Processes Governing the Static Stability in Simulations of the Last Glacial Maximum

Processes controlling the static stability, in particular, extratropical static stability, in a real cold climate, the Last Glacial Maximum (LGM), are investigated using the Paleoclimate Modeling Intercomparison Project-phase 2 (PMIP2) simulations of the LGM. Results showed that LGM extratropical dry static stability in the Northern Hemisphere (NH) is about 5%-8% higher, compared to the present day. Two main theories accounting for LGM extratropical thermal stratification, the dry baroclinic adjustment theory and moist convection adjustment theory, are tested in the PMIP2 simulations of the LGM, respectively. It is found that the dry baroclinic adjustment theory outperforms the moist convection adjustment theory in predicting LGM extratropical dry static stability in the NH, especially for the winter season. This is primarily attributable to that, during the LGM, the amount of water vapor in extratropical atmosphere is relatively smaller and the Equator-to-Pole temperature gradient is greater such that baroclinic eddies become the dominant stabilizing effect on extratropical thermal stratification. Our results support previous idealized GCM studies that extratropical static stability does not change monotonically with the global mean surface temperature.