The modulating influence of convectively coupled equatorial waves (CCEWs) on the variability of tropical precipitation

Abstract

A detailed examination of the Tropical Rainfall Measuring Mission (TRMM) daily estimates, merging high-quality (HQ)/infrared (IR) precipitation from 1998 to 2009, revealed the modulating influence of convectively coupled equatorial waves (CCEWs), including Kelvin, n = 1 equatorial Rossby (ER), mixed Rossby-gravity (MRG), and tropical depression (TD)-type waves, on the variability of tropical precipitation. Consistent with inviscid β-plane shallow water theory, the wave-induced convergence zones are found to be an active location for precipitation. Modulated precipitation by ER waves exhibits slow westward phase progression of approximately −4.8 m s−1, and it is distributed as a symmetric pair of off-equatorial maxima and a weak equatorial peak. MRG waves show an asymmetrically modulated precipitation distribution and a faster phase progression of about −16.1 m s−1 followed by enhanced symmetrical gyres. Peak precipitation within TD-type waves originates in the trough axis of off-equatorial vortex trains and propagates to the west at approximately −9.5 m s−1. Regarding Kelvin waves, typical positive precipitation anomalies travelling in the east–west direction occur in the maxima of low-level wind convergences, with an approximate phase progression of about 15.4 m s−1. Variability of tropical precipitation due to CCEWs behaves relatively varied over different seasons and locations. TD-type waves exhibit more predominant impacts than other waves, with maximum impacts of up to 9.75 ± 4.54% of the total precipitation variance during boreal summer, which is three times higher than the MRG peak (2.90 ± 0.82%). On the other hand, Kelvin and n = 1 ER waves each have more prominent effects during boreal winter; up to 6.99 ± 3.30 % and 3.77 ± 1.79 %, respectively. On average, our results suggest that, although being less dominant than other tropical oscillations (e.g. ENSO, MJO), these four types of CCEWs can considerably affect precipitation by contributing up to 16–20% of the total intraseasonal (2.5–72 days) precipitation variance in the tropics.