Potential Role of Aerosols in Causing the Global Warming Hiatus through Affecting Pacific SST: A Test with CESM1

The global mean surface temperature increase since end of 1990s have been stable, despite of the continuous increase in greenhouse gas concentrations. There have been compelling evidences suggesting that the cooling is mostly orientating from eastern tropical Pacific Ocean, as an internal variability mode due to heat exchange between deep ocean and the atmosphere. The temporal cooling is tied to negative phase of PDO (pacific decadal oscillation). However, there has been recent work suggesting that the transition of PDO from positive to negative phase might be facilitated by increase in the Asia cooling aerosols.

To test this hypothesis, we conducted CESM1-CAM5 fully coupled model simulations in which only external forcing considered is anthropogenic aerosols, from 1850 to 2020 for three ensemble members. The aerosol-only forcing indeed leads a negative PDO-like SST trends for the 15-year period beginning at about 1998. By diagnosing changes of the radiation budget, sea level pressure and surface temperature, we demonstrate that the changes in anthropogenic aerosol could affect sea surface pressure by inducing a heating or cooling in the North Pacific, and thus contribute to the PDO variability. We conclude that the increase in anthropogenic aerosol over East Asia and the decrease over North America and Europe could explain part of the negative PDO during the 15-year period since 1998. However, the mechanisms and relative contribution of each of these regions to the PDO transition remain unclear. We plan to conduct more simulations by only perturbing the anthropogenic aerosols over limited regions (e.g. Asia vs. Europe).