Diagnostic evaluation of effects of vertical mixing on meridional overturning circulation in an idealized ocean

Abstract

In this study, diagnostic equations are proposed to quantitatively evaluate meridional overturning circulation (MOC) simulated in ocean general circulation models (OGCMs). Applicability of the equations is illustrated by revisiting the MOC simulated in an idealized ocean. The simulations with surface differential heating/cooling show that, for certain horizontal distribution of vertical diffusivity, the stronger vertical mixing does not intensify the MOC while it makes the deeper water less dense. This result, which is in marked contrast to the widely accepted idea that the stronger vertical mixing promotes upwelling and intensifies the MOC by making the deeper water less dense, was investigated using the diagnostic equations. It was found that geostrophy dominates the MOC, and the geostrophic flow normal to lateral boundaries induced intense upwelling/downwelling along the boundaries. These results indicate that the primary role played by the vertical mixing on the large-scale MOC is to change hydrostatic pressure fields (geostrophic flow fields), rather than to promote upwelling. The simulation with localized cooling on the other hand showed that the ageostrophic flows significantly contribute to small-scale features of the MOC, while the geostrophic flows determine large-scale structure of the MOC. The proposed equations will thus be useful to quantitatively diagnose the MOC dynamics in realistic OGCMs.