An Enhanced La Niña-like Forced Response by Reducing Mean State Biases in CESM2

Most Earth system models (ESMs) predict an “El Niño-like” trend in the equatorial Pacific, with a local maximum in sea surface temperature (SST) warming in the eastern equatorial Pacific, as a consequence of greenhouse gas effects. However, observations show the opposite, “La Niña-like” pattern over recent decades, with cooling in the eastern equatorial Pacific. This discrepancy, highlighted in multiple recent studies, suggests that ESMs may be incorrectly simulating the equatorial Pacific’s response to greenhouse gas warming. Since the state of the equatorial Pacific is known to influence tropical cyclone activity, this uncertainty propagates to future tropical cyclone activity projections, rendering even the direction of change uncertain in some regions. One hypothesis for the erroneous trend in the historical SST pattern is that it is caused by the well-known bias in the equatorial cold tongue that is common to most models (Seager et al. 2019). In this project we aim to test this hypothesis, and investigate one possible avenue for the development of alternative projections in which future Pacific SST trends are made consistent with recent historical ones. This will enable us to explore the range of possible changes in tropical cyclones in the future using a storyline approach. As the first step, we use flux adjustment to reduce the mean state bias in a coupled ESM, and assess whether the resulting simulated historical tropical Pacific SST trends better align with those observed. Specifically, we conduct ensemble simulations using the coupled Community Earth System Model version 2 (CESM2) with around 2° atmosphere and 1° ocean resolution setup. The flux adjustments are computed using a methodology akin to the one outlined by Vecchi et al. (2013). Our results show that the flux adjustments effectively reduce CESM2’s bias in tropical Pacific SST. Assessment of the tropical Pacific SST trends is ongoing. Additionally, tropical cyclone activities in the CESM2 simulations will be analyzed through tropical cyclone genesis indices as well as synthetic storms downscaled using the Columbia Hazard model (CHAZ). In this presentation, we will present simulations of tropical Pacific SST and tropical cyclones from the flux-adjusted CESM2 alongside its unadjusted, control counterpart.