Direct numerical simulation of the diffusive convection and assessment of its impact on Artic [Arctic] climate change

Abstract

This thesis focuses on the numerical modeling of the oceanic double-diffusive convection, an important small scale mixing process which is driven by the two orders of magnitude difference in diffusivities of heat and salt in seawater. This study explores the diffusive regime of double-diffusion which is realized when cold and fresh water overlies the warm and salty water. The aim of the research was to quantify the double diffusive transport in both smooth gradients and thermohaline staircases, and to develop clear insight into the origin of the staircases and specify conditions for their formation. Based on the numerical process modeling, it was determined that the evolutionary pattern of staircases is controlled by the merging events in which weak interfaces gradually erode and ultimately disappear. To illustrate dynamics of these events, a theoretical framework - merging theorem - has been developed. It was numerically confirmed that the merging theorem predicts the time scale of merging events within the order of magnitude. The validity of the lab derived 4/3 flux law was tested and it was determined that its form is consistent with the numerical results, but the amplitude requires adjustment. The computed fluxes from numerical experiments were comparable to the diffusive fluxes that were inferred from the Beaufort Gyre observations (Wilson, 2007) and an order of magnitude greater than the fluxes from earlier laboratory based experiments. Although a large discrepancy in the values of diffusive fluxes exists in the earlier laboratory and field studies, the present analysis suggests that the diffusive fluxes could play an important factor in the Arctic heat budget; hence, future study in this field is recommended. This study contributes to a better understanding of global climate change, which presents a new challenge to national security. The Navy has to be concerned with the impact of climate change on naval operations, specifically in the Arctic where the melting polar ice cap may soon provide a gateway across the north, resulting in the opening of shipping lanes and borders.