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An Observational and Analytical Study of Marginal Ice Zone Atmospheric Jets

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Naval Postgraduate School Monterey United States

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Low-level atmospheric jets have been observed to occur frequently in marginal ice zones MIZs, but little research has been done on the dynamics of these features. In the fall of 2015, during the Office of Naval Research Sea State cruise in the Beaufort Sea, the research team used radiosondes and shipboard instrumentation to detect several atmospheric jets in the atmospheric boundary layer or in the capping temperature inversion just above. The three strongest jets had maximum wind speeds at elevations near 350 m to 400 m elevation one of these jets had a secondary maximum wind height at 900 m. Different theories have been suggested as reasons for the existence of MIZ jets, but in all the cases examined it appeared that the primary cause of the low-level jets was a thermal wind effect where the thermal wind opposes the geostrophic wind due to horizontal temperature changes in the atmospheric boundary layer and capping inversion. The jets were detected using rawinsonde measurements, complemented by daily runs of the ECMWF model. By comparing soundings that were perpendicular to the thermal gradients, it was possible to calculate how the geostrophic wind would vary with elevation. In most cases, the comparisons of the calculated thermal wind matched well with the observed winds in the upper part of the boundary layer, thus indicating that the low-level jets were primarily a result of a thermal wind opposing the background geostrophic wind. At the lowest levels, the observed winds speeds were less than the calculated geostrophic wind, as expected, due to friction.

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