Trend in atmospheric angular momentum in a transient climate change simulation with greenhouse gas and aerosol forcing

The change of atmospheric angular momentum (AAM) in long, transient, coupled atmosphere-ocean model simulations with increasing atmospheric greenhouse gas concentration and sulphate aerosol loading is investigated. A significant increase of global AAM, on the order of 4x10^25 kg m^2 s^-1 for 3xCO2-1xCO2, was simulated by the Canadian Centre for Climate Modelling Analysis (CCCma) coupled model. The increase was mainly contributed by the relative component of total AAM in the form of an acceleration of zonal mean zonal wind in the tropical-subtropical upper troposphere. Thus, under strong global warming, a superrotational state emerged in the tropical upper troposphere. The trend in zonal mean zonal wind in the meridional plane was characterized by (1) a tropical-subtropical pattern with two maxima near 30 degrees in the upper troposphere, and (2) a tripole pattern in the Southern Hemisphere extending through the entire troposphere and having a positive maximum at 60S. The implication of the projected increase of global AAM for future changes of the length of day will be discussed.

The CCCma coupled global warming simulation, like many previous studies, shows a significant increase of tropical sea surface temperature (SST) and includes a zonally asymmetric component that resembles El Nino SST anomalies. In the CCCma transient simulations, even though the tropical SST and global AAM both increased nonlinearly with time, the ratio of their time increments d(AAM)/d(SST) remained approximately constant at about 0.9x10^25 kg m^2 s^-1 (degree C)^-1. This number is close to its counterpart for the observed global AAM response to El Nino. We suggest that this ratio may be useful as an index for inter-comparisons of different coupled model simulations.