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Density Field along 12°N and 13°N in the Philippine Sea
Deep Ocean Circulation, Physical and Chemical Aspects edited by T. Teramoto 0 1993 Elsevier Science Publishers B.V. All rights reserved.
39
Density Field along 12"N and 13"N in the Philippine Sea Katsuto UEHARA, Keisuke TAIRA and Akira MASUDA*
Abstract
Hydrographic casts down to the bottom along two zonal sections at 12'N and 13'N (from 144'E to 127'E) were made with a CTD. Their analysis verified the existence of cold and saline abyssal water between the Mariana Ridge and the Kyushu-Palau Ridge. This result provides evidence of flow into the Philippine Sea through the gap called the Yap-Mariana Junction. The properties of deep water are variable in the West Mariana basin but quite homogeneous in the Philippine Basin, indicating the transitional nature in the West Mariana Basin and the existence of older bottom water in the Philippine Basin.
1.
Introduction
The Philippine Sea is the sea which locates to the south of Japan and comprises the westernmost part of the North Pacific Ocean (Fig. 1). For the depth below 4000 m, the Izu-Ogasawara and Mariana Ridges separate the Philippine Sea from the main Pacific Ocean and restricts the horizontal exchange of abyssal water except through the several narrow gaps. In reality, the map of the horizontal distribution of temperature, salinity, and dissolved oxygen compiled by Moriyasu (1972) using CSK data shows an apparent difference between the Philippine Sea and the main Pacific Ocean for the levels below 3000 m. In the North Pacific Ocean, including the Philippine Sea, the surface water cannot be dense enough t o form an existing abyssal water (Warren, 1983) and the abyssal water characterized by low salinity and high oxygen content are considered t o be supplied from the south (Stommel and Arons, 1960; Warren, 1981). Accordingly, abyssal water of the Philippine Sea must be supplied from outside. Additional data, including those obtained from the Indopac Expedition, have provided some information on this subject. The distribution of water characteristics shown by Mantyla and Reid (1983) and by Kaneko and Teramoto (1985) suggest a flow of bottom water into the Philippine Sea through the gap to the southwest of Guam, which is called the Yap-Mariana Junction. However, Mantyla and Reid (1983) used hydrographic data at the bottom layer only and the depth is different from station t o station while Kaneko and Teramoto (1985) analyzed the data mainly in upper 3000 m. The inflow through the Yap-Mariana Junction is not fully delineated. *Ocean Research Institute, University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164 Japan
40 In order t o detect this inflow more clearly, hydrographic observations were carried out in the southern Philippine Sea along 12"N and 13"N, from 144"E t o 127"E during R/V Hakuho-Maru Cruise KH-87-1 leg 2 of 8-17 February 1987. In this manuscript, we made an analysis of the CTD dataset obtained from this cruise. Emphasis is placed on depicting the hydrographic structure of abyssal and bottom layers in the southern Philippine Sea. The existence of distinctly cold, saline and dense bottom water in the West Mariana Basin is shown in cross sections of 12"N and 13"N. We also examined the vertical profile of each station for the depth below 3000 db. We found homogeneous distribution in the Philippine Basin and rather variable structures in the West Mariana Basin. Also, in the West Mariana Basin, water tends t o get warmer, less saline, and less dense as it gets farther from the Yap-Mariana Junction. These results provide strong evidence for the inflow of cold, saline, and dense bottom water through the Yap-Mariana Junction into the West Mariana Basin. In the
N -
30"
20"
10"
120"
130"
140"
E
0"
150"
Fig. 1. Topography around the Philippine Sea and the hydrographic stations of the Hakuho Maru Cruise KH-87-1 leg 2. Numerals represent station numbers.
41 30.
25.
20.
W
a
13
?
15.
W
Q
2
W
t-
J
2
10.
2
W
-t
2
5.
0.
34.2 34.4
34.6 34.8
35.0 35.2
SALINITY(PSU)
Fig. 2. T-S diagram of the CTD data. Temperature is shown in potential temperature referred t o 0 db pressure. Isopycnal lines of potential density referred t o 0 d b is shown as contour lines. Philippine Basin, the bottom water was slightly colder and denser in the western part than in the eastern part. This may be indicating the existence of a broad western boundary current toward the south along the eastern rise of the Philippine Trench.
2. 2.1.
0bservat ion Measurement and data analysis
The topography of the Philippine Sea and the location of the hydrographic stations are shown in Fig. 1. The Izu-Ogasawara Ridge and the Mariana Ridge
42 separate the Philippine Sea from the North Pacific Ocean at a depth greater than 3000 m. The Kyushu-Palau Ridge along 135"E divides the Philippine Sea into the Philippine Basin and the West Mariana Basin. There is a gap with a sill depth of 5000 m t o the south of Guam called the Yap-Mariana Junction, which connects the Philippine Sea with the North Pacific Ocean. Hydrographic stations were occupied with a Neil Brown Mark IIIb C T D and a Rosette multi-sampler. At each station, CTD d a t a were recorded t o the bottom layer of 3000-6000 d b and water sampling was made at 1 2 levels. A zigzag course was adopted to examine the hydrographic structure at both 12"N and 13"N. Dense CTD casts were made around 135"E to examine possible effects of the KyushuPalau Ridge on the bottom water distribution. Calibration of CTD d a t a were made by referring the d a t a obtained from sampled water and reversing thermometer to reduce the systematic error of the C T D sensor. Afterwards, the d a t a are recompiled into 2 d b interval data. For station 209 (below 4000 db) and station 222, we could not get enough number of valid d a t a for station 209 (below 4000 db) and station 222, so they are not included in the dataset. Estimated error of the dataset is less than 0.01"C for temperature and 0.0037 psu (practical salinity unit) for salinity.
2.2.
Results
Surface layers In the surface layer of upper 200 db, strong baroclinic feature is found. Geostrophic calculation shows a surface confined westward flow with intensity exceeding 20 cm/s (not shown), which indicates the North Equatorial Current (e.g., Toole et al., 1990). There is a salinity maximum exceeding 35.0 psu at about 110-130 d b in depth (Fig. 2). Beneath this thin layer, there is a salinity minimum a t the depth of 600-800 d b or around 26.5 in sigma-theta (potential density referred to 0 pressure). In the succeeding section, we will deal with the deep and bottom layers below the salinity minimum.
Cross sections of 12"N and 13"N Figures 3(a), (b), and (c) show the cross-sectional distributions along 12"N of potential temperature theta-4 referred to 4000 db, salinity S, and potential density sigma-5 referred to 5000 db, respectively. Figures 4(a), (b), and (c) show the crosssectional distributions along 13"N. Above 3500 db, which is roughly the depth of the Kyushu-Palau Ridge, no apparent difference can be seen between the water properties in the West Mariana Basin and those in the Philippine Basin. Obvious differences, however, appear below 3500 db; the most prominent feature is the marked contrast of water properties between both sides of the Kyushu-Palau Ridge around 135"E. Colder, more saline, and denser abyssal water exists in the West Mariana Basin (eastern side of the ridge), whereas it is absent in the Philippine Basin (western side of the ridge). Vertical and horizontal distributions are highly variable in the West Mariana Basin. This is an indication that the West Mariana Basin is nearer t o the source of abyssal water. Dissolved oxygen content measured
43
-4--
3000
c
4000
5000
6000
1
125"
LONGITUDE
u5
130"
135' LONGITUDE
140'
E
(12"N)
1000
2000
125'
130'
135" LONGITUDE
140'
E
Fig. 3. The distributions along 12"N cross-section: (a) potential temperature theta-4 (referred t o 4000 db), (b) salinity S, and (c) potential density sigma-5 (referred t o 5000 db). The numerals at the top denote the station numbers. from sampled water shows higher values at the bottom layer in the West Mariana Basin (Watanabe et al., 1988), indicating that abyssal water is relatively young there. In the Philippine Basin, water of slightly lower temperature and large density is found in the western bottom layer as indicated by isotherms of 1.565"C and 1.57"C in Figs. 3(a) and 4(a) along both 12"N and 13"N. This water may spread over few
44
---
-
3000
-
a
__--
E
r
2
3000
D
D
4000
2 4000
5000
5000
nil-----
6000
6000 125'
~
130"
135'
E
140'
125"
LONGITUDE
130"
135"
LONGITUDE
140'
E
u5 (13'N)
---
3000
Iz( 9
P
4000
5000
6000
125'
130"
135'
LONGITUDE
140"
E
Fig. 4. The same as Fig. 3 except along 13"N hundred kilometers, although the westernmost part of the basin was not observed. It is not clear whether this water is significantly colder, because the temperature difference is less than 0.005"C which is as large as half the measurement error. T h e bottom water in the western Philippine basin has properties closer to the bottom water in the West Mariana Basin. These features indicate the existence of a broad and weak southward flow in the western part of the Philippine Basin.
45 Vertical profile of all CTD stations Figures 5(a), (b) show the vertical profiles of theta-4, S, and sigma-5 for depth below 3000 db, respectively. The profiles are drawn with different types of lines according t o the following geographical classification: 3000
3500
a: 3 0)
4500
0 W
a: a
5000
5500
Fig. 5. The vertical distributions of (a) theta-4, (b) S, and (c) sigma5 below 3000 db: - -, on the south-eastern flank of the Mariana Ridge; - _ - , in the West Mariana basin; - - -,in the eastern part of the Philippine Basin; and - in the western part of the Philippine Basin. An error bar is inserted in each figure.
46
I44E-127E
P0
TE
NT I A
Fig. 5 .
L D E N S I T Y ( k g Irn
1 )
(continued)
(1) the southeastern flank of the Mariana Ridge (stations 202 and 204; outside of the Philippine Sea), (2) near the shallowest portion of the Mariana Ridge (stations 201 and 2031, ( 3 ) the West Mariana Basin (stations 205-210), (4) near the Kyushu-Palau Ridge (stations 211-213, 008, and Oll), which is about 3500 m deep, ( 5 ) eastern part of the Philippine Basin (stations 214-219), (6) western part of the Philippine Basin (stations 220-223).
For stations 201 and 203, profiles are not included in the figures because depths are shallower than 2000 db. The profiles for stations 222 and 209 (below 4000 db) are not available because the raw data are erroneous. Above 4000 db, deep water southeast of the West Mariana Ridge (stations 202 and 204) is the coldest and most saline when compared at the same depth. The water retains the characteristics of the abyssal water from the south. The vertical profiles of region (1) are distinguished from those inside the Philippine Sea (regions 3 t o 6) at depths shallower than 4000 db. In the Philippine Sea, the profiles for stations in the West Mariana Basin (regions 3 and 4) are separated from those in the Philippine Basin (regions 5 and 6) below 3500 db. The depth corresponds t o the sill depth of the Kyushu-Palau Ridge, which divides the Philippine Sea into the two basins. The water in the Philippine Basin is remarkably homogeneous below 4000 db. It is warmer, less saline, and less dense compared t o the water in the West Mariana Basin. This implies that the Philippine Basin is far from the source of abyssal water. On the other hand,
47 2.0
1.5
1.0
0.5
0.0
- 0.5 Depth (db)
1000
2000
3000
4060
5000
6000
Depth ( d b )
Fig. 6. The vertical distributions of stability for the stations (a) 205210, (b) 214-223 for the depth below 1000 db, respectively. Stability is defined in terms of the Brunt-V ais a1 a frequency (cph). Five adjacent data (10 d b in depth) were fitted t o calculate a density gradient. Positive value represents the stable state, i.e., the density gets denser as the depth increases.
48 the water in the West Mariana Basin has a large variation within a relatively small zonal scale. In the West Mariana Basin, water properties are changing rapidly from those in the North Pacific Ocean to those in the Philippine Basin. It is to be noted that station 206 rather than 205, has a vertical distribution similar t o those at stations 202 and 204 in the western North Pacific Ocean. This is most clear for salinity profiles as shown in Fig. 5(b). Station 206 is nearest to the Yap-Mariana Junction which is located at around 11"N and 139"E (Fig. 1). Accordingly, we can state that abyssal water flows into the Philippine Sea through the Yap-Mariana Junction. When we compare the vertical profiles for regions (5) and (6) in the Philippine Basin, rather small though systematic difference can be found in the temperature and density profiles. The western side of the Philippine Basin is slightly colder and denser than the eastern side as discussed in the previous section.
3. Summary and Discussion CTD measurements were made down to the bottom along 12"N and 13"N in the Philippine Sea. The most important result is that the existence of cold and saline water was verified in the deep region of the West Mariana Basin below 3500 db. Water of these properties has been inferred t o enter the Philippine Sea through the Yap-Mariana Junction in previous data analyses by Mantyla and Reid (1983) and Kaneko (1984). At station 206, which is the station nearest to the Yap-Mariana Junction, we observed the coldest, most saline, and densest bottom water in the Philippine Sea. The marked contrast of water properties between the West Mariana Basin and the Philippine Basin can be ascertained from the instrumental accuracy. Also both the horizontal and vertical gradients are compared between the east and west sides of the Kyushu-Palau Ridge. The deep water has a variable structure in the West Mariana Basin, but is homogeneous in the Philippine Basin. We may say, therefore, that the present result gives evidence of flow into the Philippine Sea through the Yap-Mariana Junction. The vertical profiles of stability of the water in the West Mariana Basin and the Philippine Basin for the depth below 1000 d b are shown in Figs. 6(a) and (b), respectively. In the Philippine basin, the stability is almost neutral below the depth of 4200 db. This suggests that although the difference of the water properties between the West Mariana Basin and the Philippine Basin become apparent from 3500 d b (Fig. 5a and b), there may be a gap at the Kyushu-Palau Ridge with a depth of 4200 d b and the water underneath retains the characteristic of the sill depth. In the West Mariana Basin, on the other hand, we can see some signals showing the variable nature of water properties: the variations of the stability between the stations are large compared with those in the Philippine Basin, and the stability does not come to zero value (i.e., there are potential density gradients at most depths) except for the depth below 5000 d b at station 206. It is not clear from our dataset whether the homogeneous distribution of density below 5000 d b reflects the sill depth of the Yap-Mariana Junction because there is only one station which has d a t a deeper than 5000 d b in the West Mariana Basin. The water density in the western part of the Philippine Basin increases slightly
t o the west (Figs. 3, 4 and 5 ) . This might be interpreted as a broad western boundary current flowing equatorward. It is impossible t o substantiate this speculation at present. At least, t h e variation is quite small, beyond t h e present precision of data.
Acknowledgement We would like t o thank Prof. T. Teramoto, Dr. M. Kawabe, a n d Mr. S. Kitagawa for their support and discussions during t h e study. We also thank the crews a n d t h e participants of t h e cruise of t h e R/V Hakuho Maru.
References Kaneko, I., 1984. Structure of mid-depth water in the Philippine Sea. Ph. D. Thesis, University of Tokyo, Tokyo, 97 pp. Kaneko, I., and T. Teramoto, 1985. Sea water exchange between the Shikoku-Philippine Basin and the western North Pacific Basin. In: Ocean Characteristics and their Changes, K. Kajiura (ed.), Koseisha-Koseikaku, pp. 54-77. Mantyla, A. W., and J. L. Reid, 1983. Abyssal characteristics of the World Ocean Waters. Deep-sea Res., 30, 805-833. Moriyasu, S., 1972. Deep waters in the western North Pacific. In: Kuroshio - Its Physical Aspects, H. Stommel and K. Yoshida (ed.), Univ. of Tokyo Press, Tokyo, pp. 387-408. Stommel, H., and A. B. Arons, 1960. On the abyssal circulation of a world ocean - 11. An idealized model of the circulation pattern and amplitude in oceanic basins. Deep-sea Res., 6, 217-244. Teramoto, T., and K. Taira, 1988. Preliminary Report of The Hakuho Maru Cruise KH-87-1, Ocean Research Institute, University of Tokyo, 38 pp. Toole, J. M., R. C . Millard, Z. Wang, and S. Pu, 1990. Observations of the Pacific North Equatorial Current Bifurcation at the Philippine Coast. J. Phys. Oceanogr., 20, 307-318. Warren, B. A., 1981. Deep circulation of the world ocean. In: Evolution of Physical Oceanography, B. A. Warren and C. Wunsch (ed.), MIT Press, Cambridge, pp. 6-41. Warren, B. A., 1983. Why is no deep water formed in the North Pacific? J. Mar. Res., 41, 327-347. Watanabe, S., S. Noriki, C. Saitoh, and Y . Watanabe, 1988. Distribution of chemical tracers in the Philippine Sea. In: Preliminary Report of The Hakuho Maru Cruise KH-87-1, T. Teramoto and K. Taira (ed.), Ocean Research Institute, University of Tokyo, pp. 30-34.
39
Density Field along 12"N and 13"N in the Philippine Sea Katsuto UEHARA, Keisuke TAIRA and Akira MASUDA*
Abstract
Hydrographic casts down to the bottom along two zonal sections at 12'N and 13'N (from 144'E to 127'E) were made with a CTD. Their analysis verified the existence of cold and saline abyssal water between the Mariana Ridge and the Kyushu-Palau Ridge. This result provides evidence of flow into the Philippine Sea through the gap called the Yap-Mariana Junction. The properties of deep water are variable in the West Mariana basin but quite homogeneous in the Philippine Basin, indicating the transitional nature in the West Mariana Basin and the existence of older bottom water in the Philippine Basin.
1.
Introduction
The Philippine Sea is the sea which locates to the south of Japan and comprises the westernmost part of the North Pacific Ocean (Fig. 1). For the depth below 4000 m, the Izu-Ogasawara and Mariana Ridges separate the Philippine Sea from the main Pacific Ocean and restricts the horizontal exchange of abyssal water except through the several narrow gaps. In reality, the map of the horizontal distribution of temperature, salinity, and dissolved oxygen compiled by Moriyasu (1972) using CSK data shows an apparent difference between the Philippine Sea and the main Pacific Ocean for the levels below 3000 m. In the North Pacific Ocean, including the Philippine Sea, the surface water cannot be dense enough t o form an existing abyssal water (Warren, 1983) and the abyssal water characterized by low salinity and high oxygen content are considered t o be supplied from the south (Stommel and Arons, 1960; Warren, 1981). Accordingly, abyssal water of the Philippine Sea must be supplied from outside. Additional data, including those obtained from the Indopac Expedition, have provided some information on this subject. The distribution of water characteristics shown by Mantyla and Reid (1983) and by Kaneko and Teramoto (1985) suggest a flow of bottom water into the Philippine Sea through the gap to the southwest of Guam, which is called the Yap-Mariana Junction. However, Mantyla and Reid (1983) used hydrographic data at the bottom layer only and the depth is different from station t o station while Kaneko and Teramoto (1985) analyzed the data mainly in upper 3000 m. The inflow through the Yap-Mariana Junction is not fully delineated. *Ocean Research Institute, University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164 Japan
40 In order t o detect this inflow more clearly, hydrographic observations were carried out in the southern Philippine Sea along 12"N and 13"N, from 144"E t o 127"E during R/V Hakuho-Maru Cruise KH-87-1 leg 2 of 8-17 February 1987. In this manuscript, we made an analysis of the CTD dataset obtained from this cruise. Emphasis is placed on depicting the hydrographic structure of abyssal and bottom layers in the southern Philippine Sea. The existence of distinctly cold, saline and dense bottom water in the West Mariana Basin is shown in cross sections of 12"N and 13"N. We also examined the vertical profile of each station for the depth below 3000 db. We found homogeneous distribution in the Philippine Basin and rather variable structures in the West Mariana Basin. Also, in the West Mariana Basin, water tends t o get warmer, less saline, and less dense as it gets farther from the Yap-Mariana Junction. These results provide strong evidence for the inflow of cold, saline, and dense bottom water through the Yap-Mariana Junction into the West Mariana Basin. In the
N -
30"
20"
10"
120"
130"
140"
E
0"
150"
Fig. 1. Topography around the Philippine Sea and the hydrographic stations of the Hakuho Maru Cruise KH-87-1 leg 2. Numerals represent station numbers.
41 30.
25.
20.
W
a
13
?
15.
W
Q
2
W
t-
J
2
10.
2
W
-t
2
5.
0.
34.2 34.4
34.6 34.8
35.0 35.2
SALINITY(PSU)
Fig. 2. T-S diagram of the CTD data. Temperature is shown in potential temperature referred t o 0 db pressure. Isopycnal lines of potential density referred t o 0 d b is shown as contour lines. Philippine Basin, the bottom water was slightly colder and denser in the western part than in the eastern part. This may be indicating the existence of a broad western boundary current toward the south along the eastern rise of the Philippine Trench.
2. 2.1.
0bservat ion Measurement and data analysis
The topography of the Philippine Sea and the location of the hydrographic stations are shown in Fig. 1. The Izu-Ogasawara Ridge and the Mariana Ridge
42 separate the Philippine Sea from the North Pacific Ocean at a depth greater than 3000 m. The Kyushu-Palau Ridge along 135"E divides the Philippine Sea into the Philippine Basin and the West Mariana Basin. There is a gap with a sill depth of 5000 m t o the south of Guam called the Yap-Mariana Junction, which connects the Philippine Sea with the North Pacific Ocean. Hydrographic stations were occupied with a Neil Brown Mark IIIb C T D and a Rosette multi-sampler. At each station, CTD d a t a were recorded t o the bottom layer of 3000-6000 d b and water sampling was made at 1 2 levels. A zigzag course was adopted to examine the hydrographic structure at both 12"N and 13"N. Dense CTD casts were made around 135"E to examine possible effects of the KyushuPalau Ridge on the bottom water distribution. Calibration of CTD d a t a were made by referring the d a t a obtained from sampled water and reversing thermometer to reduce the systematic error of the C T D sensor. Afterwards, the d a t a are recompiled into 2 d b interval data. For station 209 (below 4000 db) and station 222, we could not get enough number of valid d a t a for station 209 (below 4000 db) and station 222, so they are not included in the dataset. Estimated error of the dataset is less than 0.01"C for temperature and 0.0037 psu (practical salinity unit) for salinity.
2.2.
Results
Surface layers In the surface layer of upper 200 db, strong baroclinic feature is found. Geostrophic calculation shows a surface confined westward flow with intensity exceeding 20 cm/s (not shown), which indicates the North Equatorial Current (e.g., Toole et al., 1990). There is a salinity maximum exceeding 35.0 psu at about 110-130 d b in depth (Fig. 2). Beneath this thin layer, there is a salinity minimum a t the depth of 600-800 d b or around 26.5 in sigma-theta (potential density referred to 0 pressure). In the succeeding section, we will deal with the deep and bottom layers below the salinity minimum.
Cross sections of 12"N and 13"N Figures 3(a), (b), and (c) show the cross-sectional distributions along 12"N of potential temperature theta-4 referred to 4000 db, salinity S, and potential density sigma-5 referred to 5000 db, respectively. Figures 4(a), (b), and (c) show the crosssectional distributions along 13"N. Above 3500 db, which is roughly the depth of the Kyushu-Palau Ridge, no apparent difference can be seen between the water properties in the West Mariana Basin and those in the Philippine Basin. Obvious differences, however, appear below 3500 db; the most prominent feature is the marked contrast of water properties between both sides of the Kyushu-Palau Ridge around 135"E. Colder, more saline, and denser abyssal water exists in the West Mariana Basin (eastern side of the ridge), whereas it is absent in the Philippine Basin (western side of the ridge). Vertical and horizontal distributions are highly variable in the West Mariana Basin. This is an indication that the West Mariana Basin is nearer t o the source of abyssal water. Dissolved oxygen content measured
43
-4--
3000
c
4000
5000
6000
1
125"
LONGITUDE
u5
130"
135' LONGITUDE
140'
E
(12"N)
1000
2000
125'
130'
135" LONGITUDE
140'
E
Fig. 3. The distributions along 12"N cross-section: (a) potential temperature theta-4 (referred t o 4000 db), (b) salinity S, and (c) potential density sigma-5 (referred t o 5000 db). The numerals at the top denote the station numbers. from sampled water shows higher values at the bottom layer in the West Mariana Basin (Watanabe et al., 1988), indicating that abyssal water is relatively young there. In the Philippine Basin, water of slightly lower temperature and large density is found in the western bottom layer as indicated by isotherms of 1.565"C and 1.57"C in Figs. 3(a) and 4(a) along both 12"N and 13"N. This water may spread over few
44
---
-
3000
-
a
__--
E
r
2
3000
D
D
4000
2 4000
5000
5000
nil-----
6000
6000 125'
~
130"
135'
E
140'
125"
LONGITUDE
130"
135"
LONGITUDE
140'
E
u5 (13'N)
---
3000
Iz( 9
P
4000
5000
6000
125'
130"
135'
LONGITUDE
140"
E
Fig. 4. The same as Fig. 3 except along 13"N hundred kilometers, although the westernmost part of the basin was not observed. It is not clear whether this water is significantly colder, because the temperature difference is less than 0.005"C which is as large as half the measurement error. T h e bottom water in the western Philippine basin has properties closer to the bottom water in the West Mariana Basin. These features indicate the existence of a broad and weak southward flow in the western part of the Philippine Basin.
45 Vertical profile of all CTD stations Figures 5(a), (b) show the vertical profiles of theta-4, S, and sigma-5 for depth below 3000 db, respectively. The profiles are drawn with different types of lines according t o the following geographical classification: 3000
3500
a: 3 0)
4500
0 W
a: a
5000
5500
Fig. 5. The vertical distributions of (a) theta-4, (b) S, and (c) sigma5 below 3000 db: - -, on the south-eastern flank of the Mariana Ridge; - _ - , in the West Mariana basin; - - -,in the eastern part of the Philippine Basin; and - in the western part of the Philippine Basin. An error bar is inserted in each figure.
46
I44E-127E
P0
TE
NT I A
Fig. 5 .
L D E N S I T Y ( k g Irn
1 )
(continued)
(1) the southeastern flank of the Mariana Ridge (stations 202 and 204; outside of the Philippine Sea), (2) near the shallowest portion of the Mariana Ridge (stations 201 and 2031, ( 3 ) the West Mariana Basin (stations 205-210), (4) near the Kyushu-Palau Ridge (stations 211-213, 008, and Oll), which is about 3500 m deep, ( 5 ) eastern part of the Philippine Basin (stations 214-219), (6) western part of the Philippine Basin (stations 220-223).
For stations 201 and 203, profiles are not included in the figures because depths are shallower than 2000 db. The profiles for stations 222 and 209 (below 4000 db) are not available because the raw data are erroneous. Above 4000 db, deep water southeast of the West Mariana Ridge (stations 202 and 204) is the coldest and most saline when compared at the same depth. The water retains the characteristics of the abyssal water from the south. The vertical profiles of region (1) are distinguished from those inside the Philippine Sea (regions 3 t o 6) at depths shallower than 4000 db. In the Philippine Sea, the profiles for stations in the West Mariana Basin (regions 3 and 4) are separated from those in the Philippine Basin (regions 5 and 6) below 3500 db. The depth corresponds t o the sill depth of the Kyushu-Palau Ridge, which divides the Philippine Sea into the two basins. The water in the Philippine Basin is remarkably homogeneous below 4000 db. It is warmer, less saline, and less dense compared t o the water in the West Mariana Basin. This implies that the Philippine Basin is far from the source of abyssal water. On the other hand,
47 2.0
1.5
1.0
0.5
0.0
- 0.5 Depth (db)
1000
2000
3000
4060
5000
6000
Depth ( d b )
Fig. 6. The vertical distributions of stability for the stations (a) 205210, (b) 214-223 for the depth below 1000 db, respectively. Stability is defined in terms of the Brunt-V ais a1 a frequency (cph). Five adjacent data (10 d b in depth) were fitted t o calculate a density gradient. Positive value represents the stable state, i.e., the density gets denser as the depth increases.
48 the water in the West Mariana Basin has a large variation within a relatively small zonal scale. In the West Mariana Basin, water properties are changing rapidly from those in the North Pacific Ocean to those in the Philippine Basin. It is to be noted that station 206 rather than 205, has a vertical distribution similar t o those at stations 202 and 204 in the western North Pacific Ocean. This is most clear for salinity profiles as shown in Fig. 5(b). Station 206 is nearest to the Yap-Mariana Junction which is located at around 11"N and 139"E (Fig. 1). Accordingly, we can state that abyssal water flows into the Philippine Sea through the Yap-Mariana Junction. When we compare the vertical profiles for regions (5) and (6) in the Philippine Basin, rather small though systematic difference can be found in the temperature and density profiles. The western side of the Philippine Basin is slightly colder and denser than the eastern side as discussed in the previous section.
3. Summary and Discussion CTD measurements were made down to the bottom along 12"N and 13"N in the Philippine Sea. The most important result is that the existence of cold and saline water was verified in the deep region of the West Mariana Basin below 3500 db. Water of these properties has been inferred t o enter the Philippine Sea through the Yap-Mariana Junction in previous data analyses by Mantyla and Reid (1983) and Kaneko (1984). At station 206, which is the station nearest to the Yap-Mariana Junction, we observed the coldest, most saline, and densest bottom water in the Philippine Sea. The marked contrast of water properties between the West Mariana Basin and the Philippine Basin can be ascertained from the instrumental accuracy. Also both the horizontal and vertical gradients are compared between the east and west sides of the Kyushu-Palau Ridge. The deep water has a variable structure in the West Mariana Basin, but is homogeneous in the Philippine Basin. We may say, therefore, that the present result gives evidence of flow into the Philippine Sea through the Yap-Mariana Junction. The vertical profiles of stability of the water in the West Mariana Basin and the Philippine Basin for the depth below 1000 d b are shown in Figs. 6(a) and (b), respectively. In the Philippine basin, the stability is almost neutral below the depth of 4200 db. This suggests that although the difference of the water properties between the West Mariana Basin and the Philippine Basin become apparent from 3500 d b (Fig. 5a and b), there may be a gap at the Kyushu-Palau Ridge with a depth of 4200 d b and the water underneath retains the characteristic of the sill depth. In the West Mariana Basin, on the other hand, we can see some signals showing the variable nature of water properties: the variations of the stability between the stations are large compared with those in the Philippine Basin, and the stability does not come to zero value (i.e., there are potential density gradients at most depths) except for the depth below 5000 d b at station 206. It is not clear from our dataset whether the homogeneous distribution of density below 5000 d b reflects the sill depth of the Yap-Mariana Junction because there is only one station which has d a t a deeper than 5000 d b in the West Mariana Basin. The water density in the western part of the Philippine Basin increases slightly
t o the west (Figs. 3, 4 and 5 ) . This might be interpreted as a broad western boundary current flowing equatorward. It is impossible t o substantiate this speculation at present. At least, t h e variation is quite small, beyond t h e present precision of data.
Acknowledgement We would like t o thank Prof. T. Teramoto, Dr. M. Kawabe, a n d Mr. S. Kitagawa for their support and discussions during t h e study. We also thank the crews a n d t h e participants of t h e cruise of t h e R/V Hakuho Maru.
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