Currents in Luzon Strait Obtained from CTD and Argo Observations and a Diagnostic Model in October 2008

Abstract

On the basis of Acoustic Doppler Current Profiler (ADCP) measurements of current and hydrographic data obtained in October 2008, a diagnostic model of ocean circulation with a modified inverse method is used to study the circulation in Luzon Strait. Satellite-based geostrophic velocity calculated from the merged absolute dynamic topography is also used and compared with the in situ data. The analyses reveal that during the period of observations there were two branches of the Kuroshio current, and the maximum velocity of the current reached 175 cm s −1 at the surface. The main stream of the Kuroshio was located at 122°10 ′ E to 122°40 ′ E near the southern boundary of the study region and confined to the upper 700 m, while its western branch was located at 121°00 ′ E to 121°35 ′ E and confined to the upper 400 m. The Kuroshio water intruded into the South China Sea through the northern boundary of the study region east of 120°45 ′ E. At 1000 m depth, the flow was dominated by southwestward or westward flow in the area north of 20°30 ′ N, while the flow was mostly southeastward in the area south of 20°30 ′ N. A cyclonic eddy was identified in the region east of Luzon Strait. At 1500 m depth, cyclonic eddies were seen on both sides of Luzon Strait. The volume transport across the longitudinal section 120°45′ E between about 20.00°N and 21.20°N south of Taiwan during the period of observations was about 4 × 10 6 m 3 s −1 . One Argo float was tracked as it moved across Luzon Strait, reflecting the westward flow at both the surface and 1000 m depth.RÉSUMÉ [Traduit par la rédaction] En partant des mesures de courant par profileur de courant à effet Dopler (ADCP) et des données hydrographiques recueillies en octobre 2008, nous étudions la circulation dans le détroit de Luçon en nous servant d'un modèle diagnostique de la circulation de l'océan combiné à une méthode inverse modifiée. Nous nous servons également de la vélocité géostrophique mesurée par satellite, calculée à partir de la topographie dynamique absolue totale, pour la comparer avec les données in situ. Les analyses révèlent que pendant la période d'observation, il y avait deux ramifications du Kuroshio et que la vélocité maximale du courant a atteint 175 cm s −1 à la surface. La principale ramification du Kuroshio était située entre 122°10 ′ E et 122°40 ′ E à proximité de la limite sud de la région à l'étude et elle était confinée aux 700 premiers mètres; sa ramification ouest était située entre 121°00 ′ E et 121°35 ′ E et elle était confinée aux 400 premiers mètres. Le Kuroshio se déversait dans la mer de Chine occidentale en traversant la limite nord de la région à l'étude à l'est de 120°45 ′ E. À 1000 m de profondeur, le courant s'écoulait essentiellement en direction sud-ouest ou ouest dans le secteur au nord . Nous avons suivi une bouée Argo dérivant au gré du courant dans le détroit de Luçon, ce qui a permis d'établir qu'il s'écoulait en direction ouest à la surface et à une profondeur de 1000 m.

Seasonal and ENSO-related interannual variability of subsurface fronts separating West Philippine Sea waters from South China Sea waters near the Luzon Strait

“…At 121°E, the longitudinal range of high-salinity waters is 20.5-21.7°N. Model results and Argo trajectories also confirm that the intrusion is northwestward (Yuan et al, 2012a).…”

“…This suggests that, if the front were a wall, it would tilt towards the west. As is known, there is a net flow of upper waters of the WPS into the SCS, while there is a net outflow of intermediate from the SCS (Chen and Huang, 1996;Qu, 2002;Yuan et al, 2012aYuan et al, , 2012b. Why waters with a salinity of 34.15 are not encountered even as far east as 124°E in spring is unknown.…”

“…But in reality no pure SCS Tropical Water has been observed near the front. Since the WPS Tropical Water tends to intrude into the SCS (Qu, 2002;Qu et al, 2004;Wang et al, 2012;Yuan et al, , 2012aYuan et al, , 2012bYuan et al, , 2014aYuan et al, , 2014b, the following observation results and discussion are mainly concerned with the westward movement of these highsalinity waters. The longitudinal width of these waters in various seasons is compared at 121°E.…”

Variation in the Kuroshio intrusion: Modeling and interpretation of observations collected around the Luzon Strait from July 2009 to March 2011

“…A current mooring system was deployed at M site (Figure ) from March to April 2002, and both observed and modeled currents showed that the Kuroshio intrusion flowed into the SCS through the central LS [ Yuan et al ., ]. Based on the observed results from the Long Ranger 75 K and 150 K, ADCPs deployed at two mooring stations, M2 and M1 (see Figure ) from 25 April to 8 October 2008, the observed subtidal currents and the modeled currents both indicated that (1) a part of the Kuroshio intrusion flows northwestward through the upper layer at section 120°36′E between 20°20′N and 21°20′N in the LS during the spring 2008 [ Yuan et al ., ]; (2) the western branch of Kuroshio flows northwestward (or northward) through the upper layer at section 120°40′E between 20°20′N and 21°00′N in the LS during October 2008 [ Yuan et al ., ]. Based on these findings, the mooring station N2 site (Figure ) is scientifically chosen in the present study to investigate the Kuroshio intrusion.…”

“…Detailed seasonal dynamics of the Kuroshio intrusion were recently described using a three‐dimensional diagnostic model with modified inverse methods (MIM) [ Yuan et al ., , ; Liao et al ., ]. Numerical results from these investigations indicated that the northward Kuroshio current east of the LS extends downward to about 700 m, and a westward intrusion from the Kuroshio occurs above depths of 400 m in the LS [ Yuan et al ., , ; Liao et al ., ; Wang et al ., ].…”