On the Fluctuating Buoyancy Fluxes Simulated in a OGCM

Abstract

Abstract Subgrid-scale fluctuations with zero means have generally been neglected in ocean modeling, despite their potential role in affecting the oceanic state following Hasselmann's seminal paper on stochastic climate models and series of studies conducted thereafter. When representing effects of these fluctuations in a stochastic parameterization, knowledge of basic properties of these fluctuations is essential. Here, the authors quantify these properties using hourly output of a simulation performed with a global OGCM. This study found that fluctuating buoyancy fluxes are strong in the sense that their strengths are up to one order of magnitude larger than the magnitudes of the respective mean eddy fluxes and that the fluctuations originate not only from mesoscale eddies and tropical instability waves but also from near-inertial waves, especially in the low- and midlatitude oceans. It is this wave contribution that makes the basic properties of fluctuations distinctly different from those expected from mesoscale eddies. The geographical distribution of fluctuation intensity differs from that of mesoscale eddy activity and is strongest in the low- and midlatitude oceans complemented by additional and secondary maxima in the Gulf Stream, the Kuroshio, and the Southern Ocean. The seasonality in most of the low- and midlatitude oceans, characterized by stronger fluctuations in winter than in summer, is just the opposite of that of mesoscale eddies. In the tropical oceans, the correlation length scales reach 500 km in the zonal direction but only about 30–40 km in the meridional direction, reflecting near-inertial waves with nearly zonally oriented wavecrests. Overall, these results provide an important basis for stochastically describing the effects of subgrid-scale fluctuations.