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Ocean Surface Topography from Space
Studies of Ocean Surface Tides and Internal Tides with Satellite Altimetry


Richard Ray - (NASA Goddard Space Flight Center)

  Prof. Gary Egbert
Dr. Edward Zaron
(NASA Goddard Space Flight Center)
(NASA Goddard Space Flight Center)

This proposal builds on our current OSTST investigation, studying ocean surface tides and internal tides with satellite altimeter data, other types of data, and numerical modeling and assimilation. We propose to continue to produce highly-accurate global tidal atlases by multi-satellite empirical mapping as well as data assimilation. These will include some experimental solutions that specifically address problems related to

TOPEX/Poseidon and Jason altimetry, including the 59-day error in global mean sea level, as well as minor constituents which have been traditionally neglected.

Two major additional thrusts of this investigation are

  1. to clarify the amount by which low-mode internal tides remain phase-locked with the astronomical potential and
  2. to obtain better constraints, and thereby a deeper understanding, of the role that tidal energy plays in deep-ocean diapycnal mixing.
The stationarity of internal tides is of critical importance for assessing our ability to remove internal-tide variance from future SWOT data, and it will also further inform item (2). Stationarity will be investigated by analyzing the long 24-year time series of satellite altimetry, in part by spectral methods (tidal cusps, etc.) and by some specially designed filtering techniques that at present look encouraging. Our efforts will also be guided by analyzing outputs of one or more ocean-circulation models that include forcing by the tide-generating potential. A model ocean has its limitations, but it can be sampled without aliasing, and it can help resolve the physical processes that generate temporal variability. In addition, we will compare altimetry with several kinds of in situ data, including seafloor pressure and inverted echo sounding data. Our compilation of bottom-pressure tidal data has already proved extremely valuable in several studies, and we plan to improve it further and exploit it here in several applications.

In the sixteen years since we (Egbert and Ray) deduced barotropic dissipation from T/P altimetry, our tidal elevation fields have improved markedly. It is thus timely to revisit the subject, with a goal toward reducing noise levels and increasing spatial resolution. Combined with an assessment of baroclinic energy-flux divergence, determined from our multi-satellite mapping of the baroclinic tide, and our growing understanding of baroclinic tidal coherence, the dissipation estimates place constraints on the amount of energy available for ocean mixing. We propose to concentrate especially on locations of about two dozen major ocean mixing experiments, recently reviewed by Waterhouse and colleagues; our tidal-energy estimates should place tighter constraints on their interpretations. An ultimate goal of such work is to help the community improve mixing parameterizations needed in ocean-circulation and climate models.

Our efforts toward improved tidal models help nearly all other altimeter users, and indeed the much wider geodetic, geophysical, and oceanographic communities. Further extension to some very small constituents (e.g., terdiurnal tides) will clarify how the ocean responds to different, but exactly known, forcing. Incorporation of small waves like M3 could also help other geodetic missions such as GRACE.

Finally, we propose to support the OSTST in its calibration and validation efforts with the new Jason-3 data and to help lay the foundations for high-resolution altimetry expected from both Jason-CS/Sentinel-6 and SWOT.

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