The Adjustment of the Coupled Climate Model HadGEM1 toward Equilibrium and the Impact on Global Climate

Abstract

Abstract Coupled climate models generally have a small residual radiative flux at the top of the atmosphere. In the Met Office climate model, Hadley Centre Global Environmental Model version 1 (HadGEM1), it is incoming (heating the planet) and reduces over a 350-yr period from 0.4 to 0.1 W m−2. The process of the adjustment in HadGEM1 is examined and is shown to be linked to excessive heat gain. In the tropical and South Atlantic, cold, fresh Antarctic Intermediate Water is replaced by anomalously warm, salty intermediate water. The loss of Antarctic Intermediate Water in the South Atlantic is related to a weak Agulhas retroflection. The erosion is enhanced in the tropical Atlantic by strong upwelling. The warm, salty anomalies are advected northward outcropping in the North Atlantic subpolar gyre. In the outcrop zone, sea surface temperature and salinity are increased, which lead to an increase in global mean surface temperature and a reduction in the sea ice area. This adjusts the top of the atmosphere balance via increased outgoing longwave radiation and is partly offset by a decrease in outgoing shortwave radiation. The increased surface salinity triggers convection in the Labrador Sea and leads to a strong flushing of the thermohaline circulation. These results demonstrate that adjustment time scales for coupled climate models can be in excess of 350 yr. The potential implications of the adjustment time scale of climate models need to be considered when planning scenario and sensitivity experiments, as model drifts can be nonlinear.