The Dynamics of Warm and Cold Climates

The atmospheric dynamics of five different climate simulations with the GISS GCM are compared to investigate the changes that occur as climate warms or cools. There are two ice age simulations, the current and doubled CO2 climates, and a simulation of the warm Cretaceous. These climates have a range of global average surface air temperature of 13°C. The results are compared with those of other models, as well as to paleoclimate and recent observations. The study shows that many zonally averaged processes do not change systematically as climate changes. In particular, the January Hadley cell, jet stream, mean precipitation patterns and total atmospheric transport show surprisingly little variation among the different climate simulations. While eddy energy increases as climate cools, the effective eddy forcing of the mean zonal wind and temperature fields is not significantly greater. All these features result from balances between competing factors, and while individual processes differ in the cold and warm climates, there is much compensation. Additional results show that the relative humidity remains fairly constant as climate changes. The ratio of stationary to transient eddy kinetic energy also remains relatively constant. Eddy energy transports increase in colder climates, primarily due to changes in the stationary eddy transports. Cloud cover decreases as climate warms due to decreases in low-level clouds. The lapse rate in all the simulations follows the moist adiabatic ,value at low latitudes, and is close to the critical baroclinic adjustment value at upper midlatitudes. The latitudinal temperature gradients at midlatitudes of both the sea surface temperature and the vertically integrated air temperature are very similar in the diverse climates. It is speculated that this is due to the properties of the water molecule, and is the cause for much of the observed compensation.