Collaborative Research: Response of the upper tropical Pacific Ocean to greenhouse gas forcing in observations and models

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The tropical Pacific Ocean is a well-established driver of global climate anomalies and the largest oceanic source of 𝐶𝑂2 flux into the atmosphere. Its response to rising greenhouse gases (GHGs) will strongly influence future global climate means and extremes and the carbon cycle. Climate models simulate a warming in this region for the last several decades, which is not seen in the observational record. There is not a clear explanation of this difference or what it means for future projections. This work will be a uniquely detailed analysis of the tropical Pacific Ocean response to GHG-driven thermodynamic and dynamic forcing focusing on comparisons to observed and reanalyzed changes in ocean circulation and temperature. While much research has focused on the tropical atmosphere’s response to rising GHGs there has been much less work on the tropical oceans. This project will therefore by an important intellectual advance in understanding, in the context of radiatively-forced climate change, the ocean side of the coupled tropical atmosphere-ocean system. Determining how the tropical Pacific responds to rising GHGs is critical to projecting changes in regional climates worldwide and climate-carbon feedbacks that influence atmospheric 𝐶𝑂2. The work will advance knowledge important to climate change mitigation and adaptation. The work will support two early-career researchers (postdoctoral and graduate) and a female faculty member to make fundamental advances in understanding how a key component of the climate system responds to anthropogenic forcing. The lead PI is well integrated into the community researching evolving drought risk over North America, for which the equatorial Pacific is a key driver, and its societal impacts. Results will be communicated there to identify errors and narrow uncertainties regarding near-term hydroclimate projections under GHG-induced changeAmidst the global warming accompanying the sharp rise in GHGs since circa 1960 observations show little warming or even cooling in the equatorial Pacific cold tongue. In contrast, models within successive Coupled Model Intercomparison Projects (CMIPs) simulate warming of the cold tongue. It has been argued that this discrepancy is due to strong internal variations in nature rather than a response to GHGs. Yet various studies all show it is unlikely that CMIP models can match the observations, though they differ on just how unlikely. On the other hand, it has been hypothesized that rising GHGs lead to strengthening of the zonal SST gradient, Walker circulation and trades, a dynamically-shoaled thermocline and enhanced cooling by upwelling. Models, it is argued, have a largely opposite response due to biases in simulating the tropical Pacific atmosphere-ocean system. This project will address how the temperature, currents and thermal structure of the upper tropical Pacific Ocean respond to rising GHGs. It is a deep investigation with observations and models of the period to date for which models can be evaluated. The work is organized around hypotheses that address the questions: 1) what are the relative roles of thermodynamic and dynamic processes in determining the tropical Pacific Ocean response to rising GHGs? 2) do biases in climate models (excessive cold tongue, overdeveloped southern convergence zone, too-warm eastern subtropical stratus cloud and upwelling regions) lead to biased SST responses in upwelling and subducting regions and biased transport pathways into the Equatorial Undercurrent? 3) do model biases influence the response of tropical Pacific SSTs to GHG forcing? The main goal is to better understand the active role of nature’s ocean in the response to rising GHGs. Models are primary tools for this investigation. In response to rising GHGs the work will examine: how heat added at the surface is mixed down and transported in the interior after subduction in the subtropics; how changes in wind stress impact currents, upwelling, thermal structure and SST; how mean wind stress influences interior pathways between the equator and the subtropics and, hence, the temperature of water upwelling in the cold tongue. The responses will be decomposed with experiments with ocean models using European Center for Medium Range Weather Forecasts reanalyses for validation. The experiments will impose, in various combinations, modeled and observed fields, GHG-induced heating, and reanalyzed and CMIP6 mean and anomalous wind stresses. Heat budgets, tracers, Explanatory Artificial Intelligence methods and causal pathway analysis will reveal the mechanisms of ocean responses to observed and CMIP6 wind stress trends, how this depends on the mean ocean state being perturbed, and the relative roles of passive and dynamical ocean processes in determining trends in tropical Pacific SSTs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.