Near-Surface Frontal Zone Trapping and Deep Upward Propagation of Internal Wave Energy in the Japan/East Sea
Total Page:16
File Type:pdf, Size:1020Kb
900 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 33 Near-Surface Frontal Zone Trapping and Deep Upward Propagation of Internal Wave Energy in the Japan/East Sea ANDREY Y. S HCHERBINA AND LYNNE D. TALLEY Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California ERIC FIRING AND PETER HACKER Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii (Manuscript received 20 December 2001, in ®nal form 15 October 2002) ABSTRACT The full-depth current structure in the Japan/East Sea was investigated using direct velocity measurements performed with lowered and shipboard acoustic current Doppler pro®lers. Rotary spectral analysis was used to investigate the three-dimensional energy distribution as well as wave polarization with respect to vertical wave- numbers, yielding information about the net energy propagation direction. Highly energetic near-inertial down- ward-propagating waves were found in localized patches along the southern edge of the subpolar front. Between 500- and 2500-m depth, the basin average energy propagation was found to be upward, with the maximum of relative difference between upward- and downward-propagating energy lying at about 1500-m depth. This difference was most pronounced in the southeastern part of the basin. 1. Introduction western boundary current along the coast of the Korean peninsula. It separates from the coast at 378±398N, The Japan Sea (also known as the East Sea) is a where it meets the southward-¯owing North Korea Cold marginal sea of the northwestern Paci®c, separated from Current (NKCC). This con¯uence forms the subpolar the open ocean by the islands of Honshu, Hokkaido, front that extends eastward as a series of meanders and and Sakhalin (Fig. 1a). It consists of three fairly deep mesoscale eddies (Fig. 1c). basins: the Japan basin in the north being the deepest The Tsushima Current, joined by a few countercurrent (over 3500-m depth), and the Ulleung and Yamato ba- branches of the EKWC, continues northeastward along sins (over 2000-m depth) in the southwest and southeast the coast of Japan, generally following the jagged to- respectively, separated by the Yamato rise at about 398N, pography of the coastline and continental slope, giving 1358E. The connection with the open ocean is through four straits less than 200 m deep: Tsushima strait in the rise to high mesoscale eddy variability in the Yamato south and Tsugaru, Soya and Tatar straits in the north- Basin. The out¯ow through Tsugaru Strait provides a east. small but important input of warm saline water into the The circulation in the Japan Sea consists of two well- Kuroshio±Oyasho mixed water region east of Honshu de®ned gyres, separated by the subpolar front at about (Talley 1991). 408N. A branch of the Kuroshio Current enters through Long-term mooring observations (Takematsu et al. Tsushima Strait as two jets along either side of Tsushima 1999b) have shown a high level of internal wave energy Island: the East Korea Warm Current (EKWC) through in the Japan Sea with a sharp peak at the inertial period the western channel (major part of the in¯ow) and the of about 18 h. The most energetic inertial oscillations Tsushima Current (TC) through the eastern one (Perkins were observed in moorings located in the central parts et al. 2000). Both total in¯ow and relative transports of of the Japan Sea and in the vicinity of the subpolar the branches vary signi®cantly in time around a mean front. Unfortunately, the sparse mooring coverage could value of 2 Sv (Sv [ 106 m3 s21). The EKWC forms a not provide much detail regarding the spatial variations in the near-inertial wave ®eld. However, such variations might be anticipated, given the strong mesoscale vari- Corresponding author address: Andrey Shcherbina, Physical ability in the region which can modulate the internal Oceanography Research Division, Scripps Institution of Oceanog- wave ®eld (e.g., Mooers 1975a). raphy, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0230. In summer 1999 a comprehensive hydrographic sur- E-mail: [email protected] vey of the Japan Sea was conducted with the objective q 2003 American Meteorological Society Unauthenticated | Downloaded 09/28/21 12:20 PM UTC APRIL 2003 SHCHERBINA ET AL. 901 FIG. 1. (a) Map of the Japan/East Sea. Numbers indicate (1) Sakhalin Island, (2) Noto Peninsula, (3) Korean Peninsula, (4) Oki Island, and (5) Tsushima Island. (b) Cruise tracks and station positions for R/V Revelle (24 Jun±17 Jul 1999) and R/V Professor Khromov (22 Jul± 11 Aug 1999). (c) AVHRR sea surface temperature snapshot (courtesy of R. Arnone and S. Ladner, Naval Research Laboratory, Stennis Space Center). (d) Dynamic height at the sea surface relative to 500 m. Also shown is the cross-frontal section used in subsequent ®gures (gray line). of obtaining ``a complete synoptic view of the vertically eddy structures. A distinctive feature of the project was layered structure and major components of the circu- the extensive use of shipboard and lowered acoustic lation'' of the basin. The survey included a series of Doppler current measurements to gather a ®rst (to our ®eld experiments to study local features including the knowledge) comprehensive set of velocity pro®les cov- Tsushima and Liman Currents, the subpolar front, and ering most of the Japan Sea basin and extending to the Unauthenticated | Downloaded 09/28/21 12:20 PM UTC 902 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 33 bottom. In this paper, the velocity measurements are station drift of the Khromov occasionally led to the tilt analyzed to infer the horizontal and vertical distribution of the LADCP package beyond the measurable range of internal wave energy in the basin, with particular of 208, invalidating fragments of a few pro®les. As a attention paid to variations in the internal wave ®eld in result, errors in excess of 30 cm s21 in the absolute the frontal zone. velocity were found on several Khromov stations. Be- cause of a large number of factors affecting the cal- culation procedure, such errors are sometimes dif®cult 2. Data to recognize and quantify. The shear data, however, were The Japan Sea survey was made with two ships. The unaffected by these problems. A total of 19 LADCP southern part of the Japan Sea was covered by R/V pro®les (10 for the ®rst cruise and 9 for the second one) Revelle (Scripps Institution of Oceanography, United were excluded from the analysis because of severe gaps States) between 24 June and 17 July 1999; the survey in shear or inconsistencies between the up- and down- was continued in the northern and northeastern regions casts. by R/V Professor Khromov (Far Eastern Regional Hy- R/V Revelle is also equipped with a 150-kHz ship- drometeorological Research Institute, Russia) from 22 board ADCP that provided underway velocity mea- July to 11 August 1999. A total of 203 stations were surement with 5-min sampling interval and 8-m vertical occupied with CTD/Rosette sampling for temperature, resolution. For most of the cruise the concentration of salinity, oxygen, nutrients, chloro¯urocarbons, and oth- scatterers in the water allowed reliable velocity mea- er tracers (Fig. 1b). surements to about 300-m depth. The ADCP data were The CTD/Rosette used on most stations (with the processed using the Common Oceanographic Data Ac- exception of three casts during severe weather condi- cess System (CODAS) software package. No shipboard tions) was equipped with a lowered acoustic Doppler ADCP data were available during the Khromov cruise. current pro®ler (LADCP). With the exception of four stations inside eddies in the Japan Basin, every station extended to within 20 m of the bottom, with most within 3. Trapped internal waves in the subpolar 10 m. front region The LADCP is a self-contained broadband 150-kHz a. Background RD Instruments ADCP mounted on a standard rosette frame. This con®guration provides top-to-bottom ve- The subpolar front in the Japan/East Sea extends locity pro®ling with each CTD cast (Firing and Gordon across the basin at about 408N and is visible in both 1990). As the instrument is being lowered and then temperature and salinity ®elds. The front is observed hoisted back at the station, the ADCP records water throughout the year, but its latitude varies with season. velocities within its range approximately every second. In the synoptic sea surface temperature ®eld (Fig. 1c) As the ADCP used lacks a pressure sensor, the record from the Advanced Very High Resolution Radiometer is synchronized with the CTD pressure time series. The (AVHRR) on 8 June 1999, about two weeks prior to set of velocity pro®les, overlapping in depth, is con- the start of the survey, the front is clearly visible at its verted to vertical shear and averaged in 5-m depth bins. summer position between 398 and 408N. This satellite This procedure is necessary to remove the time varying image also shows the complex structure of the frontal (but constant for each individual pro®le) bias in the zone, consisting of a number of meanders and mesoscale velocity data, induced by the movement of the package eddies. Such structure is characteristic of all synoptic through the water. The shear pro®le is then integrated images of the front. The front is apparent in the CTD and combined with the GPS navigation data during the survey also. Dynamic height at the sea surface relative cast to get an absolute velocity pro®le (Fischer and Vis- to 500 m (Fig. 1d) shows the front at 408N looping to beck 1993). Instrument characteristics prevent reliable the south at 1358E. measurements within about 45 m from the bottom or In course of the survey, two overlapping cross-frontal 30 m from the surface.