Observations and models of upper ocean response to atmospheric forcing: Wind-driven flow, surface heating and near-inertial internal wave interactions with mesoscale currents.

摘要

Three studies consider the upper ocean response to atmospheric forcing using moored observations and simple dynamical models. Observations from the Long Term Upper Ocean Study (LOTUS) and the Frontal Air-Sea Interaction Experiment (FASINEX) are used to examine the upper ocean momentum balance, response to surface heating and the influence of laterally sheared mesoscale currents on near-inertial internal wave propagation.; Low frequency currents (time scales longer than 10 days) observed during FASINEX exhibit geostrophic balance in the seasonal pycnocline, but wind-forced shear becomes important near the surface. Knowledge of the geostrophic shear enables us to distinguish between wind-driven currents forced directly by turbulent shear-stress and those produced indirectly through Ekman pumping. The observed directly-driven response satisfies the Ekman transport relation, penetrates well below the average mixed-layer depth, spirals to the right and decays with depth.; At time scales longer than 1.5 days, a one-dimensional balance between time rate of change of heat and mixing is consistent with the observed response to surface heating. The observations satisfy the vertically integrated heat balance, exhibit downward phase propagation and decay with depth. One-dimensional models cannot explain the observations at higher frequencies, which show upward phase propagation and anticlockwise phase shifts in the integrated heating. An idealized model in which the upper ocean responds to buoyancy forcing through mixing and internal wave generation suggests an explanation for these features, where vertical advection produces upward phase propagation and phase shifts in the integrated response.; During frontal events, observed near-inertial currents have short horizontal scales and show no clear phase propagation patterns. The observations often exhibit anisotropic current ellipses elongated in the cross-front direction, where inertial bandpass buoyancy correlates with major-axis currents. Although linear, monochromatic plane waves cannot describe these features, models of near-inertial internal waves reflecting off laterally sheared frontal jets produce mode-like structures with short horizontal scales, little horizontal phase propagation and anisotropic current ellipses aligned primarily across the front. Depending on the incidence angle of the wave and the degree to which it reflects, buoyancy may correlate with one or both current ellipse components, consistent with the observations.