Biennial rainfall relationship could be observed in the tropical eastern hemisphere associated with the tropospheric biennial oscillation, and the tropical western hemisphere. In the tropical western hemisphere, previous studies have revealed that a biennial relationship exists between the interannual rainfall anomalies over Central America (CA) and equatorial South America (ESA). Understanding the behavior and mechanisms of such a biennial rainfall relationship has important applications, due to the importance of rainfall variability to agriculture, economies and society in these regions.