Multi-Decadal Indian Ocean Variability Linked to the Pacific

The Indian Ocean has sustained robust surface warming in recent decades, with warming rates exceeding those of other tropical ocean basins. However, it remains unclear how multi-decadal variability in upper-ocean thermal characteristics has contributed to these Indian Ocean trends. Using high-resolution ocean model hindcasts building on the ocean/sea-ice numerical Nucleus for European Modelling of the Ocean (NEMO) framework forced with atmospheric forcing fields of the Coordinated Ocean Reference Experiments, the characteristics of Indian Ocean temperature changes are explored. Sensitivity experiments, where interannual atmospheric variability is restricted to thermal or wind-stress forcing only, support the interpretation of forcing mechanisms for the evolution of temperature characteristics across the Indian Ocean, focusing on the top 700m. Simulated temperature changes across the Indian Ocean in the hindcasts are consistent with those recorded in observational products and ocean reanalyses.

Temperatures and heat content exhibit extensive subsurface cooling for much of the tropical Indian Ocean since the 1950s, associated with a shoaling thermocline. Here, we link upper-ocean temperature trends in the Indian Ocean to multi-decadal remote Pacific wind changes associated with the Interdecadal Pacific Oscillation/Pacific Decadal Oscillation. We find that low-frequency variations in the depth of the eastern Indian Ocean thermocline pre-condition Indian Ocean Dipole (IOD) events: the frequency of positive (negative) IOD events was significantly enhanced during decades with a shallower (deeper) thermocline background state. Multi-decadal Pacific wind forcing has also masked increases in Indian Ocean heat content due to thermal forcing since the 1960s. However, wind and thermal forcing both contribute positively to Indian Ocean heat content since the turn of the century. Drastic increases in the heat content in coming decades are therefore likely; in fact, they have been implicated to play a role in the recent warming hiatus.