Predictable Mode of Tropical Intraseasonal Variability in Boreal Summer

Abstract

This work uses a 19-year ensemble hindcast of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the average predictable time (APT) method to detect the most predictable tropical intraseasonal variability (ISV) mode. The first and most predictable mode (APT1) of tropical ISV is similar to a joint merger of the two Madden–Julian Oscillation (MJO) modes with more weight on the second mode and is characterized by a triple pattern with two positive centers in the equatorial western Indian Ocean and central Pacific Ocean and a negative center over the Maritime Continent. The APT1 doubles the skillful prediction period made by the MJO defined by a correlation skill of 0.5 (approximately 25 days in the ECMWF model), demonstrating its potential to become a skillful prediction target and to offer powerful sub-seasonal prediction sources.The underlying physical process and predictability source of the APT1 are further analyzed. The APT1 is very similar to the pattern triggered by the most predictable tropical intraseasonal sea surface temperature (SST) anomalies mode, suggesting its oceanic origin. Tropical ocean-atmosphere interaction plays a critical role in the APT1 by enhancing the evolution of tropical convection cells under WES (Wind-Evaporation-SST) and Bjerknes feedbacks. The internal atmospheric processes also have an important impact on the formation and maintenance of the APT1.

Extra predictability from a seamless approach for Asian summer monsoon precipitation from days to weeks

“…Correspondingly, the OLR SVD1 shows a tripole‐like pattern, with three relatively large centres located in the equatorial IO, the MC, and the equatorial western and central Pacific (Figure 6b). This pattern is very similar to the most predictable mode of the tropical intraseasonal variability in the boreal summer, which stems from the tropical interaction between the ocean and atmosphere during the evolution of tropical convection cells (Li and Tang, 2021a). This suggests that as the lead time and average window increase, only the most predictable intraseasonal signals remain to provide the predictability for the ASM precipitation.…”

“…similar to the first two OLR modes of the multiple‐variable empirical orthogonal functions (EOFs: e.g. Wheeler and Hendon, 2004; Li and Tang, 2021a). This indicates that MJO is a key source of the precipitation in the ASM region.…”

Predictable Pattern of Precipitation Over Asian Summer Monsoon Regions

“…To determine the timescales of each IMF, we conduct a spectrum analysis (Li & Tang, 2021; Zangvil, 1977) for each IMF (not shown). The results show that IMF1–IMF5 have significant periods of 3–8 weeks, 6–13 weeks, 1–2 years, 2–4 years, and 10 years, respectively, ranging from intraseasonal to decadal scales.…”