Trend in Atmospheric Angular Momentum in a Transient Climate Change Simulation with Greenhouse Gas and Aerosol Forcing

Abstract

The authors investigate the change of atmospheric angular momentum (AAM) in long, transient, coupled atmosphere?ocean model simulations with increasing atmospheric greenhouse gas concentration and sulfate aerosol loading. A significant increase of global AAM, on the order of 4 ? 1025 kg m2 s?1 for 3 ? CO2?1 ? CO2, was simulated by the Canadian Centre for Climate Modelling and 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° in the upper troposphere, and 2) a tripole pattern in the Southern Hemisphere extending through the entire troposphere and having a positive maximum at 60°S. The implication of the projected increase of global AAM for future changes of the length of day is discussed. The CCCma coupled global warming simulation, like many previous studies, shows a significant increase of tropical SST and includes a zonally asymmetric component that resembles El Niño 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 ?AAM/?SST remained approximately constant at about 0.9 ? 1025 kg m2 s?1 (°C)?1. This number is close to its counterpart for the observed global AAM response to El Niño. It is suggested that this ratio may be useful as an index for intercomparisons of different coupled model simulations.