and Determann, 1987; Robin et al, 1983; Holland et al, 2009) indicate that marine ice can reach appreciable thicknesses, and that the ice pump is active under shelves where the water entering the cavity is near freezing

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