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Atmospheric boundary layer coupling to midlatitude mesoscale sea surface temperature anomalies /

by Thum, Nicolai.

Abstract (Summary)
This thesis examines the mechanisms that couple the monthly-averaged atmospheric boundary layer (ABL) to open-ocean sea surface temperature (SST) perturbations on scales of 50-500 km. The observed positive correlation between surface wind speed anomalies and SST anomalies is successfully simulated using the Weather Research and Forecasting (WRF) model. In numerical experiments with idealized SST fronts, the cross-frontal surface wind acceleration in the cold-to-warm case and deceleration in the warm-to- cold case are found over narrow transition zones co-located with the narrow regions of large frontal SST changes. In the transition zone, horizontal momentum is redistributed vertically in the ABL by turbulence and convection. The largest pressure adjustments, on the other hand, take place over a much broader region downstream from the SST front. In the cold-to-warm transition zone the model simulates an unstable thermal internal boundary layer (TIBL) in the lower part of the ABL. As the TIBL grows, higher velocity air aloft is incorporated into the TIBL, accelerating the flow. Over the warm-to-cold transition zone, the momentum boundary layer collapses, and vertical mixing of momentum by turbulence and convection ceases in the upper part of the ABL. The WRF model is also applied to open-ocean ABL flow across idealized sinusoidal SST anomalies having scales similar to those observed in the Agulhas return current region. The simulated horizontal pressure gradient force anomalies are crucial to the response over the entire domain, and the vertically integrated momentum budget is found to be approximately linear. A linear diagnostic model is therefore developed which successfully predicts the observed phase and amplitude of the ABL wind, pressure and temperature response to the SST anomalies, with largest quantitative discrepancies found in the perturbation wind component perpendicular to the mean wind direction. By using the divergence and vorticity budgets, the diagnostic model shows that differences in the vertical structure of the perturbation wind components down and across the mean wind can explain the differences in the coupling coefficients for the divergence and curl as functions of downwind and crosswind SST gradients, and as functions of the angle between the SST gradient and the mean wind.
Bibliographical Information:

Advisor:

School:Oregon State University

School Location:USA - Oregon

Source Type:Master's Thesis

Keywords:turbulent boundary layer meteorology ocean temperature

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