Intensity of the IOD is represented by anomalous SST gradient between the western equatorial Indian Ocean (50E-70E and 10S-10N) and the south eastern equatorial Indian Ocean (90E-110E and 10S-0N). This gradient is named as Dipole Mode Index (DMI). When the DMI is positive then, the phenomenon is refereed as the positive IOD and when it is negative, it is refereed as negative IOD.
Since, IOD is a coupled ocean-atmosphere phenomenon it can also be represented by any other atmospheric (pressure, OLR) or oceanographic (sea surface height) as well
SST DMI dataset (monthly from 1958 to 2012) derived from HadISST dataset
SST DMI dataset (monthly from 1871 to 1997) derived from HadISST dataset (without time filter)
SST DMI dataset (from 1958 to 1999, as in Nature paper of Saji et al., 1999)
SST DMI dataset (Yearly time series averaged from June to November)
SST DMI dataset (from 1871 to 1997) derived from GISST dataset
SST DMI dataset (from 1856 to Present) derived from Kaplan (1998)
SST DMI dataset (from 1982 to Present) derived from NOAA OI SST data (base period 1970-2005)
IOC UNESCO website link for all important indices
SST in the two representative boxes of the Indian Ocean Dipole