On the contribution of transient diabatic processes to ocean heat transport and temperature variability

Abstract Time-varying processes contribute to ocean heat transport and are important to understand for accurate climate modeling. While past studies have quantified time-varying contributions to advective transport, less attention has been given to diabatic processes such as surface forcing and mixing. Using a global eddy-permitting ocean model we quantify the contribution of time-variable processes to meridional and diathermal (warm-to-cold) heat transport at different timescales using a temporal eddy-mean decomposition performed in the temperature-latitude plane. Time-varying contributions to meridional heat transport occur predominantly at mesoscale eddy-dominated mid-latitudes and in the tropics, associated with the seasonal cycle and Tropical Instability Waves. The seasonal cycle is a dominant driver of surface flux- and mixing-driven diathermal heat transports. Non-seasonal (and non-diurnal) processes contribute up to about 10% of the total. We show that transient contributions to diathermal heat transport can be interpreted as sources of Eulerian temperature variance. We thus extend recent work on the drivers of temperature variability by evaluating the role of mixing. Mixing dampens seasonal and diurnal temperature variability, except near the equator where it can be a source of seasonal variability. At mesoscale timescales mixing drives variability within and near the base of the boundary layer, the mechanisms of which are explored using a column model. We suggest that climate models that don’t resolve the mesoscale may be missing the rectified heat transport associated with high-frequency diabatic processes, in addition to the adiabatic eddy fluxes that are commonly parameterized.