Atmospheric Forcing of the South Pacific Sub-tropical Counter-current and Associated Mesoscale Eddy Activity

Seth Travis, University of Hawaii at Manoa, Honolulu, HI, United States
Prior work has shown that mesoscale eddy activity variability on interannual to decadal time scales in the South Pacific Sub-tropical Counter-current (STCC) is caused by changes in the strength of baroclinic instabilities, which is related to variations in the magnitude of regional zonal shear and stratification (Travis and Qiu 2017). Eddy activity, as observed through eddy kinetic energy (EKE), was found to be strongly correlated to the baroclinic growth rate and has significant longitudinal variation. The regional changes in shear and stratification were further decomposed to find the relative changes due to variability in the temperature and salinity fields, finding that interannual variability is controlled by shifts in both parameters. Temperature and salinity patterns in the ocean can be driven by local heat and moisture fluxes and by wind-forced advection of temperature and salinity gradients. It is expected that local changes, such as those caused by changes in the South Pacific Convergence Zone (SPCZ), would be a dominant driver of surface forcing, whereas basin-scale wind pattern shifts could alter gyre circulation and the advection of anomalous water properties into the region. This work examines the relative importance of each forcing term on the state of the ocean and subsequently, how these changes impact how baroclinically unstable the region is. Through these connections, changes in eddy activity in the STCC can be linked to changes in atmospheric forcings. Lastly, connections of these forcings to larger climatic patterns (ENSO, PDO, SAM) will be explored.