Observations of Deep Current at the Western Boundary of the Northern Philippine Basin

Abstract

One-year time series of current velocities and hydrographic parameters based on four deep moorings deployed east of the Luzon Strait are employed to study the deep current at the western boundary (DCWB) of the northern Philippine Basin. While the mean current is relatively weak, the DCWB is highly variable on an intraseasonal time scale, with dominant periods ranging between 30 and 80 days. During the period of observation (October 2011–October 2012), the DCWB reversed its direction at early April, and pointed southward (−2.4 cm/s) in summer/autumn and northward (1.7 cm/s) in winter/spring. This annual reversal of the DCWB is consistent with the water property distribution in the deep Philippine Basin, with relatively cold and fresh water to the north and relatively warm and salty water to the south. The moored time series also allow for discussion on the stratification of the deep Luzon Strait, which indicates the lower interface of Pacific deep water capable of furnishing the deepwater overflow in the Luzon Strait.

Water‐Mass Properties and Circulation in the Deep and Abyssal Philippine Sea

“…As seen in Figure 5, water-mass properties in the upper layers, including the Western Pacific Warm Pool (θ > 28.00°C), the Subtropical Mode Water (potential vorticity < 2.2 × 10 −10 m −1 s −1 , 16.00°C < θ < 19.50°C), the South Pacific Tropical Water (S > 34.900), the North Pacific Tropical Water (S > 34.900), and the North Pacific Intermediate Water (S < 34.200), have been reviewed by Oka et al (2018). At depths below 1,500 m, the Philippine Sea is filled with the Mixed Deep Water (1.20°C < θ < 2.00°C) between ∼1,500 m and ∼4,000 m, and the LCDW (θ < 1.20°C) below 4,000 m. As for the Mixed Deep Water, water-mass shows colder/warmer and fresher/saltier characters in the high/low latitudes around 2000 m (Figure 5; Reid, 1997;Zhou et al, 2018). Given the fact that the high SiO 2 tongue at 2,000 m extends westward to ∼135°E (Reid, 1997;Talley & Joyce, 1992), the SiO 2 maximum core lying at 27°N, 2,000 m is related to the propagation of the Mixed Deep Water which comes from northeastern Pacific (Figures S2 and S3) and crosses the gaps on the Izu-Ogasawara Ridge (Kaneko et al, 2001…”

“…Considering it relatively colder, saltier, O 2 higher and SiO 2 lower properties than the northern one, they may have different origins. The results of anti-cyclonic circulation pattern and different origins of two maximum SiO 2 cores are consistent with analyses ofKaneko et al (2001) andKaneko et al (1998) but needs more observations to verify.Since notable variability of current has been indicated in the deep Pacific (e.g.,Yanagimoto et al, 2010;Zhou et al, 2018), analysis of the repeated observations along 137°E has substantially improved our understanding of the deep currents in the Philippine Sea. Accompanied by a spatial structure of the LCDW and deep currents applying inverse method, our analysis has revealed the existence of an abyssal current connecting the eastern and western parts of the Philippine Sea between 22.5°N-29.5°N at the Kyushu-Palau Ridge, transporting the LCDW from the Yap Mariana Junction westward into the northern Philippine Basin.…”

Seasonal Difference of the Deep Currents in the Luzon Strait during October 2008-January 2009 and July-August 2009

“…The cyclonic and anti-cyclonic meander with different subtidal current directions plays an important role in the seasonal difference of the deep current in the LS. Comparing the observed current velocities in the DCWB of the northern Philippine Basin [19] with the observed deep current in the LS, we believe that the seasonal difference of the deep current in the LS is connected with the deep current at the DCWB of the northern Philippine Basin [19] and is also connected with the overflow near the central Bashi Channel and Luzon Trough [15].…”

“…The seasonal difference of the deep current in the LS may be connected with the deep current observed at the western boundary (DCWB) of the northern Philippine Basin [19] and the deep overflow near the central Bashi Channel and Luzon Trough [15]. It was found that a similar seasonal variation in the DCWB in the northern Philippine Basin [19]. They pointed out that the DCWB reverses its direction in April, with a southward flow (−2.4 cm/s) in summer/autumn and a northward flow (1.7 cm/s) in winter/spring.…”

Accuracy and Long-Term Stability Assessment of Inductive Conductivity Cell Measurements on Argo Floats

“…Salinity in the northern part of the Philippine Sea is lower and more variable as compared to the area to the south (Figs. 11c,f) (Zhou et al 2018), and the northward trajectory of the Argo float resulted in gradual increase of the disagreement between the measurements and the references (Figs. 9a,b).…”