Multidecadal variability in the transmission of ENSO signals to the Indian Ocean

Abstract

Since 1980, transmission of El Nino-Southern Oscillation (ENSO) signals into the Indian Ocean involves an equatorial, and a subtropical North Pacific (NP) Rossby wave pathway. We examine the robustness of the amount of energy that leaves the Pacific via each of the pathway using the Simple Ocean Data Assimilation with the Parallel Ocean Program (SODA-POP) reanalysis and a multi-century coupled model control experiment. We find that in the pre-1980 period, little ENSO signal is transmitted to the Indian Ocean and does not involve the subtropical NP pathway. Such multidecadal variability is periodically produced by the climate model. Examinations reveal that when ENSO is weak as determined by Nino3.4, their meridional extent is narrow, the associated discharge-recharge does not involve the subtropical NP pathway; further, weak ENSO events have a low signal-to-noise ratio, making the transmission hard to detect. The dynamics of multidecadal variability in ENSO strength awaits further investigation.