Convective Couplings with Equatorial Rossby Waves and Equatorial Kelvin Waves. Part I: Coupled Wave Structures

Abstract

Abstract This study investigates precipitation amounts and apparent heat sources, which are coupled with equatorial Kelvin waves and equatorial Rossby waves, using TRMM PR level 2 data products. The synoptic structures of wave disturbances are also studied using the ERA5 dataset. We define the wave phase of equatorial waves based on FFT-filtered brightness temperature and conduct composite analyses. Rossby waves show a vertically upright structure and their upright vortices induce large-amplitude column water vapor (CWV) anomalies. Precipitation activity is almost in phase with CWV, and thus is consistent with a moisture mode. Kelvin waves, on the other hand, indicate a nearly quadrature phase relationship between temperature and vertical velocity, like gravity wave structure. Specific humidity develops from near the surface to the middle troposphere as the Kelvin wave progresses. A clear negative CWV anomaly also does not exist despite the existence of negative precipitation anomalies. Convective activity corresponds well with its tilting structure of moisture and modulates the phase relationship between temperature and vertical motion. For both wave cases, apparent heat sources can amplify available potential energy despite the difference of coupling mechanisms of these two waves; precipitation is driven by CWV fluctuation for the Rossby wave case, and by buoyancy-based fluctuations for the Kelvin wave case. These can be observational evidence of actual coupling processes that is comparable to previous idealized studies. Significance Statement A coupling mechanism between equatorial waves and convective activity is a significant issue in tropical meteorology. While many previous idealized studies suggested some instability mechanisms, their true roles are not yet clear because detailed precipitation characteristics are not well investigated. We aim to quantify precipitation and synoptic-scale wave disturbances, and compare equatorial Rossby waves and equatorial Kelvin waves, which should have different instability coupling modes between each other, in order to shed light on a convectively coupling mechanism. We found that precipitation is actually driven by column moisture in Rossby waves and by dynamical fluctuation in Kelvin waves. Despite these competing mechanisms, similar top-heavy heating can maintain convectively coupled disturbances. Our observational results will support and improve theoretical studies.