Currents in the Luzon Strait obtained from moored ADCP observations and a diagnostic calculation of circulation in spring 2008

Based on current measurements at two mooring stations M2 from 25 April to 12 June, 2008 and M1 from 24 April to 11 June, CTD and wind data obtained in the Luzon Strait (LS), a three-dimensional diagnostic model with modified inverse method was used to study the regional circulation from April 23 to 26, 2008 (a La Nina year). The main results with the aid of diagnostic computation, T-S curve analysis and satellite geostrophic currents revealed the following: (1) Part of the Kuroshio flowed northwestward through the upper layer above 400 m of mooring stations M2 and M1 in the LS, respectively. (2) In the layer above 400 m the Kuroshio intrusion flowed northwestward through the longitudinal section at 120°53′E in the LS, and most continued to flow northwestward across the longitudinal section at 120°30′E. (3) In the northeastern region the circulation in the layer from 400 m to 1200 m was mainly dominated by the southeastward current, which came from the northern boundary at 21°17′N, instead of the Kuroshio. Finally, the southeastward current flowed eastward across the longitudinal section at 120°53′E in the LS into the Pacific in the layer from 400 m to 1200 m. (4) In the southern region the northward current with the South China Sea water flowed through the southern boundary from 20°00′N to 20°10′N, then it turned cyclonically and flowed westward across the southern part of longitudinal section at 120°30′E. (5) In comparison with the observed currents, the speeds of modeled currents at the 150, 200, 300 and 400 m of the computed point (120°30.54′E, 21°2′N) were 35.75, 25.50, 15.27 and 12.67 cm/s, respectively; the mean value of modeled currents at above four observed levels was 22.30 cm/s, while the mean of observed currents values at the 150, 200, 300 and 400 m levels of mooring station M2 (120°30.332′E, 20°59.961′N) were 33.08, 27.64, 13.18 and 11.67 cm/s, respectively; the mean value of observed currents at the above four observed levels was 21.39 cm/s. This signifies that the mean value of modeled currents in the upper layer above the 400 m levels was overestimated by 4.3%. (6) After comparison of the observations, the adjusted net westward modeled volume transports (VT) across the longitudinal section at 120°53′E were 4.66 and 2.66 × 106 m3 s−1, respectively, in the entire depth and the upper layer above 500 m levels of the LS during spring 2008. This indicates that the modeled VT of the westward Kuroshio intrusion across the upper layer of LS in during spring 2008 was much less than that in spring 1992 (an El Nino year) and normal years. That is to say, the westward intruding VT of Kuroshio across the upper layer of the LT may be closely related to the El Nino (or La Nina) phenomenon, being higher during El Nino and lower during La Nina, and it may be also related to the mesoscale variability in the LS.

► To study the inter-annual variability of the volume transport (VT) through the Luzon strait (LT). ► In comparison with the observed currents, the modeled currents in the upper layer were overestimated by 4.3%. ► The VT of Kuroshio across the LT is related to El Nino (or La Nina), being higher during El Nino and lower during La Nina. ► From the modeled currents, the vertical structure of the Luzon Strait transport possesses a sandwich structure.