Intraseasonal modulation of Wyrtki jet in the eastern Indian Ocean by equatorial waves during spring 2013

Intraseasonal modulation of Wyrtki jet in the eastern Indian Ocean by equatorial waves during spring 2013

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Yongliang Duan¹^,², Hongwei Liu³^,⁴^,⁵, Lin Liu¹^,²^,^*, Weidong Yu⁶^,⁷^,⁸

Acta Oceanologica Sinica | 2020, 39(7) : 11 - 18

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Acta Oceanologica Sinica | 2020, 39(7): 11-18

• Environment and Ocean-Atmosphere Interaction in the Indian Ocean •

Intraseasonal modulation of Wyrtki jet in the eastern Indian Ocean by equatorial waves during spring 2013

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Yongliang Duan¹^,², Hongwei Liu³^,⁴^,⁵, Lin Liu¹^,²^,^*, Weidong Yu⁶^,⁷^,⁸

Affiliations

¹ Center for Ocean and Climate Research, First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China

² Laboratory for Regional Oceanography and Numerical Modeling, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266061, China

³ Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

⁴ Laboratory for Ocean and Climate Dynamics, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266071, China

⁵ Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China

⁶ School of Atmospheric Sciences, Sun Yat-Sen University, Zhuhai Campus, Zhuhai 519082, China

⁷ Key Laboratory of Tropical Atmosphere-Ocean System (Sun Yat-Sen University), Ministry of Education, Zhuhai 519082, China

⁸ Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519082, China

Published: 2020-07-25 doi: 10.1007/s13131-020-1576-2

Outline

Abstract

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A strong spring Wyrtki jet (WJ) presents in May 2013 in the eastern equatorial Indian Ocean. The entire buildup and retreat processes of the spring WJ were well captured by two adjacent Acoustic Doppler Current Profilers mounted on the mooring systems. The observed zonal jet behaved as one intraseasonal event with the significant features of abrupt emergence as well as slow disappearance. Further research illustrate that the pronounced surface westerly wind burst during late-April to mid-May, associated with the active phase of a robust eastward-propagating Madden–Julian oscillation in the tropical Indian Ocean, was the dominant reason for the rapid acceleration of surface WJ. In contrasting, the governing mechanism for the jet termination was equatorial wave dynamics rather than wind forcing. The decomposition analysis of equatorial waves and the corresponding changes in the ocean thermocline demonstrated that strong WJ was produced rapidly by the wind-generated oceanic downwelling equatorial Kelvin wave and was terminated subsequently by the westward-propagating equatorial Rossby wave reflecting from eastern boundaries of the Indian Ocean.

Key words

Wyrtki jet / equatorial wave dynamics / Madden–Julian oscillation / tropical Indian Ocean

Cite this Article

Yongliang Duan, Hongwei Liu, Lin Liu, Weidong Yu. Intraseasonal modulation of Wyrtki jet in the eastern Indian Ocean by equatorial waves during spring 2013[J]. Acta Oceanologica Sinica, 2020 , 39 (7) : 11 -18 . DOI: 10.1007/s13131-020-1576-2

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1 Introduction

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The surface circulation of the tropical Indian Ocean (TIO) is primary modulated by the seasonal monsoon winds (Schott and McCreary, 2001; Schott et al., 2009). During monsoon transition seasons, the strong eastward surface Wyrtki jets (WJs), forced directly by the semiannual westerly equatorial winds, take place at boreal spring and fall seasons (Wyrtki, 1973; O’Brien and Hurlburt, 1974; Han et al., 1999; McPhaden et al., 2015; Liu et al., 2016). Previous studies on the WJ dynamics, mostly based on the theoretical and linear/nonlinear numerical models (e.g., Yoshida, 1959; O’Brien and Hurlburt, 1974; Cane, 1980; Reverdin and Cane, 1984; Reverdin, 1987; Han et al., 1999, 2001, 2004, 2011; Nagura and McPhaden, 2010a, b; Nyadjro and McPhaden, 2014), have greatly advanced our understanding for the general features and relevant mechanisms of WJs. Due to the variability of zonal wind forcing over TIO, equatorial surface WJs perform complex activities ranging from intraseasonal to decadal time scales (e.g., Reppin et al., 1999; Vinayachandran et al., 1999, 2007; Grodsky et al., 2001; Manghnani et al., 2003; Senan et al., 2003; Sengupta et al., 2001; Nagura and McPhaden, 2008, 2010a, b; Qiu et al., 2009; Chu, 2010; Gnanaseelan et al., 2012; Joseph et al., 2012; McPhaden et al., 2015; Duan et al., 2016; Sachidanandan et al., 2017; Wu et al., 2018). In addition, the equatorial wave dynamics have also well reproduced the seasonal circulation system in the TIO (Jensen, 1993; Han et al., 1999, 2011; Yuan and Han, 2006; Nagura and McPhaden, 2010a; Chen et al., 2015a, b; Huang et al., 2018a, b). In particular, the semiannual WJs’ dynamics controlled by the equatorial waves has been widely described, and it is found that the WJ variations are mainly controlled by the wind-forced Kelvin/Rossby wave and boundary-generated Rossby wave (Cane, 1980; Luyten and Roemmich, 1982; McPhaden, 1982; Han et al., 1999, 2001; Iskandar and McPhaden, 2011; Nagura and McPhaden, 2010a, b, 2012, 2016; Iskandar et al., 2016; Chen et al., 2017).

The recently international community made great effort to make up the observation over the TIO with the joint works from global countries and scientists during last two decades. As the successful implementation of Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA; McPhaden et al., 2009) and Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/Dynamics of the Madden-Julian Oscillation (DYNAMO) international field campaign (Zhang and Yoneyama, 2017), kinds of in-situ observations collected by numerous instrument, which provided higher spatial and temporal resolution as well as long term monitoring, encourage international community to investigate multi-scale variability of the WJs and the associated air-sea interactions (e.g., Masumoto et al., 2005; Iskandar and McPhaden, 2011; Nagura and McPhaden, 2012; McPhaden and Foltz, 2013; Moum et al., 2014, 2016; Jensen et al., 2015; Rao et al., 2016; Shinoda et al., 2013a, b, 2017). In particular, taking advantage of the long zonal velocity time series records from equatorial RAMA Acoustic Doppler Current Profilers (ADCPs) at 80.5°E and 90°E, Iskandar and McPhaden (2011) have shown that the near-surface layer responds directly to intraseasonal zonal wind stress forcing and subsequently propagates eastward in the form of oceanic Kelvin waves. Using an 8-year record from the same ADCP at 90°E, Rao et al. (2016) have provided a comprehensive description of the observed zonal flow and reviewed the governing mechanisms. In our previous paper, based on the ADCP observation at 85°E and numerical experiments, we have briefly reported that the anomalous WJs activities in 2013 primary result from the equatorial zonal wind anomalies associated with the strong intraseasonal oscillation events (Duan et al., 2016). However, all of their works mainly focused on the intraseasonal variability and the buildup process of the WJs, and detailed WJ evolution and the direct in-situ observation evidence of modulation of equatorial waves on the WJ was still insufficiency. Therefore, in this paper, we examine the detailed buildup and termination of the spring WJ in 2013 with two sub-surface Acoustic Doppler Current Profilers (ADCP) moorings in the eastern equatorial Indian Ocean (EqIO), and reveal the strong modulation of the equatorial waves on the WJ evolution.

2 Data and methods

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2.1 Data

The analysis in the present study mainly utilizes daily average zonal velocity data from a RAMA upward-looking ADCP mooring at 0°, 80.5°E and a equatorial ADCP mooring deployed by the First Institute of Oceanography at 85°E. The 80.5°E data are available in 5 m vertical resolution from the surface down to 200 m depth, while the 85°E data are available in 10 m vertical resolution from April 5, 2013 to April 18, 2014 (Duan et al., 2016). Data shallower than about 30 m are discarded as they are contaminated by acoustic signals reflected off the surface. It is well worth to point out that we don’t utilize the RAMA ADCP data at 0°, 90°E, which clearly displays an inconsequent WJ patterns, and the explanations are given in the Section 3.1.

Daily ocean temperature profiles at 80.5°E and 90°E are taken to describe the thermocline depth variability along the equator. And the Cross-Calibrated Multi-Platform (CCMP) V2.0 Level-3.5 (L3.5) gridded surface winds (Atlas et al., 2011) with a (1/4°) × (1/4°) spatial resolution was used to reveal the governing mechanism of the surface zonal jet.

The daily gridded sea level anomaly (SLA) data obtained from French Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO) data center was applied to analysis the modulation of the equatorial waves on the WJ evolution. This reference delayed-time product, available with a (1/4°) × (1/4°) spatial resolution, merged data from ERS-1/2, TOPEX/Poseidon, Jason, and Envisat altimeters (Ducet et al., 2000). And Ocean Surface Current Analysis Real-time (OSCAR) provides the near-surface ocean current at a 15 m depth, combining geostrophic, Ekman and Stommel shear dynamics, and a complementary term from the surface buoyancy gradient (Bonjean and Lagerloef, 2002). The data is available as 5-day averages on a global (1/3°) × (1/3°) grid.

2.2 Methods

In order to reveal the dominated roles of equatorial wave dynamics in the WJ, the sea level decomposition method, following Boulanger and Menkes (1995), is adopted to separate the meridional modes of lone equatorial waves as follows:

(1)

$h\left(x,y,t\right)=\sum\limits _{0}^{n}{r}_{n}\left(x,t\right){R}_{n}\left(y\right),$

(2)

$ {r}_{n}={\int }_{-L}^{+L}\left(u{R}_{n}^{u}+h{R}_{n}^{h}\right){\mathrm{d}}y, $

(3)

$ L = {\left( {c/ \beta } \right)^{1/2}},$

where

$ {r}_{n} $

are the wave coefficients calculated by the method and

$ {R}_{n} $

are longwave sea level structures (n=0 refers to Kelvin wave, n≥1 refers to the corresponding long Rossby wave mode; see also Figs 3a, b). The nondimensionalized zonal current

$ u $

and SLA

$ h $

are scaled with

$ c $

and

$ {c}^{2}/g $

respectively, where

$ c $

=2 m/s is the baroclinic mode phase speed and

$ g $

=9.81 m/s². For the calculation, the OSCAR data are first interpolated on to a daily regular grid as same as that of SLA data. See the detailed assumptions and projections method in the appendix of Boulanger and Menkes (1995). Following previous studies (Le Blanc and Boulanger, 2001; Nagura and McPhaden, 2010a, b, 2012, 2016; Iskandar and McPhaden, 2011), only the Kelvin wave and first meridional baroclinic Rossby wave were considered in the following context and high rank baroclinic wave modes were ignored.

3 Results

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3.1 Observed spring WJ onset and termination

Figure 1 displays the evolution of the OLR, rainfall and surface zonal winds near the equator (5°N–5°S). The active atmospheric convection associated with a robust Madden-Julian Oscillation (MJO; Madden and Julian, 1971, 1972) was found in the TIO, initiating at the end of April and then propagating eastward along the equator. In association with the active convection phase, a pronounced westerly wind burst (WWB) presented in the eastern basin at early May, and the strongest zonal winds exceeding 12 m/s were observed around 80°–90°E. From mid-May, atmospheric convection was largely suppressed, and corresponding surface zonal winds became much weaker.

Numerous studies have already demonstrated that WWB associated with the MJO event could produce strong equatorial jet within few days (Wyrtki, 1973; McPhaden, 1982; Schiller and Godfrey, 2003; Senan et al., 2003; Sengupta et al., 2007; Nagura and McPhaden, 2012; Shinoda et al., 2013a; Moum et al., 2014; Duan et al., 2016; Rao et al., 2016). Duan et al. (2016) pointed out that the spring WJ in 2013 was also largely modulated by WWB. Figure 2 illustrates the detailed evolution of surface zonal winds and the upper ocean current observed by two adjacent ADCP moorings in the eastern EqIO. Before the arrival of MJO-induced WWB, presence of an eastward flowing equatorial undercurrent (EUC) in the depth range of 80−150 m during April-early May is the dominant feature. And a very weak eastward surface current overlying the EUC was observed (Fig. 2d). Then an eastward jet at 80.5°E accelerated rapidly from < 0.5 m/s to 1.0 m/s within three days in early May. The main WJ structure (indicated by the threshold of 1.0 m/s for the zonal velocity) was located in the upper 110 m layer, with the zonal current speed in excess of 1.0 m/s between 5−22 May and a maximum of 1.76 m/s at May 14. The observed spring jet behaved just as a single intraseasonal event, consistent with the simulated results of Sengupta et al. (2007).

The evolution of spring WJ at 85°E presented similar features as that at 80.5°E (Fig. 2e, see also Table 1). The main jet structure was located in the upper 120 m layer, with a zonal velocity > 1.0 m/s between May 6−22 and a maximum of 1.83 m/s at May 16. Both dates of maximum zonal currents at 80.5°E and 85°E emerged near the end of the active phase of the MJO. Unlike the rapid accelerations, the zonal jet at 80.5°E and 85°E both experienced a subsequent slow weakening and shoaling process associated with the following relaxation of westerly wind forcing. The zonal current at 80.5°E reversed from eastward to westward at 10 June, while the jet termination at 85°E was 6 June, 4 days before the jet termination at 80.5°E. In addition, the WJ deceleration was clearly in advance of the reversal of the westerlies, indicating that the WJ termination may not be caused by surface wind forcing.

Moreover, in sharp contrast to that at 80.5°E and 85°E, there was no significant zonal jet at 90°E, although the overlying surface wind forcing is not weaker than the former sites at early May (Figs 2c and f). Only a much weaker eastward zonal current, with a peak zonal velocity of ~0.8 m/s at 8 May, was found in the upper 50 m layer. It should also be noted that the eastward current reverses its direction at 12 May, much earlier than those two western sites, although the local zonal winds were still eastward and greater than 7 m/s. However, all the outputs of the available open-accessed ocean reanalysis systems (e.g., HYCOM, SODA3, GODAS, ECMWF ORAS4, ECCO/ECCO2) and our simulated results (Duan et al., 2016) show a strong and long-lived zonal jet at 90°E (figures not shown). And the surface zonal current from OSCAR also indicated a zonal jet in the upper 30 m (Fig. 1d), which may be more consist with the current observations at 80.5°E and 85°E.

Another interesting phenomenon is the pronounced changes in the ocean thermocline. The temporal evolutions of upper-ocean temperature structure of the uppermost 200 m water column at 80.5°E and 90°E are also shown in Figs. 2g and h . During the suppressed MJO phase in mid-April, the thermocline, represented by the 25°C isotherm (D25), at 80.5°E was at the depth of about 80 m. Then it significantly deepened by about 30 m and lasted for about 17 d, which is produced by the strong zonal jet during the active MJO phase. And the upper layer temperature at 80.5°E decreased by almost 1.1°C (Fig. 2g), resulting from the combination of atmospheric cooling from above and enhanced vertical mixing from below (e.g., Schiller and Godfrey, 2003; Han et al., 2004; McPhaden and Foltz, 2013; Moum et al., 2014, 2016; Pujiana et al., 2018; Pujiana and McPhaden, 2018). The changes in the ocean thermocline at 90°E were ever more evident, deepening by almost 50 m at early May and persisting for more than one month (Fig. 2h). This great isopycnal movement also suggests that a strong zonal jet should present at 90°E. In addition, since the MJO-induced rainfall at 90°C is much stronger than 80.5°E and 85°E (Fig. 1b), the net fresh water flux could decrease the mixed layer by enhancing the near-surface stratification and then strengthen the surface WJ (Han et al., 1999; Masson et al., 2003). Furthermore, we have also examined the respective correlation between the zonal pressure gradient force (zonal gradient of SLA between three moorings and the eastern EqIO (1°S–1°N, 95°–100°E)) and the 80–140 m-averaged zonal current, and the current observations at 80.5°E and 85°E are quite consistent with previous studies (Chen et al., 2015a, 2019; Rao et al., 2016), but not that at 90°E. So, all these above results suggest that the ADCP data at 90°E may be incorrect and we thus neglect it in this paper.

3.2 Role of equatorial waves on the WJ evolution

As revealed by in-situ observations, since the spring WJ termination did not result from the surface wind forcing, the dynamical processes in the ocean interior may thus play the key role. Current observations (Figs 2d and e) illustrated there were significant baroclinic in the upper layer of the eastern EqIO. The lead-lag correlation coefficient of the zonal jets between 80.5°E and 85°E reaches 0.84 with an almost 3-day lag. Then the observed eastward phase propagation of the WJ was approximately 1.92 m/s, which is broadly commensurate with the phase speed of the baroclinic mode equatorial oceanic Kelvin waves (Le Blanc and Boulanger, 2001; Sengupta et al., 2007; Rao et al., 2016). In the surface layer, the buildup process of the eastward flowing WJ was accompanied with the downward isopycnal movements (Fig. 2g), showing the typical downwelling Kelvin wave responses (Webber et al., 2010; Iskandar and McPhaden, 2011; Moum et al., 2014; Rao et al., 2016). In the thermocline, the D25 signal at 80.5°E leading that at 90°E with a phase difference of about 6 days translates into propagation speed of about 2.02 m/s (Figs 2g and h), which also broadly agrees with the eastward propagation speed of the Kelvin wave along the equator.

Previous studies have reported that the zonal phase of seasonal WJ propagates mainly to the west in response to zonally propagating surface zonal winds (Molinari et al., 1990; Han et al., 1999; Qiu et al., 2009; Nagura and McPhaden, 2010a, b, 2016). However, the present moored records display that the spring WJ event in 2013 presented a distinct eastward-propagating pattern in the eastern EqIO, which results from the strong modulation of WWB-forced Kelvin wave. In order to further address the roles played by the wind-driven equatorial wave dynamics, the longitude-time diagram of SLA and corresponding decomposed Kelvin and Rossby waves modes are shown in Figs 3c and e. In response to the strong westerly winds occurring in late April (Fig. 1c), a prominent eastward propagation of the positive SLA is well captured in the eastern EqIO. The propagation speed is roughly 2.31 m/s, which is quite close to the phase speed of the Kelvin wave in this region (Nagura and McPhaden, 2010b; Iskandar et al., 2016). As the wind-forced equatorial Kelvin wave prevails over the eastern TIO, strong convergence by the zonal jet took place along the eastern boundary of the TIO, and then a significant SSH increase (~30 cm) was found along the coast of Sumatra in mid-May (Figs 3c and 4c).

By using a two-layer model, O’Brien and Hurlburt (1974) firstly noted that the reflected effect of eastern boundary of Indian Ocean was able to terminate WJ event. Analysis of altimeter data in 2013 had shown that the reflection of equatorial Kelvin wave into Rossby waves at the eastern boundary clearly. Figure 3c displays the westward propagation signal with speed 0.7 m/s, which matches well with the phase speed of the first baraclinic Rossby wave in this region (Le Blanc and Boulanger, 2001; Sengupta et al., 2007; Webber et al., 2012; Chen et al., 2017; Shinoda et al., 2017). Figure 3e further displays that projected first baraclinic Rossby wave westward propagation movement throughout the entire eastern basin from mid-May to late June, which is well consistent with the dates of the ocean zonal velocity reversal from eastward to westward observed by ADCP moorings (Figs 2d and e, Table 1).

A combination of the SLA and corresponding ocean surface current is also able to present more clearly the general diagram of how the equatorial waves modulating WJ termination in 2013. It displays that the rapid acceleration of eastward surface jet in early May was evident in almost the entire eastern basin, which is associated with the downwelling Kelvin wave propagates eastward from early- to mid-May (Figs 4a and c). And the large-scale eastward zonal currents became generally weaker from late May, when the westward propagating Rossby waves reflected from the Sumatra coast began to gradually decelerate the strong jet from east to west (Figs 4d and f). During mid- to late June, the westward-propagating Rossby wave can be further confirmed by the deepening of the thermocline from the RAMA moorings at 1.5°S, 80.5°E and 1.5°N, 80.5°E (figure not shown here).

4 Conclusions

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Based on in-situ observations from two equatorial ADCP moorings at 80.5°E and 85°E, we have examined the entire evolution of spring WJ in 2013 and revealed the dominated governing role of equatorial waves on the WJ onset and termination in the present study. From late April to mid-May, a robust MJO propagates eastward across the TIO and causes a pronounced surface WWB in the eastern basin. The equatorial wave analysis demonstrated that the eastward-propagating Kelvin wave, induced by the WWB, produced the strong equatorial jet in early May. The observed zonal jet presented thus as one intraseasonal event with a distinct eastward-propagating pattern in the eastern EqIO. In addition, the subsequent reflected Rossby waves radiating from eastern boundaries of the TIO not only modulated slightly the WJ strength/structure as proposed by previous studies but played a crucial role in the jet termination. The corresponding changes in the ocean thermocline also confirmed these strong modulations of equatorial waves on the WJ.

Acknowledgements

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We sincerely appreciate the data freely used in this study, including the RAMA data obtained from https://www.pmel.noaa.gov/tao/drupal/disdel/, the altimeter data from http://www.aviso.altimetry.fr/en/data.html, the CCMP sea wind product from http://data.remss.com/, and the OSCAR data from http://apdrc.soest.hawaii.edu/data/. This work was also supported by Southern Marine Science and Engineering Guangdong Laboratory, Zhuhai.

Funding

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The Basic Scientific Fund for National Public Research Institutes of China under contract Nos 2019Q03 and 2017S02; the National Natural Science Foundation of China under contract Nos 41706032, 41406012, 4187060841, 41876028 and 41676020; Taishan Scholars Programs of Shandong Province under contract No. tsqn201909165; the National Program on Global Change and Air-Sea Interaction under contract Nos GASI-IPOVAI-03, GASI-IPOVAI-02, GASI-02-IND-STSaut and GASI-02-IND-STSwin; the NSFC-Shandong Joint Fund for Marine Science Research Centers under contract No. U1606405; the Ao-Shan Talents Cultivation Program supported by Pilot National Laboratory for Marine Science and Technology (Qingdao) under contract No. 2017ASTCP-OS01.

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Year 2020 volume 39 Issue 7

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doi: 10.1007/s13131-020-1576-2

Receive Date：2020-01-08
Online Date：2026-03-31
Published：2020-07-25

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Received：2020-01-08
Accepted：2020-02-04

Funding

The Basic Scientific Fund for National Public Research Institutes of China under contract Nos 2019Q03 and 2017S02; the National Natural Science Foundation of China under contract Nos 41706032, 41406012, 4187060841, 41876028 and 41676020; Taishan Scholars Programs of Shandong Province under contract No. tsqn201909165; the National Program on Global Change and Air-Sea Interaction under contract Nos GASI-IPOVAI-03, GASI-IPOVAI-02, GASI-02-IND-STSaut and GASI-02-IND-STSwin; the NSFC-Shandong Joint Fund for Marine Science Research Centers under contract No. U1606405; the Ao-Shan Talents Cultivation Program supported by Pilot National Laboratory for Marine Science and Technology (Qingdao) under contract No. 2017ASTCP-OS01.

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