Predictability of tropical rainfall and waves: Estimates from observational data

For tropical rainfall, there are several potential sources of predictability, including synoptic-scale convectively coupled equatorial waves (CCEWs) and intraseasonal oscillations such as the Madden-Julian Oscillation (MJO). In prior work, predictability of these waves and rainfall has mainly been explored using forecast model data. Here, the goal is to estimate the intrinsic predictability using, instead, mainly observational data. To accomplish this, Tropical Rainfall Measuring Mission (TRMM) data are decomposed into different wave types using spectral/Fourier filtering. The predictability of MJO rainfall is estimated to be 22-31 days, depending on longitude, as measured by the lead time when the pattern correlation skill drops below 0.5. The predictability of rainfall associated with convectively coupled equatorial Rossby waves, Kelvin waves, and a background spectrum or nonwave component is estimated to be 8-12, 2-3, and 0-3 days, respectively. Combining all wave types, the overall predictability of tropical rainfall is estimated to be 3-6 days over the Indian and Pacific Ocean regions and on equatorial synoptic and planetary length-scales. For comparison, outgoing longwave radiation (OLR) was more predictable than rainfall by 5-10 days over these regions. Wave-removal tests were also conducted to estimate the contribution of each wave type to the overall predictability of rainfall. In summary, no single wave type dominates the predictability of tropical rainfall; each of the types (MJO, CCEWs, and nonwave component) has an appreciable contribution, due to the variance contribution, length of decorrelation time, or a combination of these factors.