[A-OS18] Physical Oceanography (General)

[AOS18-16] Short-term variability of the Kuroshio in response to an impinging anticyclonic eddy

*Ying-Chih Fang¹, Dai-Jyun Yu¹, Wei-Jou Huang², Wen-Chuan Wu¹, Yen-Huei Lin¹, Huan-Jie Shao³, Bing-Rong Jiang³ (1.Department of Oceanography, College of Marine Sciences, National Sun Yat-sen University, Kaohsiung, Taiwan, 2.Institute of Oceanography, National Taiwan University, Taipei, Taiwan, 3.New Ocean Researcher 3 Marine Instrument Center, National Sun Yat-sen University, Kaohsiung, Taiwan)

Keywords:Kuroshio, Mesoscale eddy, Impingement, Short-term variability

Impingement of mesoscale eddies onto the Kuroshio is a common feature of the North Pacific Gyre. Previous studies have shown that these anticyclonic (cyclonic) eddies have the potential to strengthen (weaken) the Kuroshio by sharpening (flattening) the horizontal isopycnal gradient. However, underwater variability of the Kuroshio during such eddy-impingement events has not been closely studied, especially over short periods of several days. This study analyzes shipboard hydrographic and velocity measurements from two ~160-km long transects along 21.8°N southeast of Taiwan, traversing the Kuroshio at the time of an impinging anticyclonic eddy (AE). The outgoing eastbound and incoming westbound transects are about 1.5 days apart, so the data provides a close view of a short-term change of the Kuroshio. The Ertel potential vorticity (PV) analysis using data collected in the eastbound transect shows a negative PV layer near depths where the AE is poised to collide with the Kuroshio. This negative PV physically indicates instability and exchange of material and is mainly the result of a horizontal velocity shear between the Kuroshio and AE. Intruding salinity is also observed in this negative PV water layer, supporting the notion that mixing between AE-sourced water and the Kuroshio is taking place. The westbound transect then indicates that the Kuroshio is substantially strengthened during the eddy impingement period. The easternmost extent of the Kuroshio also retreats ~10 km to the west. The Kuroshio in this westbound transect turns to the northeast with a flow speed increase of ~40%, reoriented from the largely northward direction in the eastbound one. The observed vertical flow shear also increases 90% over that of the eastbound transect. On the basis of the thermal wind relation and the observed vertical shear, this strengthening appears to occur when the AE is as near as 10 – 20 km to the Kuroshio.