Decadal and Interannual Variability of the Indian Ocean Dipole

Abstract: This study investigates the decadal and interannual variability of the Indian Ocean Dipole (IOD). It is found that the long-term IOD index displays a decadal phase variation. Prior to 1920 negative phase dominates, but after 1960 positive phase prevails. Under the warming background of the tropical ocean, a larger warming trend in the western Indian Ocean is responsible for the decadal phase variation of the IOD mode. Due to reduced latent heat loss from the local ocean, the western Indian Ocean warming may be caused by the weakened Indian Ocean westerly summer monsoon. The interannual air-sea coupled IOD mode varies on the background of its decadal variability. During the earlier period (1948--1969), IOD events are characterized by opposing SST anomaly (SSTA) in the western and eastern Indian Ocean, with a single vertical circulation above the equatorial Indian Ocean. But in the later period (1980--2003), with positive IOD dominating, most IOD events have a zonal gradient perturbation on a uniform positive SSTA. However, there are three exceptionally strong positive IOD events (1982, 1994, and 1997), with opposite SSTA in the western and eastern Indian Ocean, accompanied by an El Nino event. Consequently, two anomalous reversed Walker cells are located separately over the Indian Ocean and western-eastern Pacific; the one over the Indian Ocean is much stronger than that during other positive IOD events.

Keywords:
• Indian Ocean dipole,
• ENSO,
• decadal phase variation,
• interannual variability