Abstract
AbstractConvectively coupled equatorial Kelvin waves (CCKWs) are high‐impact tropical weather systems that can lead to severe flooding over the Maritime Continent. Here, a vorticity budget for CCKWs over the Indian Ocean is constructed using reanalysis data, to identify the basic mechanisms of eastward propagation and growth. The budget is reasonably well closed, with a small residual/subgrid‐scale term. In the lower troposphere, CCKWs behave like strongly modified theoretical equatorial Kelvin waves. Vortex stretching, from the divergence of the Kelvin wave acting on planetary vorticity (the term), is the sole mechanism by which the vorticity structure of a theoretical Kelvin wave propagates eastward. In the lower and middle troposphere, this term is also the key mechanism for the eastward propagation of CCKWs but, due to subtleties in its structure and phasing linked to a combination of modal structures, it also contributes to growth. Unlike in the theoretical Kelvin wave, other vorticity source terms also play a role in the propagation and growth of CCKWs. In particular, vortex stretching from relative vorticity (the term) is the largest source term, and this leads strongly to growth, through interactions between the background and perturbation vorticity and divergence. Horizontal vorticity advection by the background flow contributes to propagation, and also acts to retard the growth of the CCKW. The sum of the source terms in this complex vorticity budget leads to eastward propagation and growth of CCKWs. The structure and vorticity budget of CCKWs in the upper troposphere is quite unlike that of a Kelvin wave, and appears to arise as a forced response to the lower‐tropospheric structure. The implications for numerical weather prediction and climate simulations are discussed.
Highlights
Coupled equatorial Kelvin waves (CCKWs) are tropical weather systems that propagate eastward along the equator (Takayabu 1994; Dunkerton and Crum 1995; Wheeler and Kiladis 1999) and cause a typical increase in rainfall rate of 10 mm day-1 (Baranowski et al 2016a)
Convectively coupled equatorial Kelvin waves (CCKWs) have a strong impact on monsoon precipitation (Mekonnen et al 2009), and are a major component of the envelope of convective systems that make up the active phase of the Madden–Julian Oscillation (MJO; Guo et al 2014; Haertel et al 2015; Kikuchi et al 2018)
The vorticity tendency on the left hand side of Equation 1 is balanced by the sum of several vorticity source terms on the right, and can be considered as a vorticity budget when applied to a specific system, such as a theoretical equatorial Kelvin wave, or an observed CCKW
Summary
Coupled equatorial Kelvin waves (CCKWs) are tropical weather systems that propagate eastward along the equator (Takayabu 1994; Dunkerton and Crum 1995; Wheeler and Kiladis 1999) and cause a typical increase in rainfall rate of 10 mm day-1 (Baranowski et al 2016a). They are one type of a class of convectively coupled equatorial waves (Kiladis et al 2009).
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