Predictable Mode of Tropical Intraseasonal Variability in Boreal Summer

This work uses a 19-yr 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 tripole 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 subseasonal 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.

Automated summary

This study uses a 19-year ensemble hindcast from ECMWF and the APT method to identify the most predictable tropical intraseasonal variability mode. The APT1 mode is characterized by a tripole pattern with longer skillful prediction period, potentially originating from oceanic influences. Tropical ocean-atmosphere interaction and internal atmospheric processes play crucial roles in the formation and maintenance of the APT1 mode.