1. Introduction
Mesoscale oceanic eddies and fronts, as complex ocean dynamic phenomenon, are energetic contributors to mixing. The oceanic fronts are the boundary between warm and cold water masses, with very significant differences in temperature, density and salinity [
1]. There are different kinds of oceanic fronts, such as the large-scale meandering front, buoyancy forced coastal water mass fronts and the interface region between the deep-ocean and the coastal regions [
2]. Fronts of Kuroshio is a type of oceanic front caused by intensified current led to a strong gradient of sound in the water column, which show a notable effect on sound propagation. The dynamic ocean sound speed leads to the change of long-range acoustic propagation travel time, the movement of the sound channel axis or/and convergence zone and the horizontal refraction of rays path [
3,
4].
Oceanic fronts are observed in the global ocean, such as the Antarctic gyre, the Gulf of Mexico, the region between Hawaii and Alaska and the Kuroshio extension area. Lots of work has been done in different parts of the ocean and indeed, one of the topics on the research of the front is the mechanism of the formation and extinction of the front. To survey the generation and to evaluate vertical motions of the near-surface front, Mahadevan analysed model simulations of the oceanic front [
5]. Sokolov et al. [
6] used the expendable bathythermograph (XBT) and satellite altimeter to identify the subtropical front near Antarctica and South Australia. The analysis of the front position helps to obtain variation information in the annual sea surface temperature. In the long-term observation of the temperature gradient of fronts in the western North Pacific, Levine et al. [
7] summarised the influence of front on topography at different frontal zones. Yang et al. [
8] generalized heat and energy transfer of fronts near the continental shelf in the South China Sea, especially the mass exchange and energy distribution in the vertical direction. The other research topic is the seasonal variation in front. Nakamura et al. [
9] identified the reasons for the seasonal variations in salinity of the oceanic front in the vertical mixing layer of the western North Pacific. Nakamura proved that this variation in winter is mainly caused by strong eddies and deep-ocean cyclonic. Also, the mixing layer energy enhances in the middle of oceanic fronts, due to the existence of near-inertial internal waves in winter. The westward movement of frontal current affected by the invasion of the Kuroshio in the Luzon Strait is analyzed by Liu et al. [
10]. The generation of temperature fronts was observed in summer, using the satellite remote sensing processed data. Xie et al. [
11] outline that the season has no evident impact on the horizontal position of the front while the vertical structure in the Southwest Pacific. By measurement and statistics, they also concluded that the depth of the oceanic fronts in the subtropical zone in winter was greater than that in summer. The oceanic fronts on the continental shelf are observed by Tan et al. [
12] in both spring and summer. Also, they modelled the three-dimensional current field of the oceanic fronts and obtained its dynamic mechanism and formation reason.
The horizontal asymmetry character resulted in the complication of acoustic propagation in the area of frequently occurs current. Therefore, many studies and researches have been done on sound wave propagation through fronts. Seismic waves reflection has been observed at the boundary between two water masses with different thermodynamic characteristics. Nakamura et al. [
13] analysed the multi-channel seismic exploring data, inversed sound speed of the horizontal variation of the temperature front. It showed sound wave is blocked by variations of horizontal temperature using mathematically sound propagation models. Neubert et al. [
14] choose acoustic rays theory to deduce the average coherence intensity of sound wave multiple paths caused by random oceanic front. Tang et al. [
12] also discussed the sound propagation characteristics responses to the intensity variability of oceanic front in the South China Sea. Acoustic shadow zones are formed when the sound waves pass through the warm side of the oceanic front. Liu et al. [
15] made a preliminary summary of sound propagation of acoustic pressure resulted from typical mesoscale phenomena, using the parabolic model FOR3D [
16] to simulate the impact of mesoscale eddies and oceanic fronts on the sea surface acoustic channel and converging zone. In the Yellow sea, coupled normal mode theory used as a numerical method for traveltime tomography, estimated the subsurface velocity structure and inversed the thermocline significant changes caused by current quantitatively [
17]. Due to the limitation of experimental conditions, they study sound propagation without considering sound wave horizontal refraction. The three-dimensional thermocline inversion remains to be further explored. Jian et al. [
18] provided the difference of propagation loss of acoustic pressure with and without oceanic front by parabolic method. Guo et al. [
19] studied the travel time of sound waves and distribution of sound pressure in the ocean of Taiwan.
Furthermore, researchers have paid extensive attention to Kuroshio fronts and perform a big amount of numerical and experimental studies. Chen et al. [
20] applied the ray theory to simulate the sound propagation process of front and found a remarkable change of sound propagation at the surface sound channel [
21]. Rainville et al. [
22] explained the gradient of temperature arouse the signal variety in the East China Sea and verified by joint experiment processed data in 2001. Shapiro investigated propagating characteristics along with different directions of oceanic front in different seasons of the same year.
Although scientists have carried out a relatively comprehensive study on the western North Pacific front by comparing of oceanic fronts exists or not and obtained certain conclusions in the aspects of sound pressure transmission loss and time arrival structure, there is a lack of discussion and analysis on the three-dimensional sound field of the front. This paper discusses the degree of sound wave horizontal deflection with several depths of source and receiver and reveals strong horizontal refraction because of the oceanic front, moreover explains the anomalous distribution of the acoustic field, which is helping to design vertical spacing in an array and positions of the acoustic stations of acoustic tomography. This paper analyses numerically and experimentally how oceanic fronts affect three-dimensional acoustic propagation in the western North Pacific. As follows in
Section 2, a three-dimensional oceanic front environment was modelled and used to resolve the sound propagation and the forecasting process of the sound transmission based on the ray theory. And then, the sound propagation experiments in the Kuroshio is introduced, and the comparison between the measured results and numerical results are carried out in
Section 3. In
Section 4, the numerical results of the three-dimension acoustic propagation and the horizontal refraction process are described, while conclusions and discussions are drawn in
Section 5.
3. Comparison between Experimental and Numerical Simulation Results
The Kuroshio Current, one of the strongest surface oceanic currents in the whole ocean, intersects with the polar circulation and forms a large meander structure. The temperature and salinity distribution are extremely complex, especially in regions influenced by strong frontal currents. Nagai analysed the disturbance and temperature and salt distribution structure directly observed in the Kuroshio front sea area from the oceanographic perspective. Ocean energy dissipation is mainly influenced by the non-geotropic current and internal wave, with the changing of the temperature gradient [
31]. The dynamic phenomena in the western North Pacific were identified and tracked, such as the size, life cycle and distribution of cyclonic and anticyclonic as respectively indicated in
Figure 4a–d. The number of cold eddies and warm eddies in the western North Pacific are basically in the same magnitude, and the number of cyclonic is slightly larger.
From May to July 2019, a joint experiment of ocean acoustics and physical oceanography was carried out. The moving vessel profiler (MVP300) and conductivity-temperature-depth (CTD) measured the temperature/salinity gradients and sound speed profile. After four voyages along the ship trajectory, crossing the oceanic front, temperature and salinity data were collected continuously, which provided high-resolution environmental parameters for sound propagation study.
Figure 5 shows the topography of the experimental area in the western North Pacific, the path of MVP300 (red lines) and the position of shipboard CTD cast (green rhomb) on 8 June.
Figure 6 shows the sound speed of the ocean surface. The green rhomb and red lines are the locations of the CTD stations and MVP stations, respectively. Typical sound speed-depth curves were observed by underwater CTD in the western North Pacific, from 0 m to 2000 m in depth and deep oceans below 2000 m provided by remote sensing which is shown in
Figure 7. Afterwards,
Figure 8 illustrates four sound speed profiles in the water column collected by MVP.
We measured temperature variation in the horizontal direction, temperature and salinity consistently varied across the entire 3-D block, influenced by the warm Kuroshio current. The horizontal temperature gradient and salinity gradient between both sides of the oceanic front are large enough to detect the front, indicated in
Figure 9a,b, respectively. The front has tracked to determine the placement of the receiving array for the following acoustic experiment. It is predicted that the frontal position located at 150.6° East longitude.
Putting the relationship of depth, salinity and temperature into the Equation (
3) and calculating the variation of sound speed profile,
Figure 10a shows a layered structure of the sound velocity in the water column. Due to warm-water mass intrusion, sound velocity changes in both surface layer and thermocline.
Figure 10b exhibits sound velocity profiles at different longitude in order to describe sound speed layer structure more clearly. There is a minimum sound speed in SSP that varies from 1470 m/s to 1455 m/s at a depth of roughly from 300 m to 100 m. Meanwhile, the maximum sound speed and the sound speed near the seabed reach up to 1547 m/s.
The horizontal distribution of sound speed was obtained through interpolation. The horizontal and vertical coordinates are marked with longitude and latitude. The sound velocity distribution at a depth of 250 m is enlarged in
Figure 11. It is intuitively noticed that the change of sound velocity of the oceanic front change dramatically below the mixing layer. In
Figure 11, the geodetic longitude and latitude coordinates are converted to a Cartesian coordinate system, and the position (149° E, 39° N, z) replaced by the point (0, 0, z). This figure shows the three-dimensional temperature field at positions
km,
km and
km with a depth of 200 m. The solid black lines in the middle are the isotherm distribution of the temperature on the section. Vertical sections are the interpreted temperature profile in the distribution of uniform distance, and three layers are shown with the map of interpreted temperature at 0, 100 m and 200 m depths in
Figure 12.
The following
Figure 13 is a diagram of the sound propagation experiment configuration on the western North Pacific. The receiver system uses the ranges between a beacon fixed on the seabed and hydrophones on the submerged buoys aided by depthometers. There is a vessel in motion against the oceanic front. Melt cast explosives based on Trinitrotoluene (TNT) is used as high power and low-frequency underwater acoustic source. The transmission loss of sound pressure is acquired through theoretical calculations and experimental measurements. Two types of explosives are selected whose explosion depth is 100 m and 200 m. Moreover, more explosives occur on the side of the cold water as sound sources. The broadband explosives were thrown into the ocean every 1.7 km within 3 h from a vessel speed of 12 knots. The position of the blue rectangle is the vertical array of 10 self-contained underwater acoustic recorders, equally spaced from 50 m to 300 m.
Figure 14 is one of the hydrophones used in this experiment, and the specifications of the hydrophones are shown in
Table 1.
The actual depth of the vertical array influenced by ocean currents is not consistent with the preset values. After the depth of the vertical array was corrected, the receiving signals at the same depth were rearranged. The sampling frequency of the instrument is 10 kHz. To ensure the integrity of the received signal, the time window for intercepting the signal is selected according to the actual signal amplitude. The source signal spectrum was distributed from 5 Hz to 500 Hz, and the recorded signals were band-pass filtered with a Butterworth filter with a centre frequency of 120 Hz. The average coherence intensity of the sound wave is Equation (
10). The pressure signal is also selected to have a peak frequency at 120 Hz and a 3 dB cut-off frequency from 106 Hz to 135 Hz. Ray theory is used to analyze the sound field and obtain broadband mean acoustic intensity by averaging the frequency points within the bandwidth in the interference field by Equation (
10), where
I is the sound intensity of the centre frequency of
,
is the half of frequency bandwidth.
In this deep-sea sound propagation experiment, the acoustic reciprocity theorem was generalized for multiple sources and a fixed receiver array. In the sound field due to the source at explosive point, sound pressure received at any other hydrophones is the same as that would be produced at the explosive point if the source was placed at the depth of hydrophones. The results of signal processing received by different depth hydrophones are given by
Figure 15. It can be seen from
Figure 15a that there are no obvious differences for experimental results with different sound waves propagate direction (toward the cold side and the warm side), when the sound source was located in the mixed layer (75 m). However, it reflects the great differences that exist between the sound field propagated towards the warm and cold side of the ocean-front centre with three different depths of the acoustic sources located near the sound channel axis (150 m, 175 m and 200 m) is represented by
Figure 15b–d. On the cold-water mass side, the distribution of the sound field is abnormal. There is a drop in the curves (after 3 km from the receiver array). On the contrary, transmission loss on the warm side of the oceanic front follows the law of reciprocal attenuation of the distance. These intuitive phenomena can be observed through the corrected experimental data and the sound pressure transmission loss curve drawn by simulation. Eventually, the agreement of experiment and modelling is satisfactory. At the bottom of the equipment, the float balls system is heavy enough for the vertical line hydrophone array to remain essentially in the experimental design. Although, the error may come from the fluctuation of the line array caused by the flow of the oceanic front.
Figure 15 showed the calibrated depth of receiver hydrophone depends on the depth measurement. The standard deviation of explosion-signal receiver depths is within the allowed error range.
The whole depth of sound pressure transmission loss diagram is illustrated in
Figure 16, simulated acoustic source located at depth of 175 m, the centre of the oceanic fronts is situated at the right cold side (red line in
Figure 16). The warm water is on the right side of the front and the other side is cold. The sound field distribution on both sides of the oceanic front is discrepant. It is evident to observe that some acoustic rays turn downward until facing the centre of the oceanic front when the wave propagates to the right side, the other ray turning points. But on the other hand, there are no phenomena observed when the wave propagates to the left side. It can be concluded that the existence of the oceanic front does act as an acoustic lens that could strongly change the directions of acoustic propagation.
The data received on the hydrophone at 175 m depth were analysed, compared, and discussed here with the two-dimensional modelled result.
Figure 17 indicates transmission loss generated by explosions at the depth of 200 m. The experimental data on the warm water side is in good agreement with the curve. Nevertheless, the 2-D simulation results at the side of cold water do not agree well with the experimental data. It is doubtful that the exciting sound pressure field at this depth is greatly affected by the horizontal refraction, which will be discussed in
Section 4.
Designing a band-pass filter with a centre frequency of 200 Hz, and a 3 dB bandwidth of 50 Hz, comparing the difference in transmission loss between the two depths of explosions, the abnormal distribution of the propagation loss at 4 km and 8 km from hydrophone arrays can be observed by
Figure 18. The rapid fall was also measured, as a result of the 200 Hz filtered experimental data. According to the propagation loss curve generated by the explosions at depth of 200 m, it can also be noticed that some data at some certain distance from the receiving vertical array does not match well.