Abnormal local warming after reducing solar constant under special paleogeography configurations

In the deep past the solar constant was lower than present. In this study, we employ the Community Earth System Model version 1.2.2 (CESM1.2.2) to examine the effect of reduced solar constant on the climate of the past 250 million years. Two groups of slice experiments (fixed pre-industrial solar constant versus decreased solar constant) with 26 different paleogeography configurations and with fixed CO₂ concentration (2800 ppmv) were run. In all the experiments, the global-mean surface temperature decreases as reducing the solar constant, consistent with previous studies. However, there is significant local surface warming in the experiments of 70 Ma, 90 Ma, and 150 Ma. The warming has a magnitude of about 2 K and occurs in the sub-polar ocean region. At 70 Ma and 90 Ma, the dominant mechanism is atmospheric teleconnection between tropics and sub-polar regions. Cooling in the tropics induces abnormal atmospheric waves, and the waves propagate to the sub-polar region and subsequently change surface winds there. The abnormal winds cause an increased poleward ocean heat transport, warming the regional surface. The second mechanism is that the reducing solar constant shifts the westly jets equatorward and causes an eastern wind anomaly, which also explains the warming at 150 Ma. Moreover, positive thermohaline feedback related to enhanced surface evaporation enhances the thermohaline circulation and thereby acts to further warm the surface. The phase of the atmospheric waves and their propagation path strongly depend on the land-sea configurations, so the abnormal local warming occurs in special periods only.