Multiscale Processes and Nonlinear Dynamics of the Circulation and Upwelling Events off Monterey Bay

Abstract

Abstract The nonlinear multiscale dynamics of the Monterey Bay circulation during the Second Autonomous Ocean Sampling Network (AOSN-II) Experiment (August 2003) is investigated in an attempt to understand the complex processes underlying the highly variable ocean environment of the California coastal region. Using a recently developed methodology, the localized multiscale energy and vorticity analysis (MS-EVA) and the MS-EVA-based finite-amplitude hydrodynamic instability theory, the processes are reconstructed on three mutually exclusive time subspaces: a large-scale window, a mesoscale window, and a submesoscale window. The ocean is found to be most energetic in the upper layers, and the temporal mesoscale structures are mainly trapped above 200 m. Through exploring the nonlinear window–window interactions, it is found that the dynamics underlying the complex surface circulation is characterized by a well-organized, self-sustained bimodal instability structure: a Bay mode and a Point Sur mode, which are located near Monterey Bay and west of Point Sur, respectively. Both modes are of mixed types, but they are distinctly different in dynamics. The former is established when the wind relaxes, while the latter is directly driven by the wind. Either way, the wind instills energy into the ocean, which is stored within the large-scale window and then released to fuel temporal mesoscale processes. Upon wind relaxation, the generated mesoscale structures propagate northward along the coastline, in a form with dispersion properties similar to that of a free thermocline-trapped coastal-trapped wave. Between these two modes, a secondary instability is identified in the surface layer during 15–21 August, transferring energy to the temporal submesoscale window. Also studied is the deep-layer flow, which is unstable all the time throughout the experiment within the Bay and north of the deep canyon. It is observed that the deep temporal mesoscale flow within the Bay may derive its energy from the submesoscale window as well as from the large-scale window. This study provides a real ocean example of how secondary upwelling can be driven by winds through nonlinear instability and how winds may excite the ocean via an avenue distinctly different from the classical paradigms.