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Observations of the Korea Strait bottom cold water
Continental Shelf Research 22 (2002) 821–831
Note
Observations of the Korea Strait bottom cold water Donald R. Johnson*, William J. Teague Naval Research Laboratory, Ocean Sciences Division, Stennis Space Center, MS 39529, USA Received 18 May 2001; received in revised form 5 September 2001; accepted 20 September 2001
Abstract In this study, a set of 13 bottom mounted acoustic Doppler current profilers with temperature and pressure sensors were deployed in the Korea Strait for 11 months in 1999–2000. The overall purpose of the project was to understand the linkages between the East Asian marginal seas. Our focus in this paper is on an intrusion of bottom cold water from the East Sea which appears on the western side of the Korea Strait (Korea Strait Bottom Cold Water, KSBCW). In contrast to previous work, we did not find a yearly cycle in its appearance with maximum in summer and minimum in winter. Rather, sustained intrusions occurred in May/June and again in December/January. Bottom currents during the times of its appearance showed only limited advective intrusion with recycling back to the East Sea. Monthly scale bottom temperature records in the intrusion area were negatively correlated with the cross-strait bottom pressure anomaly, a measure of geostrophic transport through the Strait. Our hypothesis is that when geostrophic transport is low the bottom cold water intrudes, and when it is high the cold water is prevented from intruding. Prediction of the bottom cold water appearance, then, would depend on prediction of the geostrophic transport. r 2002 Published by Elsevier Science Ltd.
1. Introduction The East Sea (also known as the Japan Sea) is contained in one of four semi-enclosed marginal basins rimming the northwestern Pacific Ocean. The greatest depth of the East Sea is about 3700 m, which gives it many characteristics of a deep ocean basin. It connects to the East China Sea in the south, to the Sea of Okhotsk in the north, and to the Pacific Ocean in the northeast through relatively narrow, shallow straits. The most striking feature of the East Sea is the contrast of water masses across the polar front, which separates the *Corresponding author. Tel.: +1-228-688-4691; fax: +1228-688-5997. E-mail address: [email protected] (D.R. Johnson).
Sea into two regimes (Preller and Hogan, 1998). There is a cold northwestern regime affected by sea ice processes, river run-off and deep convection, and a warm, chlorophyll rich southeastern regime fed by Kuroshio waters entering from the East China Sea through the Korea Strait. Sverdrup et al. (1942) compare the East Sea to the Arctic Mediterranean (Greenland–Iceland– Norwegian Sea) and to the Labrador Sea where warm, saline water enters on the eastern sides and cool, diluted water flows out on the western sides. They recognize that the main difference is ‘‘ythat no great outflow of cold water takes place from the Japan Sea; the cold water on the western side is mainly part of an eddy’’. Interestingly, however, below the warm waters passing northward through the Korea Strait (Tsushima Warm Water, TWW),
0278-4343/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 2 7 8 - 4 3 4 3 ( 0 1 ) 0 0 0 9 9 - 1
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831
20 00
0 180 1400
60100 0 0
150
25
0 15
N1 N2 N3 C1 N4 N5 N6 5
125 100
12
0
10
KYUSHU
25 50
50 75 5 12
25
75
0
400
15
5
127
125
5 17
12
32 126
200115070525 400107152550
33
0 10 25
50 7 5
100
S1 S2 S3 S4 S5 S6
50 100
34
1600 1200 800 400 17 5
125 100
75
200
35
0
The Korea Strait, which connects the East Sea to the East China Sea, is about 140 km wide at its narrowest constriction, with a sill depth of about 140 m (Fig. 1). Although there appears to be general agreement that the warm, salty water which flows through the Strait originates in the Kuroshio Current, there are differences of thought on its pathway to the Strait (Suda and Hidaka, 1932; Beardsley et al., 1985; Lie and Cho, 1994; Katoh et al., 1996). Despite the limited size of the Korea Strait, this water (now the TWW and the current called the Tsushima Current, TC) has a major impact on the hydrography and circulation of the East Sea. Upon exiting the Korea Strait into the East Sea, the TC has been observed to exhibit complex branching (Beardsley et al., 1992) as it meanders south of the Polar Front toward the Tsugaru Strait where part of the water exits to the Pacific Ocean. In the north of the basin, beyond the Tsugaru Strait, another part joins cold water from the Sea of Okhotsk and then recirculates to the south
KOREA
36
10
2. Background
37
100
a ‘pool’ of cold bottom water has been found, hugging the western side of the Strait (Korea Strait Bottom Cold Water, KSBCW). This pool has been of considerable interest to regional oceanographers who have sought to understand its physical characteristics and its seasonal variations. While many hydrographic surveys (e.g., Cho and Kim, 1998) have been made, and several model studies have been conducted (e.g., Park et al., 1995), there have been few current measurements to guide the investigations. As part of a joint effort to study the East Asian marginal seas, 13 bottom mounted acoustic Doppler current profilers (ADCPs) with temperature sensors were moored in the Korea Strait for 11 months in 1999–2000. The objectives of this paper are to examine these bottom temperature and current profiles in the context of the bottom cold water intrusion into the strait, to present the flow and temperature distribution characteristics associated with the cold water and to search for correlations with potential forcing mechanisms.
Latitude
822
128 129 130 Longitude
131
132
Fig. 1. Mooring locations in the Korea Strait. Bathymetry in meters.
along the coast of Russia and the east coast of Korea, forming a large cyclonic circulation pattern around the deepest part of the basin. The cold water regime is called the East Sea Proper Water (ESPW). It was recognized early (Schott, 1935) that the shallow connection to the Sea of Okhotsk meant that the cold deep water must have been formed in-situ by excessive winter cooling. A salinity-minimum, oxygen-maximum layer is formed by a sinking mixture of water at the polar front (Moriyasu, 1972). This intermediate water separates the ESPW from the overlying warm water regime (Kim et al., 1991). It has been hypothesized (Cho and Kim, 1994) that this salinity minimum layer water (called the SML) is the source of the cold bottom water found in the Korea Strait. Using the Bernoulli energy function model of Stommel et al. (1973) and observed bottom currents in the Strait of 7–20 cm/s, Cho and Kim (1998) suggested that the depth from which water can be lifted from the quiescent basin is 163–181 m, which is appropriate for SML water, and not much below sill depth. In the Korea Strait, the bottom cold water pool appears to be confined toward the western side and lifted upward toward the Korean coast. This uplift has been the subject of several studies. Seung (1986)
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831
suggested that it could be due to the geostrophic adjustment to an increase in the TC, above the cold water. Byun (1987) theorized that upwelling favorable winds along the west side of the Strait could tilt the interface upward. Park et al., 1995, used a hydraulic model to demonstrate that the sloping bottom near the Korean coast could make the southward flowing cold water bank against the slope. Cho and Kim (1998) conducted bi-monthly CTD surveys in 1991 in the northern portion of the Strait and the Ulleung Basin. From these surveys, the KSBCW appeared only within about 50 km of the Korean coast and with a thickness of 20–50 m. Intrusion into the Strait appeared to vary seasonally with a maximum penetration of about 100 km (about half way through the Strait) occurring in August. They suggested that a bottom homogeneous layer in the cold water pool could be mixed by strong currents of about 1 m/s.
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consisted of moorings N2–N6 and S1–S6. The moorings were retrieved and refurbished in midOctober 1999, and redeployed until March 2000. Additional moorings were deployed at site N1 and at C1 for the second set. The moorings were in trawl resistant housings which each contained a 300 kHz ADCP by RDI and a Seabird pressure gauge. Both instruments had temperature sensors. Sample intervals varied between 15 min and 1 h with vertical current bins of 4 m starting at about 6–10 m above the bottom. Since the Korea Strait is relatively shallow, currents may be affected by variations in the wind fields. Gridded wind stress analysis were provided by the Navy Operational Global Atmospheric Prediction System (NOGAPS) (Hogan and Brody, 1993; Rosmond, 1992; Hogan and Rosmond, 1991).
4. Results
3. Observations In this study, an array (Fig. 1) of 13 bottommoored ADCPs with temperature sensors were deployed (Table 1) in two sets between May 1999, and March 2000 (Perkins et al., 2000; Teague et al., 2001, accepted). The first set was deployed between May 1999, and mid-October 1999, and
Table 1 Mooring locations and depths Mooring
Latitude (N)
Longitude (E)
Depth (m)
N1 N2 N3 N4 N5 N6 S1 S2 S3 S4 S5 S6 C1
35.35 35.20 35.01 34.84 34.67 34.50 34.32 34.13 33.93 33.74 33.54 33.35 34.85
129.55 129.67 129.99 130.21 130.43 130.65 127.90 128.12 128.34 128.56 128.78 129.00 129.26
122 142 132 127 130 118 59 89 113 107 152 115 103
In Fig. 2, the bottom temperature records are shown from each of the moorings along the northern N-line. If we choose a temperature of 101C to be the upper limit of KSBCW (Lim and Chang, 1969), then only the two western moorings, N1 and N2, show extended periods when the bottom temperature is below this limit. At moorings N3–N5, shorter pulses of low temperature water, lasting less than 1 week, appear in the records. These pulses are prominent in the center of the Strait, weaken toward the east side and disappear completely at N6, near the coast of Honshu. From Fig. 2, no obvious yearly cycle is apparent in the bottom temperature time series on the western side of the Strait where the intrusions occur. The clearest intrusions (longest periods of sustained cold water) at N2 occur in May/June and again in December/January. From August through November, the temperature record at N2 is very ‘noisy’. It seems likely that during this period the sharp horizontal temperature gradient bounding the cold water pool is located in the vicinity of N2 and that tidal currents and other short period current variations are advecting this gradient across the instrument. This, then, would
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831
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Bottom Temperature Temp (deg C)
15
Temp (deg C)
15
Temp (deg C)
15
Temp (deg C)
15
Temp (deg C)
15
Temp (deg C)
20
15
N1
10 5 0 20
N2
10 5 0 20
N3
10 5 0 20
N4
10 5 0 20
N5
10 5 0 20
N6
10 5 0
May June July Aug Sep Oct 1999
Nov Dec Jan Feb Mar 2000
Fig. 2. Bottom temperature time series along the northern line of moorings (N-line). See Fig. 1 for location. The 101C temperature is marked with a horizontal line as the upper limit to KSBCW.
set the eastern boundary of the cold water near N2 for most of the year, with stronger cold water intrusion in late spring and in mid-winter filling the western side out to a position between N2 and N3. In order to estimate the penetration distance of KSBCW into the Strait, temperature time series
are shown in an along-strait line on the western side at moorings N2, C1 and S2. These three time series of bottom temperature are shown in Fig. 3. Bottom water of less than 101C reaches to C1 during the December/January intrusion noted at N2. But the noisiness of the record during this period and the lowest temperature of only 61C,
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825
North/South Bottom Temperature 20
Temp (deg C)
N2 15 10 5 0 20
Temp (deg C)
C1 15 10 5 0 20
Temp (deg C)
S2 15 10 5 0 May June July 1999
Aug
Sep
Oct
Nov
Dec
Jan Feb 2000
March
Fig. 3. Bottom temperature time series along the western side of the Korea Strait. See Fig. 1 for location. The 101C temperature is marked with a horizontal line.
observed during one of the brief low temperature episodes, suggest that this location is near the front of the intrusion, with a horizontal temperature gradient boundary being advected across the mooring. The strongest low temperature pulses along the S-line are observed during this period at S2, but not reaching less than 101C during the December/January intrusion. The lower temperature water in late February at this location reaches to just below 101C, but may be more related to a seasonal trend in the TWW (compare to the time
series at N6) than to the KSBCW reaching this far south. As previously noted, although low temperature pulses occurred on the middle to mid-eastern side of the Strait, the major intrusions on the western side in which we have our immediate interest, appeared to have durations of monthly and submonthly scale. For this reason, we can eliminate noise in the records by examining monthly scale variations. The monthly averaged bottom temperature at mooring N2 is shown in Fig. 4 together
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831
Temp (deg C)
14 12 10 8 6 4
Cm/S
826
10 0 −10 −20 −30
Cm/S
20
Bottom Temperature N2
X-strait current N2
Along-strait current N2
10 0
dynes/cm^2
−10 0.02 0.00 − 0.02 − 0.04 − 0.06 − 0.08
Meters
0.02
Along-strait Wind Stress
Along-strait Pressure diff
0.00 − 0.02 − 0.04
Meters
0.030
X-strait Pressure diff
0.015 0.000 −0.015 −0.030 May June July Aug Sep Oct 1999
Nov Dec
Jan Feb Mar 2000
Fig. 4. Monthly averaged bottom temperature at mooring N2 (upper) together with monthly averaged cross-strait currents and alongstrait currents at 10 m (solid line), 20 m (dashed line) and 30 m (dash-dotted line) above the bottom, monthly averaged along-strait wind stress, and monthly averaged along-strait and cross-strait bottom pressure anomaly differences.
with monthly averaged cross-strait (positive toward the SE) and along-strait currents (positive toward the NE) at 10, 20 and 30 m above the bottom, monthly averaged along-strait wind stress, and monthly averaged along-strait and cross-strait bottom pressure anomaly differences.
Table 2 shows the correlations, r; of bottom temperature at N2 with these parameters. With 11 data points, the only correlation that reached significance (>99%) was between bottom temperature and the cross-strait pressure difference anomaly. This latter parameter was formed by
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831 Table 2 Correlation with monthly bottom temperature at N2. U and V are cross- and along-strait current components, tx and ty are cross- and along-strait wind stress components. Pressure anomaly differences are formed from N6–N2 (cross-strait) and N2–S2 (along-strait) r
Parameter U at 10 m V at 10 m U at 30 m V at 30 m tx ty Along-strait pressure difference Cross-strait pressure difference
Temp (deg C)
15
0.47 0.54 0.25 0.29 0.10 0.04 0.32 0.92
Bottom Temp at N2
10
5
0 40
Bottom Along-Strait Current at N2
Cm/S
20
0
− 20
May June July Aug 1999
Sep
Oct
Nov
Dec
Jan Feb M 2000
Fig. 5. Daily averaged (light line) and monthly averaged (dark line) bottom temperature at mooring N2 (upper). Daily averaged (light line) and monthly averaged (dark line) alongstrait currents at 10 m above the bottom (lower).
taking the monthly average bottom pressures at N6 and N2, removing the mean from each and subtracting N2 from N6. The pressure anomaly difference represents a measure of geostrophic transport through the Straits with an error due to the uncorrelated pressure variations occurring in the vertical region between the geopotential levels defined at each instrument, and an error due to uncorrelated flow along the unsampled sides of the
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Straits between the ends of the mooring line and each coast (comparison with ADCP measured total transport is given in Fig. 6). The along-strait pressure difference was formed in the same manner with the pressure anomaly at S2 subtracted from that at N2. The positive relationship between cross-shelf pressure anomaly variations and bottom temperature means that higher bottom temperatures occur with increased transport, and lower temperatures occur during decreased transport. The squared value of this correlation (r2 ) means that 85% of the variance in bottom temperature can be explained by this relationship. The negative correlation (r ¼ 0:32) of bottom temperature with alongstrait pressure anomaly variations implies the same relationship with transport through the Strait, although at this level it is not a highly significant statistic. It is interesting that although the correlations of bottom temperature to wind stress and bottom temperature to currents are both low, the correlation of along-strait wind stress (upwelling favorable against the Korean coast) to cross-shelf currents at N2 is r ¼ 0:74: This significant (>99%) and negative correlation indicates an upwelling relationship involving bottom currents. This has been suggested as a mechanism for lifting the cold water up against the Korean coast (Byun, 1987). But the low relationship with temperature indicates that this mechanism may be only marginally involved with cold water intrusions. Since monthly means may obscure shorter storm scale events, we also show the bottom temperature at N2 together with along-strait currents 10 m above the bottom at N2 (Fig. 5). These records were averaged daily to eliminate tidal currents and are overlain with the monthly averages. The bottom southward currents are clearly impulsive on storm scales of a few days. In the December/January time of sustained low bottom temperature, southward current pulses of characteristically 10 cm/s dominate. However, during the May/June intrusion of cold water, there was almost no southward flow. The overall correlation between daily averaged temperature and current records was only r ¼ 0:25; hence with 299 data points it is not statistically significant.
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831
Total Transport
4
0.04
0.02
S-Line Anomaly
0.00 2 N-Line May June July 1999
Aug
Sep
Oct
Nov
Dec
Jan Feb M 2000
− 0.02
Pressure Anomaly (m)
828
− 0.04
Fig. 6. Total transport across S-Line and N-Line as determined by ADCP along-strait current measurements together with cross-strait bottom pressure anomaly (dark line).
Correlation of daily geostrophic transport, as defined above, with temperature was also only r ¼ 0:26: This is rather surprising and indicates horizontal diffusion must be an important component of the appearance of the cold pool, occurring when monthly scale transport is reduced. Since monthly scale geostrophic transport appears to be a large influence on the appearance of the bottom cold pool, we need to validate it by comparison to total transport as measured by the ADCP along-strait currents. In Fig. 6, we show the monthly geostrophic transport anomaly, as previously defined by the across-strait bottom pressure anomaly, together with monthly average total transports along the N line and the S line. The comparison is quite good, and gives us some confidence in its use as a prediction tool. The differences between total and geostrophic transport are principally due to friction, i.e., local wind forcing which is embedded in the total transport. In Fig. 7a and b, we show plan views of currents averaged over all observations where the bottom temperature was either high or low. In the case of moorings N1 and N2, we chose 101C as a separation point. In the case of moorings N3–N6, we fit a third degree polynomial to the curves and took low temperature to mean observations when temperature was 21C below the fitted curve. This effectively isolated the low temperature pulses on the middle to mid-eastern side of the Strait. Referring to Fig. 7, we note that currents at C1 are always toward the East Sea; at N1 and N2, a
northward flow occurs during high temperature times and a cross-strait flow toward the Korean coast during low temperature times. At N3 and N4, bottom currents are relatively weak, but point cross-strait toward the coast of Japan (opposite to that at N1 and N2) during times of low temperature pulses. To further examine this relationship between flow and the KSBCW, Fig. 8 shows hodographs of monthly averaged currents from 10 to 100 m above the bottom at 10 m intervals at mooring N2. May and December are months of low bottom temperatures at N2, and October and March are months of relatively high bottom temperatures. During May and December, the currents higher in the water column are weak, and the bottom currents are relatively strong and pointed crossstrait toward the Korean Coast. During October and March the currents higher in the water column are stronger and the bottom currents are weak and pointed more along-strait. It seems clear, from Figs. 7 and 8, that characteristic bottom flows during times of low temperature do not support the concept of a strong advecting flow from the East Sea, but rather a small penetration and a recirculation back to the East Sea.
5. Summary KSBCW appearance in the Korea Strait was examined with an 11 month long array of bottom mounted ADCPs together with temperature and
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831 37
37
5 10 15 20 Velocity cm/s
5 10 15 20 Velocity cm/s
36
36
N1 N2 C1
N4 N5 N6
34
C1
N3 N4 N5 N6
34
33
33
(a)
N1 N2
35
N3
Latitude
Latitude
35
32 126
829
127
128
129 130 Longitude
131
32 126
132
(b)
127
128
129
130
131
132
Longitude
Fig. 7. (a) Currents at 10 m above bottom averaged over time of high bottom temperature (see text for definitions). (b) Currents at 10 m above bottom averaged over time of low bottom temperature.
pressure sensors. The KSBCW as a cold bottom water (o101C) intrusion appeared to be confined to the western side of the Strait and uplifted against the Korean coast. From bottom temperature time traces there was no obvious yearly cycle in cold water intrusions into the strait. Sustained intrusions occurred in May/June and again in December/January. Bottom currents during times of bottom cold water appearance showed no advective intrusion on a monthly averaged scale and only limited advective intrusion on a daily averaged scale. Rather, during low temperature episodes, bottom currents were directed toward the Korean coast on the west side and toward the coast of Japan in the middle and mid-eastern side of the Strait. This gave the appearance of a short adjective intrusion and a recirculation back into the East Sea. Of course, with gaps in mooring sites across the Straits, we might have missed some of the intrusion. But with moorings in the main low bottom temperature locations, this does not seem like a reasonable alternative. On a monthly scale, the bottom temperature signal from the moorings on the western side of the Strait correlated negatively with the cross-strait
bottom pressure anomaly. This was the only statistically significant correlation with bottom temperature among the various parameters, including currents and winds. The cross-strait bottom pressure anomaly is related to geostrophic transport variations through the Strait. The implication of the negative correlation is that the cold bottom water intruded when geostrophic transport to the East Sea was reduced and was flushed out when this transport increased. With fairly strong tides (Teague et al., accepted) to increase horizontal diffusion in the bottom layer, we can hypothesize that bottom cold water intrusions are the product of relatively weak advection augmented by horizontal diffusion. As stated in the introduction, a comparison with the Arctic Mediterranean and the Labrador Sea is probably appropriate in terms of warm inflow and cold outflow. The contradistinction is that inflow into the East Sea is high in comparison to the size of the opening (depth and width of the Korea Strait) and limits the size of the cold water outflow. If this hypothesis is correct, then the appearance of KSBCW can be predicted from the geostrophic transport through the strait. This can be tested
830
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10
May
Dec
20
20
10 100
100 −20
0
0
20
−20
0
0
−20
−20
Oct 100
Mar
20
20
10 100
10 −20
0
20
0
20
−20
−20
0
0
20
− 20
Fig. 8. Hodographs of monthly averaged currents from 10 to 100 m above the bottom at 10 m intervals at mooring N2. May and December are months of bottom temperature at N2 less than 101C. October and March are months with temperature higher than 101C.
with long time series of bottom pressure records on each side of the strait and temperature records on the west side.
ment 0602435N (NRL-SSC contribution JA/7330/ 01/0067).
References Acknowledgements This work was supported by the Office of Naval Research as part of the Basic Research Projects ‘‘Linkages of Asian Marginal Seas’’ and ‘‘Japan East Sea DRI’’ under Program Element 0601153N, and as part of the Navy Ocean Modeling Program (NOMP) under Program Ele-
Beardsley, R.C., Limeburner, R., Yu, H., Cannon, G.A., 1985. Discharge of the Changjiang (Yangtze River) into the East China Sea. Continental Shelf Research 4, 57–76. Beardsley, R.C., Limeburner, R., Kim, K., Candela, J., 1992. Lagrangian flow observations in the East China, Yellow and Japan Seas. La Mer 30, 297–314. Byun, S.-K., 1987. Appearance of cold water at the sea surface near Ulsan: theoretical model of upwelling. KORDI Report (BSPE 00052-142-1), 59pp.
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831 Cho, Y.-K., Kim, K., 1994. Two modes of salinity minimum water in the Ulleung Basin. La Mer 32, 33–40. Cho, Y.-K., Kim, K., 1998. Structure of the Korea Strait Bottom Cold Water and its seasonal variation in 1991. Continental Shelf Research 18, 791–804. Hogan, T.F., Brody, L.R., 1993. Sensitivity studies of the navy global forecast model parameterizations and evaluation of improvements to nogaps. Montreal Weather Review 121 (8), 2373–2395. Hogan, T.F., Rosmond, T.E., 1991. The description of the navy operational global atmospheric prediction system. Montreal Weather Review 119 (8), 1786–1815. Katoh, O., Teshima, K., Abe, O., Fujita, H., Morinaya, K., Nakagawa, N., 1996. Process of the Tsushima Current formation revealed by ADCP measurements in summer. Journal of Oceanography 52, 491–507. Kim, C.H., Lie, H.-J., Chu, K.-S., 1991. On the intermediate water in the southwestern East Sea (Sea of Japan). In: Takano, K. (Ed.), Oceanography of Asia Seas. Elsevier, Amsterdam, pp. 129–141. Lie, H.-J., Cho, C.-H., 1994. On the origin of the Tsushima Warm Current. Journal of Geophysical Research 99, 25 081–25 091. Lim, D.B., Chang, S., 1969. On the cold water masses in the Korea Strait. Journal of Oceanological Society of Korea 4, 71–82. Moriyasu, S., 1972. The Tsushima Current. In: Stommel, H., Yoshida, K. (Eds.), Kuroshio, its Physical Aspects. University of Tokyo, Tokyo, pp. 353–369. Park, Y.-G., Cho, Y.-K., Kim, K., 1995. A hydraulic model of the Korea Strait bottom cold current. Journal of Oceanography 51, 713–727.
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Perkins, H., Teague, W.J., Jacobs, G.A., Chang, K.-I., Suk, M.-S., 2000. Currents in Korea–Tsushima Strait during summer 1999. Geophysical Research Letters 27 (19), 3033–3036. Preller, R.H., Hogan, P.J., 1998. Oceanography of the Sea of Okhotsk and the Japan/East Sea. In: Allan R. Robinson, Kenneth H. Brink (Eds.), The Sea. John Wiley & Sons, New York, N.Y. Vol. 11, pp. 429–481. Rosmond, T.E., 1992. The design and testing of the Navy Operational Global Atmospheric Prediction System. Weather and Forecasting 7 (2), 262–272. Schott, G., 1935. Geographie des Indischen und Stillen Ozeans. Hamburg, Verlag von C. Boysen, 413pp (In German). Seung, Y.H., 1986. On the response of coastal water to the intensification of East Korea Warm Current along the East Coast of KoreaFA theoretical consideration. Journal of Oceanological Society of Korea 21, 229–235. Stommel, H., Bryden, H., Mangelsdorf, P., 1973. Does some of the Mediterranean outflow come from great depth? Pure and Applied Geophysics 105, 879–889. Suda, K., Hidaka, K., 1932. The results of the oceanographical observations on board R.M.S. Syunpu Maru in the southern part of the Sea of Japan in the summer of 1929. Part 1. Journal of Oceanographical Importance on Marine Observations 3, 291–375 (in Japanese).. Sverdrup, H.U., Johnson, M.W., Fleming, R.H., 1942. The Oceans. Prentice-Hall, Inc., Englewood Cliffs, NJ, 1060pp. Teague, W.J., Jacobs, G.A., Perkins, H.T., Book, J.W., Chang, K.-I., Suk, M.-S. Low frequency current observations in the Korea Strait. Journal of Physical Oceanology, accepted. Teague, W.J., Perkins, H.T., Jacobs, G.A., Book, J.W., 2001. Tide observations in the Korea–Tsushima Strait. Continental Shelf Research 21, 545–561.
Note
Observations of the Korea Strait bottom cold water Donald R. Johnson*, William J. Teague Naval Research Laboratory, Ocean Sciences Division, Stennis Space Center, MS 39529, USA Received 18 May 2001; received in revised form 5 September 2001; accepted 20 September 2001
Abstract In this study, a set of 13 bottom mounted acoustic Doppler current profilers with temperature and pressure sensors were deployed in the Korea Strait for 11 months in 1999–2000. The overall purpose of the project was to understand the linkages between the East Asian marginal seas. Our focus in this paper is on an intrusion of bottom cold water from the East Sea which appears on the western side of the Korea Strait (Korea Strait Bottom Cold Water, KSBCW). In contrast to previous work, we did not find a yearly cycle in its appearance with maximum in summer and minimum in winter. Rather, sustained intrusions occurred in May/June and again in December/January. Bottom currents during the times of its appearance showed only limited advective intrusion with recycling back to the East Sea. Monthly scale bottom temperature records in the intrusion area were negatively correlated with the cross-strait bottom pressure anomaly, a measure of geostrophic transport through the Strait. Our hypothesis is that when geostrophic transport is low the bottom cold water intrudes, and when it is high the cold water is prevented from intruding. Prediction of the bottom cold water appearance, then, would depend on prediction of the geostrophic transport. r 2002 Published by Elsevier Science Ltd.
1. Introduction The East Sea (also known as the Japan Sea) is contained in one of four semi-enclosed marginal basins rimming the northwestern Pacific Ocean. The greatest depth of the East Sea is about 3700 m, which gives it many characteristics of a deep ocean basin. It connects to the East China Sea in the south, to the Sea of Okhotsk in the north, and to the Pacific Ocean in the northeast through relatively narrow, shallow straits. The most striking feature of the East Sea is the contrast of water masses across the polar front, which separates the *Corresponding author. Tel.: +1-228-688-4691; fax: +1228-688-5997. E-mail address: [email protected] (D.R. Johnson).
Sea into two regimes (Preller and Hogan, 1998). There is a cold northwestern regime affected by sea ice processes, river run-off and deep convection, and a warm, chlorophyll rich southeastern regime fed by Kuroshio waters entering from the East China Sea through the Korea Strait. Sverdrup et al. (1942) compare the East Sea to the Arctic Mediterranean (Greenland–Iceland– Norwegian Sea) and to the Labrador Sea where warm, saline water enters on the eastern sides and cool, diluted water flows out on the western sides. They recognize that the main difference is ‘‘ythat no great outflow of cold water takes place from the Japan Sea; the cold water on the western side is mainly part of an eddy’’. Interestingly, however, below the warm waters passing northward through the Korea Strait (Tsushima Warm Water, TWW),
0278-4343/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 2 7 8 - 4 3 4 3 ( 0 1 ) 0 0 0 9 9 - 1
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831
20 00
0 180 1400
60100 0 0
150
25
0 15
N1 N2 N3 C1 N4 N5 N6 5
125 100
12
0
10
KYUSHU
25 50
50 75 5 12
25
75
0
400
15
5
127
125
5 17
12
32 126
200115070525 400107152550
33
0 10 25
50 7 5
100
S1 S2 S3 S4 S5 S6
50 100
34
1600 1200 800 400 17 5
125 100
75
200
35
0
The Korea Strait, which connects the East Sea to the East China Sea, is about 140 km wide at its narrowest constriction, with a sill depth of about 140 m (Fig. 1). Although there appears to be general agreement that the warm, salty water which flows through the Strait originates in the Kuroshio Current, there are differences of thought on its pathway to the Strait (Suda and Hidaka, 1932; Beardsley et al., 1985; Lie and Cho, 1994; Katoh et al., 1996). Despite the limited size of the Korea Strait, this water (now the TWW and the current called the Tsushima Current, TC) has a major impact on the hydrography and circulation of the East Sea. Upon exiting the Korea Strait into the East Sea, the TC has been observed to exhibit complex branching (Beardsley et al., 1992) as it meanders south of the Polar Front toward the Tsugaru Strait where part of the water exits to the Pacific Ocean. In the north of the basin, beyond the Tsugaru Strait, another part joins cold water from the Sea of Okhotsk and then recirculates to the south
KOREA
36
10
2. Background
37
100
a ‘pool’ of cold bottom water has been found, hugging the western side of the Strait (Korea Strait Bottom Cold Water, KSBCW). This pool has been of considerable interest to regional oceanographers who have sought to understand its physical characteristics and its seasonal variations. While many hydrographic surveys (e.g., Cho and Kim, 1998) have been made, and several model studies have been conducted (e.g., Park et al., 1995), there have been few current measurements to guide the investigations. As part of a joint effort to study the East Asian marginal seas, 13 bottom mounted acoustic Doppler current profilers (ADCPs) with temperature sensors were moored in the Korea Strait for 11 months in 1999–2000. The objectives of this paper are to examine these bottom temperature and current profiles in the context of the bottom cold water intrusion into the strait, to present the flow and temperature distribution characteristics associated with the cold water and to search for correlations with potential forcing mechanisms.
Latitude
822
128 129 130 Longitude
131
132
Fig. 1. Mooring locations in the Korea Strait. Bathymetry in meters.
along the coast of Russia and the east coast of Korea, forming a large cyclonic circulation pattern around the deepest part of the basin. The cold water regime is called the East Sea Proper Water (ESPW). It was recognized early (Schott, 1935) that the shallow connection to the Sea of Okhotsk meant that the cold deep water must have been formed in-situ by excessive winter cooling. A salinity-minimum, oxygen-maximum layer is formed by a sinking mixture of water at the polar front (Moriyasu, 1972). This intermediate water separates the ESPW from the overlying warm water regime (Kim et al., 1991). It has been hypothesized (Cho and Kim, 1994) that this salinity minimum layer water (called the SML) is the source of the cold bottom water found in the Korea Strait. Using the Bernoulli energy function model of Stommel et al. (1973) and observed bottom currents in the Strait of 7–20 cm/s, Cho and Kim (1998) suggested that the depth from which water can be lifted from the quiescent basin is 163–181 m, which is appropriate for SML water, and not much below sill depth. In the Korea Strait, the bottom cold water pool appears to be confined toward the western side and lifted upward toward the Korean coast. This uplift has been the subject of several studies. Seung (1986)
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831
suggested that it could be due to the geostrophic adjustment to an increase in the TC, above the cold water. Byun (1987) theorized that upwelling favorable winds along the west side of the Strait could tilt the interface upward. Park et al., 1995, used a hydraulic model to demonstrate that the sloping bottom near the Korean coast could make the southward flowing cold water bank against the slope. Cho and Kim (1998) conducted bi-monthly CTD surveys in 1991 in the northern portion of the Strait and the Ulleung Basin. From these surveys, the KSBCW appeared only within about 50 km of the Korean coast and with a thickness of 20–50 m. Intrusion into the Strait appeared to vary seasonally with a maximum penetration of about 100 km (about half way through the Strait) occurring in August. They suggested that a bottom homogeneous layer in the cold water pool could be mixed by strong currents of about 1 m/s.
823
consisted of moorings N2–N6 and S1–S6. The moorings were retrieved and refurbished in midOctober 1999, and redeployed until March 2000. Additional moorings were deployed at site N1 and at C1 for the second set. The moorings were in trawl resistant housings which each contained a 300 kHz ADCP by RDI and a Seabird pressure gauge. Both instruments had temperature sensors. Sample intervals varied between 15 min and 1 h with vertical current bins of 4 m starting at about 6–10 m above the bottom. Since the Korea Strait is relatively shallow, currents may be affected by variations in the wind fields. Gridded wind stress analysis were provided by the Navy Operational Global Atmospheric Prediction System (NOGAPS) (Hogan and Brody, 1993; Rosmond, 1992; Hogan and Rosmond, 1991).
4. Results
3. Observations In this study, an array (Fig. 1) of 13 bottommoored ADCPs with temperature sensors were deployed (Table 1) in two sets between May 1999, and March 2000 (Perkins et al., 2000; Teague et al., 2001, accepted). The first set was deployed between May 1999, and mid-October 1999, and
Table 1 Mooring locations and depths Mooring
Latitude (N)
Longitude (E)
Depth (m)
N1 N2 N3 N4 N5 N6 S1 S2 S3 S4 S5 S6 C1
35.35 35.20 35.01 34.84 34.67 34.50 34.32 34.13 33.93 33.74 33.54 33.35 34.85
129.55 129.67 129.99 130.21 130.43 130.65 127.90 128.12 128.34 128.56 128.78 129.00 129.26
122 142 132 127 130 118 59 89 113 107 152 115 103
In Fig. 2, the bottom temperature records are shown from each of the moorings along the northern N-line. If we choose a temperature of 101C to be the upper limit of KSBCW (Lim and Chang, 1969), then only the two western moorings, N1 and N2, show extended periods when the bottom temperature is below this limit. At moorings N3–N5, shorter pulses of low temperature water, lasting less than 1 week, appear in the records. These pulses are prominent in the center of the Strait, weaken toward the east side and disappear completely at N6, near the coast of Honshu. From Fig. 2, no obvious yearly cycle is apparent in the bottom temperature time series on the western side of the Strait where the intrusions occur. The clearest intrusions (longest periods of sustained cold water) at N2 occur in May/June and again in December/January. From August through November, the temperature record at N2 is very ‘noisy’. It seems likely that during this period the sharp horizontal temperature gradient bounding the cold water pool is located in the vicinity of N2 and that tidal currents and other short period current variations are advecting this gradient across the instrument. This, then, would
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831
824
Bottom Temperature Temp (deg C)
15
Temp (deg C)
15
Temp (deg C)
15
Temp (deg C)
15
Temp (deg C)
15
Temp (deg C)
20
15
N1
10 5 0 20
N2
10 5 0 20
N3
10 5 0 20
N4
10 5 0 20
N5
10 5 0 20
N6
10 5 0
May June July Aug Sep Oct 1999
Nov Dec Jan Feb Mar 2000
Fig. 2. Bottom temperature time series along the northern line of moorings (N-line). See Fig. 1 for location. The 101C temperature is marked with a horizontal line as the upper limit to KSBCW.
set the eastern boundary of the cold water near N2 for most of the year, with stronger cold water intrusion in late spring and in mid-winter filling the western side out to a position between N2 and N3. In order to estimate the penetration distance of KSBCW into the Strait, temperature time series
are shown in an along-strait line on the western side at moorings N2, C1 and S2. These three time series of bottom temperature are shown in Fig. 3. Bottom water of less than 101C reaches to C1 during the December/January intrusion noted at N2. But the noisiness of the record during this period and the lowest temperature of only 61C,
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825
North/South Bottom Temperature 20
Temp (deg C)
N2 15 10 5 0 20
Temp (deg C)
C1 15 10 5 0 20
Temp (deg C)
S2 15 10 5 0 May June July 1999
Aug
Sep
Oct
Nov
Dec
Jan Feb 2000
March
Fig. 3. Bottom temperature time series along the western side of the Korea Strait. See Fig. 1 for location. The 101C temperature is marked with a horizontal line.
observed during one of the brief low temperature episodes, suggest that this location is near the front of the intrusion, with a horizontal temperature gradient boundary being advected across the mooring. The strongest low temperature pulses along the S-line are observed during this period at S2, but not reaching less than 101C during the December/January intrusion. The lower temperature water in late February at this location reaches to just below 101C, but may be more related to a seasonal trend in the TWW (compare to the time
series at N6) than to the KSBCW reaching this far south. As previously noted, although low temperature pulses occurred on the middle to mid-eastern side of the Strait, the major intrusions on the western side in which we have our immediate interest, appeared to have durations of monthly and submonthly scale. For this reason, we can eliminate noise in the records by examining monthly scale variations. The monthly averaged bottom temperature at mooring N2 is shown in Fig. 4 together
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831
Temp (deg C)
14 12 10 8 6 4
Cm/S
826
10 0 −10 −20 −30
Cm/S
20
Bottom Temperature N2
X-strait current N2
Along-strait current N2
10 0
dynes/cm^2
−10 0.02 0.00 − 0.02 − 0.04 − 0.06 − 0.08
Meters
0.02
Along-strait Wind Stress
Along-strait Pressure diff
0.00 − 0.02 − 0.04
Meters
0.030
X-strait Pressure diff
0.015 0.000 −0.015 −0.030 May June July Aug Sep Oct 1999
Nov Dec
Jan Feb Mar 2000
Fig. 4. Monthly averaged bottom temperature at mooring N2 (upper) together with monthly averaged cross-strait currents and alongstrait currents at 10 m (solid line), 20 m (dashed line) and 30 m (dash-dotted line) above the bottom, monthly averaged along-strait wind stress, and monthly averaged along-strait and cross-strait bottom pressure anomaly differences.
with monthly averaged cross-strait (positive toward the SE) and along-strait currents (positive toward the NE) at 10, 20 and 30 m above the bottom, monthly averaged along-strait wind stress, and monthly averaged along-strait and cross-strait bottom pressure anomaly differences.
Table 2 shows the correlations, r; of bottom temperature at N2 with these parameters. With 11 data points, the only correlation that reached significance (>99%) was between bottom temperature and the cross-strait pressure difference anomaly. This latter parameter was formed by
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831 Table 2 Correlation with monthly bottom temperature at N2. U and V are cross- and along-strait current components, tx and ty are cross- and along-strait wind stress components. Pressure anomaly differences are formed from N6–N2 (cross-strait) and N2–S2 (along-strait) r
Parameter U at 10 m V at 10 m U at 30 m V at 30 m tx ty Along-strait pressure difference Cross-strait pressure difference
Temp (deg C)
15
0.47 0.54 0.25 0.29 0.10 0.04 0.32 0.92
Bottom Temp at N2
10
5
0 40
Bottom Along-Strait Current at N2
Cm/S
20
0
− 20
May June July Aug 1999
Sep
Oct
Nov
Dec
Jan Feb M 2000
Fig. 5. Daily averaged (light line) and monthly averaged (dark line) bottom temperature at mooring N2 (upper). Daily averaged (light line) and monthly averaged (dark line) alongstrait currents at 10 m above the bottom (lower).
taking the monthly average bottom pressures at N6 and N2, removing the mean from each and subtracting N2 from N6. The pressure anomaly difference represents a measure of geostrophic transport through the Straits with an error due to the uncorrelated pressure variations occurring in the vertical region between the geopotential levels defined at each instrument, and an error due to uncorrelated flow along the unsampled sides of the
827
Straits between the ends of the mooring line and each coast (comparison with ADCP measured total transport is given in Fig. 6). The along-strait pressure difference was formed in the same manner with the pressure anomaly at S2 subtracted from that at N2. The positive relationship between cross-shelf pressure anomaly variations and bottom temperature means that higher bottom temperatures occur with increased transport, and lower temperatures occur during decreased transport. The squared value of this correlation (r2 ) means that 85% of the variance in bottom temperature can be explained by this relationship. The negative correlation (r ¼ 0:32) of bottom temperature with alongstrait pressure anomaly variations implies the same relationship with transport through the Strait, although at this level it is not a highly significant statistic. It is interesting that although the correlations of bottom temperature to wind stress and bottom temperature to currents are both low, the correlation of along-strait wind stress (upwelling favorable against the Korean coast) to cross-shelf currents at N2 is r ¼ 0:74: This significant (>99%) and negative correlation indicates an upwelling relationship involving bottom currents. This has been suggested as a mechanism for lifting the cold water up against the Korean coast (Byun, 1987). But the low relationship with temperature indicates that this mechanism may be only marginally involved with cold water intrusions. Since monthly means may obscure shorter storm scale events, we also show the bottom temperature at N2 together with along-strait currents 10 m above the bottom at N2 (Fig. 5). These records were averaged daily to eliminate tidal currents and are overlain with the monthly averages. The bottom southward currents are clearly impulsive on storm scales of a few days. In the December/January time of sustained low bottom temperature, southward current pulses of characteristically 10 cm/s dominate. However, during the May/June intrusion of cold water, there was almost no southward flow. The overall correlation between daily averaged temperature and current records was only r ¼ 0:25; hence with 299 data points it is not statistically significant.
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831
Total Transport
4
0.04
0.02
S-Line Anomaly
0.00 2 N-Line May June July 1999
Aug
Sep
Oct
Nov
Dec
Jan Feb M 2000
− 0.02
Pressure Anomaly (m)
828
− 0.04
Fig. 6. Total transport across S-Line and N-Line as determined by ADCP along-strait current measurements together with cross-strait bottom pressure anomaly (dark line).
Correlation of daily geostrophic transport, as defined above, with temperature was also only r ¼ 0:26: This is rather surprising and indicates horizontal diffusion must be an important component of the appearance of the cold pool, occurring when monthly scale transport is reduced. Since monthly scale geostrophic transport appears to be a large influence on the appearance of the bottom cold pool, we need to validate it by comparison to total transport as measured by the ADCP along-strait currents. In Fig. 6, we show the monthly geostrophic transport anomaly, as previously defined by the across-strait bottom pressure anomaly, together with monthly average total transports along the N line and the S line. The comparison is quite good, and gives us some confidence in its use as a prediction tool. The differences between total and geostrophic transport are principally due to friction, i.e., local wind forcing which is embedded in the total transport. In Fig. 7a and b, we show plan views of currents averaged over all observations where the bottom temperature was either high or low. In the case of moorings N1 and N2, we chose 101C as a separation point. In the case of moorings N3–N6, we fit a third degree polynomial to the curves and took low temperature to mean observations when temperature was 21C below the fitted curve. This effectively isolated the low temperature pulses on the middle to mid-eastern side of the Strait. Referring to Fig. 7, we note that currents at C1 are always toward the East Sea; at N1 and N2, a
northward flow occurs during high temperature times and a cross-strait flow toward the Korean coast during low temperature times. At N3 and N4, bottom currents are relatively weak, but point cross-strait toward the coast of Japan (opposite to that at N1 and N2) during times of low temperature pulses. To further examine this relationship between flow and the KSBCW, Fig. 8 shows hodographs of monthly averaged currents from 10 to 100 m above the bottom at 10 m intervals at mooring N2. May and December are months of low bottom temperatures at N2, and October and March are months of relatively high bottom temperatures. During May and December, the currents higher in the water column are weak, and the bottom currents are relatively strong and pointed crossstrait toward the Korean Coast. During October and March the currents higher in the water column are stronger and the bottom currents are weak and pointed more along-strait. It seems clear, from Figs. 7 and 8, that characteristic bottom flows during times of low temperature do not support the concept of a strong advecting flow from the East Sea, but rather a small penetration and a recirculation back to the East Sea.
5. Summary KSBCW appearance in the Korea Strait was examined with an 11 month long array of bottom mounted ADCPs together with temperature and
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831 37
37
5 10 15 20 Velocity cm/s
5 10 15 20 Velocity cm/s
36
36
N1 N2 C1
N4 N5 N6
34
C1
N3 N4 N5 N6
34
33
33
(a)
N1 N2
35
N3
Latitude
Latitude
35
32 126
829
127
128
129 130 Longitude
131
32 126
132
(b)
127
128
129
130
131
132
Longitude
Fig. 7. (a) Currents at 10 m above bottom averaged over time of high bottom temperature (see text for definitions). (b) Currents at 10 m above bottom averaged over time of low bottom temperature.
pressure sensors. The KSBCW as a cold bottom water (o101C) intrusion appeared to be confined to the western side of the Strait and uplifted against the Korean coast. From bottom temperature time traces there was no obvious yearly cycle in cold water intrusions into the strait. Sustained intrusions occurred in May/June and again in December/January. Bottom currents during times of bottom cold water appearance showed no advective intrusion on a monthly averaged scale and only limited advective intrusion on a daily averaged scale. Rather, during low temperature episodes, bottom currents were directed toward the Korean coast on the west side and toward the coast of Japan in the middle and mid-eastern side of the Strait. This gave the appearance of a short adjective intrusion and a recirculation back into the East Sea. Of course, with gaps in mooring sites across the Straits, we might have missed some of the intrusion. But with moorings in the main low bottom temperature locations, this does not seem like a reasonable alternative. On a monthly scale, the bottom temperature signal from the moorings on the western side of the Strait correlated negatively with the cross-strait
bottom pressure anomaly. This was the only statistically significant correlation with bottom temperature among the various parameters, including currents and winds. The cross-strait bottom pressure anomaly is related to geostrophic transport variations through the Strait. The implication of the negative correlation is that the cold bottom water intruded when geostrophic transport to the East Sea was reduced and was flushed out when this transport increased. With fairly strong tides (Teague et al., accepted) to increase horizontal diffusion in the bottom layer, we can hypothesize that bottom cold water intrusions are the product of relatively weak advection augmented by horizontal diffusion. As stated in the introduction, a comparison with the Arctic Mediterranean and the Labrador Sea is probably appropriate in terms of warm inflow and cold outflow. The contradistinction is that inflow into the East Sea is high in comparison to the size of the opening (depth and width of the Korea Strait) and limits the size of the cold water outflow. If this hypothesis is correct, then the appearance of KSBCW can be predicted from the geostrophic transport through the strait. This can be tested
830
D.R. Johnson, W.J. Teague / Continental Shelf Research 22 (2002) 821–831
10
May
Dec
20
20
10 100
100 −20
0
0
20
−20
0
0
−20
−20
Oct 100
Mar
20
20
10 100
10 −20
0
20
0
20
−20
−20
0
0
20
− 20
Fig. 8. Hodographs of monthly averaged currents from 10 to 100 m above the bottom at 10 m intervals at mooring N2. May and December are months of bottom temperature at N2 less than 101C. October and March are months with temperature higher than 101C.
with long time series of bottom pressure records on each side of the strait and temperature records on the west side.
ment 0602435N (NRL-SSC contribution JA/7330/ 01/0067).
References Acknowledgements This work was supported by the Office of Naval Research as part of the Basic Research Projects ‘‘Linkages of Asian Marginal Seas’’ and ‘‘Japan East Sea DRI’’ under Program Element 0601153N, and as part of the Navy Ocean Modeling Program (NOMP) under Program Ele-
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