Response of atmosphere-ocean circulation to aerosol patterns of the 1970s and 2000s

The spatial distribution of anthropogenic aerosols was substantially different in the 1970s and 2000s. In the 1970s, a maximum in anthropogenic aerosol optical depth (τ_a) was situated over the North Atlantic, North America, and Europe. By the 2000s, the τ_a maximum has shifted to East Asia. That difference in the global pattern of τ_a has implications for the regional radiative effects and the regional climate. Here, we show how the differences in the global pattern of τ_a favour changes in the North Atlantic sea-surface temperature (NASST). To this end, we perform several hundred years of fully coupled atmosphere-ocean simulations with the CMIP6 model MPI-ESM1.2. Our simulations use observationally informed anthropogenic aerosol optical properties and a prescribed effect on the cloud albedo. All our simulations are sufficiently long to eliminate the impact of model-internal variability on the results.

Contrasting the output of the simulation using τ_a of the 1970s against the output from the pre-industrial control without τ_a largely shows the expected decrease in NASSTs, which is due to the surface cooling by the aerosol effects on radiation and clouds. Surprisingly, we also see an unusual warming response in the eastern sub-polar gyre - the same region that became known as the North Atlantic Warming Hole (NAWH). The described NASST responses reversed as we go from the anthropogenic aerosols of the 1970s to the 2000s, namely a relative decrease of the NASST in the region of the NAWH and an increase away from it. Analysing the atmosphere and ocean dynamics in the model output reveals that these NASST responses are driven by changes in the circulation of the model. The signal in the NAWH is explained by the oceanic meridional heat convergence that is primarily attributed to the gyre heat transport and not the overturning circulation that is often in focus of research. Our diagnostics also show a latitudinal shift of the jet stream over the North Atlantic being closer to the equator in the 1970s than in the 2000s, which is consistent with the response of the sub-polar gyre via the coupling by the wind stress at the ocean surface. Our results imply that the regional reduction in anthropogenic aerosols over the North Atlantic between the 1970s and 2000s favoured (1) a stronger NAWH cooling signal, (2) a stronger Arctic amplification measured by the NASST, and (3) a latitudinal shift of the mean position of the jet stream. Sensitivity tests with the model indicate that more than two-thirds of these signals are associated with aerosol effects on clouds. It highlights the need to better constraint aerosol-cloud interactions for an assessment of circulation responses to changes in atmospheric composition. 

 

Reference:

Fiedler, S., and  Putrasahan, D., 2021: How does the North Atlantic SST pattern respond to anthropogenic aerosols in the 1970s and 2000s? Geophysical Research Letters,  48, e2020GL092142. https://doi.org/10.1029/2020GL092142.