Title: Northern Hemisphere summertime regional circulation trends driven by anthropogenic aerosol forcing
Abstract
Reanalysis data show that the Northern Hemisphere summertime circulation has undergone significant changes with important implications for surface weather and its extremes in the satellite era. In particular, midlatitude jets and storm tracks have weakened, and stationary wave trends exhibit distinct regional patterns. Previous work showed that aerosol forcing is partly responsible for the midlatitude jet weakening. Here we utilize the simulations from the Detection and Attribution Model Intercomparison Project (DAMIP) to investigate the impact of aerosol forcing on the storm track weakening and stationary wave trends. The DAMIP simulations show that aerosol forcing contributes to about half of the storm track weakening in the zonal mean and dominates in the Pacific. We show that aerosol-induced shortwave radiation trends over land strengthen land-to-ocean energy contrast and land-to-ocean energy export. Consequently, energy converges poleward of oceanic storm tracks, demanding weaker poleward energy transport, and the storm tracks weaken particularly over the Pacific. The DAMIP simulations also show that the simulated stationary wave trends over Eurasia are attributable to aerosol forcing. However, this attribution is challenged as the DAMIP simulations and more generally CMIP6 models fail to capture the stationary wave trends in reanalysis. We discuss potential reasons for the underestimated stationary wave trends in the climate models. Our results highlight the importance of aerosol forcing for summertime circulation and related regional impacts.
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