Abstract
Abstract Adjoint methods of sensitivity analysis were applied to the California Current using the Regional Ocean Modeling Systems (ROMS) with medium resolution, aimed at diagnosing the circulation sensitivity to variations in surface forcing. The sensitivities of coastal variations in SST, eddy kinetic energy, and baroclinic instability of complex time-evolving flows were quantified. Each aspect of the circulation exhibits significant interannual and seasonal variations in sensitivity controlled by mesoscale circulation features. Central California SST is equally sensitive to wind stress and surface heat flux, but less so to wind stress curl, displaying the greatest sensitivity when upwelling-favorable winds are relaxing and the least sensitivity during the peak of upwelling. SST sensitivity is typically 2–4 times larger during summer than during spring, although larger variations occur during some years. The sensitivity of central coast eddy kinetic energy to surface forcing is constant on average throughout the year. Perturbations in the wind that align with mesoscale eddies to enhance the strength of the circulation by local Ekman pumping yield the greatest sensitivities. The sensitivity of the potential for baroclinic instability is greatest when nearshore horizontal temperature gradients are largest, and it is associated with variations in wind stress concentrated along the core of the California Current. The sensitivity varies by a factor of ∼1.5 throughout the year. A new and important aspect of this work is identification of the complex flow dependence and seasonal dependence of the sensitivity of the ROMS California Current System (CCS) circulation to variations in surface forcing that was hitherto not previously appreciated.
Highlights
The California Current System (CCS) is a complex eastern boundary current (Hickey 1979, 1998), and there remain many unanswered questions concerning the underlying dynamics of the circulation (Miller et al.1999)
An adjoint method has been used to explore variations in the sensitivity to surface forcing of coastal sea surface temperature (SST) and upwelling, eddy kinetic energy (EKE) and the potential for baroclinic instability of the complex circulation patterns that develop in the CCS
3) identification and appreciation of the often complex nature of spatial variations in the circulation sensitivities to the surface forcing that are intimately tied to the structure of the underlying ocean circulation via wind-induced wave development and the evolution of forcing-induced perturbations controlled by localized barotropic and baroclinic processes and instabilities; and 4) a clear separation of the circulation sensitivity to wind stress versus wind stress curl—a topic that has been much discussed in the oceanographic literature (e.g., Enriquez and Friehe 1995)
Summary
The California Current System (CCS) is a complex eastern boundary current (Hickey 1979, 1998), and there remain many unanswered questions concerning the underlying dynamics of the circulation There have been numerous modeling and diagnostic studies of the CCS, and many indicate that local variations in the surface wind play an important role in controlling the circulation (e.g., Allen 1980; McCreary et al 1987; Brink 1991; Batteen 1997; Oey 1999; Di Lorenzo 2003). While some fundamental aspects of the CCS circulation have been documented and are well understood, such as the role of the alongshore wind stress in promoting upwelling and in establishing cross-shelf pressure gradients that drive the primary current systems, quantitative questions remain about the sensitivity of the circulation to the timing and structure of variations in the forcing.
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