Simplified heat budgets can yield misleading diagnoses of ocean temperature change

Noel Brizuela, Sarah M Kang, Malte F. Stuecker , Chia-Ying Lee, Feng Jiang

Temporal changes in upper ocean temperature (𝑇) are crucial indicators of climate variability, change, and its environmental impacts. Ocean heat budgets (OHBs) used to understand these changes are a standard tool across multiple disciplines, but the vast majority of OHBs in published research rely on approximations that are rarely documented and tested. Here, we use output from the CESM2 model to perform exact and approximate OHBs of the Ni˜no3.4 region and compare their results. Exact OHBs are computed using specialized variables that directly reflect the model’s implementation of tracer equations, while approximate OHBs use standard variables to recreate the redistribution of heat by oceanic processes. Our results highlight that major drivers of heating can hide within small residual terms and thus go unacknowledged by simplified OHBs. We also find that reconstructions of advective heating exaggerate the role of advection in the growth of ENSO events in this model (by ∼25% and ∼100% for El Nino and La Nina, respectively). Lastly, we show that eddy parameterizations can dominate multidecadal changes in 𝑇 and thus represent a poorly-explored source of long-term change. Parameterized processes help shape 𝑇 across the global ocean, but their contributions go unnoticed in simplified OHBs. To curb incomplete views of the oceanic temperature balance, we propose that future model intercomparison projects save and report values of advective and parameterized heating exactly as implemented by each model.