Diapycnal mixing transience and the meridional overturning circulation

Abstract

Diapycnal mixing of ocean waters is crucial to the dynamics and associated heat transport of the meridional overturning circulation, yet uncertainty exists regarding the distribution and physical mechanisms of this mixing. This study uses a highly-idealized, single-hemisphere model of buoyancy-forced flow to examine the examine the effects of the transience of diapycnal mixing on the MOC. The strength of the MOC was found to be insensitive to mixing transience when mixing occurred uniformly on basin boundaries. For mixing that was highly localized in space, a ten-fold increase in transience, as compared with the time-invariant control, resulted in a decrease by about 20% of MOC mass and heat transport. The degree of sensitivity in the highly localized case is likely to be a strong function of the surface restoring timescale for temperature. The circulation dynamics associated with transient mixing displayed large-scale, complex oscillations that increased in amplitude with the transience of mixing. Attempts to quantify the relationship between mixing transience, MOC strength, and the power expended in mixing were inconclusive and merit further investigation.