Investigating recent decadal trends in the Pacific westerly jet in response to various atmospheric forcings using CMIP6 model results and reanalysis data

Abstract. The strength and location of the North Pacific westerly jet (NPWJ) affects weather and trans-Pacific pollution transport as it triggers and directs atmospheric river events toward North America. We used four reanalysis datasets and eight Coupled Model Inter-comparison Project Phase 6 (CMIP6) models to investigate the characteristics and changes of the NPWJ during 1980&nash;2019. The NPWJ climatologic core seasonally swings between north and south, being most southerward (~33° N) in winter and most northward (~45° N) in summer, as shown by the observation-based reanalysis data. All data provide strong evidence for the weakening (up to -0.45 and -0.68 ms^-1decade^-1) and northward shift (0.2° and 1.0°) of the NPWJ in summer and autumn during the study period. Various atmospheric forcing experiments performed by the CMIP6 models further reveal aerosol forcing being the main driver, which can be traced back to the spatially inhomogeneous anthropogenic aerosol emission changes that increase in Asia and decrease in Europe. When we apply Earth system climate models to investigate the feedback between atmospheric forcings and atmospheric dynamical fields on decadal scales, two points should be noted. First, there is a need to include interactive chemistry in the CMIP6 model simulations to bring the dynamical fields closer to those based on observational data. Second, in addition to the well-mixed greenhouse gases, anthropogenic aerosols, and natural forcings proposed in the Detection and Attribution Model Intercomparison Project (DAMIP) single-forcing simulations, time-varying ozone radiative forcing is also important to climate change.