Satellite Estimates of Mode-1 M2 Internal Tides Using Nonrepeat Altimetry Missions

Abstract

Abstract Previous satellite estimates of internal tides are usually based on 25 years of sea surface height (SSH) data from 1993 to 2017 measured by exact-repeat (ER) altimetry missions. In this study, new satellite estimates of internal tides are based on 8 years of SSH data from 2011 to 2018 measured mainly by nonrepeat (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. 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. The denser ground tracks of NR8yr make it possible to examine the impact of window size in the first-round plane wave analysis. Internal tides mapped using six different windows ranging from 40 to 160 km have almost the same results on global average, but smaller windows can better resolve isolated generation sources. The impact of time coverage is studied by comparing NR8yr160km and ER25yr160km, which are mapped using 160-km windows in the first-round plane wave analysis. They are evaluated using independent satellite altimetry data in 2020. NR8yr160km has larger model variance and can cause larger variance reduction, suggesting that NR8yr160km is a better model than ER25yr160km. Their global energies are 43.6 and 33.6 PJ, respectively, with a difference of 10 PJ. Their energy difference is a function of location. Significance Statement Our understanding of internal tides is mainly limited by the scarcity of field measurements with sufficient spatiotemporal resolution. Satellite altimetry offers a unique technique for observing and predicting internal tides on a global scale. Previous satellite observations of internal tides are mainly based on 25 years of data from exact-repeat altimetry missions. This paper demonstrates that internal tides can be mapped using 8 years of data made by nonrepeat altimetry missions. The new dataset has shorter time coverage and denser ground tracks; therefore, one can examine the impact of window size and time coverage on mapping internal tides from satellite altimetry. A comparison of models mapped from the two datasets sheds new light on the spatiotemporal variability of internal tides.