Collaborative Proposal: Exploring hypotheses of Southern Hemisphere westerly wind changes on Southern Ocean circulation and biogeochemistry

The circulation of the Southern Ocean plays a prominent role in regulating Earth's climate, in particular connecting the deep ocean to the surface. This connection holds particular importance for the Earth's carbon cycle as upwelling of waters from the deep ocean brings stored carbon of the deep to the surface, eventually entering into the atmosphere as carbon dioxide. Increased upwelling is thought to have brought about the initial rise in atmospheric CO2 concentrations during the last deglaciation, caused by changes to the surface westerlies and resulting changes to the wind-driven upwelling. This project investigates a recent hypothesis for how the westerlies changed during this time period affects Southern ocean circulation and carbon cycle, through the use of a state-of-the-art climate model. The predictions of this hypothesis will be contrasted against those made by the 'annular-mode' view for westerly changes that dominates current thinking, and both compared against available paleoproxy observations. The research provides concrete tests to each hypothesis, and the process will help advance our understanding of how southern hemisphere westerlies affect the ocean climate and carbon cycle. The project will also enhance collaboration between two US-based scientists with an overseas collaborator based in Taiwan.

Previous studies have largely assumed that paleo-westerly changes were zonally uniform, analogous to the so-called 'annular mode' behavior that dominates year-to-year variations in the southern hemisphere westerlies today. However, in reality there are insufficient paleoproxy data to confirm this interpretation, and in a recent study one of the PIs proposed an alternative hypothesis for westerly changes through the modulation of the South Pacific Split Jet. The Split Jet is a regional feature of the South Pacific whereby the core of the westerlies split into two around Australia, and this split extends across the Pacific. Based on dynamical reasoning and comparison to paleoproxy records, the PI argued that the Split Jet feature weakened during Heinrich events, in particular Heinrich event 1 that preceded the last deglaciation. In this study, the consequences for a weaker Split Jet on the Southern ocean circulation and biogeochemistry will be investigated through idealized simulations using the ocean component of the Community Earth System Model version 1 with a simple ocean biogeochemistry module. Focus will be placed on understanding changes to the ocean surface temperature and frontal locations, changes to the upwelling and carbon fluxes. These simulations will be compared and contrasted against simulations where annular-mode like westerly changes is imposed. Through comparison between these simulations with available ocean paleoproxies, an assessment can be made as to which westerly configuration was more likely. If the Split Jet hypothesis turns out to be more correct, it has strong implications for our conceptualization and understanding of Southern ocean climate and circulation changes.