Representing the Indian Ocean Dipole

Abstract

Purpose. This paper offers an alternative representation of the Indian Ocean Dipole. Instead of the zonal gradient of equatorial sea surface temperature, the new index uses tropical sub-surface temperatures (T100). Methods and Results. The space-time character of the new index is defined by empirical orthogonal function analysis in the domain 20°S–5°N, 35°–120°E. The spatial pattern reflects an inherent zonal dipole with a temporal score that correlates with atmospheric empirical orthogonal function modes that describe the Walker circulation and basin-scale convection. Statistical regressions are conducted in the period 1979–2019 to evaluate the traditional Dipole Mode Index and the new T100 index, and the association with East Africa climate and Pacific Nino3.4 SST. These demonstrate improved performance of the T100 index with ~ 30% higher r2 explained variance. Conclusions. Whereas the old index tracks feedback between equatorial sea surface temperature / zonal wind / surface fluxes, the new index tracks coupling between south Indian Ocean Rossby waves / anticyclonic curl / thermocline oscillations