How accurate did GCMs compute the insolation at TOA for AMIP-2?

[1] Monthly averages of solar radiation reaching the Top of the Atmosphere (TOA) as simulated by 20 General Circulation Models (GCMs) during the period 1985–1988 are compared. They were part of submissions to AMIP-2 (Atmospheric Model Intercomparison Project). Monthly averages of ISCCP-FD (International Satellite Cloud Climatology Project – Flux Data) are considered as reference. Considerable discrepancies are found: Most models reproduce the prescribed Total Solar Irradiance (TSI) value within ±0.7 Wm⁻². Monthly zonal averages disagree between ±2 to ±7 Wm⁻², depending on latitude and season. The largest model diversity occurs near polar regions. Some models display a zonally symmetric insolation, while others and ISCCP show longitudinal deviations of the order of ±1 Wm⁻². With such differences in meridional gradients impacts in multi-annual simulations cannot be excluded. Sensitivity studies are recommended.