Poleward-propagating near-inertial waves enabled by the western boundary current

초록

Near-inertial waves (NIWs) exist everywhere in the ocean except at the equator; their frequencies arelargely determined by the local inertial frequency,f. It is thought that they supply about 25% of theenergy for global ocean mixing through turbulence resulting from their strong current shear andbreaking; this contributes mainly to upper-ocean mixing which is related to air-sea interaction, typhoongenesis, marine ecosystem, carbon cycle, and climate change. Observations and numerical simulationshave shown that the low-mode NIWs can travel many hundreds of kilometres from a source regiontoward the equator because the lower inertial frequency at lower latitudes allows their free propagation.Here, using observations and a numerical simulation, we demonstrate poleward propagation of typhoon-induced NIWs by a western boundary current, the Kuroshio. Negative relative vorticity, meaninganticyclonic rotational tendency opposite to the Earth’s spin, existing along the right-hand side of theKuroshio path, makes the local inertial frequency shift to a lower value, thereby trapping the waves. Thisnegative vorticity region works like a waveguide for NIW propagation, and the strong Kuroshio currentadvects the waves poleward with a speed ~85% of the local current. This finding emphasizes thatbackground currents such as the Kuroshio and the Gulf Stream play a significant role in redistribution ofthe NIW energy available for global ocean mixing.