Remote Sensing of Ocean Internal Waves: An Overview

Transcription

1 Journal of Coastal Research West Palm Beach, Florida May 2012 Remote Sensing of Ocean Internal Waves: An Overview Victor Klemas School of Marine Science and Policy University of Delaware Newark, DE 19716, U.S.A. klemas@udel.edu ABSTRACT KLEMAS, V., Remote sensing of ocean internal waves: an overview. Journal of Coastal Research, 28(3), West Palm Beach (Florida), ISSN The oceans are density stratified because of vertical variations in temperature and salinity. Oceanic internal waves can form at the interface (pycnocline) between layers of different water density and propagate long distances along the pycnocline. Internal waves on continental shelves are important because they can attain large amplitudes and affect acoustic wave propagation, submarine navigation, nutrient mixing in the euphotic zone, sediment resuspension, crossshore pollutant transport, coastal engineering, and oil exploration. Internal waves induce local currents that modulate surface wavelets and slicks, causing patterns of alternating brighter and darker bands to appear on the surface. The surface patterns can be mapped by satellites using synthetic aperture radar (SAR) or visible imagers. The objectives of this article are to discuss methods for remotely studying and mapping ocean internal waves and to present examples illustrating the application of satellite remote sensing. ADDITIONAL INDEX WORDS: Ocean internal waves, remote sensing, satellite oceanography. INTRODUCTION AND BACKGROUND The water column in the ocean is frequently not homogeneous, but stratified, with low-density, warmer water residing on top of high-density, colder water. The boundaries that separate water layers of different density and temperature are called pycnoclines and thermoclines, respectively. Internal waves (IWs) or packets of IWs can form at the interface between layers of different water density and travel long distances along the pycnocline. The IWs are especially common over the continental shelf regions of the oceans and where brackish water overlies salt water at the outlets of large rivers. The period of IWs approximates the period of the tides, suggesting that the IWs may be generated by strong tidal currents flowing over sharply varying bottom topography, such as shelf breaks and shallow sills. Internal waves in the open ocean and on continental shelves can attain amplitudes in excess of 50 m and strongly influence acoustic wave propagation, submarine navigation, nutrient mixing in the euphotic zone, sediment resuspension, crossshore pollutant transport, coastal engineering, and oil exploration (Duda and Preisig, 1999; Ebbesmeyer, Coons, and Hamilton, 1991; Shanks, 1987). Studies of IWs are of particular interest to companies using oil-drilling rigs and other vulnerable structures in the ocean. The U.S. Navy has been investigating the nature and causes of internal waves and their effect on acoustic waves used for detecting subsurface objects; IWs have also been suspected of causing the sinking of several submarines (Pinet, 2009). DOI: /JCOASTRES-D received 26 August 2011; accepted in revision 20 September Coastal Education & Research Foundation 2012 Since ancient times, mariners sailing the oceans have observed internal waves in the form of alternating darker and brighter bands, with increased sea-surface roughness. The IWs cause local currents that modulate surface wavelets and slicks. Because subsurface internal waves produce this strong surface signature, they can be mapped with satellite and airborne sensors. Satellite synthetic aperture radars (SARs) have become the most important sensors for observing internal waves. The IWs have also been studied using multispectral satellite images and space-shuttle photographs. The objectives of this article are to discuss the most-effective methods for remotely observing ocean IWs and to present examples illustrating the application of remote sensing to studies of IW generation and propagation. OCEANIC INTERNAL WAVES The oceans are density stratified because of vertical variations in temperature and salinity. As the temperature decreases toward the bottom of the ocean, it causes the density of the water to increase toward the bottom. Ocean stratification is primarily determined by the action of turbulent mixing of water masses because of wind stress and heat exchange at the air sea interface. Stratification retards vertical mixing of water masses, reduces vertical transport of nutrients, and affects propagation of acoustic signals in the ocean. The pycnocline is a zone that vertically separates water masses having different densities (Mirie and Pennell, 1989; Pinet, 2009). The IWs can form at the interface between water layers of different density and propagate along the pycnocline. Oscillations are more easily set up at an internal interface than at the sea surface because the difference in density between two water layers is much smaller than between water

2 Remote Sensing of Ocean Internal Waves 541 Table 1. An overview of synthetic aperture radar (SAR) satellites. Adapted from Susanto, Mitnik and Zheng (2005). Satellite ERS-1 and ERS-2 RADARSAT ENVISAT Sensor SAR SAR ASAR Launch Date 17 July April November March 2002 Frequency (GHz) Wavelength (cm) Polarization 1 VV HH VV, HH, VH, HV Incidence angle 20 26u 10 59u 15 45u Swath width (km) Ground resolution (m) V and H are the vertical and horizontal polarization of the SAR signal, respectively. The first letter denotes polarization of the transmitting signal. The second letter represents polarization of the receiving signal. and air. Hence, less energy is required to generate internal waves than is required to generate surface waves of similar amplitude. Surface waves can be up to 18 m high, whereas IWs can reach heights of 100 m, depending on the thickness of the upper water layer. The IWs travel more slowly than surface waves of similar amplitude because the difference in density between two water masses is much less than it is between air and water. The IWs typically have wavelengths from hundreds of meters to tens of kilometers and have periods from several minutes to several hours. Internal waves are trapped in the pycnocline of stratified oceans. Internal waves can exist as self-reinforcing solitary waves (solitons) or as trains of such waves. Because of a balance between nonlinearity and dispersion, IWs can propagate a long distance along a pycnocline without changing their waveform. (Hsu and Liu, 2000; Klymak and Moum, 2003; Pinkel, 2000; Susanto, Mitnik, and Zheng, 2005). Internal waves can be generated by disturbances, such as ocean storms or strong tidal currents flowing over sharply varying bottom topography, including shelf breaks, sills, and shallow banks. Each disturbance can generate a single IW or an entire packet of such waves. The period of the internal wave packets approximates the period of the tides, suggesting a cause-and-effect relationship between the packets and the tides. Ship and mooring data have been collected along many coasts and have been used to describe the nonlinear internal wave field and the background oceanographic conditions that form the propagation waveguide on continental shelves (Shroyer, Moum, and Nash, 2011). Once generated, internal waves can reverse polarity, refract, undergo constructive and destructive interference, become unstable, and break (Cai, Long, and Gan, 2002; Cummins et al., 2003; Farmer and Armi, 1999; Fett and Rabe, 1977; Gerkema and Zimmerman, 1995; Haury, Briscoe, and Orr, 1979; Hibiya, 1990; Holloway et al., 1997; Small, 2001). Tidally generated IWs, which are usually nonlinear and dispersive, frequently evolve as trains of internal waves. The resulting wave packets normally consist of several solitary waves. These wave packets can be divided into two types: single-wave IW packets, containing only one IW with or without an oscillating tail, or multiple-wave packets composed of a group of rank-ordered IWs. The amplitude of the wave crests and their separation distance decreases toward the tail end of the wave packet. A single-wave IW can disintegrate into a packet of rank-ordered smaller internal waves while propagating onto a shoaling shelf (Liu et al., 1998; Zhao et al., 2004; Zheng et al., 2001a). Internal wave theories predict that, if the depth of the upper layer is smaller than the depth of the lower layer, the resulting internal waves are waves of depression, i.e., they push the pycnocline down. The leading edge of an IW of depression is associated with a convergent surface flow region and the trailing edge with a divergent region. At the front of the internal wave, the amplitude of the Bragg waves is increased, whereas at the rear it is decreased. This is the reason why, on SAR images, the front section of an internal wave of depression is bright and the rear section is dark (Alpers, 1985; Alpers, Brandt, and Rubino, 2008). Furthermore, a number of researchers have observed IWs change polarity from depression to elevation internal waves (Orr and Mignerey, 2003; Shroyer, Moum. and Nash, 2007; Zhao et al., 2003). Internal waves are an important mechanism for the transport of momentum and energy within the ocean (Helfrich and Melville, 1986; Lamb, 1994). In the context of vertical mixing processes, internal waves may influence local biological productivity and transport plankton and fish larvae to other areas (Holligan, Pingree, and Mardell, 1985). REMOTE SENSING OF OCEANIC INTERNAL WAVES Since the launch of Seasat (in 1978), the European Remote Sensing (ERS) satellites 1 and 2, and the Canadian RADAR- SAT, SARs have been used to map many internal wave patterns on continental shelves and the open ocean. A summary of recent satellites equipped with SARs is presented in Table 1. As shown in Table 1, the spatial resolution of these SAR systems ranges from 8 m to 100 m, with a typical resolution of 25 m (Hsu and Liu, 2000; Liu et al., 1998; Purkis and Klemas, 2011). Satellite SARs have become the most-important sensors for observing ocean internal waves, being capable of all-day, all-weather observations with a controlled geometry (Zheng and Alpers, 2004). Radar antenna beams point at right angles to the aircraft/satellite flight path and are characterized by

3 542 Klemas Figure 1. Drawing of the flow field associated with internal waves and the consequent modulation of surface waves (Robinson, 2004). (With permission of Springer Science & Business Media.) their range resolution (along the main axis of the beam) and azimuth resolution (across the radar beam). The SARs have a major advantage over real-aperture, side-looking airborne radars (SLARs) because SARs can attain not only a high range resolution, which depends on using short radar pulses, but also a fine azimuth resolution, even from satellite orbit. This is accomplished by using the Doppler effect. Objects on the ground experience different frequency shifts in relation to their distances from the satellite track. Thus, objects at the leading edge of the radar beam reflect a pulse with an increase of frequency (relative to the transmitted frequency) because of their position ahead of the satellite, and those at the trailing edge of the antenna beam experience a decrease in frequency. This is similar to the increase and decrease of the pitch of a train whistle as it passes a stationary observer. As a result, it is possible to compare the frequencies of transmitted and reflected signals to determine the nature and amount of the frequency shift. Knowledge of the frequency shift permits the assignment of reflections to their correct positions on the image and the synthesis of the effect of a long antenna with a very high azimuth resolution (Campbell, 2007; Cracknell and Hayes, 2007; Elachi and van Ziel, 2006; Martin, 2004). As shown in Figures 1 and 2, IWs displace the pycnocline/ thermocline vertically and generate internal currents that alternately converge and diverge on the surface, capturing slicks and modulating sea-surface roughness and small-scale waves (1 10 cm) caused by local winds (Robinson, 2004). Because the wavelength of the radar pulses is selected so that it geometrically matches the wavelengths of the small-scale gravity capillary waves on the ocean surface, a constructive (resonant) reflection condition called Bragg scattering results, enhancing the returned radar signal. According to Bragg scattering theory, the amplitude of small-scale sea-surface waves that obey the Bragg resonance condition (Bragg waves), determines the backscattered radar power. Resonant or Bragg scattering is the most-important mechanism in the interaction of an electromagnetic wave (e.g., a radar pulse) and small-scale (gravity capillary) sea-surface waves. Because of this interaction of the Bragg waves with the IW-induced surface currents, the Bragg wave amplitude increases in convergent flow regions and decreases in divergent flow. As a result, the radar signatures of IWs consist of alternating bright and dark bands (Alpers, Brandt, and Rubino, 2008; Ikeda and Dobson, 1995; Li, Clemente-Colon, and Friedman, 2000; Robinson, 2004). Figure 2. Processes by which subsurface ocean phenomena modulate the radar-measured roughness to produce a signature on a radar image (Robinson, 2004) (With permission of Springer Science & Business Media.) Subsurface ocean phenomena can also modulate surface currents to capture biological material and natural oils on the surface. As shown in Figure 2, a visible, wavelike surface pattern may result from the influence of features such as internal waves on the surface current. When the crests of the internal wave pass below, water in the surface layer essentially flows down the sides of the crest and pools into the area overlying the wave s troughs. Because surface water is diverging over the crests and converging over the troughs, biological material and natural oils on the surface are also alternately dispersed and concentrated in a similar wave pattern. These natural slicks calm the water surface and change how it reflects light and radar waves. Thus, the surface slicks can also reveal the presence of the underlying internal waves (Alpers, 1985; Apel, 2003; Robinson, 2004). In satellite images, IWs can be identified as periodically alternating darker and brighter stripes (Figure 3). A satellite image typically takes a snapshot of a vast horizontal twodimensional field of internal wave packets, each separated by several kilometers and containing several wave crests (Jackson, 2004). As shown in Figure 3, the amplitude of the wave crests and their separation distance usually decreases toward the tail end of the wave packet. The IW parameters, such as characteristic half width, crest length, number of waves, propagation direction, distance between neighboring packets, distance between neighboring waves, and wave speed, can be determined from one or a series of satellite images (Porter and Thompson, 1999; Zhao et al., 2003; Zheng et al., 2001b). By using these IW parameters in models, oceanographers have

4 Remote Sensing of Ocean Internal Waves 543 Figure 3. Seasat SAR image of the surface of the Gulf of Mexico, showing numerous ocean features, including internal waves, currents, and rainstorms. Courtesy: NASA. been able to derive important physical information, such as the oceanic mixed-layer depth (Brandt, Alpers, and Backhaus, 1996; Jones, 1995; Kara, Rochford, and Hurlburt, 2003; Li, Clemente-Colon, and Friedman, 2000; Porter and Thompson, 1999). Numerous ocean features are resolved (Figure 3), including rainstorms, IWs, and currents, all of which modify the roughness of the water s surface and, hence, have a backscatter signature in the microwave spectrum. The Seasat was launched on 26 June 1978, but failed 105 days later. Airborne imagers and shore-based radars have been used to study IWs along the coast (Weidemann et al., 2000). For instance, shore-based, X-band, marine radar observations from the rock of Gibraltar, as well as airborne SAR observations, contributed to clarifying the generation and propagation characteristics of internal waves in the Strait of Gibraltar. The X-band marine radar had a wavelength of 3.2 cm, a 100-m range resolution, and a 1-degree azimuth resolution. It was able to detect surface roughness features of the internal waves out to a range of 15 km (Richez, 1994; Watson and Robinson, 1990). More recently, range resolutions of tens of meters have been obtained with new shore- and ship-based X-band radars. (Braun, Ziemer, and Bezuglov, 2007; Ziemer, 2008). Internal waves and their surface manifestations have also been observed by shore-based high-frequency (HF) radars, notwithstanding their poorer spatial resolution. For instance, IW-driven surface currents were studied with HF radars in the Florida Keys. The surface-current observations from HF radar revealed that not only the low-frequency and tidal currents were resolved by the measurement, but also the higherfrequency motions related to internal wave activity (Shay, 1997). The HF radars operate in the 3 30 MHz frequency range and use a ground-wave propagation mode of the electromagnetic waves. Depending on the operating frequency selected, HF radars can attain ranges of 300 km and spatial resolutions of 0.3 to 1.5 km (Gurgel and Schlick, 2008). The HF radars are now being integrated within coastal-monitoring networks (Klemas, 2009). However, to observe individual waves in a field of internal wave packets, shore-based HF radars cannot compete with the high-resolution and wide coverage of satellite SARs. The IWs have also been investigated using multispectral satellite images and space shuttle photographs (Artale et al., 1990; Klemas, Zheng, and Yan, 2001; Zheng, Yan, and Klemas, 1993). For instance, by interpreting two space shuttle photographs taken with a Linhof camera on 8 June 1991, researchers recognized 34 IW packets on the continental shelf of the Middle Atlantic Bight (Zheng, Yan, and Klemas, 1993). The internal wave field had a three-level structure: packet groups with average wavelengths of 17.5 km, packet groups with average wavelengths of 7.9 km, and waves with average wavelengths of 0.6 km. Using the finite depth theory, the investigators derived the maximum amplitude of internal waves as 5.6 m, the phase speed as 0.42 m/s, and the period as 23.8 minutes. The frequency distribution of the internal waves was triple-peaked at Hz, Hz, and Hz. Substituting the statistical results of the number of waves in a packet into the fission law, it was determined that the upper and lower edges of the shelf break were the primary and secondary generation sources of the internal waves, respectively. Calculations showed that the group period of the IWs was 12.5 hours, coinciding with that of the local semidiurnal tides. This confirmed that the tides were a dominant, generating force for IWs on the continental shelf (Zheng, Yan, and Klemas, 1993). CASE STUDY: SATELLITE INVESTIGATIONS OF INTERNAL WAVES IN THE LOMBOK STRAIT The Indonesian seas have a complex coastline geometry and bathymetry, narrow passages, stratified waters, and strong tidal currents. Thus, they provide ideal conditions for the generation of high-amplitude IWs. Internal waves are commonly observed in the Lombok Strait, which separates the Indonesian islands of Bali and Lombok. The Lombok Strait is one of the outflow straits of the Indonesian Throughflow (ITF), which transport water from the Pacific to the Indian Ocean (Murray and Arief, 1988). The strait contains channels as deep as 1400 m and sill crests in the ITF at depths of about 250 m (Susanto, Mitnik, and Zheng, 2005). Because of the presence of stratified waters, the rough topography, and the strong tidal currents in the Lombok Strait, strong IWs are generated, which can be clearly observed by radar and optical imagers on satellites, such as ERS-1/2, RADARSAT SAR, and ENVISAT. To determine the characteristics of internal waves in the Lombok Strait, Susanto, Mitnik, and Zheng (2005) chose two consecutive images taken during the ERS-1/2 tandem mission. The first image was taken by ERS-1 on 23 April 1996, and the second image was taken 24 hours later by ERS-2 on 24 April. As shown in Figure 4, packets of IWs propagating both northward and southward are clearly seen in both images. The northward-propagating internal wave packet in the left image contains more than 25 IWs. Within that wave packet, the wavelengths seem to be monotonically decreasing from about 5 km in the front to

5 544 Klemas suggested that internal waves generated in the sill area of the Lombok Strait propagated in three different patterns: southward only, northward only, and in both directions. As a result, they confirmed that the background current, in association with the ITF, stratification, and tidal conditions, controls the internal wave generation and propagation direction in this region (Susanto, Mitnik, and Zheng, 2005). SUMMARY AND CONCLUSIONS Figure 4. Internal waves in the Lombok Strait imaged by ERS-1 and ERS-2 SAR systems. Adapted from Susanto, Mitnik and Zheng, Credits: European Space Agency. 2 km in the rear. The SAR images were taken after the maximum spring tide, as measured by shallow pressuregauge arrays deployed at both sides of the strait and by tidegauge data. Analysis of the two consecutive satellite SAR images showed that the internal waves were generated by the interaction of successive tidal flows with the sill south of the Lombok Strait. The tidal flow there is predominantly semidiurnal, based on current measurements using moorings deployed near the sill (Murray et al., 1990). By measuring the distance between successive wave packets and their distances to the sill, investigators were able to calculate the average wave speed. The distances between packets on the images taken on the two dates were 88.1 km and 87.6 km, respectively. Thus, the average propagation velocity was about 1.96 m/s. The number of the waves per packet for the two dates was 25 and 23, average wavelengths were 3.10 km and 3.8 km, and the halfwidths were 1.99 km and 2.75 km, respectively. The length of the crest-lines of the internal waves was considerably more than 100 km. Based on the SAR images, the investigators also Internal waves on continental shelves are important because they can attain large amplitudes and affect acoustic wave propagation, submarine navigation, nutrient mixing to the euphotic zone, sediment resuspension, cross-shore pollutant transport, coastal engineering, and oil exploration. Studies of internal waves are of particular interest to the U.S. Navy and companies using oil-drilling rigs or other vulnerable structures in the ocean. The U.S. Navy has been investigating the nature, causes, and effects of internal waves ever since they were suspected of causing the sinking of several submarines. Internal waves form at the interface (pycnocline) separating layers of different water densities and propagate along a pycnocline over long distances. Internal waves can be generated by strong tidal currents flowing over sharply varying bottom topography, including shelf breaks, sills, and shallow banks. The period of the internal wave packets approximates the periods of the tides, suggesting the tides are the generating mechanism. Once generated, IWs can refract, undergo constructive and destructive interference, become unstable, and break. The IWs typically have wavelengths from hundreds of meters to tens of kilometers and periods from several minutes to several hours. The IWs are especially common over the continental shelf regions of the oceans and where brackish water overlies salt water at the outlets of large rivers. Internal waves displace the pycnocline/thermocline vertically and generate internal currents that converge and diverge on the surface, capturing slicks and modulating sea surface roughness and small-scale waves (1 10 cm). This causes patterns of alternating brighter and darker bands to appear on the ocean surface. The modulated surface wavelets produce a strong resonant (Bragg) backscatter of the radar pulses, so that the surface patterns can be mapped by satellites using a SAR. Satellite SARs are capable of all-day, all-weather observations with a controlled geometry. Satellite SARs have been mapping internal waves for decades, starting with Seasat in Internal waves have also been studied using airborne sensors, multispectral satellite images, and space shuttle photographs. A case study of internal waves in the Lombok Strait, Indonesia, was used to illustrate how internal wave parameters, such as characteristic half-width, crest length, number of waves, propagation direction, distance between neighboring packets, distance between neighboring internal waves, and wave speed, can be determined from one or a series of satellite SAR images. By using these IW parameters in models, oceanographers have been able to derive important physical information, such as the oceanic mixed-layer depth.

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