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.