Summer Kuroshio Intrusion through the Luzon Strait confirmed from observations and a diagnostic model in summer 2009

•Summer Kuroshio Intrusion across Luzon Strait is firstly confirmed in summer 2009.•Modeled result agrees reasonably with the Argo float and Mooring observations.•Kuroshio intrusion in summer 2009 was resulted by weak upstream Kuroshio due to ENSO.•Kuroshio intrusion in October 2008 was resulted by the seasonal monsoon winds.

Based on current measurements recorded at Mooring Station N2 (20°40.441′N, 120°38.324′E), hydrographic data and Argo observations in the period starting from July 2009, a diagnostic model with a modified inverse method is used to study the circulation in the Luzon Strait (LS). A number of new circulation features in the LS are found as follows. (1) Both observed and modeled currents show that the intruded Kuroshio flow northwestward through the LS into the South China Sea (SCS) in the upper 400 m during July 2009. (2) The diagnostic model confirms that the Kuroshio is located in the area east of 121°20′E and west of 122°20′E at 20°00′N. There is a meso-scale cyclonic eddy at both the surface and 1000 m depth in the area west of the Kuroshio near 20°00′N. The meso-scale cyclonic eddy forced the Argo float 1 to make the cyclonic trajectory. Then, the Argo float 1 went into the area of the western Kuroshio. The Argo float 1 was tracked as the flow moved northwestward into the SCS across the LS in July and August, 2009, reflecting the northwestward flow at both the surface and 1000 m depth, which coincided with modeled currents. These results confirm the northwestward Kuroshio intrusion into the SCS across the LS in summer 2009 for the first time. (3) From the dimensional analysis for the equation of stream function, it is seen that the joint effect of the baroclinity and relief (JEBAR) and the β-effect are two important mechanisms on the Kuroshio intrusion into the SCS in this period. This summer Kuroshio intrusion results from the weaker upstream Kuroshio transport in summer 2009 (El Niño initiating period) due to inertia effects and is associated with a weak volume transport across the LS (2.15 × 106 m3 s−1 westward). (4) After comparison of the dynamics of the Kuroshio intrusion during October 2008 and summer 2009, it is clear that these were influenced by the seasonal variability due to the monsoon winds and the interannual variation resulting from the weak upstream Kuroshio transport affected by the ENSO due to inertia effects.