Direct Observations of Near‐Inertial Wave ζ‐Refraction in a Dipole Vortex. Issue 21 (5th November 2020)

• Record Type: Journal Article
• Authors: Thomas, Leif N.; Rainville, Luc; Asselin, Olivier; Young, William R.; Girton, James; Whalen, Caitlin B.; Centurioni, Luca; Hormann, Verena
• Is Part Of: Geophysical research letters. Volume 47:Issue 21(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-11-05
• Subjects: wave‐mean flow interactions; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL090375
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Generated at large horizontal scales by winds, near‐inertial waves (NIWs) are inefficient at radiating energy without a shift to smaller wavelengths. The lateral scales of NIWs can be reduced by gradients in the Coriolis parameter ( β ‐refraction) or in the vertical vorticity ( ζ ‐refraction) or by strain. Here we present ship‐based surveys of NIWs in a dipole vortex in the Iceland Basin that show, for the first time, direct evidence of ζ ‐refraction. Differences in NIW phase across the dipole were observed to grow in time, generating a lateral wavelength that shrank at a rate consistent with ζ ‐refraction, reaching ∼40 km in 1.5 days. Two days later, a NIW beam with an ∼13 km horizontal and ∼200 m vertical wavelength was detected at depth radiating energy downward and toward the dipole's anticyclone. Strain, while significant in strength in the dipole, had little direct effect on the NIWs. Plain Language Summary: Winds blowing over the ocean generate waves, not just the familiar ones that ride along the surface, but another type which travels down into the deep sea. These so‐called internal waves can transmit a large amount of wind energy downward and thus are thought to play an important role in sustaining the deep branch of the ocean circulation. When these internal waves are formed, however, they are ineffective at transporting energy because their wavelengths are large, being set by the wind's ∼1, 000 km footprint on the ocean. In order for the waves to efficiently radiate energy downward, their wavelengths must shrink. Theory predicts that the interaction of these waves with ocean vortices can lead to a contraction in wavelength and enhanced energy transport. Here we describe observations made in a pair of counterrotating vortices in the Iceland Basin, which confirm this prediction for the first time in the ocean. Ship‐based measurements were used to track the evolution of internal waves generated by the passage of a storm. Within 2 days of their formation, the waves' wavelength shrank to ∼40 km. Two days later, the waves were observed traveling downward and into the clockwise‐swirling vortex, in line with theory. Key Points: We report the first observations of the reduction in lateral scale of near‐inertial waves by vorticity gradients As their scale shrinks, waves rapidly radiate downward and into regions with anticyclonic vorticity Strain in the background flow field had little direct effect on the evolution of the near‐inertial waves