Explaining the Equatorial Pacific Thermocline Response to Climate Change with a Model Hierarchy

Matt Luongo, Shang-Ping Xie, Ian Eisenman, Shantong Sun, Kyle C. Armour

Most studies of the equatorial Pacific response to anthropogenic forcing have focused on patterns of sea surface temperature (SST) change. However, similar SST patterns can be consistent with a range of different subsurface responses, each with differing physical and biogeochemical implications. While historical observation and climate model mismatches have been suggested in the literature, we show that model simulations can largely capture the observed 1958-2020 subsurface temperature trend in the equatorial thermocline. We then analyze a hierarchy of idealized model simulations, consisting of fully-coupled, mechanically-decoupled, ocean-only, and reduced gravity models, to understand which ocean dynamics contribute to this response. We show that the response of the thermocline to idealized climate change can be explained by a combination of decadal Bjerknes-like momentum dynamics and radiatively-forced buoyancy-driven dynamics. We further decompose the buoyancy-driven pattern into a pattern driven by remote, subtropical SST forcing and a pattern driven by local, equatorial SST forcing. The remote-SST-forced pattern of thermocline warming shows the signature of dynamic and thermodynamic subtropical cell adjustments. Meanwhile, increased stratification in the local-SST-forced pattern both coherently shoals the thermocline and relaxes thermocline tilt to largely cool the thermocline. Considered together, we recreate the long-term subsurface equatorial Pacific response to idealized greenhouse gas forcing as a linear combination of (i) wind-stress-driven changes, (ii) remote buoyancy-driven changes, and (iii) local buoyancy-driven changes. To conclude we discuss implications for recent temperature trends, revisit canonical theories of the ocean dynamical thermostat, and show the insensitivity of forced responses to forcing geography.