Diversity of strong negative Indian Ocean dipole events since 1980: characteristics and causes

In this study, the diverse features of four strongest negative Indian Ocean dipole ( n IOD) (1996, 1998, 2010 and 2016) since 1980 are quantitatively examined. The contributions of air-sea feedback to La Niña-related sea surface temperature (SST) anomalies (SSTA) are identified and their association with surface winds as well as different time-scale variations is compared. The growth of SSTA in Dipole Mode Index (DMI) indicates one event (1996) being a dipolar pattern and the other three (1998, 2010 and 2016) being monopolar patterns during their mature phases. During 1996 case, SSTA in west (IOD-W) and east (IOD-E) poles are both significant and comparable though with different physical origins. Effective Bjerknes-feedback dominated in accumulating warm-water in IOD-E and Madden–Julian oscillation (MJO) activity constructively enhances this IOD-E warming, while the heat loss induced by the wind-evaporation-SST (WES) and cloud-radiation-SST feedback lead to a fast and profound cooling in IOD-W. Developing on a non-La Niña background, the IOD-E warming is initiated with positive heat flux anomalies that lead the linear temperature advection by 1-month(s), indicating this dipole pattern as a local independent event that possibly triggered by interior-basin disturbances in the Indian Ocean (IO). Three monopolar cases all exhibited above 3.0 standard deviations warm peaks over IOD-E but with extremely weak signals in IOD-W. Though the Bjerknes feedback and heat-flux play crucial roles over the three monopolar cases with the strongest in 2016, the initial zonal winds are first established through atmospheric teleconnection without the leading involvement of heat flux. The concurrent extreme La Niña in 1998 and 2010 triggered strong westerly wind anomalies over the IO and favored the rapid growth of these monopolar SSTA, but La Niña influence stronger in 2010. The damping effect of nonlinear advection and the triggering effect of long-stalled MJO convective anomalies played important roles in the 1998 and 2016 monopolar cases.