Graduation date: 2007
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.