Observational studies on association between eastward equatorial jet and Indian Ocean dipole

Abstract

Association between weakening/strengthening of the eastward equatorial jet (EEJ) in both seasons and the Indian Ocean dipole (IOD) was investigated using two independent observational datasets (October 1992 to September 2007): (a) the dipole mode index I(t) and (b) the 5-day Ocean Surface Current Analyses-Realtime (OSCAR) obtained from satellite altimetry and scatterometer data, which has strong seasonal variability, with the EEJ occurrence in spring and fall, shown from the time-longitude cross-section of equatorial zonal velocity (1°S-1°N). The association is detected in two ways. First, time series of averaged zonal velocity over (1°S-1°N, 42°E-100°E) U(t) shows a close association to the dipole mode index: positive IOD events (1994, 1997, 2006) correspond to negative U (westward equatorial current), and negative IOD events (1994, 1995, 1999, 2005) correspond to positive U (eastward equatorial current). Second, the EEJ weakening/strengthening is represented by the streamfunction anomaly relative to its climatological monthly mean fields. The streamfunction anomaly is further analyzed using the empirical orthogonal function (EOF) method. The first EOF mode accounts for 55% of the variance with corresponding principal component A ⁽¹⁾(t) showing evident pattern of EEJ strengthening and weakening. The correlation coefficient between I(t) and A ⁽¹⁾(t) is around 0.49. This may confirm the linkage in some sense (only EOF-1 considered) between the positive (negative) IOD events and the weakening (strengthening) of the EEJ. The dipole pattern of lag-correlation between the sea surface temperature anomaly and U confirms the connection between the EEJ weakening/strengthening and the IOD events.