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Tyler D. Hennon, Matthew H. Alford, and Zhongxiang Zhao

Abstract

Though unresolved by Argo floats, internal waves still impart an aliased signal onto their profile measurements. Recent studies have yielded nearly global characterization of several constituents of the stationary internal tides. Using this new information in conjunction with thousands of floats, we quantify the influence of the stationary, mode-1 M2 and S2 internal tides on Argo-observed temperature. We calculate the in situ temperature anomaly observed by Argo floats (usually on the order of 0.1°C) and compare it to the anomaly expected from the stationary internal tides derived from altimetry. Globally, there is a small, positive correlation between the expected and in situ signals. There is a stronger relationship in regions with more intense internal waves, as well as at depths near the nominal mode-1 maximum. However, we are unable to use this relationship to remove significant variance from the in situ observations. This is somewhat surprising, given that the magnitude of the altimetry-derived signal is often on a similar scale to the in situ signal, and points toward a greater importance of the nonstationary internal tides than previously assumed.

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Tyler D. Hennon, Stephen C. Riser, and Matthew H. Alford

Abstract

This study examines the global variability of the internal wave field near a depth of 1000 m using data from a set of 194 Argo floats equipped with Iridium communications, capable of measuring hourly temperature and pressure during the park phase of their 10-day cycles. These data have been used to estimate vertical isotherm displacements at hourly intervals, yielding a global measure of the heaving due to internal gravity waves. The displacement results have been employed to examine the global variability of these waves and how the displacement power spectrum compares to the canonical Garrett–Munk spectrum. Using the data, the authors find correlations between internal wave intensity and seafloor roughness, proximity to the seafloor, and the magnitude of the local barotropic velocity. The measurements also show large seamount-trapped waves at high latitudes and coastally trapped subinertial waves. These observations provide a rough global census of the nature of these waves that can ultimately be used in studies of ocean mixing.

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Arnaud Le Boyer, Matthew H. Alford, Robert Pinkel, Tyler D. Hennon, Yiing J. Yang, Dong Ko, and Jonathan Nash

Abstract

Despite sufficient wind forcing, internal waves in the South China Sea do not exhibit the strong near-inertial wave (NIW) peak that is typical in most of the world oceans. Using data from 10 contemporaneous moorings deployed in summer 2011, we show that strong isopycnal vertical tidal displacements transfer most of the near-inertial (NI) kinetic energy (KE) to frequencies higher than the inertial frequency in an Eulerian reference frame. Transforming to an isopycnal-following reference frame increases the KE at NI frequencies, suggesting the presence of NIWs. However, the projection onto a semi-Lagrangian coordinate system still underestimates the expected NI peak. To fully resolve NIWs requires the use of time-dependent vertical wavenumber–frequency spectra because the intrinsic frequency of the NIWs varies substantially, owing to Doppler shifting by lateral mesoscale flows. Here, we show NIW intrinsic frequency variations of ±0.2 cpd within few days, of similar magnitude as the observed variations of relative vorticity associated with the meandering Kuroshio.

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Arnaud Le Boyer, Matthew H. Alford, Nicole Couto, Michael Goldin, Sean Lastuka, Sara Goheen, San Nguyen, Andrew J. Lucas, and Tyler D. Hennon

Abstract

The Epsilometer (“epsi”) is a small (7cm diameter × 30cm long), low-power (0.15 W) and extremely modular microstructure package measuring thermal and kinetic energy dissipation rates, χ and ε. Both the shear probes and FP07 temperature sensors are fabricated in house following techniques developed by Michael Gregg at the Applied Physics Laboratory / University of Washington (APL/UW). Sampling 8 channels (2 shear, 2 temperature, 3-axis accelerometer and a spare for future sensors) at 24 bit precision and 325 Hz, the system can be deployed in standalone mode (battery power and recording to microSD cards) for deployment on autonomous vehicles, wave powered profilers, or it can be used with dropping body termed the “epsi-fish” for profiling from boats, autonomous surface craft or ships with electric fishing reels or other simple winches. The epsi-fish can also be used in real-time mode with the Scripps “fast CTD” winch for fully streaming, altimeter-equipped, line-powered rapid-repeating near-bottom shipboard profiles to 2200 m. Because this winch has a 25ft boom deployable outboard from the ship, contamination by ship wake is reduced 1-2 orders of magnitude in the upper 10-15 m. The noise floor of ε profiles from the epsi-fish is ~ 10−10 W kg−1. This paper describes the fabrication, electronics and characteristics of the system, and documents its performance compared to its predecessor, the APL/UW Modular Microstructure Profiler (MMP).

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