Response of the Benguela upwelling system to four decades of global warming

The Benguela upwelling system (BUS) stands out as one of the most productive marine systems in the world's oceans, providing significant ecological and economic value. This productivity primarily arises from the upwelling process, where equator-ward alongshore winds, coupled with Earth's rotation, drive the offshore movement of shallow coastal waters, causing the ascent of deep, cold, nutrient-rich waters, thereby fuelling primary production. Given the importance of the BUS, understanding the potential impacts of ongoing climate warming is crucial. Climate warming may alter atmospheric pressure gradients, potentially intensifying upwelling-favourable winds, a concept known as the "Bakun hypothesis". However, this hypothesis remains debated within the scientific community. The objectives of this dissertation were to determine whether there has been an intensification of the winds driving the BUS and, if so, to identify the principal physical and biogeochemical changes resulting from this intensification. Physical alterations examined include Ekman offshore transport, while biogeochemical changes encompass three major aspects: Chlorophyll-a concentrations as a proxy for algal blooming, food web structures, and dissolved oxygen dynamics within the system, focusing on hypoxic water masses. Previous studies addressing similar questions have faced limitations due to the system's complexity and the scarcity of observational data covering long periods to address long-term trends. To overcome these challenges, this dissertation investigates the BUS over four decades from 1980 to 2020, using a highly resolved coupled physical-biogeochemical model via the NEMO-BFM modeling suites, employing a nesting approach with a horizontal resolution of 1/16° over the BUS domain, nested from a global ocean domain with a resolution of 1/4°.