THERMAL PLUMES AND INTERNAL SOLITARY WAVES GENERATED IN THE LOMBOK STRAIT STUDIED BY ERS SAR

THERMAL PLUMES AND INTERNAL SOLITARY WAVES GENERATED IN THE LOMBOK STRAIT STUDIED BY ERS SAR Leonid Mitnik 1, Werner Alpers 2, and Lim Hock 3 1 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 2 Institute of Oceanography, University of Hamburg, Troplowitzstr. 7 /III, D-22529 Hamburg, Germany Phone: 49.40.42838 5432, Fax: 49.40.42838 5713, E-Mail: alpers@ifm.uni-hamburg.de 3 Centre for Remote Imaging, Sensing and Processing, National University of Singapore, Lower Kent Road Singapore 119260, E-mail: phylimh@leonis.nus.edu.sg (t) 0065.874.3220 (f) 0065.775.7717 ABSTRACT The Lombok Strait separates the Indonesian islands Bali and Lombok and is the second most important strait through which water is exchanged between the Pacific Ocean and the Indian Ocean. The south end of the Lombok Strait is split by a small island and on each side of the island are rugged sills with an average depth of about 200 m. On the sill, tidal current speeds up to ±3.5 m/s are encountered. Variable southward flow of warm Pacific waters through the strait into the Indian Ocean forms a well-developed thermal plume south of the sill. The position of the front associated with this plume has been identified on several ERS SAR images. Its extent and shape are determined by the water transport through the strait and characteristics of the South Java Current which in turn depend both on the annual cycle of the monsoon winds and prolonged disturbances in wind field. Another oceanic phenomenon that has been detected on ERS SAR images is the internal solitary waves that are tidally generated at the sill and propagate in both directions. The waves propagating northward into the Java Sea have always a quite regular circular shape while the ones propagating southward into the Indian Ocean have a very irregular shape. Physical explanation for this behavior is presented. We attribute this mainly to the strongly horizontally varying current field south of the sill. Since the sea surface manifestations of the internal waves vary very strongly with time, we hypothesize that the internal wave characteristics south of the Lombok Strait depend on the stratification of the water body and the strength of the water flow through the strait. Thus, we suggest that ERS SAR imagery showing internal waves south of the strait may also be used for obtaining information on the throug-strait flow variations. 1. INTRODUCTION The Indonesian throughflow from the Pacific to the Indian Ocean plays an important role both in the regional circulation around the western equatorial Pacific and the eastern Indian Oceans and in the global oceanic circulation [1,2]. The waters of the Pacific Ocean have a lower salinity and higher temperature than the waters of the Indian Ocean. In terms of cross-sectional area available for the transport of Pacific waters into the Indian Ocean, the Lombok Strait is the second most important channel through the lower Indonesian Archipelago. It separates the Indonesian islands Bali and Lombok. The water depth in the Lombok Strait is typically between 800 and 1000 m, except in the southern section where the small island Nusa Penida is located. The western channel of the Lombok Strait, called Badung Strait, has a cross sectional area which is less than one-fourth the cross sectional area of the main channel which is located between the islands Nusa Penida and Lombok. In this channel a very irregularly shaped sill is located which has a maximum depth of about 350 m [3]. As was shown in [4], a pressure gradient between the Pacific and the Indian Ocean is the forcing mechanism that controls the circulation in the Indonesian throughflow. This gradient is mainly caused by the monsoon. But also the passage of tropical cyclones over the Timor Sea contributes to this pressure gradient [5]. This implies that the circulation and the seasonal water mass distribution are subject to large seasonal variations [6]. During El Niños the lower sea level in the western Pacific diminishes the transport through the Lombok Strait. In particular during the El Niño event in 1997 the transport of Pacific waters into the Indian Ocean was lower than normal [7]. Most of the data on the water properties and on the currents in the Lombok Strait area originate from 7 current meter moorings and from 234 CTD casts which were carried in this strait between January 1985 and March 1986 [3,5] and more recently from three oceanographic programs of the Indonesian seas study, focusing on the period April 15 to June 15, 1997 [8,9]. The most important known facts about the Lombok Strait and surrounding waters are the following:

The current in the Lombok Strait is bi-directional with respect to the northeast-southwest direction. A southward flow in the uppermost 200 m persists throughout the year. Between July and September the sustained flow speed in the north strait can exceed 70 cm/s. Dramatic flow reversals to the north reaching 75 cm/s at the 35 m level were observed between 15 February and 15 March as well as the last ten days of April 1985. Weak currents were measured in the period mid October 1985 to mid January 1986. The maximum tidal current velocity measured on the shallow (150 m) east flank of the sill in the south strait was 350 cm/s. In the north strait, amplitudes of tidal current velocity ranged from 20 to 50 cm/s over the spring-neap cycle. In the northern section of the strait, the surface isothermal and isohaline layers, which are usually 30-50 m thick, overlay a strong thermocline which extends to a depth of about 400 m. The lower salinity in the surface layer results from the intense rainfall in this region. The southward motion of Pacific waters in the strait in June 1985 is clearly discernible in the distribution of the thermohaline properties in the surface layer (Fig.1) [5]. In particular, the isotherms show the penetration of warm (29 C) surface water into the northern section of the strait. South of the sill a welldeveloped thermal plume intrudes over 30 km into the Indian Ocean. Temperature gradients are steepest on the western side of the plume, which is attributed to the presence of the South Java Current which flows eastward during northeast monsoon. The temperature drop is approximately 3 C over a distance 15 km (Fig.1, left). The salinity distribution on the 10-db surface shows a similar, but less pronounced difference from north to south (Fig.1, right). Recent year-long current observations (March 1997 to March 1998) south of Java have confirmed semi-annually reversing behavior of the South Java Current and also revealed the abrupt change of flow direction caused by the southward propagating coastally trapped Kelvin wave (CTKW) for the period May 16 to May 30 1997 [8,9]. Eastward propagating downwelling Kelvin wave presents the oceanic adjustment to the wind forcing in the equatorial western Indian Ocean which excites the CTKW. The fluctuations in geostrophic velocity through Lombok Strait were estimated for the period April 15 to June 15 1997 using the pair pressure gauges located at the northern strait entrance. Anomalous northward flow was observed from May 17 to the beginning of June, with maximum northward flow between May 29 and 31 [9] and resulted from the impact of a Kelvin wave. Very likely that both local and remote atmospheric forcing will influence on the location of the Pacific water plume south of the Lombok Strait and on spatial gradients of hydrological characteristics on plume s boundary. To our knowledge the variations of surface circulation in the Lombok Strait area and the interaction of the strong tidal currents with the sill have not been studied before in detail. Thus ERS SAR images provide first insights into the largescale dynamics of the Lombok Strait. On these images oceanic fronts, current boundaries, and internal waves become visible due to the modulation of the surface roughness by variable currents associated with these phenomena. Fig.1. Temperature (left) and salinity (right) on the 10 db surface [5]

2. ERS SAR IMAGES The SAR images used in this paper were received and processed by the Singapore ground station. For the period from April 1996 to March 2000, we found on the website of the Singapore ground station more than 80 ERS-1/2 SAR and 5 Radarsat SAR quick-look images of the Lombok Strait and the surrounding waters. The precision processed ERS SAR images have a spatial resolution of 25 m, but the quick-look images, which are placed on the websites of the ground stations, have a lower geometric and also a lower radiometric level resolution. Current-induced radar signatures in the waters north and south of the Lombok Strait (Flores Sea and Indian Ocean) could already be delineated on many quick-look ERS SAR images. We have selected several ERS SAR strips of quicklook images which were acquired on 25 December 1996, 10 January, 30 May, 27 August and 5 November 1997, and 15 December 1999 for precision processing and have analyzed them in detail. We found that the radar signatures of oceanic fronts and internal waves are highly variable. Unfortunately, no simultaneously collected in situ data are available for validating our interpretations of the ERS SAR images. However our interpretations are in an agreement with in situ observations reported in [3,5]. Furthermore, the common features were detected on SAR images of the Lombok Strait and on SAR images of other sea areas, for which in situ data are available (see for example [10-14]). All except one (track 196) ERS SAR images were acquired during descending tracks (tracks 146 and 418) (see Fig. 2). 3. TRANSPORT THROUGH THE LOMBOK STRAIT A yearlong current meter record of upper-ocean flow in the Lombok Straits shows that the 1985 average transport in the Indian Ocean is 1.7 Sv. The peak transport was observed in July and August (3.8-4 Sv) during east monsoon, followed by a minimum less than 0.5 Sv at the end of year [3]. The measurements and simulations have also demonstraited that relatively short periods of northward transport are also observed [3,9]. ERS-2 SAR image (Fig.3) acquired on 27 August 1997 (track 418) covers the Flores Sea (above), Bali and Nusa Penida islands, the Lombok Strait and the Indian Ocean (below). The images gives an idea of the radar signatures of intrusion of the warmer Pacific waters into the Indian Ocean when southward flow reaches maximum. The Pacific waters intrudes into the Indian Ocean both through the western channel (Badung Strait) and through the eastern channel (Lombok Strait). They look brighter against the background Indian Ocean waters. The main amount of water enters through the deeper and broader eastern channel and forms well developed front. The southernmost boundary of the plume is at approximately 100 km from the sill. The western and eastern boundaries of the plume begins near Nusa Penida and south of Lombok island, correspondingly. The plume formed by water flow through the Badung Strait is at approximately 70 km from Nusa Penida. Island wake south of Nusa Penida is characterized by decrease brightness. Two packets of nonlinear solitary waves propagate to the south and to the west from the western plume. Fig.2. The locations of ERS-1/2 SAR images over the Lombok Strait area. 146 and 418 are descending tracks and 196 is an ascending track.

ESA 1997 Fig.3. ERS-2 SAR image acquired on 27 August 1997 at 02:32 UTC (above) showing intrusion of the Pacific waters into the Indian Ocean during maximum southward transport as well as packets of nonlinear solitary waves. Fig.4. ERS-2 SAR image acquired on 5 November 1997 at 02:32 UTC (right) showing intrusion of the Pacific waters into the Indian Ocean and packets of nonlinear solitary waves which are generate by the sill and propagate to the south of it in the Indian Ocean and to the north of it in the Flores Sea. ESA 1997

ESA 1996 ESA 1997 Fig.5. ERS-2 SAR image acquired on 25 December 1996 at 02:32 UTC (track 418) showing intrusion of the Indian Ocean waters into the Flores Sea and a northward propagating packet of internal waves. Fig.6. ERS-2 SAR image acquired on 10 January 1997 at 02:29 UTC (track 146) showing probable intrusion of the Indian Ocean waters into the Flores Sea and packets of nonlinear solitary waves propagating both in the Flores Sea and in the Indian Ocean.

ESA 1999 ESA 1997 Fig.7. ERS-2 SAR image acquired on 15 December 1999 at 02:32 UTC (track 418) showing intrusion of the Pacific waters into the Indian Ocean through the Lombok Strait and two packets of internal waves propagating in the Flores Sea. The first packet crosses the Kanegan Island and some coral reefs. Fig.8. ERS-2 SAR image acquired on 30 May 1997 at 02:29 UTC (track 146) during intrusion of the Indian Ocean waters into the Pacific [9] and two packets of internal waves propagating in the Flores Sea.

4. INTERNAL WAVES The ERS SAR images depicted in Fig.3 Fig.8 at 146 and 418 tracks show the Lombok Strait, the island Bali and Lombok (western and eastern boundaries of the strait), the small island Nusa Penida inside the strait, the Flores Sea and the Indian Ocean. Packets of solitary waves propagating nortward in the Flores Sea (Fig.4 - Fig.8) or southward in the Indian Ocean (Fig.3,4 and Fig.6) or in both directions (Fig. 4 and Fig. 6) are clearly visible on the images. They are generated by the interaction of strong tidal currents with the sill between the islands Nusa Penida and Lombok. The radar signature of the internal solitary waves (i.e., their radar contrast against the background) depends on wind speed and direction. In this region the wind field is highly variable because of the presence of high mountains on Bali and Lombok. Interaction of air flow with the mountains forms the extended island wakes (wind shadow areas). Direction and velocity of the main water flow (to the south or to the north) also exert some action on the radar contrasts of the internal solitons since determine relative wind velocity relative to moving water surface. On the ERS SAR image of 5 November 1997 (Fig. 4) a northward-propagating internal solitary wave packet with more than 12 solitons can be delineated. The leading soliton has already reached the shallow water region where coral reefs are encountered. The length of the wave packet is difficult to determine since the leading soliton of the second packet (located probably near dark curve bands approximately 70 km south of the leading crest of the first packet) intrudes the rear of the first packet. Likely it is equal to approximately 80 km as follows from analysis of some other SAR images of the considered area. The length of several crests in the packet exceeds 110 km. They are reliably detected within the whole swath. Within the wave packet, the wavelength decreases monotonically. The maximum wavelength is approximately 6.5 km. Sea surface manifestations of another internal solitary wave packet are also visible on the image. It may be assumed that the observed two packets were formed by the interaction of successive semi-diurnal tidal flows with the sill between the Lombok and Nusa Penida islands. Somewhat different pattern of surface manifestations of internal solitons in the Flores Sea is visible on ERS-2 SAR image acquired on 15 December 1999 (Fig.7). On this day the solitons were imaged at a later phase of the tidal cycle. Several solitons crossed the Kangean Island and some coral reefs at the top of the image (see Fig.2). Their crests are not visible in the shallow waters. The crests of more than 15 solitons can be delineated at the right-hand side at the top of the image. Their radar contrasts decrease and dissipate at the left-hand side. The eastern part of the second northward propagating wave packet also has the larger radar contrast compare to the western part. Internal solitons in the Flores Sea on the ERS-2 SAR image for 10 January 1997 at track 146 (Fig. 6) are observed against the variable background caused not only by the surface wind variations but by the presence of rain cells, natural slicks and, probably, intrusion of the Indian Ocean waters as well. The imprints of internal solitons are well defined within relatively narrow darker band at the top of the image where their wavelength Λ changes from about 5 km to about 3.5 km and then below near left-hand side of the image where Λ decreases from about 2 to about 0.7 km. (Some interruption in their surface expressions is likely due to increased brightness of a background in the intruded water area). The surface imprints of the solitons from the succeeding tide are visible as low contrast semicircular bands to the north of Lombok Strait. The distance between the leading solitons in two wave packets in Fig.6 and Fig.7 is about 115 km and thus the propagation velocity is about 2.6 m/s in a case of semidiurnal tide. On 25 December 1996 the solitons were imaged by ERS-2 SAR at an earlier phase of the tidal cycle. The leading crest of the first packet consisting of more than 15 solitons is at about 100 km from the north Lombok Strait. The circular feature in the Lombok Strait is likely a soliton formed in the sill area by the succeeding tide. The distance between the leading solitons in these packets is about 110 km (the propagation velocity is about 2.5 m/s). A narrow contrast band between islands Bali and Kangean and further north marks front boundary between water masses with different hydrological characteristics. Darker area northeast of Bali is island wake. Decreased wind speed within the wake improves visibility of soliton crests. The ERS-2 SAR image for 30 May 1997 shown in Fig.8 at track 146 was acquired during maximum anomalous northward flow of the Indian Ocean waters into the Flores Sea between May 29 and 31[9]. However, signatures which can be assigned to the plume front of Indian Ocean water are absent on the image. The low contrast brightness boundaries near Lombok Island result very likely from disturbaces of wind field by mountains. Two packets of nonlinear internal solitary waves propagate northward into the Flores Sea. The first packet contains more than 10 rankordered solitons with monotonically decreasing distance between them (from about 6.5 km to about 2 km). The second packet consists of 4 solitons only. The distance between the leading solitons in these packets is about 85 km (the propagation velocity is about 1.9 m/s).

The radar signature of the southward propagating internal wave (Fig.4) packet is smaller than the one of the northward propagating wave packet. About 7 crests are seen on the SAR image. Their wavelength decreases from about 2 to less than 1 km. Narrow bright bands south of Lombok and Nusa Penida are likely the boundaries of a plume of the warmer Pacific waters which intrudes over 25 km into the Indian Ocean (see Fig. 1 which shows a well-developed thermal plume detected by CTD data). 5. CONCLUSIONS The analysis of ERS-1/2 SAR images of the Lombok Strait and the adjacent waters in the Flores Sea and in the Indian Ocean have revealed sea surface manifestations of two important phenomena: intrusion of the Pacific waters into the Indian Ocean and vice versa and nonlinear internal solitary waves which are generated by the interaction of the strong tidal flow with topographic features in the sill area between the islands Lombok and Nusa Penida. From published in situ observations follows that the thermohaline characteristics of the Pacific and Indian Ocean waters differ significantly that forms the plume fronts ion their boundaries and is favorable for its delineation on some ERS SAR images. The internal solitary waves propagate both to the north and to the south. The propagation velocity in the Flores Sea is about 1.8-1.9 m/s however can reach about 2.5 m/s. The speed was estimated from the distance between the leading solitons of two successive wave packets and the period of the semi-diurnal tide. The analysis of ERS SAR images has shown that the radar signatures of internal solitary waves as well as of the plume front vary strongly. The radar signatures depend both on remote and local conditions like the large-scale pressure field, the presence and development stage of an El Niño, the characteristics of the South Java Current, the phase of the tidal cycle, and the wind speed and direction. It is obvious that further in situ and space observations and advanced modeling studies are necessary for deeper understanding of the circulation in the Lombok Strait area, its interannual, seasonal and shorter period variations. Difference in water properties in combination with strong current is the bases for estimation of water transport using surface manifestations of the plume. Systematic SAR observations (especially from the future Envisat satellite) will then allow to gain more insight into the variation of the water exchange between the two oceans through the Lombok Strait and to study the peculiarities of internal wave generation and propagation north and south of the strait. ACKNOWLEDGEMENTS. This study was carried out within ESA Contract No. 13370/98/I-C Implementation of a Package for Global Oceanography with ERS SAR. We thank the people from the ERS Help and Order Desk for their support. The authors are indebted to the personnel of the Center of Remote Imaging, Sensing and Processing, National University of Singapore for their help in data acquisition and processing. We are grateful to Vyacheslav Dubina of the Pacific Oceanological Institute, FEB RAS for assistance in SAR image processing. REFERENCES [1] A.L. Gordon, Interocean exchange of thermocline water, J. Geophys. Res., vol. 91, pp. 5037-5046, 1986. [2] W.J. Schmitz, Jr., On the interbasin-scale thermohaline circulation, Rev. Geophys., vol. 33, pp. 151-173, 1995. [3] Murray, S.P., and D. Arief, 1988, Throughflow into the Indian Ocean through the Lombok Strait, January 1985- January 1986, Nature, 333, 444-447. [4] K. Wyrtki, Indonesian throughflow and the associated pressure gradient, J. Geophys. Res., vol. 92, pp. 12941-12946, 1987 [5] S.P. Murray, D. Arief, J.C. Kindle, and H.E. Hurlburt, Characteristics of circulation in an Indonesian Archipelago strait from hydrography, current measurements and modeling results, in The Physical Oceanography of Sea Strait. L.J. Pratt, Ed., NATO ASI Series, vol. 318. Dordrecht, Boston, London: Kluwer Academic Publishers, pp. 3-23. 1990 [6] C. Coatanoan, N. Mentzl, M. Fieux, and B. Coste, Seasonal water mass distribution in the Indonesian throughflow entering the Indian Ocean, J. Geophys. Res.,vol. 104, pp. 20801-20826, 1999. [7] J.C Chong, J. Sprintall, S.Hautala, W. Morawitz, N.A. Bray, and W. Pandoe Shallow throughflow variability in the outflow straits of Indonesia, Geophys. Res. Letters, 27, pp. 125-128, 2000. [8] J. Sprintall, J. Chong, F. Syamsudin, W. Morawitz, S. Hautala, N. Bray, and S. Wijffels, Dynamics of the South Java Current in the Indo-Australian Basin, Geophys. Res. Letters, vol. 26, pp. 2493-2496, 1999. [9] J. Sprintall, A.L. Gordon, R. Murtugudde, and R.D. Susanto, A semiannual Indian Ocean forced Kelvin wave observed in the Indonesian seas in May 1997, J. Geophys. Res., vol. 105, pp. 17217-17230, 2000. [10] A.R. Osborne, and T.L. Burch, Internal solitons in the Andaman Sea, Science, vol. 208, pp. 451-460, 1980.

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