Modulation of the Convectively Coupled Kelvin Waves by the MJO over Different Domains

Abstract

In this study we have examined the modulation of convectively coupled Kelvin waves (CCKWs) by different Madden–Julian oscillation (MJO) states over the Indian, Pacific, and Atlantic Ocean domains. Convectively active CCKW events associated with active MJO convection, suppressed MJO convection, and quiescent MJO states were derived using wavenumber–frequency-filtered outgoing longwave radiation (OLR) indices over the three domains. Composite analysis of CCKW events during different MJO states indicates that the amplitude and phase speed of CCKW are modulated by the MJO state. CCKW amplitude is stronger (weaker) and it propagates relatively slower (faster) and more (less) eastward when the MJO amplitude is strong (weak). The phase speed of CCKW is much slower over the Indian Ocean domain, whereas it propagates relatively faster over the Atlantic Ocean domain. It is hypothesized that the observed difference in CCKW phase speeds is related to the gross moist stability (GMS). The clear linear relationship observed between GMS and CCKW phase speeds over the different domains and during different MJO states and the observed differences in CCKW vertical structures support this hypothesis. It is found that CCKWs exhibit a baroclinic vertical structure over the Indian and Pacific Ocean domains and a barotropic vertical structure over the Atlantic Ocean. Planetary-scale convection associated with the MJO reduces the static stability allowing for baroclinic modes to prevail, which in turn reduces the GMS and the effective equivalent depth, eventually slowing down the CCKW phase propagation. The results suggest that CCKW events may be treated as a mixed-moisture mode.