Small-scale dynamics of the under-ice boundary layer
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In ice covered polar regions, the interaction between ocean, ice and atmosphere is an important component in the complex climate system. Exchange of heat, mass and momentum occurs across the boundary layers, both in the ocean and in the atmosphere, hence understanding the involved processes are crucial in order to determine future climate. In this thesis dynamics and thermodynamics of the under-ice boundary layer are investigated based on measurements of turbulent fluxes in close proximity to the ice/ocean interface and microstructure profiling of the upper ocean. The topic is addressed in four papers which focus on exchange processes at the ice/ocean interface as well as regional measurements of turbulence and turbulent fluxes in ice covered areas around Spitsbergen and in the Weddell Sea. High rates of melting are often encountered as sea ice drifts into water with temperatures well above freezing, which may be typical of the marginal ice zones. It has been shown in previous studies that these melting rates are limited by double diffusive effects in a thin layer close the ice/ocean interface. In this study, turbulent fluxes from the under-ice boundary layer are used to show that double diffusive effects are important for the melting rates and show that the strength of this double diffusion is close to the range suggested by previous studies. It is also shown that by not considering double diffusive effects at the boundary, melting rates are overestimated by up to several cm per day. By analyzing the conditional statistics of the Reynolds stress in the boundary layer it is found that the main fraction of the stress comes from high turbulence events, so called “sweeps” and “ejections”, which is consistent with boundary layer flows in other environments. Closest to the ice, the sweeps are found to be more intense than further away from the interface, which can be related to the observed increase in friction velocity with depth. The West Spitsbergen Current transports Atlantic Water, which is the main source of salt and heat to the Arctic. This study presents measurements obtained during 6 drifting experiments northwest of Spitsbergen where the West Spitsbergen Current enters the Arctic and which also is an area of substantial air/sea/sea exchange. Heat fluxes within or in close proximity of the main branches of the West Spitsbergen Current are O(100) W m-², due to high mixed layer temperatures and large ice drift. Over the shelf areas high mixing and turbulent fluxes are observed due to tidal effects and interaction with topography. Heat fluxes averaged over the different water masses found in the area show that turbulent heat flux decreases with increasing distance from the surface. Hence it indicates that Atlantic Water is not the main source for vertical mixing of heat. A major contribution to mixed layer heat content is found to be horizontal advection and entrainment of water from below. In the far Southern Atlantic, the Weddell Sea is another important site of ocean and atmosphere interaction. In the area of Maud Rise, a topographic feature in the eastern Weddell Sea, the water column is only weakly stable making it susceptible to deep convection. This study presents wintertime mixed layer turbulence measurements obtained during two ice drift over the Maud Rise. Heat fluxes were comparable to earlier studies in the same area and could be estimated from the mean properties of the mixed layer. The under-ice roughness was estimated to be very smooth and comparison with a one dimensional steady state model suggests that this is a local effect and not representative for the entire ice floe. The main source of turbulent kinetic energy is velocity shear from the ice, however in some periods horizontal heterogeneity in water masses, ice topography and open water fraction can affect the stability and introduce upstream sources of turbulent kinetic energy.
Paper I: Geophysical Research Letters 36, Sirevaag, A., Turbulent exchange coefficients for the ice/ocean interface in case of rapid melting, L04606. Copyright 2009 The American Geophysical Union. Reproduced with permission. Submitted version. The published version is available at: http://dx.doi.org/10.1029/2008GL036587Paper II: Journal of Physical Oceanography 39(12), Sirevaag, A.; Fer, I., Early spring oceanic heat fluxes and mixing observed from drift stations north of Svalbard, pp. 3049–3069. Copyright 2009 American Meteorological Society. Full text not available in BORA due to publisher restrictions. The published version is available at: http://dx.doi.org/10.1175/2009JPO4172.1Paper III: Sirevaag, A.; McPhee, M. G.; Morison, J. H.; Shaw, W. J.; Stanton, T. P., 2009, Wintertime mixed layer measurements at Maud Rise, Weddell Sea. Reproduced with permission. Submitted version.Paper IV: Geophysical Research Letters 31, Fer, I.; McPhee, M. G.; Sirevaag, A., Conditional statistics of the Reynolds stress in the under-ice boundary layer, L15311. Copyright 2004 The American Geophysical Union. Reproduced with permission. Accepted version. The published version is available at: http://dx.doi.org/10.1029/2004GL020475
PublisherThe University of Bergen