Indirect evidence for substantial damping of low‐mode internal tides in the open ocean

Abstract

A global high-resolution ocean circulation model forced by atmospheric fields and the M 2 tidal constituent is used to explore plausible scenarios for the damping of low-mode internal tides. The plausibility of different damping scenarios is tested by comparing the modeled barotropic tides with TPXO8, a highly accurate satellite-altimetry-constrained tide model, and by comparing the modeled coherent baroclinic tide amplitudes against along-track altimetry. Five scenarios are tested: (1) a topographic internal wave drag, argued here to represent the breaking of unresolved high vertical modes, applied to the bottom flow (default configuration), (2) a wave drag applied to the barotropic flow, (3) absence of wave drag, (4) a substantial increase in quadratic bottom friction along the continental shelves (with wave drag turned off), and (5) application of wave drag to the barotropic flow at the same time that quadratic bottom friction is substantially increased along the shelves. Of the scenarios tested here, the default configuration (1) yields the most accurate tides. In all other scenarios (2-5), the lack of damping on open ocean baroclinic motions yields baroclinic tides that are too energetic and travel too far from their sources, despite the presence of a vigorous mesoscale eddy field which can scatter and decohere internal tides in the model. The barotropic tides are also less accurate in the absence of an open ocean damping on barotropic motions, that is, in scenarios (3) and (4). The results presented here suggest that low-mode internal tides experience substantial damping in the open ocean.

Frequency content of sea surface height variability from internal gravity waves to mesoscale eddies

“…The impacts of the wave drag in damping the barotropic and baroclinic tides are described in Ansong et al . [] and the impacts of the wave drag on the model barotropic and baroclinic tidal energy budget are described in Buijsman et al . [].…”

“…As expected and consistent with Ansong et al . [], the nonstationarity of the tidal signal increases as the record length increases. As a time saving measure, the global maps of HYCOM12 and HYCOM25 SSH variance are constructed from output subsampled at intervals.…”

“…Here we complement the literature on SSH variance in altimetry and tide gauges with an examination of SSH output in two new global simulations of the HYbrid Coordinate Ocean Model (HYCOM) [ Chassignet et al ., ]. The new HYCOM simulations are forced by both tidal and atmospheric fields [ Arbic et al ., ; Shriver et al ., ; Ansong et al ., ; Buijsman et al ., ; Ngodock et al ., ] and therefore have the potential to realistically simulate SSH variance on a global scale over periods from hours to years. As a hybrid coordinate model, HYCOM also has the potential to accurately model both coastal and open‐ocean sea level variance.…”

Semidiurnal internal tide energy fluxes and their variability in a Global Ocean Model and moored observations

“…[] showed that the barotropic and low‐mode baroclinic tides from simulations forced by tides and atmospheric fields compare more closely to satellite altimeter observations when a parameterized internal wave drag is applied to the bottom flow. In contrast to our previous HYCOM tide simulations [ Arbic et al ., ; Richman et al ., ; Shriver et al ., ; Timko et al ., ; Stammer et al ., ; Buijsman et al ., ; Müller et al ., ; Ansong et al ., ] which were run in purely forward (non‐data‐assimilative) mode, the simulations used here incorporate an Augmented State Ensemble Kalman filter (ASEnKF) [ Ngodock et al ., ] to reduce barotropic tidal SSH errors. The M 2 sea surface elevation error, computed over grid points equatorward of 66° and having seafloor depths exceeding 1000 m, of the HYCOM ASEnKF simulations used here is 2.6 cm [ Ngodock et al ., ].…”

“…[] and Ansong et al . [] compared the barotropic and internal tide sea surface elevation signals in HYCOM with those in altimeter‐constrained products. Timko et al .…”