Linking extreme precipitation in Southeast Asia to equatorial waves

Abstract

Equatorially trapped waves, such as Kelvin Waves, Equatorial Rossby Waves and Westward‐moving Mixed Rossby–Gravity (WMRG) Waves, play a major role in organising tropical convection on synoptic to sub‐seasonal time‐scales. These waves have the potential to provide an important source of predictability for high‐impact weather in Southeast (SE) Asia and the Tropics more widely. To aid understanding of the role played in high‐impact weather by such waves, the observed statistical relationship between identified equatorial waves and heavy rainfall in SE Asia is examined for the period 1998–2016. Increases in the amount of precipitation and the likelihood of extreme precipitation in SE Asia are linked to all three types of waves that are included in analysis; Kelvin, equatorial Rossby and WMRG waves. There is both increased mean rainfall and increased probability of occurrence of heavy rainfall on days when high‐amplitude waves propagate over SE Asia. In particular, heavy precipitation can be up to three times more likely in regions of SE Asia during equatorial waves, including Malaysia, Indonesia and the Philippines. Kelvin waves have a large influence on heavy rainfall over Indonesia, WMRG and Kelvin waves impact Malaysia rainfall, and equatorial Rossby and WMRG waves are linked to increased rainfall over the Philippines. Based on this study it can be concluded that the probability of extreme precipitation in this region is dependent on equatorial wave activity. Therefore, the skill in probabilistic prediction of extreme precipitation in SE Asia would be expected to be conditional on the skill in equatorial wave prediction, and the modelled relationship between equatorial waves and convection.

Impacts of convectively coupled equatorial waves on rainfall extremes in Java, Indonesia

“…The dominant role of Kelvin‐wave‐induced rainfall variability in Indonesia is consistent with the findings of previous studies (Lubis and Jacobi, 2015; Ferrett et al ., 2019), albeit here with a detailed regional analysis based on surface rain‐gauge data and a gridded rainfall product. This study emphasizes that the probability of extreme precipitation over Java Island increases on days when the equatorial wave wet phase is occurring.…”

“…(2020) showed that high‐amplitude CCEWs play a significant role in enhancing daily and diurnal rainfall in Southeast Asia. However, most of the changes in rainfall intensity and extremes reported in these studies are confined over the ocean rather than the land especially Java, likely due to biases in the satellite data (see figure 4 in Ferrett et al ., 2019 and figure 5 in Sakaeda et al ., 2020). Previous studies have found that the TRMM satellite tends to underestimate torrential rainfall over land regions in Indonesia on a sub‐daily‐to‐daily time scale, owing to the inadequate detection capability of the satellite, especially during the rainy season (e.g., As‐Syakur et al ., 2011; Vernimmen et al ., 2012; Prasetia et al ., 2013; Giarno et al ., 2018; Sekaranom et al ., 2018).…”

“…In particular, using Tropical Rainfall Measuring Mission (TRMM) satellite‐based rainfall data, Ferrett et al . (2019) and Sakaeda et al . (2020) showed that high‐amplitude CCEWs play a significant role in enhancing daily and diurnal rainfall in Southeast Asia.…”

“…CCEWs play a substantial role in tropical rainfall variability via their coupling with convection (e.g., Matsuno, 1966; Takayabu, 1994; Wheeler and Kiladis, 1999; Yang et al ., 2003; Kiladis et al ., 2009; Kim and Alexander, 2013; Lubis and Jacobi, 2015; van der Linden et al ., 2016; Ferrett et al ., 2019; Sakaeda et al ., 2020). The three well‐established classes of CCEWs, including Kelvin waves, equatorial Rossby waves (ER), and mixed Rossby‐gravity (MRG) waves, have been found to have significant impacts on tropical rainfall (e.g., Cho et al ., 2004; Sobel et al ., 2004; Lin et al ., 2006; Masunaga et al ., 2006; Mounier et al ., 2007; Kim and Alexander, 2013; Lubis and Jacobi, 2015; van der Linden et al ., 2016; Ferrett et al ., 2019; Schlueter et al ., 2019; Sakaeda et al ., 2020). Lubis and Jacobi (2015) found that, on average, CCEWs can significantly affect rainfall in the tropics by contributing up to 16–20% of the total intraseasonal (2.5–72 days) rainfall variance.…”

“…Wave‐induced convergence (divergence) zones are found to be collocated with regions of enhanced (suppressed) rainfall, consistent with inviscid‐ β ‐plane shallow‐water theory (Matsuno, 1966; Lubis and Jacobi, 2015). More recently, studies have shown that the frequency and intensity of daily extreme rainfall are influenced by CCEWs (van der Linden et al ., 2017; Ferrett et al ., 2019; Subudhi and Landu, 2019; Sakaeda et al ., 2020). In particular, using Tropical Rainfall Measuring Mission (TRMM) satellite‐based rainfall data, Ferrett et al .…”