Jie Sun and Zhaohua Wu
The mixed Rossby-gravity (MRG) waves are one type of tropical waves that have been studied since 1960s, marked by the seminal works of Yanai and of Matsuno in 1960s. They are highly spatiotemporally inhomogeneous, which makes the task of identifying and isolating MRG waves in noisy data a very challenging one. However, due to their strong interactions with other tropical waves or convectively coupled systems such as Madden-Julian oscillation and tropical cyclones, improved diagnoses of MRG waves and their climatology are urgently needed.
Here we report our effort in identifying and isolating MRG waves and in obtaining the evolution of individual events and their climatology. In this study, we take advantage of highly spatiotemporal local multi-dimensional ensemble empirical mode decomposition (MEEMD) method and the known spatial structures of MRG waves based on tropical wave theory to build a new (semi-automatic) analysis tool for isolating individual MRG events and synthesizing the climatological features of MRG waves. This new approach overcomes many drawbacks of previous tropical wave diagnoses and leads to significantly fewer misinterpretations of MRG waves and their climatology. In addition, this new approach has great potential for diagnosing other types of tropical waves and their interactions.
We quantify the spatiotemporal inhomogeneity of MRG waves and search for their origin. We found that individual MRG waves in tropical troposphere rarely have their local wavelength exceeding 4000 kilometers (with a zonal wavenumber smaller than 9). However, the spatial preference of occurrences of MRG waves in tropical Indian Ocean, western tropical Pacific, and Amazon suggests that MRG are tied closely to persisting convective heating and are thermally forced waves in a dissipative tropical atmosphere. We also show that it is exactly this high spatial inhomogeneity of the activity of MRG waves that leads to the incorrect identification of large wavelength (corresponding to small wavenumber) MRG waves using zonal Fourier expansion. The dispersion relationship diagram for tropical MRG waves, which uses local zonal wavenumber and frequency instead of Fourier decomposition based global domain wavenumber and frequency, is constructed and compared to the tropical wave theory.