Blocked Drainpipes and Smoking Chimneys: Discovery of New Near-Inertial Wave Phenomena in Anticyclones

Abstract

Time-varying winds blowing over an eddying ocean generate near-​inertial waves (NIWs) that tend to be trapped in anticyclones. Such anticyclones have been termed inertial chimneys in the past but have recently been renamed inertial drainpipes, given their propensity to funnel NIW energy downward to the deep ocean. Here, we present evidence of a semi-blocked inertial drainpipe where downward-​propagating NIWs trapped in an anticyclone are partially reflected off the permanent pycnocline, returned toward the surface, and dissipated at the top of the seasonal pycnocline in a submesoscale filament of anticyclonic vorticity. Observations made on the northern rim of an anticyclone in the Iceland Basin include a high-​resolution survey of velocity, hydrography, and microstructure. Upward-propagating NIWs were observed in a salty, submesoscale filament of anticyclonic vorticity near the edge of the eddy, potentially trapped there. Above the filament and at the top of the seasonal pycnocline, turbulence was enhanced over what could be attributed to local winds and surface cooling. Ray tracing suggests the filament could have channeled and focused trapped upward-propagating NIWs, acting as an inertial chimney in a truer sense of the term, possibly intensifying the wave energy sufficiently to sustain the observed turbulence. Numerical simulations of NIWs in anticyclonic vorticity and stratification representative of the observations suggest that the upward-propagating NIWs could have been generated by a wind event 12 days prior and reflected off a sharp jump in stratification at the base of the anticyclone. Here, the transition between the weakly stratified winter mixed layer and the permanent pycnocline partially reflects downward-​propagating NIWs, limiting the inertial drainpipe effect.