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Based on direct current measurements from two separated cruises in October 2008-January 2009 and July-August 2009, we obtained a valuable deep current observation of the Luzon Strait (LS). Rectified wavelet power spectra analysis (RWPSA) and the geostrophic current calculation are used to study the deep current. We find that the deep current differs in different seasons. The current is strongest in autumn (October-November) and weaker in summer (July-August) and in winter (December-January). 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. The observed seasonal difference of the deep current in the LS is connected with the deep current observed at the western boundary of the northern Philippine Basin and is also linked with the overflow near the central Bashi Channel and Luzon Trough. The RWPSA of the long observation suggests the dominant periods of 8 d, 19 d in the deep current. The dynamical cause of the resulting velocity distribution at 1850 and 1760 m is the pressure field and bottom topography steering. The observed deep current agrees well with the geostrophic current calculation.

The water exchange between the Pacific water and the South China Sea (SCS) through the Luzon Strait (LS) plays a very important role in the circulation of the SCS. Overall, the exchange can be categorized into three different layers, namely, the upper layer above 400 m, the middle layer between 500 and 1200 m, and the deep layer. In the upper layer above 400 m in the LS, a considerable number of studies have discussed the westward intrusion of the Kuroshio into the SCS through the LS (e.g., [^{6} m^{3}∙s^{−1}) [

Most previous current measurements in the LS used the shorter observational time periods (less than one month), except [

In terms of the analysis, the rectified wavelet power spectra analysis (RWPSA) was first proposed by [

Based on the ADCP deployed at the mooring station N2 (20˚40.441'N, 120˚38.324'E) of the LS in depths of 50 - 300 m between July 7, 2009 and April 10, 2011, and the TDs at around 340 and 365 m between July 9, 2009 and July 9, 2011, a longer period variation of the Kuroshio into the LS was investigated [

Based on a long mooring observation in the deep Bashi Channel and Luzon Trough, the deep-water overflows are estimated to be 0.83 ± 0.46 Sv and 0.88 ± 0.77 Sv at the Bashi Channel and the Luzon Trough, respectively [

The present study represents the first investigation of the deep current in the LS over a long period (a total of 122 d from two separated cruises) using a direct measurement. This long dataset is very valuable and unique, because it reveals the existence of seasonal difference of the deep current in the LS. The RWPSA also suggests that the dominant periods of variability are similar to some dominant periods observed in the upper ocean. Section 2 presents the characteristics of deep currents in the LS during the two cruises. Section 3 analyzes the dominant period for the time series using RWPSA. The direct measurement is also compared with the geostrophic current calculation to confirm the dynamical processes in Section 4. Finally, the conclusion is given in Section 5.

During the second cruise, the mooring station N2-2 was located at 20˚40.441'N, 120˚38.324'E, which is the same as the location of the mooring station N2 in [

During the first cruise, the observed velocity vectors were sampled at a time interval of 20 min.

January 2009 are (u, v) = (−2.8, 1.6), (−3.5, 1.2), (−0.4, 0.2), and (−0.9, 0.7) (unit: cm/s), during 15 days in October, all of November, December, and 8 days in January, respectively. The corresponding maximum values of the subtidal current velocity were (Vmax, θ) = (10.2, 271˚), (9.2, 277˚), (7.0, 286˚), and (9.9, 270˚) (unit: cm/s, in the clockwise direction from the north), respectively. In general, they were all westward. During the entire observation period, the averaged subtidal current velocity was (u, v) = (−2.0, 0.9) (unit: cm/s), which was northwestward. This suggested an overall tendency of the Pacific water intrusion northwestward into the SCS at 1850-m depth through the LS during the observation period of October 16, 2008 to January 8, 2009.

During the second cruise, the observed velocity vectors are sampled at a time interval of 30 min. The observed daily subtidal currents at the mooring station N2-2 (1760-m depth) are shown in

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 [

The seasonal difference of the deep current in the LS is also consistent with the deep overflow observation near the central Bashi Channel and Luzon Trough [

The RWPSA can effectively correct the distorted wavelet spectra in the standard wavelet analysis [

shown in the top right (log2 base). Two dominant periods are found in our study. 1) A period of approximately 8 d was found with the dimensionless PSD magnitude ~35. This period can also be found for the zonal velocity at the almost same location in the LS between depths of 50 and 200 m [

The 8-d period seems to be consistent with the variation observed in the upper 300 m. They suggested that the 8-d period in the upper ocean of the LS is related to both the clockwise and anticlockwise meanderings of the Kuroshio [

Based on the hydrographic data obtained from July 4 to 11, 2009 [

f v = 1 ρ 0 ∂ p ∂ x (1)

f u = − 1 ρ 0 ∂ p ∂ y (2)

The hydrostatic equations are

p = ρ 0 g ζ − g ∫ 0 z ( ρ − ρ 0 ) d z (3)

In Equations (1)-(3), u and v are the eastward and northward velocity components, respectively. A constant reference density ρ 0 = 1032.5 kg / m 3 , ρ and p are the density and pressure anomalies, respectively, Coriolis parameter f = 5.0 × 10 − 5 s − 1 , ζ is the sea surface elevation, and the other notations are standard. According to the dynamical method from [

ζ = − 1 ρ 0 ∫ − H 0 ( ρ − ρ 0 ) d z (4)

where H is accepted as a reference level, i.e., H = 2400 m, which is the deepest sill in the Bashi Channel [

p = − g ∫ − H z ( ρ − ρ 0 ) d z (5)

From the hydrographic data obtained during July 4 to 11, 2009 [

From Equations (1), (2), and (5), we can compute the pressure anomalies and, thus, the velocity vectors at 1850 and 1760 m in the LS in July 2009 (see Figures 4(a)-(d)).

Now, we consider using the reference level H = 2150 m, i.e., the water depth at two stations N1-1 and N1-2.

flow nearly northeastward at about 20˚30'N and then turn counterclockwise, flowing northwestward in the area 20˚35'N to 20˚45'N. In other words, the current velocity vectors flow nearly along the isobaths. We compare the geostrophic current calculation with the direct observed current velocity at the depth of 1760 m. At the depth of approximately 1760 m at station N2-2, the observed current velocity (V, θ) = (2.2 cm/s, 281.3˚) from July 7 to August 12, 2009; the computed current velocity (V, θ) = (3.0 cm/s, 298˚) for a reference level H = 2400 m, and (V, θ) = (3.0 cm/s, 275˚) for H = 2150 m in July 2009. This suggests that: 1) computed velocity vectors agree well with each other when the two different reference levels are used; and 2) regardless of the reference levels, the computed and observed results basically agree with each other. This shows that the estimated currents are similar if the reference level is changed to H = 2150 m (see

The above velocity vector distribution naturally can be explained from the pressure fields. From

Moreover, at the depth of 1850 m in July 2009, we compare the geostrophic current velocity station at N2-1 with that at station N2-2 to ensure data consistency between these two stations. The current velocity at station N2-1 is (V, θ) = (2.4 cm/s, 308˚), and the current velocity at station N2-2 is (V, θ) = (2.5 cm/s, 304˚). This similarity suggests that these two locations are close to each other. Similarly, very similar current velocities can be found at 1760 m during the same observational period—current velocity (V, θ) = (2.7 cm/s, 303˚) at station N2-1 versus (V, θ) = (3.0 cm/s, 298˚) at station N2-2. These results support the combination of data from these mooring stations in the analysis.

Finally, we compare the geostrophic current calculation with the direct observed current velocity at the depth of 1760 m. At this depth at station N2-2, the observed current velocity (V, θ) = (2.2 cm/s, 281.3˚) from July 7, 2009 to August 12, 2009, and the computed current velocity (V, θ) = (3.0 cm/s, 298˚) in July 2009. This shows that the computed and observed results basically agree with each other.

Based on the direct current measurements at mooring station N2-1 from October 2008 to January 2009 and at station N2-2 from July to August 2009, we present a very valuable dataset of deep current observations in the LS. The long period of observation (a total of 122 d from two separated cruises) is extremely unique. The hydrographic data obtained at station N2-2 from July 4 to 11, 2009 is also used to compute the velocity vector distribution at 1850 and 1760 m and verify the direct current measurements. Our results show a clear seasonal difference in the subtidal deep current velocity, ( u ¯ , v ¯ ) = (−2.8, 1.6), (−3.5, 1.2), (−0.4, 0.2), (−0.9, 0.7), and (−2.2, 0.4) (unit: cm/s) during the period from October 16 to 31; November and December, 2008; January 1 to 8, 2009; and July 7 to August 12, 2009, respectively. The current is strongest in autumn (October-November) and weaker in summer (July-August) and winter (December-January). 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 [

The long dataset provides us an opportunity to further analyze the dominant periods associated with the deep current in the LS using the RWPSA. The zonal velocity has dominant periods of 8 d and 19 d, and the meridional velocity has dominant periods of 8 d, 14 d, and 28 d. These periods with much weaker dimensionless PSD are consistent with the many dominant periods observed in the upper 300 m [

Furthermore, the geostrophic current calculation suggests that the deep currents flow nearly northward at about 20˚30'N and then turn counterclockwise, flowing northwestward in the area 20˚35'N to 20˚45'N. The currents are expected to flow nearly along the isobaths, for example, the current velocity vectors flow along the 3000-m isobaths toward the SCS. Thus, we conclude that the subtidal velocity distribution at 1850 and 1760 m results partially from the bottom topography effect.

The above velocity vector distribution can be naturally explained from the pressure fields. The positive west-east pressure gradient results in the northward current velocity, while the positive south-north pressure gradient results in the associated westward current. Thus, the dynamical cause of the resulting subtidal velocity distribution at 1850 and 1760 m is mainly due to the pressure field. Finally, the observed deep current also agrees well with the geostrophic current calculation, confirming that the strong geostrophic balance still holds for the deep current in the LS.

This study was supported by the National Key R & D Program of China (2017YFC1404200), the National Program on Global Change and Air-Sea Interaction (GASI-IPOVAI-04), the China Ocean Mineral Resources Research and Development Association Program (DY135-E2-3-02), and the National Nature Science Foundation of China (41830540). It was also funded by the Project of State Key Laboratory of Satellite Ocean Environment Dynamics (SOEDZZ1802, SOEDZZ1803 and SOEDZZ1805).

The authors declare no conflicts of interest regarding the publication of this paper.

Yuan, Y.C., Guan, W.B., Yang, C.H., Tseng, Y.-H. and Wang, H.Q. (2019) Seasonal Difference of the Deep Currents in the Luzon Strait during October 2008-January 2009 and July-August 2009. Atmospheric and Climate Sciences, 9, 284-297. https://doi.org/10.4236/acs.2019.93020