Determining Causation of Hiatuses in a Large Ensemble

We present an energy balance over the surface and ocean mixed layer, using a large ensemble of coupled model simulations of the historical period. The large ensemble allows us to cleanly isolate internal variability in the energy budget and to explore its role in surface warming hiatuses.

The observed hiatus in surface temperature warming from 1998-2012 deviated from most model projections. Amongst the explanations for the disparity is quasi-random internal variability, leading to an anomalous increase in ocean heat uptake away from the surface. Locating this "missing heat" in a particular ocean basin has been the goal of many observational and modelling studies, but there is no consensus on where that heat is stored. Such studies often infer the cause of the hiatus by finding large increases of heat in a particular ocean basin, or by pointing to a vertical heat transfer to deeper ocean layers. Our findings suggest that ocean heat uptake can only partly account for hiatuses in the model: internal variability in top of the atmosphere radiation fluxes plays an equally important role in determining the surface layer energy budget.

We further find that all major ocean basins, except the Indian, have enough variability in heat content to store the “missing heat”, and do so far more frequently than hiatuses occur. It is the sum of all basins acting together – with cumulative or opposing effects – that make the ocean’s definitive contribution to the surface energy balance.