WATER DYNAMICS IN THE KURIL STRAITS AREA STUDY WITH ERS SAR ABSTRACT

WATER DYNAMICS IN THE KURIL STRAITS AREA: STUDY WITH ERS SAR Leonid Mitnik, Vyacheslav Dubina, Vyacheslav Lobanov, and Tatyana Supranovich V.I. Il‘ichev Pacific Oceanological Institute, Far Eastern Branch, Russian Academy of Sciences

43 Baltiyskaya St., 690041, Vladivostok, Russia

Phone: 07.4232.312854, Fax: 07.4232.312573, E-mail: mitnik@online.vladivostok.ru

ABSTRACT

Water circulation in the area of the Kuril straits is characterized by joint action of tidal and nonperiodic flows between the Okhotsk Sea and the Pacific Ocean. Strong tidal currents and rough bottom topography around the islands play an important role in local hydrography and are responsible for formation of fronts, rips, wakes and vortices, and packets of internal waves. ERS-1 and ERS-2 SAR images of the Yekaterina Strait which separates the islands Kunashir and Iturup and the Friz Strait between the islands Iturup and Urup in the southern Kuril Islands are analyzed together with ancillary data and calculated tidal currents to study peculiarities of water dynamics in the southern Okhotsk Sea. The SAR signatures of the ocean surface are related to the changes of water mass properties, current shear, natural surfactant concentration, and stability of the boundary layer of the atmosphere. The SAR images have allowed us to identify the submesoscale features such as tidal fronts in the strait area, eddies of 1-10 km in diameter as well as intrusion of the Okhotsk Sea waters into the Pacific Ocean. Pattern of the shear fronts on the SAR images varies with a tidal phase. However the main areas of tidal energy dissipation seem to be stable and are tightly related to the bottom topography. Typical spatial patterns of water dynamics in the Yekaterina and Friz Strait area and new fine details of the surface water structure are discussed.

1. INTRODUCTION

Satellite sensing of the ocean with synthetic aperture radar provided a new way for study of phenomena, which are characterized by high small scale and mesoscale spatial variability. ERS SAR images have a spatial resolution of 25 m within a swath width of 100 km. A SAR image represents practically instant imprint of sea surface roughness. Both the oceanic and atmospheric phenomena affect on a field of the surface roughness and thus a SAR can detect the imprints of their exposure. Application of a such powerful tool is a prime consideration for study of those dynamic areas of the World Ocean for which firm and brief description is absent in spite of multiyear observations. Straits between the Kuril Islands are among these areas.

Since the discovery of these islands, mariners mark interesting, surprising and sometimes dangerous phenomena on the sea surface such as strong currents with different directions, eddies, bands of foam, etc. [1,2]. In this respect two southern straits, the Yekaterina Strait and the Friz Strait stand out especially. Researchers point out the strong and poor explained short period water dynamics in the area [2-4]. As a rule, all above-mentioned phenomena are associated with tidal currents. They have a complex character due to interaction of tidal wave with the features of coast topography and bottom topography. However tidal current pattern in spite of its complexity has a periodic character and can be computed with astronomical data. Another important problem is ambiguity in estimate of water exchange between the Okhotsk Sea and the Pacific Ocean. A high probability of fog in summer and cloudiness in winter hampers the application of satellite sensing in visible and infrared spectral bands to study this area. Taking into account this fact and a high sensitivity of radar backscatter to the sea surface roughness variations a special program was initiated to study water dynamics in the Kuril strait area with a SAR.

The main goal of this ongoing research is detection of those relatively stable, repetitive features on the SAR images of the Yekaterina and Friz straits which are caused by only tidal current dynamics, bottom topography and coast outlines. Elimination of other factors from consideration is justified by the following reasoning. First, nonperiodic currents in the straits are in average in 2-3 times less than tidal currents and a probability of maximum velocities of these currents is approximately 2-6 % [3]. Secondly, the average constant flows around the islands are anticyclonic and roughly stable. Thirdly, the maximum radar contrasts induced by oceanic dynamics are observed at wind speed W = 3-8 m/s. At W > 7-8 m/s wind influences on tidal currents, however, the SAR images obtained at strong winds were eliminated from consideration because of the contrasts resulted from dynamic factors dissipated. Analysis of SAR images was carried out with usage of ancillary information such as NOAA AVHRR images, surface analysis maps, meteorological observations of coastal stations located on the Kuril Islands, and ship reports.

2. BACKGROUND

The Friz Strait is a trough between the islands Iturup and Urup with a depth more than 500 m. An average depth is 385 m and maximum depth is 1031 m [3]. The minimum width of the strait is approximately 40 km. The Yekaterina Strait separates the islands Kunashir and Iturup. A width of the strait is approximately 20 km. At the middle portion of the strait the depth is in the range from 82 to 437 m [5].

In all, water dynamics in the area of the islands Kunashir, Iturup and Urup is characterized by anticyclonic circulation around each of the islands. Integral flows in the Yekaterina and Friz straits direct from the Okhotsk Sea to the Pacific Ocean. During summer-autumn period, when the Soya Current is well developed, its waters can be entrained in flows along the western side of the straits. The thermal fronts with the gradients up to 1.6°C/km form in the Yekaterina Strait in this case [3]. Double-sided circulation is more stable in the Friz Strait compare to the Yekaterina Strait where it strongly disturbed by tidal currents and by season variability. Waters of the cold Oyashio Current, which flows southeast of the Southern Kuril Islands, are also entrained in circulation in the straits. The frontal zones across the straits and “cold” water patches near the southwestern coast of Iturup and Urup are among other thermodynamic features that were revealed during multiyear oceanographic expeditions [3,4].

Tidal currents affect strongly the circulation and water properties in the Yekaterina and Friz strait area. In connection with this consider the main concepts and definitions associated with tides.

An index of tidal current J

J = (V k1+V o1)/V m2,

where V k1 and V o1 are amplitudes of the main diurnal waves ,1 and 01 and .2 is amplitude of the main semidiurnal wave are used for definition of kind of tide in accordance with Duvanin classification [6].

Tide belongs to regular semidiurnal at 0 ≤ J ≤ 0.5, to irregular semidiurnal at 0.5 ≤ J ≤ 2.0, to irregular diurnal at 2.0 ≤ J ≤ 4.0, and to regular diurnal tides at J≥4.

The term "current" will denote tidal current. Other currents are defined in each particular case. The terms "flood" and "ebb" are used for definition of tidal currents directed from the Pacific Ocean in the Okhotsk Sea and vice versa, correspondingly.

Meteorological conditions are determined by monsoon circulation. The northern and northwestern winds are prevailing in winter and the southern and southeastern winds are prevailing in summer. Cyclonic circulation is typical for the Okhotsk Sea in winter with maximum of its activity in December-January. Anticyclonic circulation prevails from April to September with maximum in June-July [4].

3. DATA. SAR IMAGES

ERS-1/2 SAR images over the area under study were provided by the European Space Agency for implementing of the AO3-401 project “Mesoscale oceanic and atmospheric phenomena in the coastal area of the Japan and Okhotsk seas: Study with ERS SAR and research vessels”. SAR images in a format of CEOS Precision Image were recorded on CD-ROM-s. Procedures of pasterization and equalization were applied to the images for their visual analysis.

The Friz Strait area was covered by a frame 909 at descending tracks (local time 22 h 13 min) and by a frame 2691 at ascending tracks (local time 11 h 02 min) (Fig.1). Additionally, two ERS SAR images were obtained over the eastern part of the strait. 5 SAR images were acquired in December – May and 9 images – in June-October. Two 909 and eight 2691 frames are considered in the paper.

15 ERS SAR images were obtained over the Yekaterina Strait. All images were acquired at ascending tracks, frame 2709, (local time 11 h 02 min) (Fig.1). 6 SAR images were taken in December – May and 9 images – in June-October.

Fig.1. Bathymetric chart of the southern Kuril Region. Solid rectangles show the locations of ERS SAR frames 909 and 2691 over the Friz Strait and frame 2709 over the Yekaterina Strait. Red dots mark the points for which tidal currents were calculated.

Tidal Currents

Friz Strait

There are two points of current observations in a central portion of the strait (Fig.1). The first one is in a trough, and the second one is in the Pacific Ocean to the south of the first one. At the southern point J = 2.5 and thus tidal currents are irregular diurnal. Current variability is determined by spring and neap time and predominantly by moon declination.The variations are very complex and can be characterized by the computed values for particular dates. The maximum semidiurnal tidal currents are observed in approximately 1.5 days after new moon and after full moon and the maximum diurnal ones - in 1.5 days after the maximum moon declination. Maximum current velocity reaches 1 m/s. At the northern point J = 3.3 and thus tidal currents are mixed with a large diurnal component. Current variability is determined mainly by moon declination. Maximum currents are observed in approximately 1.5 days after the maximum moon declination. Maximum current velocity reaches 1.2 m/s. Hourly velocity and direction of tidal currents were computed for these points to compare the locations of SAR signatures with the results of the simulations.

Yekaterina Strait

There are 8 points of current observations in the strait (Fig.1). Fig.2a shows distribution of tides by their kinds. Index J changes in the strait from 4.6 to 6.8 that characterizes well-defined diurnal tides. Tidal currents with J = 6.8 are rear in the Far Eastern seas bordering Russia. Current variations follow moon declination only. Irregular diurnal currents are observed near the strait in the Okhotsk Sea and in the Pacific Ocean. Irregular semidiurnal tides are in the Okhotsk Sea further from the strait where J ≤ 2. Maximum current velocities are observed in June and December and the minimum ones in September and March. Fig.2b shows distribution of maximum possible currents velocity. Maximum velocity is near Iturup where it reaches approximately 4 m/s. In the central part of the strait maximum current velocity exceeds 2m/s. Current velocities decrease to ≤ 0.8 m/s near the strait in the Okhotsk Sea and in the Pacific Ocean. Average yearly 44°N 45°N 146°E

148°E Malokurilsk YuzhnoKurilsk Kurilsk Kunashir

Ituru

Urup Okhotsk

Sea

45°N 2691

9092709

Pacific Ocean 46°N 150oE

Yekaterina

Strait

Friz Strait

a)b)

Fig.2. Types of tides in the Yekaterina Strait area: red – diurnal tides, blue – irregular diurnal tides, green – irregular semidiurnal tides. Numbers indicate the values of index of tidal current J (a). Maximum velocity of tidal current in meters per second (b).

maximum velocities are about 0.6 maximum possible current velocity. Hourly velocity and direction of tidal currents were computed at 8 points to compare the computed values with the features revealed on the ERS SAR images.

4. OVERVIEW OF SAR IMAGES

In this study, we have analyzed 27 precision processed SAR images acquired over the Friz and Yekaterina straits by the ERS-1/2 satellites in 1992- 2000, as provided by the European Space Agency (ESA).

Friz Strait

12 ERS SAR images of the Friz Strait were used to study the features of tidal circulation in the strait. 6 SAR images were taken during flood, 4 during ebb, and 2 during the change of tidal phase. The surface imprints of the oceanic signatures were absent on 3 SAR images due to strong winds.

The linear features are often detected on SAR images. They are observed in a form of either the contrast lines or the boundaries, which separates the areas with different brightness. Such features are both in the Okhotsk Sea and in the Pacific Ocean. However, they rear cross the strait. Their length reach 50-60 km. From analysis of the location and shape of these features as well as brightness distribution of the SAR images it may be suggested that radar pattern is determined by transfer through the strait of waters with different thermal properties and by various eddy phenomena which accompany this transfer. Very likely that water dynamics in the Friz Strait area depends on the synoptic scale and mesoscale processes in the Oyashio Current and in the Okhotsk Sea area near the strait. Thus sophisticated treatment of the processes in the Friz Strait can be done only with usage of data for the broader area including the SAR images to the north and to the south of the frames 2691 and 909.

The area of the increased brightness, which is near Cape Van-der-Lind on the western Urup, is commonly occurring feature. This crescent-shaped feature begins in shallow water near the cape and points towards the Okhotsk Sea (during flood phase) or towards the Pacific Ocean (during ebb phase). The length of this feature is approximately 15-20 km. With amplification of flood tide, its shape is transformed and the bright complex radar signatures arise in the northwestern part of the strait. Fig.3 presents four stages of development of the considered process. All fragments orient north for convenience of a comparison. ERS-1 SAR image acquired on 3 June 1995 at 00:50 UTC (Fig.3a) is characterized ebb tide. The crescent- shaped feature points to the Pacific Ocean. This feature points towards the

Fig.3. Fragments of ERS SAR images of the eastern Friz Strait showing generation and evolution of eddy-like formation on Urup shelf on 3 June 1995 at 00:50 UTC (ebb) (a), on 25 September 1998 at 00:53 UTC (flood tide) (b),on 7 September 1993 at 00:53 UTC (strong flood tide) (c) and on 27 October 1992 at 00:53 UTC (the strongest flood tide) (d).

Okhotsk Sea on ERS-2 SAR image taken on 25 September 1998, at 00:53 UTC (Fig.3b) during initial stage of flood tide. At stronger flood tide, which was observed on ERS-1 SAR image acquired on 7 September 1993 at 00:53 UTC,the crescent- shaped feature is transformed in more complex and bright one and displaced to the northwest (Fig.3c). At last ERS-1 SAR image obtained on 27 October 1992, at 00:53 UTC demonstrates the large area with increased sea surface roughness which was observed during the most intensive flood tide.

Yekaterina Strait

From 15 ERS SAR images of the Yekaterina Strait 6 images fall on flood phase and 9 images on ebb phase. The diversified linear structures are clearly seen on the images. They are typical for the frontal boundaries between different water masses, for the current shift areas and convergence zones [7]. However tidal dynamics in the Yekaterina Strait manifests itself brighter than in the Friz Strait area. Tidal currents in central portion of the strait closer to Iturup have practically bi- directional character. Thus the features which are typical for both phases are distinctly traced.

During ebb phase, the linear structures are located either in transit of the strait or downstream. As a rule they are directed either crosswise the strait in its narrow or at some angle to current direction. In most cases the area with increased brightness is observed in northwestern portion of the strait narrow southeast of 20-m isobath. Its size changes from 1.5 x 3 km to 3 x 6 km (Fig.4).

a)

b)

c)d)?ESA 1995Look direction ?ESA 1998Look direction ?ESA 1992Look direction ?ESA 1993Look direction

?ESA 1992Look direction

Fig.4. ERS-1 SAR image of the Yekaterina Strait acquired on 22 April 1992 at 01:02 UTC during ebb tide.

ERS-1 SAR images shown in Fig.5 was acquired on 22 September 1995 at 01:02 UTC when maximum ebb stage was observed at the whole strait. A plume of the Soya Current warm waters and the fronts on its boundaries are clearly visible on the image.

Flood stage is characterized by the appearance of an eddy formation with diameter of approximately 13-15 km (Fig. 6). This feature is observed in several places: in the southeastern portion of the strait, south of Iturup, in narrow of the strait and in its northeastern portion. Joint analysis of several SAR images where this eddy was detected together with the calculated daily variations of current vectors (not shown) in 7 dots depicted in Fig.1 allows us to formulate the following preliminary conclusions about generation and evolution of the considered cyclonic eddy.

Tidal current in the southeastern portion of the strait is bi-directional. At sharp change of tidal phase (approximately on 180 degrees during 2 h) and under favorable dynamic conditions a spiral cyclonic eddy is formed in the area 44°15′-44°25′N, 146°50′-147°E (Fig.7a). Developing tidal flow shifts the eddy in the strait with a velocity of approximately 1.5-2.0 m/s (Fig.7b). After crossing of the narrow of the strait, movement of the eddy decreases because the velocity of tidal flow also decreases (Fig.7c). The change of tidal phase in the northeastern portion of the strait proceeds at different times. Tidal currents in the area where there is the eddy have different directions and destroy it. The incidentally oriented current shear zones which are formed during this process manifest themselves on the SAR image as the linear contrast signatures of both signs and with a length of approximately 6-9 km (Fig.7d).

?ESA 1995Look direction

Fig.5. ERS-1 SAR image of the Yekaterina Strait taken on 22 September 1995 at 01:02 UTC during intensive ebb tide.

5. CONCLUSIONS

The images obtained by satellite synthetic aperture radar have demonstrated the new possibilities to study the highly variable dynamic phenomena in the surface waters of the Yekaterina and Friz straits. Joint analysis of the ERS SAR images and the daily variations of flood-ebb currents computed for several points in the straits permitted to reveal a number of dynamic formations which are periodically generated by tidal flow. The results of more comprehensive analysis of remote and in situ data together with the tidal current simulations will improve understanding of mechanisms, which are responsible for generation and evolution of the revealed phenomena. In turn the knowledge of these mechanisms is important both for solving oceanographic problems and for development of SAR backscatter models.

ACKNOWLEDGMENTS

This study was carried out within ESA AO3-401 project “Mesoscale oceanic and atmospheric phenomena in the coastal area of the Japan and Okhotsk seas: Study with ERS SAR and research vessels”. We thank the people from the ERS Help and Order Desk for their support as well as staff of the Institute of Automation and Process Control, FEB RAS for providing NOAA AVHRR data. We are grateful to the Sakhalin Office on Hydrometeorology and Environment Control and personally to V.A. Lepekhov for providing hydrometeorological information collected on the southern Kuril stations. This work was partly supported by INTAS grant 99-0242.

?ESA 1993Look direction Fig.6. Fragment of ERS-1 SAR image of the Yekaterina Strait the size of 50 km x 80 km acquired on 25 August 1993 at 01:02 UTC during flood tide.

Fig.7. Fragments of ERS SAR images of the Yekaterina Strait showing generation and evolution of a cyclonic eddy in flood current. Formation of the eddy in the southeastern portion of the strait during the change of tidal phase 3 h before radar sensing taken on 25 August 1993 at 01:02 UTC (a). The eddy location in 6 h after transfer through the strait as observed on 27 October 1995 at 01:02 UTC (b). The begining of the eddy transformation in approximatly 9 h after its formation on SAR image acquired on 23 December 1992 at 01:02 UTC (c). Destroy of the eddy in 12 h after its formation during the change of tidal phase as observed on 3 March 1993 at 01:02 UTC (d).

a)

b)c)d)?ESA 1993Look direction ?ESA 1995

Look direction ?ESA 1992Look direction ?ESA 1993Look direction

REFERENCES

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