The dynamics of the Cape Horn Current

Abstract

The Cape Horn Current (CHC) flows along the Pacific continental slope towards Cape Horn and merges with the north and middle branches of the Subantarctic Front of the Antarctic Circumpolar Current (ACC) before entering the South West Atlantic through the Le Maire Strait. The exchange of nutrients between the Pacific and Atlantic Oceans may highly depend on the CHC strength and variability.
This thesis uses high-resolution eddy-resolving ocean models and different altimetry datasets to quantify the CHC strength and its variability based on geostrophy. The decomposition of the sea level gradient variability associated with the CHC builds an initial step for examining the primary processes driving the CHC variability. Finally, the mechanisms of the CHC variability are determined using a combination of ocean models, reanalysis and different altimetry datasets.
A strong relationship is found between the CHC strength and sea level gradient in high-resolution ocean models. This relationship holds for both mean and variability, allowing the CHC strength and variability to be estimated from the altimetry datasets for the first time. The altimetry-based estimate of the CHC is found to increase in strength by an order of magnitude from north to south. During the past 29 years of the altimetry era, the CHC has stayed stable.
The relationship between the CHC and sea level gradient provides a method to determine the drivers of the CHC via investigating the drivers of the sea level gradient. The decomposition of the sea level gradient is an initial step towards drivers of the sea level gradient identification. The decomposition shows both onshore and offshore sea level variability are associated with the Cape Horn Current variability. For onshore and offshore sea level, the sea level variability is decomposed into barotropic, baroclinic, thermosteric and halosteric components. The decomposition indicates different processes dominate on the shelf and in the deep ocean and they also change when merging with the ACC system at Cape Horn.
With the help of the relationship between the CHC strength and sea level gradient variability, and the decomposition of sea level variability, the dominant forcing of the CHC variability is finally identified. Before merging the main branch of the ACC, the barotropic Costally-Trapped Waves (CTWs) propagate poleward on the continental shelf leading to a barotropic sea level variability on the shelf of the CHC domain. Then the CTWs generate baroclinic eddies which propagate to the deep ocean advecting heat and salt offshore leading to the baroclinic sea level variability in the deep ocean. The interplay of barotropic waves on the shelf and baroclinic eddies in the deep ocean primarily drives the CHC variability. However, the primary mechanism changes slightly as the CHC moves poleward. When the CHC joins the Sub-Antarctic Front of the ACC system, the dominant baroclinic eddies in the deep ocean of the CHC variability are generated by the ACC. Compared to the primary forcing, other surface forces, such as buoyancy forcing and local wind forcing, play a minor role in influencing the CHC variability.

Date of Award	3 Oct 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Rory J Bingham (Supervisor) & Oliver D Andrews (Supervisor)