Variability in the origins and pathways of <scp>P</scp> acific <scp>E</scp> quatorial <scp>U</scp> ndercurrent water

The Pacific Equatorial Undercurrent (EUC) transports water originating from a number of distinct source regions, eastward across the Pacific Ocean. It is responsible for supplying nutrients to the productive eastern Equatorial Pacific Ocean. Of particular importance is the transport of iron by the EUC; the limiting nutrient in that region. Although the mean circulation and sources of EUC water are reasonably well understood, it is unclear how the contribution of water from these sources to the EUC vary on seasonal to interannual timescales. Here a Lagrangian analysis is applied to the eddy-resolving OFAM3 ocean simulation in order to identity variability in the makeup of the EUC over an 18 year period (1993-2010). While ENSO has an influence on the variability of source transport contributions to the EUC, the signal of increased (decreased) transport of water from the LLWBCs during El Ni&#241;o (La Ni&#241;a) periods does not translate to substantial changes in the makeup of the EUC between 165&#176;E and 140&#176;W. It is hypothesized that this is due to the large spread in travel times of water parcels as they travel from the source regions into the EUC. The consequent erosion of transport anomalies generated at the different western boundary source regions associated with ENSO may help explain why previous studies found little relationship between variability in iron fluxes off Papua New Guinea shelves and the chlorophyll response in the eastern tropical Pacific. Key Points: A Lagrangian framework is used to backtrack EUC water sources Increased LLWBC water during El Ni&#241;o is smeared out within the EUC Travel times and particle positions from individual sources exhibit large spread