Quantifying the impact of wind-stress and heat flux variability on simulated and observed ocean heat uptake

The capacity of the world's ocean to take up heat and carbon at its surface and to transport it to great depths is an important factor in setting the response of the climate system to external radiative forcing. Our ability to detect changes in oceanic heat content and attribute them to external radiative forcing crucially depends on quantifying the impact of unforced internal variability in augmenting or masking the externally forced signal. However, the extent to which atmospheric variations in wind and heat fluxes affect the upper ocean heat content remains poorly quantified. Here we present a new, process-based approach to detecting and attributing ocean heat content changes where we use the CMIP5 multi-model ensemble in combination with observational and reanalysis data to determine the impact of internal variability in (i) wind-stress forcing - expressed via ekman pumping and suction - and (ii) the net surface heat flux on historical and projected future ocean heat uptake. We find large regional differences in the relative role of the variability in these two forcings: in the tropical ocean, we show that the contribution of both wind-stress and heat flux variability on decadal ocean heat uptake trends is largely constant over time. In contrast, the projected future change in the location and strength of the mid-latitude jets under the RCP8.5 emission scenario results in an enhanced contribution of these two surface forcings to the externally forced simulated heat uptake. We thus illustrate that regional ocean-atmosphere dynamics and surface energy balances constitute a key factor in detecting the regional emergence of the forced heat uptake signal. We further use three reanalysis products as observational constraints for the model derived relations between heat content changes and the two surface forcings, which allows to quantify the contribution of wind-stress and heat flux related variability to the observed ocean heat uptake pattern since the 1980s for the first time.