Changes in aerosol effective radiation forcing from CESM2 to CESM3_development. Short wave or long wave?

[oyvindseland]

Hi

Although NorESM has a different aerosol parameterisation than CESM, the numbers for ERF have been fairly close for released versions. The same seems to hold true for NorESM3-development / CESM3 development as both are decreasing compared to NorESM2 / CESM2. My latest estimate for NorESM3-development version is -0.7 W m-2 and if I have not misunderstood the same reduction is also found in CESM3. However when I look more into the details it seem that the reason for change is not some much a reduction in cloud short wave effect but a quite large positive LW cloud effect. Is this the case also for the CESM3 development version?

What I also find interesting is that the increase in LWCF is less local to the aerosol regions than the more negative SWCF so it looks more like a circulation response than impact from the aerosols themselves. In particular there seems to be a southward shift in Atlantic and Eastern Pacific ITCZ even without the change in SST pattern as one typically finds in a coupled simulation with increased NH aerosols. As in CAM we do not in general underestimate / overestimate the precipitation in Amazon / West Africa in uncoupled simulations but the impact of the added aerosol is to increase the precipitation in the Amazon region. Do you find the same for CESM3.