Impacts of ENSO events on cloud radiative effects in preindustrial conditions: Changes in cloud fraction and aerosol emissions, wet scavenging and transport

The impacts of the El Niño-Southern Oscillation (ENSO) events on shortwave and longwave cloud radiative effects (CRESW and CRELW) and the related changes in cloud fraction and aerosol emissions, wet scavenging and transport are quantified using three 150-year simulations for the preindustrial condition from the CESM model. Compared to recent observations from Clouds and the Earth's Radiant Energy System (CERES), the model simulation successfully reproduced larger variations of CRESW over the tropical western and central Pacific, Indonesian regions, and the eastern Pacific, as well as large variations of CRELW located mainly within the tropics. The ENSO cycle is found to dominate interannual variations of cloud radiative effects, especially over the tropics. Relative to those during La Niña events, simulated cooling (warming) effects from CRESW (CRELW) during El Niño events are stronger over tropical western and central Pacific, with the largest difference exceeding 40 Wm-2 (30 Wm-2), and weaker effects of 10-30 Wm-2 over Indonesian regions and the subtropical Pacific. Sensitivity tests show that variations of cloud radiative effects are mainly driven by ENSO-induced changes in cloud fraction. However, changes in natural aerosol concentrations, primarily due to changes in wet scavenging and transport processes, also play an important role in modulating the variations of cloud radiative effects. Because of increased (decreased) precipitation in El Niño (La Niña) events, increased (decreased) wet scavenging and transport of natural aerosols offset about 10% of variations of cloud radiative effects averaged over the tropics, whereas the emission changes enhance the variations by 4-6%. Moreover, the variation in medium and high cloud fraction accounts for about 20-50% of the interannual variations of CRESW over the tropics and almost all of the variations of CRELW between 60°S and 60°N. The variation of low cloud fraction plays a dominant role in contributing interannual variations of CRESW over midlatitude oceans. Variations in natural aerosols with all processes explained 10-30% of the interannual variations of both CRESW and CRELW over tropical central pacific, Indonesian regions and tropical Indian Ocean, as well as 10-40% of the interannual variations of CRESW over the west coast of major continents.