Geostrophic Vortex Merger and Streamer Development in the Ocean with Special Reference to the Merger of Kuroshio Warm Core Rings

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  • 1 Tohoku National Fisheries Research Institute, Shiogama-shi, Miyagi, Japan
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Abstract

A simple, quasigeostrophic, two-layer model was used to study the interactions of two like-signed vortices and streamer development in the ocean. In particular, models corresponding to the observed merger of two Kuroshio warm core rings were examined in detail. A quasigeostrophic shallow-water (QGSW) f-plane model reproduced many features of the observed merger; partial mergers were prevalent because critical point vortex breaking mostly determines QGSW mergers. The prevalence of partial baroclinic vortex mergers explain the frequent development of streamers from ring to ring. The QGSW critical merger distance is considerably reduced compared with the one for two-dimensional vortex mergers, especially in the parameter range where a large vortex wins. Moreover, the boundary of “which vortex wins” is more independent of vortex size for the QGSW case than in the 2D case, suggesting that a smaller vortex can more easily win and survive against a larger vortex than in the 2D case. This may explain the frequently observed northward development of warm streamers from newly generated large warm core rings toward older and smaller warm core rings, which had experienced winter and were located to the north of the new rings. Depending on the lower-layer vorticity distribution, upper-layer vortices undergo considerable meridional movement by heton-type coupling with lower-layer opposite-signed vortices. This movement might explain the tendency of Kuroshio warm core rings to move poleward. Possible modifications of vortex merger due to the planetary β effect were also discussed.

Abstract

A simple, quasigeostrophic, two-layer model was used to study the interactions of two like-signed vortices and streamer development in the ocean. In particular, models corresponding to the observed merger of two Kuroshio warm core rings were examined in detail. A quasigeostrophic shallow-water (QGSW) f-plane model reproduced many features of the observed merger; partial mergers were prevalent because critical point vortex breaking mostly determines QGSW mergers. The prevalence of partial baroclinic vortex mergers explain the frequent development of streamers from ring to ring. The QGSW critical merger distance is considerably reduced compared with the one for two-dimensional vortex mergers, especially in the parameter range where a large vortex wins. Moreover, the boundary of “which vortex wins” is more independent of vortex size for the QGSW case than in the 2D case, suggesting that a smaller vortex can more easily win and survive against a larger vortex than in the 2D case. This may explain the frequently observed northward development of warm streamers from newly generated large warm core rings toward older and smaller warm core rings, which had experienced winter and were located to the north of the new rings. Depending on the lower-layer vorticity distribution, upper-layer vortices undergo considerable meridional movement by heton-type coupling with lower-layer opposite-signed vortices. This movement might explain the tendency of Kuroshio warm core rings to move poleward. Possible modifications of vortex merger due to the planetary β effect were also discussed.

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