Rectified Ocean Heat Uptake from Oscillatory Surface Forcing

Abstract

Abstract Ocean heat uptake is asymmetric with respect to the sign of radiative forcing. It is already known that surface cooling anomalies penetrate into the ocean faster than surface warming anomalies. Because of this asymmetry, the time-variable component of radiative forcing can induce a long-term, rectified cooling trend in ocean heat content, which is this work’s primary focus. Here, we explore this asymmetry and rectification on global and interannual scales, its implications, and its possible dependence on model parameters. We do so using a full-complexity global ocean–sea ice general circulation model and an idealized one-dimensional vertical mixing model, both forced with idealized abrupt and oscillatory surface forcing anomalies. In both models, the ocean heat uptake response to an abrupt cooling perturbation is larger than an equal-magnitude warming perturbation. This asymmetry is shown to be larger when the background vertical diffusivity is smaller, and is therefore likely model dependent. Sinusoidal oscillatory forcing with zero time mean induces a rectified cooling trend in both models, whose magnitude depends on both the diffusivity and the frequency of the interannual oscillatory forcing. The net rectified cooling can reach a rate of approximately −0.11 W m−2, which is substantial relative to the estimated anthropogenic warming rate of 0.47 W m−2 (von Schuckmann et al.). We discuss this rectification effect in the context of volcanic forcing in climate models, whose time-variable component may cause model-dependent ocean cooling in CMIP6 historical simulations, reaching 5%–30% the size of the total impact of volcanic forcing in our one-dimensional model. Correcting for this cooling may help reduce uncertainties in modeled ocean heat content evolution.