The dynamics of ocean heat transport variability

The north-south heat transport is the prime manifestation of the ocean's ro le in global climate, but understanding of its variability has been fragmen tary owing to uncertainties in observational analyses, limitations in model s, and the lack of a convincing mechanism. We review the dynamics of global ocean heat transport variability, with an emphasis on timescales from mont hly to interannual. We synthesize relatively simple dynamical ideas and sho w that together they explain heat transport variability in a state-of-the-a rt, high-resolution ocean general circulation model. Globally, the cross-eq uatorial seasonal heat transport fluctuations are close to +/-3 X 10(15) W, the same amplitude as the cross-equatorial seasonal atmospheric energy tra nsport. The variability is concentrated within 20 degrees of the equator an d dominated by the annual cycle. The majority of the variability is due to wind-induced current fluctuations in which the time-varying wind drives Ekm an layer mass transports that are compensated by depth-independent return f lows. The temperature difference between the mass transports gives rise to the time-dependent heat transport. It is found that in the heat budget the divergence of the time-varying heat transport is largely balanced by change s in heat storage. Despite the Ekman transport's strong impact on the time- dependent heat transport, the largely depth-independent character of its as sociated meridional overturning stream function means that it does not affe ct estimates of the time-mean heat transport made by one-time hydrographic surveys. Away from the tropics the heat transport variability associated wi th the depth-independent gyre and depth-dependent circulations is much weak er than the Ekman variability. The non-Ekman contributions can amount to a 0.2- 0.4 X 10(15) W standard deviation in the heat transport estimated from a one-time hydrographic survey.