Evolution of a Kuroshio warm-core ring—variability of the hydrographic structure

Abstract

The life history and structural variations of warm-core ring 86B, which detached from the Kuroshio Extension and moved northward interacting with neighboring currents over a life span of more than 3 years, were described by combining NOAA-infrared images and hydrographic data from 1986 to 1988. In the present study, interactions with neighboring currents and the atmosphere were found to strongly influence the evolution of 86B and to play a significant role in the northward transport of warm water. After the ring had detached from the main axis of Kuroshio meander, 86B was intensified by coalescence with a warm tongue extending from the Kuroshio Extension. Just before the detachment of 86B, another warm-core ring, 86A, was pushed northward as the Kuroshio meander developed. During the formation of 86B, warm water was carried northward as warm streamers through the warm tongue, 86B and 86A. Ring 86B moved intermittently northward. These northward movements were associated with the interaction with neighboring currents; they began as intrusions of warm streamers from the Kuroshio frontal disturbances or other warm-core rings and by coincidental deformation to an elliptical shape with a meridional major axis. Then 86B moved northeastward, rotating clockwise. The northward movement stopped when the supply of warm water ended, 86B then moved westward, returning to a circular shape. During the winter, when distinct warm-streamer intrusions were not observed, westward movements were dominant. Drastic structural variations after formation were brought about by wintertime convection and ring-ring interaction. During winter, not only the core temperature but also the salinity decreased and the diameter was reduced, suggesting an inward flow and mixing with Oyashio low-salinity water through wintertime convection. It was also observed that the negative relative vorticity increased, associated with the reduction of the horizontal scale and the deepening of the homogeneous layer. The ring-ring interaction occurred between 86B and the larger, westward-moving warm-core ring (OR) from offshore. A large amount of water from ring OR was transported northward and encircled 86B, conserving its vertical structure. As a result, 86B expanded and the volume of core water more than doubled, while OR decreased in size. The two rings had similar homogeneous waters and densities, and the interaction occurred between a stronger and smaller vortex and a weaker and larger one. Other developments of warm streamers were frequently observed through ring-ring or ring-Kuroshio interactions; these were related to the deserved near-surface structural changes and ring movements.