Quasi-Periodic Oscillations in a Symmetric General Circulation Model

Long lime integrations with a symmetric version of the CLAS Climate Model with hydrology have shown that the Hadley circulation has well defined strong and weak episodes. This oscillation of the Hadley circulation seems to occur in two dominant range of periodicities, one with periods between 10 and 15 days and another with periods between 20 and 40 days. This quasi-periodic behavior of the Hadley circulation is seen only when the moist convective heating is determined by dynamics. If the latent beating in the atmosphere is prescribed and kept artificially fixed, this oscillation of the Hadley circulation disappears. Thus, this oscillation of the Hadley circulation appears as a result of interactions between moist convective and dynamical processes. A wave phenomenon is seen in the lower atmosphere that propagates toward the position of maximum radiative beating. This wave perturbation has a length scale of about 15–20 degrees latitude in the north-south direction. This phenomenon has large amplitude over land and relatively small amplitude over the oceans. It is found that evaporation from the land surface plays a key role in generation and maintenance of this phenomenon. In a controlled experiment in which evaporation from the land surface was kept zero, this wave perturbation in the lower atmosphere was not observed. The relevance of them phenomena to observed low-frequency fluctuations in the tropics and to the transient behavior of the Intertropical Convergence Zone is discussed.