Isolating Mesoscale Coupled Ocean-Atmosphere Interactions in the Kuroshio Extension Region

he Kuroshio Extension region is characterized by energetic oceanic mesoscale and frontal variability that alters the air-sea fluxes that can influence large-scale climate variability in the North Pacific. We investigate this mesoscale air-sea coupling using the SCOAR (RSM-ROMS) regional eddy-resolving coupled ocean-atmosphere (OA) model that downscales the observed large-scale climate variability from 2001-2007. The model simulates many aspects of the observed seasonal cycle of OA coupling strength for both momentum fluxes and latent and sensible heat fluxes.
We introduce a new modeling approach to study the scale-dependence of two well-known mechanisms for the surface wind response to mesoscale sea surface temperatures (SST), namely, the ‘vertical mixing mechanism’ (VMM) and the ‘pressure adjustment mechanism’ (PAM). We compare the fully coupled model to the same model with an online, 2-D spatial smoother applied to remove the mesoscale SST field felt by the atmosphere. Both VMM and PAM are found to be active during the strong wintertime peak in coupling strength seen in the model and in observations. For VMM, large-scale SST gradients surprisingly generate coupling between downwind SST gradient and wind stress divergence that is often stronger than the coupling on the mesoscale, indicating their joint importance in OA interaction in this region. In contrast, VMM coupling between crosswind SST gradient and wind stress curl occurs only on the mesoscale, and not over large-scale SST gradients, indicating the essential role of the ocean mesoscale. For PAM, the model results indicate that coupling between the Laplacian of sea level pressure and surface wind convergence occurs for both mesoscale and large-scale processes, but inclusion of the mesoscale roughly doubles the coupling strength. We also found coupling between latent heat flux and SST to be significant throughout the whole runentire seasonal cycle in both the fully-coupled mode and large-scale coupled mode, with peak coupling during winter months. The atmospheric response to the oceanic mesoscale SST is studied by comparing the fully coupled run to an uncoupled atmospheric model forced with smoothed SST prescribed from the coupled run. Precipitation anomalies are found to be forced by surface wind convergence patterns that are driven by mesoscale SST gradients, indicating the importance of the ocean forcing the atmosphere at this scale. For the month of January 2001, we analyzed mesoscale precipitation anomalies and found that they collocate with mesoscale, 10m wind convergence rather than mesoscale SST anomalies, suggesting the role of VMM on mesoscale precipitation.