Understanding the Multiscale Structures in the Tropics

In tropics, the power spectrum of the outgoing longwave radiation (OLR) has three major features. They are: a) a red noise spectrum; b) spectral peaks associated with the Madden-Julian oscillation (MJO); c) the spectral peaks associated with the equatorial waves. In order to understand the multi-scale nature of the tropics, the above features need to be explained. Although we do not fully understand what sets the equivalent depth of these observed equatorial waves, Matsuno (1966) has already predicted these equatorial waves theoretically. In this study, we will try to find the minimum recipes of the red noise spectrum and the MJO. In a shallow water simulation, with parameterized convection and radiative cooling, a red noise spectrum is simulated. MJO-like signals and equatorial waves are also observed. This result suggests that convection (short term forcing) and radiation (long-term memory) could be essential recipes for the multi-scale structures in tropics. Further analysis shows that the interaction between eastward and westward moving high frequency waves could be the basic mechanism of the MJO.