Tropical circulation variability, with emphasis on the monsoon systems is examined using several variables for the 1948-1999 period. The monsoon component in the 200 hPa velocity potential (transient-χ), the deviation from the annual mean of the zonal deviation field, is the key parameter used here to obtain the monsoon modes. The Empirical Orthogonal Function (EOF) analysis of the transient-χ yields two separate modes. The first mode describes the multi-decadal variability of the monsoon systems in South America and in the western Pacific. This mode, called the multi-decadal monsoon (MDM) mode, occurs throughout the year and contains the multi-decadal variability linked to the PDO dynamics. The second mode describes the opposite interannual variations of the Asian, African and Indian Ocean region and western United States, Mexico and eastern Pacific Ocean. This mode, called here the interannual mode (IAM) bears some similarities with the tropical interannual El Niño-Southern Oscillation mode. Some aspects of these modes, including their circulation and thermodynamic characteristics are discussed here. Finally, indices based on the loading patterns of these modes are constructed. The potential of these indices for monitoring tasks are analyzed. This is a new aspect not proposed in previous work.
Monsoon systems; multi-decadal variability; interannual variability
