1245 Determination of Ocean Absolute Dynamic Topography Using the Minimum Energy State

Wednesday, 25 January 2017
4E (Washington State Convention Center )
Peter C. Chu, NPS, Monterey, CA

Geostrophic balance represents the minimum energy state in linear Boussinesq primitive equations with conservation of potential vorticity. On the base of that, a new absolute dynamical topography equation (η-equation),

  -[∂x(gH/f2)∂xη + ∂y(gH/f2)∂yη ] +η = (g/f2)(∂C/∂x - ∂B/∂y)

is derived for the absolute dynamic topography (η) with H the water depth,  f the Coriolis parameter, g the gravitational acceleration, and coefficients (B, C) determined by temperature (T) and salinity (S). The η-equation provides the constraint to identify the geostrophy for the practically produced η fields. Along with satellite observations, in-situ ocean measurements of temperature, salinity, and velocity have also rapidly advanced such that the global ocean is now continuously monitored by near 4,000 free-drifting profiling floats (called Argo) from the surface to 2000 m depth with all data being relayed and made publicly available within hours after collection (http://www.argo.ucsd.edu/). This provides a huge database of (T, S) and in turns the (B, C) coefficients for the η-equation.

The η-equation is solved numerically in this study for the North Atlantic Ocean (100oW-6oW, 7oN-72oN) on 1oX1o grids with the coefficients (B, C) calculated from the three-dimensional (T, S) data of the NOAA National Centers for Environmental Information (NCEI) World Ocean Atlas 2013 version 2 (http://www.nodc.noaa.gov/OC5/woa13/woa13data.html) and H from the NOAA ETOPO5 (https://www.ngdc.noaa.gov/mgg/fliers/93mgg01.html) using simple Dirichlet (for rigid boundary sections) and Neumann (for open boundary sections) boundary conditions. The numerical solution η agrees well with the difference between the time-averaged sea surface height and the geoid from the NASA/JPL (http://gracetellus.jpl.nasa.gov/data/dot/,  also considered as the mean dynamic topography) in most part of the North Atlantic Ocean.  It offers an alternative way to estimate the absolute dynamic topography.

Further application of the η equation method on the high-precision altimetry measurements of SSH such as the Surface Water and Ocean Topography (SWOT) is also presented. 

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