Abstract:
Observational and modeling techniques are employed to investigate the thermal and
inertial upper ocean response to wind and buoyancy forcing in the North Atlantic
Ocean. First, the seasonal kinetic energy variability of near-inertial motions observed
with a moored profiler is described. Observed wintertime enhancement and surface
intensification of near-inertial kinetic energy support previous work suggesting that
near-inertial motions are predominantly driven by surface forcing. The wind energy
input into surface ocean near-inertial motions is estimated using the Price-Weller-
Pinkel (PWP) one-dimensional mixed layer model. A localized depth-integrated
model consisting of a wind forcing term and a dissipation parameterization is developed and shown to have skill capturing the seasonal cycle and order of magnitude
of the near-inertial kinetic energy. Focusing in on wintertime storm passage, velocity
and density records from drifting profiling floats (EM-APEX) and a meteorological spar buoy/tethered profiler system (ASIS/FILIS) deployed in the Gulf Stream in
February 2007 as part of the CLIvar MOde water Dynamics Experiment (CLIMODE)
were analyzed. Despite large surface heat loss during cold air outbreaks and the drifting nature of the instruments, changes in the upper ocean heat content were found
in a mixed layer heat balance to be controlled primarily by the relative advection of
temperature associated with the strong vertical shear of the Gulf Stream. Velocity
records from the Gulf Stream exhibited energetic near-inertial oscillations with frequency that was shifted below the local resting inertial frequency. This depression
of frequency was linked to the presence of the negative vorticity of the background
horizontal current shear, implying the potential for near-inertial wave trapping in the
Gulf Stream region through the mechanism described by Kunze and Sanford (1984).
Three-dimensional PWP model simulations show evidence of near-inertial wave trapping in the Gulf Stream jet, and are used to quantify the resulting mixing and the
effect on the stratification in the Eighteen Degree Water formation region.
