A spurious positive Indian Ocean Dipole in 2017

Abstract

The Indian Ocean Dipole (IOD) is an intrinsic ocean-atmosphere coupled mode in the tropical Indian Ocean (IO). It features a sea surface temperature (SST) anomaly gradient, defined by the dipole mode index (DMI), the difference of area-averaged SSTanomaly between the western tropical IO (50°–70°E, 10°N–10°S) and the southeastern tropical IO (90°–110°E, Eq.-10°S). The IOD event tends to develop and mature in the boreal summer and fall seasons. When a positive IOD (pIOD) occurs, SST warms up in the west and cools down in the east, which sets up an east-west pressure gradient. The atmosphere hence responds to the anomalous SST gradient. Anomalous easterly winds prevail along the equator driven by the zonal pressure gradient, causing low sea level anomaly (SLA) in the eastern tropical IO. Rainless weather dominates over the Maritime Continent due to a weakened convection. Anomalies of an opposing polarity are generated during a negative IOD event.