Impact of Model Tuning on Simulated Mid-Pliocene Warming

A tuned version of HadCM3 (HadCM3t) is used to simulate the climate of the mid-Pliocene warm period (mPWP) and is compared with the original untuned version of HadCM3 (HadCM3u). After the tuning, HadCM3t performs as well as HadCM3u in simulating the preindustrial climate, but aligns better with the reconstructed mPWP sea surface temperature anomalies, primarily due to a better representation of high latitude warming. Regarding the mPWP climate anomaly relative to the preindustrial, compared to HadCM3u, HadCM3t produces stronger mid-to-high latitude warming with polar warming 2.2 times the global mean, which is higher than the 2.0 of HadCM3u. The warming of the mPWP is primarily explained by an increase in emissivity and surface albedo. The warming of the polar regions induced by emissivity and surface albedo from HadCM3t is higher by 0.6°C and 0.7°C respectively, than that from HadCM3u, leading to warmer mPWP high latitudes. The increase in shortwave radiation over the Polar Regions is driven mainly by changes in surface albedo, which lead to a temperature rise of +0.7°C, and by changes in cloud cover, which contribute an additional +0.1°C to the temperature increase. Here, the increased cloud-induced shortwave radiation change is due to decreased cloud fraction and decreased cloud scattering effect; the increased surface albedo-induced radiative effects result from the increased ice loss due to warm high latitudes. This tuning allows models like HadCM3 to better align with proxy data, offering a more reliable baseline for projecting future scenarios under similar conditions.