Observed interannual variability of the Kuroshio and Luzon Undercurrent associated with tropical and subtropical wind forcing

The interannual variability of the Kuroshio and Luzon Undercurrent (LUC) east of the Luzon Island is examined using observations from a moorings during December 2010-October 2012 and three moorings during January 2018-May 2020 deployed at 18 degrees N. Measurements from acoustic Doppler current profiler (ADCP) equipped on the moorings indicate that the interannual variations of upper-layer Kuroshio and lower-layer LUC were in phase. The interannual variability is strong in the Kuroshio but weakens substantially in the LUC. A proxy of the Kuroshio and LUC based on sea level slope is developed and validated by comparing the ADCP and satellite altimeter measurements. The proxy during 1993-2020 shows that the interannual sea level slope across the Kuroshio is primarily associated with sea level fluctuations on its eastern flank. Diagnostic analysis and nu-merical sensitivity experiments using a simplified ocean model indicate that the wind forcing in the subtropical northwestern Pacific Ocean played an essential role in the interannual variability of Kuroshio and LUC's strength and water mass properties during the observation periods in 2011 and 2018-2020, although the tropical winds dominated the interannual variability of the two boundary currents during most time of the past three decades (1993-2020). The mooring observations show that the subsurface water mass in the LUC at the depth of about 650-800 m experienced a considerable freshening event from July 2019 until at least January 2020 due to abnormally strong subtropical wind forcing.

Automated summary

The interannual variability of the Kuroshio and Luzon Undercurrent (LUC) east of the Luzon Island is examined using observations from a moorings during December 2010-October 2012 and three moorings during January 2018-May 2020 deployed at 18 degrees N. Measurements from acoustic Doppler current profiler (ADCP) equipped on the moorings indicate that the interannual variations of upper-layer Kuroshio and lower-layer LUC were in phase. The interannual variability is strong in the Kuroshio but weakens substantially in the LUC. A proxy of the Kuroshio and LUC based on sea level slope is developed and validated by comparing the ADCP and satellite altimeter measurements. The proxy during 1993-2020 shows that the interannual sea level slope across the Kuroshio is primarily associated with sea level fluctuations on its eastern flank. Diagnostic analysis and nu-merical sensitivity experiments using a simplified ocean model indicate that the wind forcing in the subtropical northwestern Pacific Ocean played an essential role in the interannual variability of Kuroshio and LUC's strength and water mass properties during the observation periods in 2011 and 2018-2020, although the tropical winds dominated the interannual variability of the two boundary currents during most time of the past three decades (1993-2020). The mooring observations show that the subsurface water mass in the LUC at the depth of about 650-800 m experienced a considerable freshening event from July 2019 until at least January 2020 due to abnormally strong subtropical wind forcing.