SPATIAL-TEMPORAL DISTRIBUTION AND CHARACTERISTICS OF INTERNAL WAVES IN THE OKHOTSK AND JAPAN SEAS

STUDIED BY ERS-1/2 SAR AND ENVISAT ASAR

L. M. Mitnik, V. A. Dubina

V.I. Il‘ichev Pacific Oceanological Institute, Far Eastern Branch, Russian Academy of Sciences 43 Baltiyskaya St., 690041, Vladivostok, Russia, E-mail: mitnik@poi.dvo.ru, dubina@poi,dvo.ru

ABSTRACT

Numerous packets of internal waves (IWs) were revealed on the ERS-1 and ERS-2 SAR and Envisat ASAR images of the Okhotsk and Japan Seas. High-resolution and quick look images were used for their detection and analysis. To improve temporal and spatial resolution, the visible images obtained by Terra and Aqua MODIS were also screened. Parameters of IWs such as number of waves in a packet, their wavelengths, a crest length, propagation velocity, etc. were estimated from the images. Propagation velocity of IWs in the open sea was computed from spacing between packets generated by successive tides as well as from displacement of internal wave signatures on Envisat and ERS-2 SAR images taken 30 min apart and on Terra and Aqua MODIS images taken in about 100 min apart. 1. INTRODUCTION The Japan and Okhotsk Seas are the semi-enclosed seas located in the Northwest Pacific Ocean and bounded by 34°N at the south and 62°N at the north. The seas are connected via the Tartar Bay and La Perouse Strait. The seas are influenced by a complex current system that includes the Tsushima Current (a small branch of the Kuroshio) that enters into the Japan Sea through the Korea Strait, the Soya Warm Current along the Hokkaido coast, as well as flows through the Tsugaru Strait, straits in the Kuril Islands, etc. The tidal flows in the Japan Sea and especially in the Okhotsk Sea are complicated. All types of tides are in the seas however only in the Okhotsk Sea each of them covers vast areas and is characterized by significant magnitude [1, 2]. Interaction of tidal flows with the bottom features such as underwater mountains and canyons and a continental shelf results in internal wave generation in waters with stable stratification. They can be particularly pronounced where pycnocline is well developed. Variable currents associated with IWs can significantly modify short wind waves with lengths < 0.5 m. In moderate wind conditions, this can be observed both by radar and visually from ships and satellites. Information on internal waves is useful for oceanographers to understand better energy transfer mechanisms across continental shelves, sediments distribution, acoustic signal variations, etc., for the off

shore oil industry and others applications. However characteristics and spatial-temporal distribution of IWs in the Japan and Okhotsk seas have not been adequately investigated: in situ observations of IWs were carried out only in several places and number of the satellite images taken by SAR and optical instruments, on which surface manifestations of IWs were clearly detected is also small [3, 5, 6, 8-10 and referenced cited there]. This study is focused on the statistical analysis and case studies of IWs detected on satellite high resolution radar and medium resolution visible images. Internal waves’ pattern registered on satellite images can be used for estimating several quantitative characteristics like wavelength change within a wave packet, length of crests waves, propagation speed and direction, etc. that permits to relate them with hydrological structure in the area under consideration.. 2. THE DATA SET

Analysis of spatial-temporal variability of IWs was performed with the usage of archive ERS-1, ERS-2 and Envisat images as well as Terra and Aqua MODIS images. SAR PRI images were provided by the European Space Agency for implementing of the AO3-401, AO 391 and AOBE.2775 projects. The SAR images were acquired in 1992-2006 and cover the sea surface between 35−62°N and 130−170°E. More than 580 PRI images are now in our database: 350 frames for the Japan Sea and 230 frames for the Okhotsk Sea. More than 1100 quick look (QL) images were also screened. IWs were detected on 135 SAR images. IWs possessingg by high radar contrast were also revealed on QL images. Detailed quantitative analysis of parameters of IWs was carried out for several cases. Original two-bite SAR PRI images were transformed in two files: one-byte raster in JPG graphic format and in a two-dimensional array of the NRCS values in a linear scale. The NRCS values were found with the usage of SAR TOOLBOX utilities developed at the ESA and input values of backscatter signal that are in PRI file. At first the resolution of both files decreased from 12.5 to 100 m for ERS-1/2 SAR images and from 75 to 300 m for Envisat ASAR images. The resulting pixels of graphic file were computed using 8 х 8 matrix and bicubic interpolation. The NRCS values were averaged

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Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

by a window, which had the same size. Graphic file was subjected to equalization by histogram correction. Correction of brightness trend as a function of incidence angle was performed if required. Such two files provide acquisition of reliable quantitative information on radar contrasts at optimal image visualization. Archive of SAR images together with metadata was integrated in the oceanographic GIS based on Internet techniques. MODIS images were downloaded through MODIS Rapid Response Project portal at the NASA Goddard Space Flight Center http://rapidfire.sci.gsfc.nasa.gov/ . True color images with resolution ∆l = 250 m were constructed by a combination of 1, 4 and 3 spectral channels. All cloudless and partly-clouded images acquired by Terra and Aqua MODIS in August-October and some images acquired in May-June and in November were analyzed for IWs detection. To eliminate false detection, the same packets of IWs or individual solitons should be revealed both on Terra and Aqua images. Time difference between Terra and Aqua acquisition is 80-110 min. 3. RESULTS

3.1. Japan Sea Internal waves were detected on 63 SAR images and observed on the almost whole sea area where a depth ranged from 30 to 3500 m. Coverage of the sea, however, was not uniform. They were revealed on the SAR images taken from May to December and on the MODIS images taken from August to October. The IWs propagated mainly on shore and off shore approximately parallel to isobaths. However some packets propagated in the directions approximately normal to the isobaths. The distance between the first and the second solitons (wavelength) in packets ranged from 100 m up to 14.5 km; number of waves in a packet reached 50. Length of leading crest varied from 4 to 250 km. Propagation speed of IWs in the open sea changed between 0.7 and 1.8 m/s. Surface manifestations of IWs on SAR images differ noticeably that reflect distinctions in physico-geographical conditions in different areas of the sea that follows from consideration of examples given below. 7 September 1997, 01:32 UTC. ERS-2 SAR image shows several packets of internal solitons (Fig. 1а). The area covered by SAR is shown by red rectangle in Fig. 2b. Packets А and В (arrows mark leading solitons of these packets) were very likely generated by interaction of westward propagating semidiurnal tidal wave with shelf. The distance between the first solitons of packets В and А ∆L = 42.6 km. Propagation speed of the first solitons relative to the bottom was estimated from relationship V = ∆L/T, where T = 12 h 25 min is

semidiurnal tidal period. For packet А, speed V = 0.95 m/s. For packet В, propagation speed (V ≈ 0.8-1.0 m/s) was estimated from the distance between the first soliton of this packet and narrow light bands C in a dark area, which are likely signatures of the next packet. Packet А consists at least of 16 solitons. The crest length of leading soliton is ≈140 km. Imprints of these solitons in the sea surface roughness field are seen completely at the center, partly to the north of the center and piecewise in two regions to the south of the center. The distance between successive solitons at the center of packet А decreases monotonically from the front to the rear from 2.5 km to 300 m (see the NRCS profile in Fig. 2d along section shown by dark arrow in Fig. 2a). For packet В, wavelength λ between successive waves decreases monotonically from ≈5 km (front area) to 300 m (rear area at the distance of ≈37 km). Wave wake with a width of about 5 km and waves with λ ≈200 m are clearly visible in the rear area. Several first solitons of packet А incident on the wake. Waves of packet A are characterized by high intensity: increment between the maximum and minimum NRCS values (between light and dark bands) reaches 8 dB that illustrates NRCS profile in Fig. 1d. Parameters of solitons in packets А and В are summarized in Tab. 1.

Table 1. Parameters of internal wave packets Parameter Symbol A B

Packet length, km L 17 42.6 Number of waves N ≥ 16 ~30 Maximum wavelength, km λmax 1.0-2.7 4.4-6 Minimum wavelength, km λmin 0.3 0.3 29 August 2000, 13:27 UTC. 7 packets of IWs are visible on ERS-2 SAR image in southwestern part of Peter the Great Bay (Fig. 2). Crest length of leading solitons in the packets is ≈8-10 км. Leading waves are expressed as dark undulatory lines. Successive solitons are distinguished only in the southern parts of leading waves and a length of their crests is 2-4 km. Maximum wavelength in the packets λmax ≈200-300 m, and minimum one λmin < 100 m. Packet C has approximately 20 waves. They are characterized by higher radar contrast compare to the others packets. The packets move along shelf edge in direction of underwater canyon, the location of which is traced by 200-m isobath at the right of the image. Canyon is the place of IWs generation. They propagate mainly onshore but also in direction opposite to a case under consideration.

Seven trains of IWs were reliably distinguished on Terra MODIS visible image taken on 15 September 2003 over the central basin where a depth exceeds 3000 m [10]. IWs propagated in several directions.

(a)

A

B (b)

(d) (c)

Figure 1. (а) ERS-2 SAR image (100 x 200 km) showing internal wave packets west of Sangar Strait, 7 September 1997 at 01:32 UTC; (b) enlarged fragment of the image shown by dark rectangle in (а); (c) and (d) NRCS profiles along

sections marked by arrows in (а) and (b), respectively.

Figure 2. (a) Fragment of ERS-2 SAR image (16 x 21 km) showing several packets of small-scale internal waves in Peter the Great Bay. (b) Chart of the Japan Sea showing depth contours (m) and the location of SAR images taken on

7 September 1997 (large red rectangle) and on 20 August 2000 (small red rectangle).

Maximum wavelength fall in the range between 3.5 and 8 km, crest lengths varied from 40 to 122 km and propagation speed changed from 0.6 to 1.6 m/s. Spatial distribution of internal waves summarizing the satellite observations is given in [10]. 3.2. Okhotsk Sea

In the Okhotsk Sea IWs were observed from April till December. They were detected both in the coastal zone (Aniva Bay, Sakhalin shelf, northern shelf) and in the open sea. They are characterized by the broad range of parameters such as wavelength, width of a packet, crest

length, etc. IWs generated by continental shelf east of Sakhalin sometimes formed a common front, length of which reached about 500 km [5]. Two elongated common fronts formed by internal wave packets propagating on shore are distinguished at the left of SAR image acquired on 28 June 1995 (Fig. 3). They were generated by two successive tidal flows. The

(a)

(a)

(b) (c)

(c) (b)

(d)

(d)

F igure 3. (a) ERS-1 SAR image (100 x 170 km) taken on28 June 1995 at 12:28 UTC showing the signatures of

internal waves along the east coast of Sakhalin generated by two consecutive tidal flows. (b), (c) and (d) Profiles of radar backscatter along transects A, B,

and C indicated by color arrows in the image.

Figure 4. Fragment ERS-2 SAR image (80 x 170 km) acquired on 4 September 2001 at 12:29 UTC, showing several internal wave packets near northern Sakhalin.

(b), (c) and (d) Profiles of radar backscatter along transects A, B, and C indicated by color arrows in the

image.

distance between the fronts varies between 30 and 40 km. The complexity of the signatures indicates that there are a large number of internal wave sources on the adjacent shelf. The complexity of signatures is typical for field of internal waves in the various parts of the Okhotsk Sea, in particular, to the north of Sakhalin (Fig. 4), near Terpeniya Cape (Fig. 5) and in the Aniva Bay (Fig. 6). Example of internal waves appearance in a quick look image is presented in Fig. 5. Terpeniya Cape is seen at the bottom of an image. IWs propagate in different directions and sometimes cross each other at right angle. Complicated shape of the leading solitons (they possess by the highest contrast against the surroundings waters) is very likely due to variability of bottom topography as well as hydrological structure in the area of Cold East Sakhalin Current [2]. MODIS 250-m resolution visible images also allowed to detect internal wave packets. In particular, 11 internal wave packets propagating mostly westward were registered on Terra and Aqua MODIS images acquired on 15 June 2005 at 01:30 and at 03:10 UTC, respectively over the Sakhalin shelf. Propagation speed estimated by the measuring displacement of the leading solitons ranged approximately from 0.6 to 1.1 m/s. Aniva Bay is characterized by the increased biological productivity and intensive fishery. Analysis of ERS-1/2 SAR and Envisat ASAR images of the Bay have shown that the IWs are rather common phenomenon in a warm period when there are strong-stratified waters to the

north of the Soya Warm Current [2, 5]. Combination of stratification, currents and variable bottom topography is favourable for internal wave generation. Essentially, the internal waves in the area were not investigated. Now they are accessible for study by joint analysis of their surface manifestations on SAR images [3, 5, 6, 8-10] together with map of bathymetry and weather maps. Several packets of the IWs propagating mainly westward and eastward are visible on Envisat ASAR image acquired on 15 August 2005 (Fig. 6a). A width of the wave packets is 1-10 km long. The length of several first crests exceeds 40 km. Within the packets, the wavelengths λ appear to be monotonically decreasing, from the front to the rear. So for packet A, λ decreases from 1.8 to 0.9 km (Fig. 6b), for packet B from 2.5 to 2.0 km (Fig. 6c), and for pocket C from 0.9 km to 0.5 km (Fig. 6d). Weak low-contrast IWs were also detected within the Soya Warm Current (Fig.6a).

(a)

Fi

(b)

(c) (d)

Figure 6. (a) Internal waves in Aniva Bay on Envisat ASAR image taken on 15 August 2005 at 00:49 UTC.

(b), (c) and (d) Profiles of radar backscatter along transects A, B, and C indicated by arrows in the image.

gure 5. Envisat ASAR quick look image (100 km x 200 ) acquired on 31 August 2005 at 00:45 UTC showing

several internal wave packets. km

4. DISCUSSION AND CONCLUSIONS IWs were observed over the northern Asian Marginal Seas: the Bering, Okhotsk, Japan and Yellow Seas [3]. The main sources of quantitative spatial information to examine these IWs are satellite data retrieved from measurements of various satellite sensors. A wide swath SAR such as Envisat ASAR, RADARSAT SAR and ALOS PALSAR is an important tool for the study of IWs over the ocean since it can contribute the high-resolution data. From above it appears that the clearly surface manifestations of the internal waves and other dynamic features were found on many ERS-1/2 SAR and Envisat ASAR images of the Japan and Okhotsk Seas. Promising sources of regularly available remotely sensed data (as opposite to occasional ASAR images) are the Terra and Aqua MORIS spectroradiometer. All these sensors are characterized by a wide swath and possess improved spatial resolution. The time series of these images permit estimating the appearance and displacement of the IWs and others dynamic features and their changes under influence of environmental factors. In particular, in this connection fusion of SAR, thermal infrared and ocean colour data should give a deeper understanding of relations between physical and biological processes since the ocean colour (chlorophyll a concentration) images visualize the near-surface circulation.

Apart of detection of the IWs, SAR images provide the most important information on the interior ocean by analysis of the IW imprints and their comparison with the average hydrological data for particular areas as well as with the results of modelling.

The database of IWs in the Japan and Okhotsk Seas was built up. It includes SAR images and Terra and Aqua visible images. The images were used to derive statistics of internal waves. All analyzed SAR and visible images in combination with data obtained earlier were summarized to produce the most complete and detailed maps showing spatial distribution of the internal waves in the Japan Sea [10] and in the Okhotsk Sea. Parameters of internal waves in two seas estimated by analysis of satellite SAR and visible images are given in Tab. 2. Table 2. Parameters of internal waves in the Japan and

Okhotsk Seas Parameters Japan Sea Okhotsk

Sea Packet length, km 50 20 Number of waves in packet ≥55 ≥25 Maximum wavelength, km 14.5 5.8 Minimum wavelength, km <0.1 <0.1 Leading crest length, km 4-250 4-105 Maximum light band width, m 500 500 Maximum dark band width, m 800 800 Propagation speed, m/s 0.7-1.8 0.6-1.4

Acknowledgments. This study has been carried out within ESA ERS project AO3-401, Envisat project AO-ID-391 and ESA-IAF Bear project AO-2775. This work is partly supported by INTAS Project 03-51-4987, RFBR Projects: 06-05-65177-а and 06-05-96076-р_vostok_а, and Russian State project: “Investigation of ocean-atmosphere system with passive and active microwave sensing from new generation of satellites.”

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2. Sea Hydrometeorology and Hydrochemistry, vol. 9, The Okhotsk Sea, Issue 1, Hydrometeorological Conditions, Eds. B.Kh. Glukhovskiy, I.P. Goptarev, and F.C. Terziev, Sankt Petersburg: Gidrometeoizdat, 1998, 343 pp. (in Russian).

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