Springtime Aerosol Indirect Forcing Across the North Pacific Ocean in Observations and Global Models

Transport of Asian aerosols across the North Pacific Ocean peaks during the spring season. A mix of aerosols is found in the maritime environment as far west as the North American West Coast, including Asian dust and pollution, local N. American pollution, and new particle formation. Aerosol transport events are arranged in plumes, with pristine maritime conditions prevailing between the plumes. Also present are complex cloud systems comprising stratocumulus decks, mixed phase cumulus, and mesoscale cyclones. The impact of Asian aerosols on cloud microphysical and radiative properties across the North Pacific Ocean basin during April 2004 is observed in aircraft and satellite observations. In-situ observations from the Cloud Indirect Forcing Experiment (CIFEX) document the decrease in cloud drop size and increase in cloud drop number concentration owing to the enhanced concentration of cloud condensation nuclei offshore of Northern California during aerosol transport events. The impact of these microphysical effects on shortwave cloud radiative forcing is quantified using blended MODIS and CERES satellite observations in the CIFEX region and across the entire ocean basin. The magnitude of the microphysical and radiative effects observed by aircraft and satellite are compared with the NOAA/GFDL AM2 and NASA GEOS global atmospheric climate models employing new physically-based parameterizations of aerosol nucleation. The ability of these models to provide quantitative estimates of aerosol indirect forcing over ocean basins is evaluated.