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Altimeter and Current Meter Observations of Internal Tides: Do They Agree?

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  • 1 National Institute of Water and Atmospheric Research, Kilbirnie, Wellington, New Zealand
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Abstract

Baroclinic tides can be observed both remotely from the Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter and in situ using current meters. However, it is rare that current meter moorings have high vertical resolution and are located under T/P ground tracks so that a direct comparison can be made between the two methods of observations. Here, data from a current meter mooring directly under a T/P ground track off the Bounty Plateau, New Zealand, are used to obtain energy estimates of the first baroclinic mode. These estimates are compared with those calculated from the T/P surface elevation. The two methods return estimates of the internal tide that are in agreement in phase and direction but have about a factor-of-2 difference in amplitude (and a factor-of-4 difference in energy); the flux estimates are 787 and 170 W m−1, respectively. Uncertainties in these estimates are relatively large, and there is a low but not negligible probability that the differences are entirely due to measurement error. However, there are other reasons that might explain the differences in the estimates. It may be that the baroclinic tide is highly modulated in time and the current meters were deployed during a period of relatively low amplitude.

Corresponding author address: Stephen M. Chiswell, National Institute of Water and Atmospheric Research, P.O. Box 14-901, Kilbirnie, Wellington, New Zealand. Email: s.chiswell@niwa.cri.nz

Abstract

Baroclinic tides can be observed both remotely from the Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter and in situ using current meters. However, it is rare that current meter moorings have high vertical resolution and are located under T/P ground tracks so that a direct comparison can be made between the two methods of observations. Here, data from a current meter mooring directly under a T/P ground track off the Bounty Plateau, New Zealand, are used to obtain energy estimates of the first baroclinic mode. These estimates are compared with those calculated from the T/P surface elevation. The two methods return estimates of the internal tide that are in agreement in phase and direction but have about a factor-of-2 difference in amplitude (and a factor-of-4 difference in energy); the flux estimates are 787 and 170 W m−1, respectively. Uncertainties in these estimates are relatively large, and there is a low but not negligible probability that the differences are entirely due to measurement error. However, there are other reasons that might explain the differences in the estimates. It may be that the baroclinic tide is highly modulated in time and the current meters were deployed during a period of relatively low amplitude.

Corresponding author address: Stephen M. Chiswell, National Institute of Water and Atmospheric Research, P.O. Box 14-901, Kilbirnie, Wellington, New Zealand. Email: s.chiswell@niwa.cri.nz

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