The Waves to Weather challenge: Do large-scale equatorial waves significantly modulate regional rainfall in southern Vietnam?

About 85% of the annual precipitation in southern Vietnam (ca. 8–12°N, 104–110°E) occurs during the southwest monsoon season lasting approximately from June to October. Flooding due to heavy rainfall affects many inhabitants along river estuaries in this region. Large-scale equatorial waves like the Madden-Julian-Oscillation (MJO) and Convectively Coupled Equatorial Waves (CCEWs) are known to modulate the larger scale convective activity, often indicated by variations in (filtered) satellite-observed outgoing longwave radiation (OLR) anomalies. To what extent these tropical modes affect regional daily rainfall anomalies in, for example, small river catchments and can be utilized to improve rainfall forecasts or to classify regional weather situations is often hard to detect from rainfall data sets. The present contribution seeks to understand and quantify the role of the MJO and CCEWs for rainfall in southern Vietnam as a whole, but also for the small Thi Vai catchment near Ho Chi Minh City (Vietnam). The NOAA OLR is considered as a measure of tropical deep convection. Prominent spectral peaks in circum-equatorial OLR between 15°S and 15°N occur in wavenumber–frequency diagrams along the dispersion curves of the solutions of the shallow water wave equations. They are interpreted as CCEWs. Meridionally averaged wave-filtered GridSat brightness temperatures between 15°S and 15°N and their time derivatives are used to define phases and amplitudes of CCEWs. This will allow determining active and inactive phases of CCEWs in the vicinity of Vietnam. Eastward propagating deep convection is also related to the 30–90-day Madden–Julian oscillation (MJO). The Real-Time Multivariate MJO (RMM) Index – based on the first two empirical orthogonal functions of the combined fields of the OLR and the zonal winds in the 850 hPa and 200 hPa height levels between 15°S and 15°N– is used for the definition of convectively active and inactive phases of the MJO. In addition, the RMM Index gives information about the approximate location of the convective activity and its intensity. TRMM 3B42 V7, APHRODITE MA V1101 data and rain gauge measurements are used to investigate the relation between tropical wave phases and amplitudes and precipitation in southern Vietnam and adjacent regions. The major findings are: (a) Precipitation amounts in southern Vietnam are higher during convectively active phases of the MJO and CCEWs. The waves differ in terms of their relative importance for rainfall enhancement. (b) For increasing amplitudes the difference between area-averaged precipitation during inactive and active phases increases. We will further test the hypothesis if precipitation amounts are higher, when multiple wave types are in their convectively active phase over the Vietnam region.