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 (CCKW) by different Madden-Julian oscillation (MJO) states
over the Indian, Pacific and Atlantic Ocean domains. Convectively active
CCKW events associated with the MJO convectively active, convection
suppressed and weak amplitude states were derived using
wavenumber-frequency filtered outgoing long wave radiation (OLR) indices
over the three domains. Composite analysis of CCKW events during
different MJO states indicate that the amplitude and phase speed of CCKW
are modulated by the MJO state. The amplitude of CCKW are stronger
(weaker) and it propagates slower(faster) and more (less) eastward when
the MJO amplitude is strong. The phase speed of CCKW is much slower over
the Indian Ocean domain, while 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 the GMS and CCKW phase speeds
over the different domains, during different MJO states and the observed
differences in CCKW vertical structures support this hypothesis. It is
found that the CCKW exhibits 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 may be treated as a mixed-moisture mode.