Upwelling, Instability, and Mixing West of the Galápagos Archipelago from Glider Observations

J K Jakoboski, New Zealand Meteorological Service (MetService), MetOcean Division, Raglan, New Zealand; Woods Hole Oceanographic Institution, Woods Hole, United States, Breck Owens, Woods Hole Oceanographic Institution, MA, United States, Robert E Todd, Woods Hole Oceanographic Institution, Physical Oceanography, Woods Hole, MA, United States, Kristopher B Karnauskas, University of Colorado at Boulder, Boulder, CO, United States; Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States and Daniel L Rudnick, Scripps Institution of Oceanography, La Jolla, CA, United States
The Galápagos Archipelago lies on the equator in the path of the eastward flowing Pacific Equatorial Undercurrent (EUC), which is centered south of the equator near 0.5oS. Just west of the archipelago exists a region of anomalously cold SST known as the Galápagos Cold Pool, which may be partially maintained by upwelling of the EUC and/or vertical mixing resulting from instability associated with the equatorial current system. A fleet of autonomous, underwater gliders, deployed just west of the Galápagos Archipelago between 2oS - 2oN along 93oW and between 2oS and the southernmost point of Isla Isabela from 2013 - 2016 collected a unique set of temperature, salinity, and velocity observations, providing the opportunity to estimate sections of Ertel potential vorticity (EPV). When the EUC reaches the Galápagos Archipelago, it bifurcates into two branches that flow north and south around the archipelago at a latitude determined by the archipelago’s topography. Strong velocity gradients associated with the EUC core exist within 2o latitude of the equator where the Coriolis parameter approaches zero. In this region, these velocity gradients can result in EPV having sign opposite that of the Coriolis parameter. The EPV field suggests whether necessary conditions are met for symmetric/inertial, barotropic and/or baroclinic instability associated with the equatorial current system. Downward integration of the net horizontal transport profile obtained from glider transects provides indicates upwelling in the upper 300 m over a 52 day time period. The horizontal gradient of EPV over the three-year and shorter-term averages suggests that the region between 2oS and 2oN above 100 m is unstable to barotropic instability, while limited regions are baroclinically unstable. Conditions conducive to inertial and symmetric instabilities are observed between the EUC core and the equator and within the south branch of the EUC.