Direct observations of the collapse in lateral scale and subsequent propagation of a near-inertial wave in a dipole vortex

Leif N Thomas, Stanford University, Stanford, CA, United States, Luc Rainville, University of Washington, Seattle, WA, United States, Craig Lee, Univ Washington, Seattle, WA, United States, Olivier Asselin, Scripps Institution of Oceanography, UC San Diego, United States, William R Young, University of California, La Jolla, CA, United States and James B Girton, Applied Physics Laboratory, University of Washington, Seattle, WA, United States
Generated at large lateral scales set by the wind's footprint on the ocean, near-inertial waves (NIWs) are inherently inefficient at radiating energy down to the abyss without a shift to smaller wavelengths. The lateral scales of NIWs can be reduced by gradients in the Coriolis parameter, f, or the vertical vorticity of the balanced flow through the beta-effect and refraction, respectively. While the beta-effect has been observed, no direct observational evidence of refraction has been reported in the literature. This is in spite of the fact that gradients in vorticity are typically much larger than beta, suggesting that refraction should be prevalent. Here we present observations from surveys from a cruise in the Icelandic Basin made as part of the Near-Inertial Shear and Kinetic Energy in the North Atlantic experiment (NISKINe) that show, for the first time, direct evidence of refraction. The surveys targeted the axial jet of a dipole vortex where the gradient in vorticity was ~100 times larger than beta and the horizontal strain was confluent. The surveys were made after the passage of an atmospheric front which accelerated a NIW in the upper 50 m. By rapidly sampling the velocity on either side of the jet, lateral differences in NIW phase could be estimated. NIW phase decreased more quickly on the cyclonic side of the jet, generating a lateral wavelength that shrank with time at a rate consistent with refraction, reaching ~30 km in 5 inertial periods. Shortly thereafter, a NIW beam with a ~30 km horizontal and ~300 m vertical wavelength was detected at depth. The beam appeared to radiate NIW energy away from the jet, down and towards the center of the anticyclone, evidencing the formation of an inertial drainpipe (a.k.a. an inertial chimney). Surprisingly, the NIW was not obviously affected by the confluent strain despite its persistence and O(0.1f) strength. Results from a companion theoretical study were used to interpret these findings and will be discussed.