Abstract

Ocean circulation in the Indian Ocean is predominantly driven by the monsoon and is responsible for convergence along the equator. As a result, upwelling is primarily restricted to the western boundary where surface waters are anomalously depleted in 14C. Here, we describe aspects of western boundary upwelling based on insights derived from the first coral radiocarbon time-series in the Indian Ocean. The absence of a distinct subannual pre-bomb Δ14C signal suggests that open and coastal upwelling are negligible off the coast of Kenya. Instead, our results suggest that upwelling from the coast of Somalia and possibly Oman are the sources of the depleted seasonal Δ14C signal. In contrast, the southern hemisphere subtropical gyre provides water enriched in 14C. We demonstrate that the coral Δ14C time-series is a tracer for meridional transport in the Indian Ocean. The Indian Ocean exhibits a shallow cross-equatorial overturning circulation cell. Our results demonstrate that the Kenyan coral radiocarbon record is responding to a western boundary limb of this cell, similar to that observed in other subtropical oceans. Therefore, while the majority of cross-equatorial transport is in the interior and eastern basin of the Indian Ocean, our results argue that the Somali Current is a distinct pathway for inter-hemispheric water mass exchange.

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