Eddy Circulation, Heat and Salt Balances, and Ocean Metabolism: Observations from a Seaglider-Mooring Array at Ocean Station Papa
Pelland, Noel Adolfson
MetadataShow full item record
Seagliders were deployed at Ocean Station Papa (OSP; 50N, 145W) in the southern Gulf of Alaska, augmenting a NOAA Ocean Climate Stations surface fluxes mooring and subsurface ADCP, from 8 June 2008 to 21 January 2010. Seagliders provided increased resolution of the three-dimensional temperature, salinity, and dissolved oxygen field surrounding the mooring, resolving horizontal gradients at scales relevant to local advection. This thesis concerns the analysis and interpretation of results from this time series, with application to 1) observations of eddy circulation unusual in comparison to typical conditions at OSP, 2) quantitative analysis of the temperature and salinity balances in the upper 200 m using least-squares methods, and 3) diagnosis of the balance of kinematics and net metabolism in the balance of dissolved oxygen. Geostrophic velocities estimated using Seaglider were moderate and poleward during 2008 before rotating counterclockwise and weakening in early 2009. Coinciding with the chance in horizontal circulation were observations of strong horizontal water mass gradients, downward isopycnal displacements, and vertical finestructure circumstantially consistent with subsurface thermohaline fronts. A likely explanation is the presence of a mesoscale anticylonic eddy; oxygen characteristics in the permanent pycnocline suggest that this feature originated to the south or west of OSP. A least-squares method was used to determine the velocity and diffusivity that best explained the integrated temperature and salinity balances. The least-squares solution velocity and diffusivity elements were in good agreement with a priori estimates or, in the case of diffusivity, with general expectations about the vertical structure and magnitude of turbulent mixing coecients in the upper ocean. Using the solution elements, the vertically- and temporally-integrated heat and salt balances were closed with mean square error that was a small fraction of the total variance. In the top 120 m, the surface heat input was balanced by cooling due to vertical advection; the resulting excess of salt was removed by horizontal advection. This combination of terms is contrary to the climatological concept of flow at OSP. The oxygen budget had similar character to the temperature budget shallower than 100 m and the salinity budget at greater depths. Horizontal advection was unimportant in the oxygen budget in the surface layer, but was a leading term deeper. Robust estimates of respiration were formed in the region deeper than the surface layer and shallower than the permanent pycnocline, giving a net apparent oxygen utilization of 1.2 mol O2 m-2 yr-1 (0.8 mol C m-2 yr-1). Given previously estimated rates of net community productivity of C in the surface layer in summer, this rate indicates that 30-50% of the organic carbon produced there during each growing season is respired before it can reach the deep ocean.
- Oceanography