The dynamics of ocean heat transport variability

The north‐south heat transport is the prime manifestation of the ocean's role in global climate, but understanding of its variability has been fragmentary owing to uncertainties in observational analyses, limitations in models, and the lack of a convincing mechanism. We review the dynamics of global ocean heat transport variability, with an emphasis on timescales from monthly to interannual. We synthesize relatively simple dynamical ideas and show that together they explain heat transport variability in a state‐of‐the‐art, high‐resolution ocean general circulation model. Globally, the cross‐equatorial seasonal heat transport fluctuations are close to ±3 × 10 15 W, the same amplitude as the cross‐equatorial seasonal atmospheric energy transport. The variability is concentrated within 20° of the equator and dominated by the annual cycle. The majority of the variability is due to wind‐induced current fluctuations in which the time‐varying wind drives Ekman layer mass transports that are compensated by depth‐independent return flows. 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 changes in heat storage. Despite the Ekman transport's strong impact on the time‐dependent heat transport, the largely depth‐independent character of its associated meridional overturning stream function means that it does not affect estimates of the time‐mean heat transport made by one‐time hydrographic surveys. Away from the tropics the heat transport variability associated with the depth‐independent gyre and depth‐dependent circulations is much weaker than the Ekman variability. The non‐Ekman contributions can amount to a 0.2–0.4 × 10 15 W standard deviation in the heat transport estimated from a one‐time hydrographic survey.