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Observations of Mode-One Nonlinear Internal Waves (NLIW) of Opposite Polarity in Changing Background Conditions
The transformation of internal waves on continental shelves is important to mass transfer, nutrient replenishment, and heat transfer. Yet, the transfer of energy from larger to smaller scale or between nonlinear internal waves (NLIW) themselves remains poorly understood. We present 1 month of through water column observations of temperature and currents on the southeast continental shelf of the Bay of Biscay, a region where internal wave dynamics have never been described. Over the shallower part of the shelf, a relatively strong baroclinic dynamic exists, with the mode-1 internal tide and NLIW generating currents more than three times the barotropic tide. The nature of these features varied greatly over the subtidal timescales, which we correlate to wind-driven currents and the associated modulation of background stratification. In addition to the well-documented processes of internal tide steepening and NLIW polarity reversal, we present novel observations of colocated elevation and depression (termed opposite polarity) NLIW. While this colocation has been previously studied theoretically, it has not been described with in situ observations to date. In agreement with theory, we observed these waves when the wind-driven dynamics resulted in double pycnocline stratification. We found that the collocated waves of depression and elevation propagate independently on the upper and lower pycnocline, respectively. We use direct estimates of wave speed to infer the potential for interaction between waves of opposite polarity and discuss the potential relevance for other regions worldwide where double pycnocline background conditions are observed.
Key Points
Internal tides and nonlinear internal waves (NLIW) produce the strongest currents on the Aquitaine shelf
Coastal wind-driven upwelling alters the cross-shelf evolution of the NLIW
First field observations of coexisting mode-1 NLIW of opposite polarity under double-pycnocline conditions
Plain Language Summary
Internal waves vertically displace surfaces of constant density (isopycnals). They can propagate long distances and reach continental shelves. High-frequency internal waves (with periods of few minutes) are usually termed nonlinear internal waves (NLIW). Coastal internal waves are a crucial driver of mixing between different ocean layers and cross-shelf transport all over the world. NLIW can displace isopycnals downward (depression waves) or upward (elevation waves). Internal waves propagate in a changing background environment, which affects their characteristics and, therefore, changes the mixing and transport they drive. In this study, we used temperature and velocity measurements to characterize the coastal internal waves between a period of 15 min and 12.42 hr (the semidiurnal tidal period) on the Bay of Biscay shelf. First, measurements indicate that internal waves at this location dominate the hydrodynamic forcing compared with the barotropic tide. We observe different internal waves depending on the background conditions. NLIW can displace isopycnals downward (depression waves) or upward (elevation waves). We have observed the coexistence of depression and elevation waves for the first time.
Keyword(s)
coastal internal waves, Bay of Biscay shelf, nonlinear internal waves, internal tide, in situ observations
Full Text
File | Pages | Size | Access | |
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Publisher's official version | 25 | 8 Mo |