International Journal of Remote Sensing: Vol 41, No 15

6/11/2020 International Journal of Remote Sensing: Vol 41, No 15

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Internal solitary wave observations in the Flores Sea using the Himawari-8 geostationary satellite

I Wayan Gede Astawa Karang , Chonnaniyah & Takahiro Osawa

To cite this article: I Wayan Gede Astawa Karang , Chonnaniyah & Takahiro Osawa (2020) Internal solitary wave observations in the Flores Sea using the Himawari-8 geostationary satellite, International Journal of Remote Sensing, 41:15, 5726-5742, DOI: 10.1080/01431161.2019.1693079 To link to this article: https://doi.org/10.1080/01431161.2019.1693079

Published online: 19 Nov 2019.

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Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=tres20 INTERNATIONAL JOURNAL OF REMOTE SENSING 2020, VOL. 41, NO. 15, 5726–5742 https://doi.org/10.1080/01431161.2019.1693079

Internal solitary wave observations in the Flores Sea using the Himawari-8 geostationary satellite I Wayan Gede Astawa Karanga,b, Chonnaniyahb and Takahiro Osawab,c aDepartment of Marine Sciences, Faculty of Marine Science and Fisheries, Udayana University, Denpasar, Indonesia; bCenter for Remote Sensing and Ocean Sciences (CReSOS), Udayana University, Denpasar, Indonesia; cCenter for Research and Application of Satellite Remote Sensing, Yamaguchi University, Ube, Japan

ABSTRACT ARTICLE HISTORY The Japanese Meteorological Agency’s geostationary satellite Received 28 February 2019 Himawari-8/Advanced Himawari Imager (AHI) images, acquired over Accepted 15 September 2019 the Flores Sea in Indonesia, observed that long internal solitary waves (ISWs) seem to be generated from the Sape Strait, which then propagates through the Java Sea, the Sumba Strait, and the Savu Sea. Shallow water wave refraction behaviour is present during the observations. These important parameters related to the ISW propagation (wavelength, soliton number, ﬁrst crest length, propagation direction, and phase speed) could also be estimated using Himawari-8 images. These observational results were also compared with Moderate Resolution Imaging Spectroradiometer (MODIS) images, Sentinel-1A Synthetic Aperture Radar (SAR) images, and the Korteweg-de Vries (KdV) equation. The geostationary satellite Himawari-8 showed great potential and was very helpful in observing the ISW propagation parameters, spatially and temporally, in the Flores Sea in detail.

1. Introduction The oceanic internal wave is a subsurface ocean phenomena. Despite this wave occurs within the sea surface, several processes in the oceans and coastal areas are affected by this wave. The internal waves (IWs) can be generated when the combination of strong currents flowing over the rough bottom topography exist. They can travel more than hundred kilometres while transporting mass and momentum. The surface manifestation of IWs can be seen because of the variations in surface currents induced by the IW’s activity below the surface, which then modulates the sea surface roughness. The IW surface signatures are often detected by satellite imagery, using satellite optical sensors and radar images. The large amplitudes and the nonlinear properties of IWs are often known as internal solitary waves (ISWs). The ISWs surface manifestation on the radar images can be identified as the pairs of adjacent bright and dark bands on a uniform background (Apel 2002). Indonesian Seas are the most suitable places for the generation of ISWs due to the complex bathymetry, which is attended by the strong current of the Indonesian

CONTACT I Wayan Gede Astawa Karang [email protected] Department of Marine Sciences, Faculty of Marine Science and Fisheries, Udayana University, Denpasar, Indonesia © 2019 Informa UK Limited, trading as Taylor & Francis Group INTERNATIONAL JOURNAL OF REMOTE SENSING 5727 throughflow (ITF) between the Pacific and Indian Oceans. These areas are characterized by strong tidal currents (Hatayama, Awaji, and Akitomo 1996; Ray and Egbert 2005), as well as prominent topographical features. The strong tidal current associated with the ITF and the complex bathymetry generate high amplitude IWs in the main channels of the ITF, such as the Lombok Strait, the Ombai Strait, the Sumba Strait, the Flores Sea, the Halmahera Sea, the Molucca Sea, the Savu Sea and the Banda Sea (Susanto, Mitnik, and Zheng 2005; Jackson 2007; Lindsey, Nam, and Miller 2018). The Flores Sea is a part of a substantial area in the Indonesian Seas that is very important for understanding the inter-ocean transport process around the sea areas. The bathymetry of the Flores Sea (Figure 1) is characterized by a sea basin, often called the ‘Flores Basin’ in the south, which approximately has a maximum depth of 5140 m. Oceanographic data in the ITF area has been widely observed from field observations and numerical simulations, but ISW observation in the Flores Sea is slightly ignored. Several studies on the dynamics of ISWs have focused on the Lombok Strait, the Banda Sea, the Ombai Strait, the Sulu Sea, the Makassar Strait, the Molucca Sea and the Celebes Sea (Jackson 2007, 2004; Karang et al. 2012; Lindsey, Nam, and Miller 2018; Matthews et al. 2011; Mitnik et al. 2000; Ningsih et al. 2008; Susanto, Mitnik, and Zheng 2005). Evidence of ISWs in the Flores Sea was captured by satellite images, as shown by Mitnik et al. (2000) and Jackson (2004). They presented the capability of SAR images from European Remote Sensing (ERS) satellites to detect ISWs in the Flores Sea, and a number of the ISW packets and the direction were clearly visible. Jackson (2004) suggested that the waves most likely originated near the island group north of Sumbawa, in the Flores Sea.

Figure 1. The map of the Flores Sea and adjacent area. 5728 I. W. G. A. KARANG ET AL.

The reflection of incident sunlight and skylight from the surfaces can produce the optical satellite images in the visible band. The specular (or near-specular) reflection of sunlight on the surface of the ocean is called sunglint (Jackson and Alpers 2010). Cox and Munk (1954) described that the viewing geometry, the sea surface roughness, and wind conditions are the component function of the observed sunglint intensity. The ISWs appearing only in the sunglint region. Satellite sunglint imagery capability has been widely used in ISW observations. Optical imagery from the Satellite Pour l’Observation de la Terre (SPOT) satellites was used by Mitnik et al. (2000) to compare the sea surface signatures of ISW visible on Synthetic Aperture Radar (SAR) images. Jackson (2007) used the sunglint images from Moderate Resolution Imaging Spectroradiometer (MODIS) to survey high frequency nonlinear internal wave occurrences around the world. The Himawari-8 geostationary satellite data, which has a 10-minute temporal resolution, is the most effective solution for observing the evolution of ISWs in detail. Lindsey, Nam, and Miller (2018) presented the possibility of the Himawari-8 geostationary satellite for tracking oceanic nonlinear IWs in the Indonesian Seas and provided the ability to calculate the propagation speed of the detected waves. Gao et al. (2018) also calculated and analysed the propagation directions and the phase speeds of the ISWs detected in the South China Sea using Himawari-8 images. These studies have proved that the new generation geostationary satellites could be useful tools to monitor and investigate oceanic IWs. Therefore, the ISWs in the Flores Sea are crucial for the prediction of large scale oceanic circulation, as well as the Indonesian Seas circulation. However, in the Flores Sea, IW measurements have not been carried out, and the characteristics of propagation and the generation site of the ISWs remain uncertain. This study discusses the characteristics of some parameters related to ISW propagation (wavelength, soliton number, first crest length, propagation direction, and phase speed) in the Flores Sea, using Himawari-8 images. The ISWs were detected in the Flores Sea and have unique characteristics.

2. Data and methods 2.1. Himawari-8 satellite The main tool for this observation is the new geostationary meteorological satellite Himawari-8. The satellite is the latest generation satellite belonging to the Japanese Meteorological Agency (JMA); it was successfully launched on 7 October 2014 and has remained in a geostationary orbit at the coordinate of 140.7 °E since 16 October 2014. This satellite is equipped with more sophisticated optical sensors, with better radiometric, spectral and spatial resolution, than other satellites in previous geostationary orbits. Himawari-8 with the principal payload, Advanced Himawari Imager (AHI), has 16 observation bands that have a spatial resolution of 500 m or 1 km in the visible and near-infrared bands, respectively, and 2 km for the infrared band. Excellence at the temporal resolution with revised time is shorter (approximately 10 minutes) if combined with superiority in spatial resolution and will provide an excellent capacity as a tool for identifying and monitoring rapidly changing weather phenomena (Bessho et al. 2016). The visible band at 640 nm or band (#3) is the band that provides the best ability (spatial resolution 500 m) as a tool for extracting and estimating parameters of ISW in the Flores Sea. INTERNATIONAL JOURNAL OF REMOTE SENSING 5729

ISWs can be detected by the Himawari-8 satellite in the sunglint area. The sunglint area is observed by satellite changes according to the angle of specular reﬂection from the surface of the water. The changes that occur are a function of surface roughness, both diurnal and seasonal, according to the location of the Sun or sensor geometry. ISW detection using Himawari-8 is not available every day in this area, although Himawari-8 can provide observation images with a temporal sampling of 10 minutes during an ~120- minute window each day over multiple days in the areas where the sunglint passes. The sunglint areas depend on the Sun/sensor locations that move meridionally with the season, and this observation requires a relatively cloud-free scene. The ISW detection and estimation of several parameters using Himawari-8 satellite imagery have been conducted in previous studies (Gao et al. 2018; Lindsey, Nam, and Miller 2018). These studies have proven the ability of Himawari-8 in the detection and estimation of certain parameters. The temporal resolution of Himawari-8 has a revisit time of 10 minutes, which can be used to estimate the ISW phase speed more realistically without the assumption of tide around this area.

2.2. Ancillary data Additional tools for this observation are used to verify the existence of ISW in the Flores Sea using images from other satellites. First, to verify the locations of ISWs in the Flores Sea, high spatialcoverageimageswerecollectedfromtheMODISdata(https://ladsweb.modaps.eosdis. nasa.gov). The MODIS sensors were launched into Earth’s orbit by National Aeronautics and Space Administration (NASA) satellites in 1999 and 2002 on board Terra and Aqua, respectively. The MODIS sensor is in a Sun-synchronous orbit at an approximately 700-km altitude, and each sensor collects data along a 2300-km wide swath, providing near-global daily coverage. The MODIS data was acquired at 36 bands in the visible and infrared regions of the spectrum, with a spatial resolution between250m(2bands),500m(5bands)and1km (29 bands). Daily reﬂectance products (Level-1B) at 250 m resolution were used as compara- tive data with Himawari-8 images. Second, to verify the estimation of the ISW phase speeds in the Flores Sea, a C-band SAR instrument were collected from Sentinel-1A images. This instrument has a spatial resolution of down to 5 m and up to 100 m. The Sentinel-1A images collected from the Copernicus website (https://scihub.copernicus.eu/). Ground Range Detected (GRD) pro- duct types, with an Interferometric Wave sensor mode on descending orbit and Vertical- Vertical (VV) polarization images were used to conﬁrm the phase speeds of the ISW in the Flores Sea. Third, General Bathymetric Chart of the Oceans (GEBCO) data were used to obtain bathymetric information from the detected ISWs in the Flores Sea.

2.3. Korteweg-de vries (KdV) theory The propagation and evolution of ISWs in the ocean has been formulated, and the wave amplitude can be expressed by the Korteweg-de Vries equation (Osborne and Burch 1980; Susanto, Mitnik, and Zheng 2005) @η @η @η @3η þ c þ αη þ β ¼ 0 (1) @t 0 @x @x @x3 5730 I. W. G. A. KARANG ET AL. where η(x,t) is the interface displacement between two layers from the unperturbed level for a function of two variables, space (x) and time (t). The first two terms of Equation (1) shown are linear and non-dispersive. The third term is nonlinear and represents finite amplitude effects. The fourth term is linear and results from weak dispersion due to the water depth. The parameters c0, α, and β are the coefficients for the linear speed, nonlinear, and dispersion effects. In a two-layer system with an upper layer thickness

(h1) and bottom layer thickness (h2) and with an upper layer density (ρ1) and bottom layer density (ρ2), the above coeﬃcients are as follows (Fan et al. 2015):

1 Δρ h1h2 2 c0 ¼ g (2) ρ h1 þ h2

3 ðÞh2 h1 α ¼ c0 (3) 2 h1h2

1 β ¼ c h h (4) 6 0 1 2 where g is the gravity acceleration, ρ is the average density of the water, and Δρ = ρ1 – ρ2. Linear parameters c0 and β have close relationships with the water depth. The variation in phase speed c0 mainly depends on the water depth (topography). The spatial distribution of the dispersion parameter β is mainly related to topographic features. In the deep ocean, β is largest and gradually decreases to zero in coastal areas, where breaking of ISWs frequently occurs (Liao et al. 2014). The steady-state independent wave solution for Equation (1) when the nonlinear eﬀect is in balance with the dispersion eﬀect is (Fan et al. 2015): x C t ηðÞ¼x; t η sech2 P (5) 0 l

η C0 0 2 x CPt Ux ¼ sech (6) h1 l where η0 is the maximum wave amplitude, Cp is the phase speed, and l is the half-width of the solitary wave. Ux is the velocity of the surface current induced by the IW in the x-direction, and the positive and negative are taken for the depression and elevation of the IWs, respectively. Both Cp and l are described as follows: η ðÞ 0 h2 h1 CP ¼ C0 1 þ (7) 2h1h2

2h h l ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 2 (8) η jj 3 0 h2 h1 The KdV equation satisfied the stereotypes of the shallow water ISW, so the surface currents of the IWs are the function of the pycnocline depth and the amplitude of the IW, according to Equation (6). INTERNATIONAL JOURNAL OF REMOTE SENSING 5731

2.4. Parameter extraction Surface manifestation of the ISW is usually characterized as isolated wave fronts (called solitons) or in collections of waves (called packets), where the number of individual waves in a packet can vary from three to a few dozen, depending on their age and distance from the generation point (Jackson 2007). The ISWs appear in the imagery as alternating bands of light and dark strips that result from variation in sea surface roughness (Alpers 1985). The ISWs detected in the Flores Sea have a curvilinear pattern (like an arc) of bright and dark bands. Leading waves in a packet have the brightest intensity and decrease to the rear. These important parameters related to ISW propagations, such as soliton number, wavelength, first crest length, propagation direction, and phase speed are significant parameters for studying the evolution of the ISWs. The soliton number is described as a number of bright/dark lines in one packet. The average wavelength between solitons in one packet is defined as the distance between two adjacent bright bands. The first crest (leading soliton) is the front of the soliton in one package and usually has the brightest intensity of all the solitons. The length of the first crest wave is measured as the distance between the two brightest ends of the soliton (Figure 2(a)). The Sentinel Application Platform (SNAP) was used for the extraction process of ISW manifestation on the sea surface and the estimation of the parameter using the Himawari-8 images. The ISW extraction results were used to estimate several ISW parameters, such as wavelength, soliton number, first crest length, direction and phase speed of the ISW.

Figure 2. (a) Sketch of ISW propagation direction estimation using the Himawari-8 image. The ‘AB’ curve is the ISW first crest. Point ‘O’ is the midpoint of the straight red line ‘AB’. The red line ‘OC’ is perpendicular to the line ‘AB’. The angle ‘a’ is the ISW propagation angle formed by the ‘OC’ line and the yellow line for the North direction (N). (b) ISW speed estimation scheme that is detected using the Himawari-8 image using the extraction of two different images of the time. The blue line is the extraction of the first wave crest (t0) and the white line is the extraction of the second wave crest (t1) after a certain time interval. 5732 I. W. G. A. KARANG ET AL.

The ISW phase speed can be estimated by dividing the distance between two extraction lines of the first crest wave at different times and the different time between the selected images (Figure 2(b)). Therefore, this estimation method does not use the tidal period assumption. ISWs detected in the Flores Sea have arc-like shapes, and some waves do not have well-defined edges, contributing to the difficulty in unambiguously selecting the same ‘point’ along with the wave at two consecutive times. The phase speed was calculated for the available timeframe each day to reduce uncertainty in the distance measurement compared to calculation every 10 minutes. Standard deviations refer to the measurement uncertainty rather than the change in the calculation results of the phase speed each 10-min observation.

3. Results and discussions 3.1. ISWs characterization using Himawari-8 images Figure 3 shows two images of the Himawari-8/AHI red light visible band (640 nm), which have a spatial resolution of ~ 500 m and were recorded on 11 October 2018 at 13:50 WITA (local time) and 13 October 2018 at 13:20 WITA. They show several bright and dark lines with a curvilinear (arc-like) shape that propagate towards the Flores Sea, Sumba Strait, Indian Ocean and the Savu Sea. In Figure 3(a), six packets of ISWs were detected over the Flores Sea and Sumba Strait, and it could be assumed that the ISW detected was generated in the Sape Strait, a small strait between the islands of West Nusa Tenggara and East Nusa Tenggara. This assumption is based on the direction of the ISW propagation, which leads to the northern and southern parts of the strait. Packet 1 is assumed to be the ﬁrst packet generated and Packet 5 is the last packet generated at the time of recording of the images by the Himawari-8 satellite. The ISW propagation to the north towards the Flores Sea has unique characteristics and has an arc-like shape. This wave packet propagation might be based on the depth of the seas. The wave packets that propagate in the Flores Sea could maintain energy up to a distance of more than 400 km (estimated distance of Packet 1 to the generation site). Meanwhile, the wave packet that propagates to the south towards the Sumba Strait, Indian Ocean, and the Savu Sea has an arc-like shape and breaks into an irregular shape because of the waves crashing into the Sumba Island. Figure 3(b) shows the broken wave packet is divided again into the west, towards the Indian Ocean (Packet 7), and to the east, towards the Savu Sea (Packet 6). Based on the location and coordinates of the Himawari-8 images (Table 1), some of the wave packets being detected on diﬀerent days have a similar location. This indicates that the wave packets occur almost in the same location every day.

3.2. ISWs parameter estimation results Figure 4(a–c) presents the extraction results of ISW patterns from two Himawari-8 images at diﬀerent times in the Flores Sea area, which are overlapped with a bathymetric contour. The ISWs are shown to propagate following the contour of the bathymetry. The ISW detected around the Flores Sea (Packet 2–5) changes approximately 2-20° from the direction of the ISW near the generation area. Packet 1 changes propagation direction almost 90°. In detail, the refraction is due to the diﬀerence in the depth along the ISW INTERNATIONAL JOURNAL OF REMOTE SENSING 5733

Figure 3. Himawari-8/AHI red band (640 nm) imagery were recorded, (a) on 11 October 2018 13:50 WITA (local time), and (b) on 13 October 2018 13:20 WITA (local time). The black arrows marked ‘1ʹ –‘7ʹ represent the ISW packet IDs that are referenced in Tables 1 and 2. 5734 I. W. G. A. KARANG ET AL.

Table 1. List of ISWs detected in Himawari-8 images acquired on 11–13 October 2018. The latitudes (Lat) and longitudes (Lon) are approximations, and time (WITA) refers to the first and the last time for each image recorded using the local time. IWs Packet ID Location Lat (°S) Lon (°E) Date Time (WITA) 1 Flores Sea 5.6 116.9 11 October 2018 13:50 2 Flores Sea 6.6 116.6 3 Flores Sea 6.9 117.2 4 Flores Sea 7.2 117.8 5 Flores Sea 7.1 118.8 6 Sumba Strait 9.3 119.4 1 Flores Sea 5.6 116.9 12 October 2018 13:20 2 Flores Sea 6.6 116.7 3 Flores Sea 6.9 117.2 4 Flores Sea 7.3 117.8 5 Flores Sea 7.2 118.8 6 Sumba Strait 9.3 119.1 7 Indian Ocean 9.2 118.0 2 Flores Sea 6.5 116.8 13 October 2018 13:00 3 Flores Sea 6.9 117.3 4 Flores Sea 7.3 117.9 5 Flores Sea 7.5 118.7 6 Sumba Strait 9.2 119.5 7 Indian Ocean 8.9 118.6 propagation area. The wave refraction characteristics are caused by changes in bathymetry. Part of the first crest of the ISW at shallower depths will propagate at a smaller speed than at deeper depths. As a result, the first crest line of the ISW will turn and try to align with the bathymetric contour. Table 1 reports all extracted data and estimated parameters based on the ISW patterns from the Himawari-8 satellite. Figure 4(a) shows the extraction results of the ISW pattern, using the two Himawari-8 images on 11 October 2018 that overlapped with the bathymetric contour. Both the extraction results have a time difference of approximately 40 minutes. Six ISW packets have been detected in these images; five packets propagate to the northern part and two packets propagate to the southern part of the generation site in the Sape Strait. Figure 4(b,c) also have the same extraction results as the characteristics in Figure 4(a), and the results of the parameter calculation are shown in Table 2. The ISW initially moves westward with an azimuth 325° angle and decreases approximately 15-25° before the wave is refracted such that the direction of the azimuth angle becomes 10°. The change in direction of the ISW propagation angle can be caused by changes in the contour of the bathymetry (Figure 4(a– c)). The ISW also has a refractive effect. Ocean waves, in general, propagate parallel to changes in the bathymetric contour. The ISW refraction event is the result of changes in speed (due to the shoaling process) and changes in wavelength. This can be seen in the ISW parameter estimation, using the Himawari-8 images (Table 2). Several ISW parameters during the propagation process in the Flores Sea can be estimated and are presented in Table 2. The summary of the ISW generation site and propagation direction can be seen in Figure 5. The ISW originates from the Sape Strait as a generation site (Figure 5, red rectangle) and will propagate in four directions (Figure 5, dashed arrows). Additionally, the ISW parameters are changed due to different depths of propagation, which turn the depth in the central part of the Flores Sea (Figure 5, black solid arrows). INTERNATIONAL JOURNAL OF REMOTE SENSING 5735

Figure 4. The results of the ISW feature extraction of two Himawari-8 images diﬀering in time on the same date, overlapping with the bathymetry contour on (a) 11 October 2018 at 13:50–14:30 WITA (Table 2.A), (b) 12 October 2018 at 13:20–14:30 WITA (Table 2.B), and (c) 13 October 2018 at 13:00–14:30 WITA (Table 2.C).

Table 2. ISW parameter estimation results (wavelength, soliton number, ﬁrst crest length, direction, and phase speed) using Himawari-8 images. Standard deviation (StD) refers to the phase speed measurement uncertainty. ISW Packet ID refers to the number of packets based on Figure 3. ISW Wave- First Crest Figure Packet length Length Direction Average Phase ID ID Dates Time (WITA) (km) Soliton (km) (°) Speed (m s−1) StD A 1 11 October 2018 13:50–14:30 3.51 11 43.03 8.51 2.01 0.32 2 3.14 6 110.11 309.70 1.97 0.34 3 2.28 5 68.47 303.33 1.97 0.30 4 3.49 2 23.61 302.89 1.24 0.47 5 4.33 1 161.35 325.45 1.37 0.08 6 1.65 4 117.59 191.25 1.72 0.09 B 1 12 October 2018 13:20–14:30 3.38 8 46.84 14.51 1.79 0.09 2 3.23 5 124.91 309.23 1.73 0.22 3 2.47 4 80.78 303.59 1.66 0.23 4 3.71 3 22.84 301.82 1.58 0.16 5 3.98 1 163.52 319.17 1.87 0.10 6 2.75 3 103.88 190.20 2.15 0.17 7 3.34 4 78.46 267.63 1.31 0.23 C 2 13 October 2018 13:00–14:30 2.98 5 89.36 314.25 1.86 0.10 3 3.23 6 66.34 301.85 1.80 0.10 4 3.49 4 20.65 300.10 1.78 0.04 5 4.32 1 125.64 316.00 2.44 0.34 6 1.99 1 62.55 144.00 2.19 0.15 7 2.76 4 35.18 317.65 1.98 0.32 5736 I. W. G. A. KARANG ET AL.

Figure 5. The ISW propagation scheme detected in the Flores Sea using Himawari-8 images. The solid arrow represents the initial wave energy, and the dashed arrows represent energy changes due to changes in depth and geometry. The red arrow indicates the changes in phase speed due to the decrease of depth.

This observation indicates that the surface signature of ISW in the Himawari-8 images originated from the semidiurnal internal tides which were refracted because of the changeq inffiffiffiffiffiffiffiffiffiffi bottom topography. The IW characteristic slope of semidiurnal internal tides γ ¼ ω2f 2 ω N2ω2 , where is the semidiurnal wave frequency and f is the inertial frequency, predicts the generation of the internal tides by steep bottom topography at a semidiurnal frequency (Nam and Park 2008; Park and Watts 2006). In the Flores Sea area, where the buoyancy frequency (N) is ~1.6 × 10−3 cycle/s (Prihatiningsih et al. 2019), the characteristic slope of semidiurnal internal tides is ~ 0.014. This result indicate that the Flores Sea has an optimal condition for generating strong semidiurnal internal tides and the existence of propagating IWs. ISWs observed using Himawari-8 (Figure 3(a–c)) are seen to propagate independently, without interacting with each other. The changes in bathymetry in the central part of the Flores Sea have an impact on the direction and speed of ISW propagation in the northern part of the generation site. While in the southern part, ISW geometry changes due to the strait length and is directly blocked by the Sumba Island, which causes the waves to break and propagate towards two directions – to the Indian Ocean and to the Savu Sea. An interesting example of wave refraction from the Himawari-8 image acquired on 11 October 2018 at 13:50 WITA is shown in Figure 6(a). Refraction of a surface manifestation of ISWs approaching the bathymetric slope is caused by changes in depth. ISWs in INTERNATIONAL JOURNAL OF REMOTE SENSING 5737

Figure 6. (a) The ISW refraction process can be seen in the Himawari-8 image acquired on 11 October 2018 at 13:50 WITA. Red arrows are the soliton wavelengths. (b) The result of ISW feature extraction (red line) overlaid with the bathymetric contour. Rectangles A and B indicate Slower and Faster speeds, respectively. shallower water will be slowed compared to those in deeper water. Thus, the soliton crests will rotate and tend to become parallel to the contour bathymetry as they approach it. The frequency of ISWs does not change along the propagation path, but their speed and wavelength change. In deeper water, the speed becomes faster and the wavelength becomes longer (Figure 6(b), rectangle A). In shallower water, the speed becomes slower and the wavelength becomes narrower (Figure 6(b), rectangle B). Theoretically, this is equivalent to assuming ISW refraction according to the KdV solution. As mentioned in Equation (2) and Equation (6), the ISW phase speed in the pycnocline area depends on the depth of the h1 and h2 (upper and bottom layer, respectively). The KdV equation might correspond to refraction effects, as illustrated in the Himawari-8 image for the case of each soliton propagation in a wave packet. The average phase speed value for an ISW package shows the opposite result. The further away from the generation site, the average speed between packets becomes larger. Figure 7(a) shows the Sentinel-1A image, with oceanic ISW characteristics of extended bright and dark features in the Flores Sea area representing the ISW packets in each train. The image shows variation in the intensity due to changes in the sea surface roughness and detects three ISW packets propagating from the Flores Sea towards the Java Sea. The Sentinel-1A image can estimate ISW phase speeds using the tidal period assumption. The average phase speed of ISWs detected on this image is 1.70 m s−1. The ISW first crest length cannot be estimated using the available SAR image due to the swath limitations of Sentinel-1A. Figure 7(b) shows the extraction of optical images and SAR on the same date at different times, overlapped with the bathymetric contour. Both of these images have a time difference of approximately 8 hours and confirmed the consistency of the shape and path of the propagation. The result of the phase speed estimation of the Himawari-8 image with a 10-min temporal resolution was approximately 1.97 m s−1 and the result from the Sentinel-1A images with a 12-day temporal resolution, using the assumption of 5738 I. W. G. A. KARANG ET AL.

Figure 7. SAR observation of an ISW in the Flores Sea (a) Sentinel-1A image with ISW patterns taken at 05:53 WITA on 11 October 2018. The red line is the distance between two consecutive first crest packets detected in the SAR image. (b) ISW feature extraction result of Sentinel-1A and Himawari-8 images differing in time on the same date and overlapping with the bathymetric contour on 11 October 2018. The red line is the extraction result of the Himawari-8 image recorded on 11 October 2018 at 14:30 WITA, and the blue line is the extraction result of the Sentinel-1A image required on 11 October 2018 at 05:53 WITA. the semidiurnal tidal period, obtained phase speeds of approximately 1.70 m s−1. SAR image also confirm that the nature of the wave refraction is detected in the image Himawari-8. Figure 8 highlights that ISWs in the Flores Sea were also captured by the MODIS-Aqua image of band (#1) (645 nm), with a spatial resolution of ~250 m. MODIS-Aqua and Himawari-8 images have almost identical patterns and occurrence areas of ISWs in the Flores Sea, or in other words, the location of the Himawari-8 observed ISWs has been verified. Both sensors are beneficial for observing ISW signatures over the sunglint area in the Flores Sea. Himawari-8 images in the present study can observe the signature of ISWs in the Flores Sea at up to seven series images a day (Table 2) which can track speed and change the direction of each soliton at a certain longitude-latitude position in a short time periods (minutes to hours). The phase speed and direction of propagation of ISW can change in a short period because of the stratification conditions of the water column and different depths so that the measurement method using Himawari-8 is very effective. However, with a one-day repetition time, there is only a single capture of the ISW signature by MODIS-Aqua per day. The changes that might occur in minutes or hours are not recorded so that the measurement of phase speed and other parameters will use assumptions. An assumption such as the tidal period approach is commonly used to calculate the phase speed but the method cannot explain the changes that occur on a much shorter scale than the tidal period. Additionally, clouds are the most common problem for ISW signatures in optical sensors such as MODIS-Aqua and Himawari-8. The case images show that both sensors have a different response to thin clouds. The Himawari-8 image is more robust to thin clouds than MODIS-Aqua. It is not only robust in response to thin clouds, with extremely high temporal resolution, but the possibility of observation of ocean surface signatures is also high due to the short period of cloud cover changes. INTERNATIONAL JOURNAL OF REMOTE SENSING 5739

Figure 8. Surface manifestation of ISWs in the Flores Sea on the optical imagery acquired at 14:50 WITA on 13 October 2018 by (a) MODIS-Aqua image and (b) Himawari-8 image.

Analytical estimation of the ISW phase speeds using KdV theory has been applied in the Flores Sea. Fan et al. (2015) describes the ISW phase speeds as a function of maximum wave amplitude (η0), linear wave coefficients (c0), and water depths (Equation (7)). Vertical motion of IWs (assumed as the maximum wave amplitude) reported 40 m in the Flores Sea (Tomascik et al. 2013), assuming the average density of seawater is ~1025 kgm−3, the density of sea surface layer in Indonesian Seas is ~1022.95 kgm−3 (Susanto, Gordon, and Sprintall 2007), upper layer thickness is ~110 m (Gordon, Ffield and Ilahude 1994), and the water depths is ~500 m. The estimated ISW phase speeds in the Flores Sea by using Equation (7) is ~1.47 m s−1. Comparison of ISW phase speeds estimation results using satellite imagery in this observation with estimation results using KdV theory are shown in Table 3. The phase speeds estimation results from the different satellite images for the ISW ID 2–3(Figure 3) are consistent with the estimation result from the KdV equation (Equation (7)). Estimation result from Himawari-8 has the closest value with the KdV estimation result. Taking advantage of real-time ISW progression provided by Himawari-8, it is possible to extract important temporal-grained features of ISW. The Himawari-8 satellite makes it possible to detect the changes in the direction, speed, and refraction of ISWs in the ocean, which is difficult when using low temporal resolution satellite images. The real-time ISW progression should be explored further in order to solve the

Table 3. ISW phase speed estimation results from the diﬀerent satellite images and the KdV equation. Estimation method Phase speed estimation for ISW ID 2–3(ms−1) Himawari-8 1.62 MODIS-Aqua 1.63 Sentinel-1A SAR 1.66 KdV equation 1.47 5740 I. W. G. A. KARANG ET AL. problems of how ISWs evolve, which can then be used as a reference and input data for ocean circulation models, particularly in areas with intensive IW occurrence, such as the Indonesian Seas.

4. Conclusions In this study, the Himawari-8 new generation geostationary satellite shows the capability to observe and analyse short and real-time ISW propagation in the Flores Sea, with an extremely high temporal resolution (10 minutes) period. Himawari-8 can even track sudden changes in ISW behaviour in the ocean, such as wave refraction and reflection, caused by the complexity of bathymetry and water stratification in the Flores Sea and in an adjacent area in which it is impossible to obtain images by using polar-orbit satellites, such as MODIS and SAR. The characteristic and propagation of ISWs in the Flores Sea are investigated. A series of sunglint images from Himawari-8 has shown that the ISWs in the Flores Sea are generated over prominent topographic features in the Sape Strait. The ISW might be generated in the Flores Sea as a result of internal tide-topography interaction, associated with the current in the Sape Strait. The parameters of ISW propagation (wavelength, soliton number, first crest length, propagation direction, and phase speed) were extracted and estimated from the Himawari-8 images. The ISW speed was locally calculated on each packet in adifferent location. The average speed was 2.03 m s−1 both to the north and the south of the generation site. It is interesting to note that the ISW phase speed changes are caused by differences in depth (KdV theory) and geometry during its propagation. The propagation processes and mechanisms of ISWs in the Flores Sea are complicated due to the complex bathymetry. The ISW packet locations and propagation directions remained stable during the time of observation. The ISWs in the Flores Sea propagate towards four directions. The ISW propagation to the north towards the Flores Sea has an arc-like shape and is divided into the west towards the Java Sea and to the north towards the Sulawesi islands. Meanwhile, the ISW that propagates to the south towards the Sumba Strait and the Savu Sea has an arc-like shape, which then breaks into an irregular shape because it crashes into the Sumba islands. The breaking ISW is then divided again into the west towards the Indian Ocean and to the east towards the Savu Sea. Additionally, the real-time information of location and propagation parameters of ISWs from Himawari-8 is a very crucial input for vertical mixing parameterizations in ocean circulation models. In the future, these instant results might be beneficial for submarine and maritime activities, to anticipate the destructive effect of IWs. Additionally, the ISW vertical displacement mechanism and the dissipation are still required in addition to numerical simulation and field observations.

Acknowledgements

Authors would like to thank the Japanese Meteorological Agency (JMA) and Center for Environmental Remote Sensing, Chiba University for providing the Himawari-8 data. Thanks for some reviewers also to conduct improvements. INTERNATIONAL JOURNAL OF REMOTE SENSING 5741

Disclosure statement

No potential conﬂict of interest was reported by the authors.

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