Improving OSCAR Currents with Level 2 Fields

Kathleen Dohan (Earth and Space Research, United States)

CoAuthors

Jonathan Lilly (NorthWest Research Associates, United States)

Event: 2016 Ocean Surface Topography Science Team Meeting

Session: Science II: From large-scale oceanography to coastal and shelf processes

Presentation type: Type Poster

Ocean Surface Current Analyses-Realtime currents (OSCAR, podaac.jpl.nasa.gov) are global ocean surface velocities calculated from sea surface height (SSH) gradients, ocean vector winds, and sea surface temperature fields using geostrophy, Ekman, and thermal wind dynamics. A main objective of the OSCAR project is to further our understanding of the mechanisms behind the transfer of momentum between the atmosphere and the ocean through the planetary boundary layer and in doing so improve the generation of surface currents from satellite data. A crucial component is capturing accurate geostrophic motions.

OSCAR uses the AVISO gridded MADT SSH fields (http://www.aviso.altimetry.fr/). The advantage to using AVISO gridded fields is clear: the data is very high quality, combines multiple altimeters, and is quick to use in the OSCAR system. Much of the mismatch between OSCAR and in situ surface current measurements is assumed to be attributable to missing physics in the simple OSCAR model. However, a significant loss of signal occurs by using the gridded SSH fields, particularly when taking gradients to calculate geostrophic currents.

We have been using local polynomial fitting mapping methods to investigate the mesoscale and sub-mesoscale signal in both altimetry and ocean vector wind fields. A main question to address is the degree to which the use of smoothed level 4 fields affects the calculation of surface currents. An important feature of local polynomial fitting over optimal interpolation methods is the order of fit: a first-order (linear) fit calculates gradients as part of the mapping.

When OSCAR is compared with global surface drifters, velocity standard deviations are underestimated by 20-50% in OSCAR over much of the oceans. Preliminary results were presented last year that showed that amplitudes of geostrophic currents steadily decrease with an increased smoothing radius of the SSH fields. The method did indeed show that one can retrieve more accurate currents by calculating directly from level 2 fields. However, the study was performed in the Gulf Stream area, which is expected to be highly geostrophic. In this presentation we extend the study to a global domain. An assessment of the legitimacy of applying geostrophy to small scale fields will be evaluated per dynamical region. Just how much can we push geostrophy to small scales, and is it regionally dependent?

 

Poster show times:

RoomStart DateEnd Date
Grande Halle Thu, Nov 03 2016,11:00 Thu, Nov 03 2016,18:00
Kathleen Dohan
Earth and Space Research
United States
kdohan@esr.org