Impacts of regional mixing on circulations in the subtropical and tropical Pacific Ocean

We investigate the sensitivity of solutions to an eddy-permitting ocean general circulation model (OGCM) to regional changes in vertical diffusion at large spatial and temporal scales. Specifically, we increase the background part of the vertical diffusion coefficient, κ_b, from κ₀ to κ₀ + δκ_b within spatially distinct subregions of the subtropical and tropical Pacific, assess the impacts of those changes, and diagnose the processes that account for them. For that purpose, we compute anomalies defined as δq ≡ q - q₀, where q denotes an arbitrary variable in a sensitivity solution with a geographical κ_b enhancement and q₀ is the same variable in the control solution. Each solution is integrated for 20 years. The initial anomaly can be largely explained as a response to δq_t ≈ δκ_bq_0zz, where the subscripts z and t denote differentiation. In particular, this equation qualitatively explains the density decrease (increase) above (below) the main pycnocline in years 1-2. This dynamical signal associated with density anomaly propagates westward as Rossby waves, equatorward along the western boundary, eastward along the equator, poleward along the eastern boundary, and then into the interior. The equilibrium state reflects this classical pathway of Kelvin and Rossby waves. When the anomaly originates in subtropics, however, this dynamical signal is weak outside the latitude band of the region where κ_b is enhanced. In contrast, anomalies in passive scalar are advected along isopycnal surfaces even to regions where the dynamical signal is weak. The most conspicuous are the negative salinity anomalies on isopycnals ("spiciness" anomalies) that are generated in the subduction regions of the Subtropical Cells in the northern and southern hemispheres and are advected to the equator, reducing salinity in the Equatorial Undercurrent (see Figure).; Annual-mean salinity anomaly (color) of a test run and the horizontal velocity vectors of the control run (arrows) on the 24.6 σ_θ isopycnal for year 20. In the control run κ_b = 10^-5 m²/s uniformly and in the test run κ_b = 5 × 10^-5 m²/s south of 10°N and east of 160°W.