A Lagrangian analysis of circulation and productivity in the tropical Pacific ocean

The aim of this thesis is to determine the source and pathway variability of water masses and their associated iron content into the Equatorial Undercurrent (EUC). This is achieved through the use of Lagrangian trajectories, where virtual particles are advected in Eulerian velocity data backwards in time from within the EUC to predefined source sections in the Western Pacific. Offline Lagrangian computations are highly sensitive to the temporal resolution of the model outputs. Therefore, a suitable resolution is first determined. Up to 9-day averaging results in statistically indistinguishable means and high correlations for volume transport and transit times from 3-day outputs for various currents. The results give confidence in using 1-day velocity outputs to examine the sources of water and iron feeding the EUC from the western Pacific, a topographically complex region.The EUC transports water originating from a number of distinct source regions including water passing through the low latitude western boundary currents (LLWBCs); north from the Mindanao Current and south from the New Guinea Coastal Undercurrent (NGCU) and New Ireland Coastal Undercurrent (NICU). The LLWBCs are potentially responsible for supplying the limiting nutrient iron to the productive Eastern Equatorial Pacific (EEP). The increased transport of water from the LLWBCs during El Niño does not translate into coherent changes in the makeup of EUC waters. The remote signals from the LLWBCs are smeared out by the large amounts of eddying from the Western Pacific to the EUC.We then develop a Lagrangian iron model, which calculates iron changes along trajectories due to exogenous inputs, scavenging, biological activity and dilution. We find that a sole NGCU iron source, as previously assumed, underestimates observations in the EUC due to high scavenging and dilution. An additional NICU iron source enhances iron concentrations up to observed levels and is able to propagate a coherent iron signal to explain how western iron sources can affect the timing and intensity of blooms in the EEP.The results highlight the importance of using combined Lagrangian and Eulerian methods as well as moving towards eddy-resolving model resolutions in order to capture important key processes for nutrient transport