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Ocean Surface Topography from Space
Global Internal Tides from Satellite Altimetry: Next-Generation Internal Tide Model and Internal Tide Oceanic Tomography


Zhongxiang Zhao - (University Of Washington, Seattle)

Satellite altimetry is the only practical technique for mapping internal tides on a global scale. However, the traditional point-wise harmonic analysis has its inherent limitations in extracting internal tides from satellite altimeter data, because of the complex nature of the global internal tide field and mesoscale contamination. We have developed a plane wave fit method to address this challenge. In this method, internal tides are extracted using data in a fitting window (about one wavelength), rather than at one single site. This mapping technique is superior to point-wise harmonic analysis for the following reasons: First, it separately resolves internal tidal waves with different propagation directions; Second, it suppresses mesoscale contamination by extracting internal tides with both spatial and temporal coherence.

Taking advantage of 20 years of multisatellite SSH measurements, we have constructed global maps of M2, S2, O1, and K1 internal tides. Recently, we have made significant progress in examining the temporal variability of the internal tide field. We divide the multiyear altimeter data into small time windows. This enables us to construct yearly internal tide fields from 1995 onward. This analysis will leads to the next-generation internal tide model, in which the amplitude and phase are time variables.

The travel time changes of internal tides are affected by the upper ocean warming. Thus, we bring up a new concept of internal tide oceanic tomography (ITOT) to monitor ocean warming on a global scale. ITOT is much like acoustic tomography (Munk et al. 1995), but that the work wave is internal tides. ITOT measures ocean temperature changes by precisely measuring travel time changes of long-range internal tides. ITOT offers a long-term, low-cost, environmentally-friendly technique for monitoring global ocean warming.

More recently, we have evaluated the performance of our internal tide model in the central North Pacific using 4 years of CryoSat-2 data. We found significant variance reduction before and after the M2 internal tide correction. The result suggests that our internal tide model is a reliable and efficient model. We propose to conduct such kind of examination on the global scale and using data from other satellites. In addition, it is also beneficial to inter-compare internal tide models constructed using different mapping techniques.

Our proposed work will focus on the following three topics:

  1. Construction of the next-generation model with time-variable amplitude and phase:
    1. Construct 20 yearly internal tide fields from 1995-2014;
    2. Construct 12 monthly internal tide fields;
    3. Quantify the temporal variations of amplitude and phase.
  2. Development of the ITOT technique for monitoring global ocean warming:
    1. Examine along-beam phase changes;
    2. Build transfer function from speed change rate to ocean temperature change;
    3. Calibrate with field measurements (e.g., Argo, XBT, and moorings).
  3. Evaluation and improvement of our internal tide model:
    1. Refine parameters in plane wave fitting region by region;
    2. Compare with other altimeter-based internal tide models;
    3. Evaluate our model using altimeter data from other satellites.
This proposal echoes the OSTST solicitation of NASA as follows. (1) This proposal utilizes over 20 years of multisatellite altimeter data (T/P, Jason-1/2, ERS-1/2, Envisat, and GFO) to construct the next-generation internal tide model. Meanwhile, altimeter data from other satellites (CryoSat-2, AltiKa, and Sentinel-3) will be used to evaluate our internal tide model. (2) Our internal tide model may be used to correct internal tide signals in SWOT as well as previous nadir-looking altimeter missions. (3) The proposed ITOT technique can infer the sea level rise caused by hydrostatic thermal expansion, separated from contribution of melting ice. ITOT supports NASA's operation and application of altimeter satellites.

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