Abstract's details

Satellite estimates of Mode-1 M2 Internal Tides using non-repeat altimetry missions

Zhongxiang Zhao (University of Washington, United States)

CoAuthors

Maarten Buijsman (University of Southern Mississippi, United States)

Event: 2022 Ocean Surface Topography Science Team Meeting

Session: Tides, internal tides and high-frequency processes

Presentation type: Type Forum only

Previous satellite estimates of internal tides are usually based on 25 years of sea surface height (SSH) data from 1993–2017 measured by exact-repeat (ER) altimetry missions. In this study, new satellite estimates of internal tides are based on eight years of SSH data from 2011–2018 measured mainly by non-repeat (NR) altimetry missions. The two datasets are labeled ER25yr and NR8yr, respectively. NR8yr has advantages over ER25yr in observing internal tides, because of its shorter time coverage and denser ground tracks. For comparison, mode-1 M2 internal tides are mapped from both datasets following the same procedure that consists of two rounds of plane wave analysis with a spatial bandpass filter in between. It is shown that mode-1 M2 internal tides can be mapped from NR8yr in fitting windows as small as 40 km, in contrast to 160-km windows required by ER25yr. The two internal tide models are labeled NR8yr40km and ER25yr160km, respectively. Evaluation using independent satellite altimeter data confirms that NR8yr40km is better than ER25yr160km. Their global energies are 49 and 34 PJ, respectively, indicating that ER25yr160km underestimates the M2 internal tide by 15 PJ (~30%). Further comparisons reveal that the 160-km (versus 40-km) window underestimates the M2 internal tide by 5 PJ (~10%) and the 25-yr (versus 8-yr) time coverage underestimates the M2 internal tide by 10 PJ (~20%). This work confirms that non-repeat (e.g., geodetic, drifting) altimetry missions (phases) can be used to observe internal tides, which offers more orbital choices for one extended altimetry mission.

The effect of window size on internal tide estimation is examined using a high-resolution HYCOM simulation in the same manner. It is confirmed that internal tides are not underestimated by the 40-km fitting window employed in this mapping technique. The effect of time-series duration on internal tide estimation is examined using the six-year-long HYCOM simulation following the same procedure. The investigation is still ongoing. It is expected that our satellite and HYCOM results will provide constraints on the percentage of the incoherent internal tide, which is a function of time-series duration. In addition, the global internal tide dissipation map is computed using the new satellite observation, whose high resolution is a key improvement, because dissipation is derived from flux divergence. The satellite derived dissipation map is compared with the dissipation map derived from the HYCOM simulation reported previously.
 
Zhongxiang Zhao
University of Washington
United States
zzhao@apl.washington.edu