Mesoscale Eddy Activity in the South Pacific Subtropical Counter-current: Decadal Variability and Bio-physical Connections

Travis, Seth
Qiu, Bo
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Mesoscale eddies are important contributors to ocean circulation, and are ubiquitous throughout the world's oceans. They are capable of transporting heat, salinity, nutrients, and phytoplankton, and are important in the transfer of energy between different scales. In the South Pacific the Subtropical Counter-current is a region of heightened eddy activity which has been little studied. The South Pacific Subtropical Counter-current (STCC) is an eastward flowing current which overlays the westward South Equatorial Current (SEC). This vertically sheared STCC-SEC system is subject to baroclinic instabilities, which gives rise to mesoscale eddies. Decadal variability of eddy activity in the western, subtropical South Pacific is examined using the past two decades of satellite altimetry data. By using ocean reanalysis data, low-frequency variations in the state of the ocean in this region are investigated. It is found that the low-frequency changes in shear and stratification simultaneously work to modulate the strength of baroclinic instabilities. These changes in the strength of the instabilities consequently affect the observed eddy activity. Using a linearization of the baroclinic growth rate, the contribution to the variability from the changes in shearing is found to be roughly twice as large as those from changes in stratification. Additionally, changes in the temperature and salinity fields are both found to have significant impacts on the low-frequency variability of shearing and stratification, for which salinity changes are responsible for 50-75% of the variability as caused by temperature changes. However, the changes in all these parameters do not occur concurrently, and can alternately work to negate or augment each other. By furthering the investigation of this system to look at the driving mechanisms leading to changes in the shear and stratification, larger drivers of overall eddy activity can be identified. The Estimating the Circulation and Climate of the Ocean, phase II (ECCO2) ocean state model is used to perform budget analyses to identify to most important mechanisms altering the temperature and salinity fields in the STCC, and subsequently, the shear and stratification. These budgets can then be related back to the linearized baroclinic growth rate to look at the impact of individual drivers on eddy activity. Variability in advective flux convergence is found to be the most consequential driver, for both shear and stratification, while direct atmospheric surface forcing through net heat flux and moisture fluxes are of approximately equal importance. Atmospheric forcings are additionally found to be related to the Interdecadal Pacific Oscillation through changes in the location and strength of the South Pacific Convergence Zone. Mesoscale eddies have been shown to have significant effects on biogeochemical cycles, as observed in local levels of near-surface chlorophyll. In the South Pacific Subtropical Counter-current, however, an inconsistent chlorophyll anomaly response and a low correlation to the presence of eddies challenges simple explanation of the mechanisms at play. Using Glob-Colour ocean color data and Aviso altimetry data, an investigation of the area found that a seasonal reversal occurs in the character of the chlorophyll anomaly within eddies (reversal from positive to negative, and vice versa). The cause of this reversal is inferred to be a seasonally-changing limiting factor within the region. Argo float profiles co-located inside and outside of eddies are used to show the coincidence of chlorophyll anomalies with seasonally changing mixed layer depths and the ability of the eddies to access deep nutrient pools. Observations of other mechanisms, such as eddy stirring or eddy-Ekman pumping, are found to be seasonally less important than the mixed layer depth change induced nutrient flux. Additionally, metrics are developed to globally identify oceanic regions in which such seasonal reversals in chlorophyll anomalies could occur.
Physical oceanography, Biological oceanography, Baroclinic, Eddies, Mesoscale, Phytoplankton
147 pages
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