An object-approach to detect convectively coupled equatorial waves

Many studies have shown that the space-time spectrum of tropical cloudiness satellite data suggest the existence of convectively coupled equatorial waves (CCEWs). More precisely, while to first order the space-time power spectrum of tropical cloudiness has characteristics of a red-spectrum, removal of an estimated background reveals peaks along the dispersion curves of equatorial shallow water modes. Spectral filtering using wavenumbers and frequencies corresponding to regions where the spectral power is above the background has become the primary way of studying CCEWs; however, the technique has been subject to a number of critiques mostly due to the ``ad hoc'' nature of the background estimation. In the present work, the existence of these waves in the tropical atmosphere is further validated using an alternative technique. Specifically, contiguous cloudy regions (CCRs) in space and time are detected on a ``synoptic-to-planetary'' filtered brightness temperature dataset where all planetary wavenumbers less than 20 and periods from 1.25 to 96 days are retained. Analysis of the identified CCRs reveals wave-like features of spatial and temporal scales consistent with some of the spectral peaks that stand above the estimated background spectrum. Objects of scales typical of Kelvin waves are the dominant eastward propagating CCRs whereas CCRs similar to westward inertia-gravity waves (WIG) are the most common among westward propagating CCRs. Although the number of CCRs similar to WIGs and eastward inertia-gravity waves (EIGs) are comparable, the spectral peak associated with the later is much weaker. Thus, this study shows that EIGs are often embedded on larger scale wave envelopes, as opposed to WIGs which most often propagate on their own. More broadly, while the spectral peaks (after the removal of the background) are well separated in the frequency and wavenumber domain, the corresponding waves are not necessarily separated in physical space. Objects at scales consistent with the MJO spectral peak are not detected, likely due to the complexity of the higher frequency waves within the MJO. As an application of the technique, trajectories, spatial scales, and phase speeds associated with CCRs similar to Kelvin waves are presented.