Reconstructing the Southern Hemisphere Westerlies and their role in changing atmospheric CO2

Abstract

This thesis investigates the impact of changing Southern Hemisphere westerly winds (SHW) on the ocean carbon cycle and our ability to robustly reconstruct these wind changes over the past millennium using paleo-proxy data. Transient and equilibrated impacts of changes in SHW wind stress on ocean circulation and carbon pumps are explored using a fully coupled Earth system climate model of intermediate complexity (UVic) while our ability to reconstruct the winds, interpreted as changes in the Southern Annular Mode (SAM), is assessed in both a pseudo-proxy (GFDL CM2.1) and a physical paleo-proxy framework. The main findings of this thesis are: (i) A southward shift of the SHW during the deglaciation in conjunction with changes in Atlantic overturning, provide a plausible mechanism for ventilation of the deep ocean and may explain recorded changes in atmospheric CO2 and Δ14 C. (ii) Small perturbations of the latitudinal position and intensity of the SHW in isolation of other forcing has the potential to significantly alter air-sea carbon exchange, caused primarily by circulation-driven changes in the physical and biological pumps. (iii) Our ability to identify such changes in the winds is limited, as reconstructions in a model framework suggest that only approximately 50% of SAM variance is captured. Maximising the size of the proxy network, calibration window length and geographic diversity of proxy source regions are all shown to aid in producing a more skilful reconstruction. (iv) Paleo-reconstructions of SAM differ significantly due to the nature of the proxy network and the index with which it is calibrated, while the reconstruction method used is less important. Despite significant differences, all reconstructed indices share significant low frequency variability in the 64-128 year space. Overall, while the model simulations suggest that changing SHW may contribute significantly to observed changes in CO2 on multi-decadal to centennial time-scales, our inability to reconstruct the winds robustly currently precludes us from making such connections