The Oceanic Fingerprint of the Forced Response to Anthropogenic Aerosols

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Human emissions to the atmosphere impact our climate in two ways: changing how efficiently our planet cools via radiation back to space through increasing greenhouse gases; and changing how much sunlight reaches Earth’s surface via the impacts of aerosols on atmospheric brightness and cloudiness. More is understood about the warming effect of greenhouse gases, which act globally, than the cooling effect of anthropogenic aerosols, which act more regionally (close to emission sites in the northern hemisphere). Quantifying how much these two opposing climate drivers are responsible for past change is also challenging, as our climate system also has natural chaotic fluctuations. The ocean is a major player in controlling the rate and form of change in Earth’s climate. This project will exploit the unprecedented global ocean observations from the Argo array and a large set of climate model runs (which capture a lot of Earth’s climate chaos) to try to tease apart the fingerprint of aerosols versus greenhouse gases on our recent past climate. Some of the questions the team will address are: Has the aerosol effect been over or underestimated in the past? How has the ocean been impacted? Ultimately the team aims to help improve climate projections and thus support better mitigation and adaptation actions by the community. The project will also support and help train a postdoctoral scientist.Anthropogenic aerosols, along with greenhouse gases (GHG), have caused substantial climate change over the past several decades, with the former much less understood and quantified. While most previous studies have focused on surface or atmospheric responses, the investigators will investigate how the ocean water masses and circulation evolve due to anthropogenic aerosol forcing in contrast to the background of internal variability and GHG-forced changes during the historical period and in future projections. To isolate externally forced signal from internal climate variability, they will exploit new large ensemble simulations using a single general circulation model with common external forcing but different initial conditions. They will focus on comparing the externally forced component and internal variability, evaluation of the performance of various spatiotemporal analysis methods in separating those signals and the robustness of their estimation. The team hopes to develop fingerprints of the ocean interior response to anthropogenic aerosol forcing found in the model runs, and then examine how likely it is to detect these fingerprints in the observations, unforced control runs, runs with other anthropogenic forcing, and runs with natural external forcing. They will attempt to identify and remove the internal variability from the observations to test whether the detectability of the fingerprint of the forced signal can be improved.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.