The Indian Ocean Dipole, Wyrtki Jets, and Their Impacts on Coastal Sea Level of India and Bangladesh

The Indian Ocean Dipole (IOD) mode, often referred to as Zonal Mode, is a coupled ocean-atmosphere mode of variability on interannual timescal (e.g., Webster et al. 1999). Its east-west dipole pattern of SST anomaly peaks in boreal fall, and has large impacts on Indonesian drought and east African flood.The seasonal cycle of monsoon and equatorial currents - particularly the semiannual spring and fall Wyrkti Jets - contributes to the seanal phase-locking of the IOD (Halkides, Han and Webster 2006). The IOD is associated with a distinct spatial pattern of sea level, which involves westward propagating Rossby waves. During a negative IOD event (e.g., 1998), the unusually strong westerly winds intensify the fall Wyrkti Jet, causing anomalous sea level rise in the eastern equatorial Indian Ocean and along the Bay of Bengal coasts, which may have important implication for sea level rise and salt water intrusion/contamination for Bangladesh and India (Han and Webster 2002). While the Wyrtki jets are primarily seasonal phenomena, its strength and eastward heat transport can also be affected by atmospheric intraseasonal oscillations (Han, Webster and coauthors, 2004). Finally, the cross-equatorial heat transport is an important component of the self-regulating, coupled ocean-atmosphere-monsoon system (Webster et al. 2002).