Growing Influence of Indian Ocean Waters in the South Atlantic Intermediate Layer over the last 30 years

Global climate change is concurrently and profoundly altering the ocean’s physical and biogeochemical environment. The intermediate water in the South Atlantic and Indian Oceans, which together constitute a critical component of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC), lies at the heart of these changes. The intermediate layer of the South Atlantic is ventilated by two primary sources: relatively fresher and younger waters (characterised by low Apparent Oxygen Utilization, AOU) originating from the Pacific Ocean, and saltier and older waters advected from the Indian Ocean via the Agulhas System. However, the extent to which the intrusion of saline, older Indian Ocean waters via the Agulhas System modulates the stability of the AMOC’s upper limb remains poorly understood. Specifically, the temporal variability and long-term contribution of the Indian Ocean waters to the South Atlantic intermediate layer remains a knowledge gap.

Here, we focus on observational evidence of and investigate the underlying mechanisms driving the influence of Indian Ocean intermediate waters on the Atlantic Ocean in a warming climate. We examine AOU-salinity covariability across decadal to multi-decadal time scales within South Atlantic intermediate water. This analysis integrates high-quality observational databases of temperature, salinity, dissolved oxygen, and water age, as well as repeat hydrographic sections, allowing us to link their observed variability to changes in circulation and mixing, while considering oxygen disequilibrium effects and the influence of the biological carbon pump in changing AOU.

We find an increasing influence of Indian Ocean water in the South Atlantic at the intermediate layer over the past 30 years. The most strongly impacted regions are identified. In addition, we quantify the impact of Indian Ocean influence on the South Atlantic and show that this signal has become progressively detectable over the past 30 years, but has not yet exceeded the level of internal variability, indicating an ongoing ‘Indianization’ of the South Atlantic intermediate layer. We identify the underlying mechanisms related to the increasingly positive phase of the Southern Annular Mode (SAM) and an associated multidecadal increase in Agulhas leakage. Finally, we will discuss the potential implications of this phenomenon for the long-term stability of the AMOC.