Observations of Hydraulic Roughness and Form Drag in the Wake of a Deep Ice Keel in the Arctic Ocean

Abstract

Decrease in Arctic Ocean perennial sea-ice has been observed in recent decades. As sea-ice continues to decline, marine traffic will increase and the United States will require a more active military presence in the Arctic. Future Arctic conditions must be forecasted with increased accuracy to allow for proper planning with regard to the nations changing role in the region. It is the goal of this thesis to contribute to the knowledge of turbulence and mixing associated with ice keels in the Arctic Ocean in order to improve the accuracy of predictive coupled ocean/ice/atmospheric models. At Applied Physics Laboratory Ice Station 2011, a 3-dimensional (3-D) sonar was used to record high resolution morphological measurements of an ice keel approximately 33 m long by 29 m across and 10 m deep. Sensors were deployed in the water column approximately 10 m from the keel to record water properties of salinity, temperature, and 3-D velocity at selected depths in the upper water column. These observations were used to make calculations of turbulence within the mixed layer, in order to gain a greater understanding of how keels affect turbulent drag and heat fluxes in the upper ocean. Results indicate that keels generate significant turbulence and mixing in the upper ocean, even during benign weather conditions in which there is little surface forcing. Keels increase the kinetic energy of the upper ocean through production of turbulent eddies during times of weak stratification and the generation of internal waves during times of strong stratification. Keel-induced turbulence and mixing may lead to entrainment of warmer water underlying the surface mixed layer that could be a contributor to ice melting. Calculation of the quadratic drag coefficient Cw also indicated that Cw varies greatly with water column stratification and ice undersurface roughness. Values as high as 0.08 were seen in the wake of a 10 m ice keel during strong stratification, and as low as 0.002 when the current was not affected by the keel during weak stratification. Most numerical models utilize a constant value of 0.0055 for |Cw|. Varying |Cw| based on ice roughness and water column structure could greatly improve model accuracy.