Instability in boundary layer between the North Equatorial Current and underlying zonal jets based on mooring observations*

WANG Fujun , FENG Junqiao WANG Qingye ZHANG Linlin HU Shijian HU Dunxin

1 Key Laboratory of Ocean Circulation and Wave, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China 2 Function Laboratory for Ocean Dynamics and Climate, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266000, China

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

Abstract Instability/stability in the North Equatorial Current (NEC) basin is studied based on data obtained from nine moorings deployed at 8.5°N, 10.5°N, 11.0°N, 12.5°N, 13.0°N, 15.0°N, 15.5°N, 17.5°N, and 18.0°N along 130.0°E during cruises in 2015-2017. In low latitudes, the Coriolis parameter and stratifi cation ratio play important roles in NEC stability, whereas velocity shear and the layer depth ratio are important for NEC stability in high latitudes. Beneath the westward NEC, eastward zonal jets occur intermittently centered around 8.5°N, 12.5°N, and 17.5°N along 130.0°E. Similar to the NEC, the main body of these zonal jets also deepens with latitude. In the boundary layer comprising the bottom NEC and upper zonal jets, the growth rate of the NEC is attributed not only to velocity shear but also to zonal jet velocity based on the longwave assumption. Based on the shortwave assumption, the growth rate is proportional to zonal jet velocity but has no relationship with velocity shear. Climatologically, the growth rate in the boundary layer is not zero at 8.5°N, 12.5°N, and 13.0°N, where the velocity shear and zonal jets are larger than at other stations. The instability also occurs at the time node when the zonal jets are strong enough, although the mean zonal jets may disappear at this station.

Keyword: instability; North Equatorial Current (NEC); zonal jets

1 INTRODUCTION

A westward fl ow called the North Equatorial Current (NEC), which is confi ned between 8°N-17°N in the surface layer of the North Pacifi c Ocean, is an important region for water mass exchange and mixing (e.g., Fine et al., 1994; Qu et al., 1998). The main body of the NEC is concentrated within a layer shallower than the 26.5-26.8 potential density ( σθ) surfaces (Qiu et al., 2015). To the east of Mindanao Island, the NEC bifurcates into the southward-fl owing Mindanao Current (MC) (e.g., Nitani, 1972; Hu and Cui, 1991; Wang et al., 2016) and the northwardfl owing Kuroshio Current (KC) (e.g., Lien et al., 2014; Qiu et al., 2014; Chen et al., 2015).

On interannual timescales, NEC transport increases (decreases) during El Niño (La Niña) periods (Qiu and Lukas, 1996; Kim et al., 2004). Seasonally, the NEC is strong in spring and weak in autumn. Dynamically, the variability of the sea surface height anomaly and the NEC is attributed primarily to local winds and Rossby waves propagated from the eastern boundary of the Pacifi c Ocean (e.g., Qiu and Joyce, 1992; Qu et al., 2008; Kashino et al., 2009; Wang et al., 2019).

Using ocean models with high resolution and low dissipation, Richards et al. (2006) demonstrated that alternating jet-like structures occur between 30°N-55°N and in the tropics. Based on an ocean model of the North Pacifi c Ocean, Nakano and Hasumi (2005) found a series of zonal jets with meridional scale of 3°N-5°N. Alternating jets have also been discovered in the deep ocean following analysis of fl oat data (Hogg and Owens, 1999). Based on profi ling fl oat temperature-salinity data, Qiu et al. (2013a and b) identifi ed three well-defi ned eastwardfl owing jets with velocity of 2-5 cm/s. The jets were centered around 9°N, 13°N, and 18°N within the 26.9-27.3 σθsurfaces along 130°E-135°E beneath the permanent westward-fl owing NEC. Zhang et al. (2017) used direct mooring observations to reveal the existence of eastward zonal jets below the NEC at 10.5°N and 13.0°N.

Using the Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) products, it has been shown that three regions of high variability exist in the North Pacifi c Ocean: the Kuroshio/Kuroshio Extension southeast of Japan, North Equatorial Countercurrent along 5°N, and the region off the Pacifi c coast of Central America around 11°N (Qiu, 1999). In addition, there is a regional maximum of sea surface height variability with a root mean square value of ＞0.10 m in the area 19°N-25°N, 135°E-175°W (Wyrtki, 1975; Aoki and Imawaki, 1996; Qiu, 1999), where the northern part of the NEC is beneath the surface eastward-fl owing North Pacifi c Subtropical Countercurrent (STCC). The instability of the STCC-NEC system has been discussed comprehensively based on baroclinic instability theory (Qiu, 1999; Kobashi and Kawamura, 2002).

If the bottom layer of the NEC and the upper layer of the zonal jets are considered a coupled system within the NEC basin, it is important to investigate its stability. Furthermore, although NEC stability has been discussed qualitatively by Qiu (1999), the key factors governing its stability remain to be determined. To address these issues, nine moorings were deployed at 8.5°N, 10.5°N, 11.0°N, 12.5°N, 13.0°N, 15.0°N, 15.5°N, 17.5°N, and 18.0°N along 130.0°E to clarify both the growth rate in the boundary layer of the NEC-zonal jets system and the relative contributions to the growth rate of diff erent NEC parameters. The remainder of this paper is organized as follows. The data obtained from the nine moorings and the AVISO products are introduced in Section 2. In Section 3, the relative contributions to NEC stability from diff erent parameters and the growth rate in the boundary layer of the NEC-zonal jets system are discussed. A short discussion and the derived conclusions are presented in Section 4.

2 DATA

2.1 Acoustic Doppler Current Profi ler (ADCP) mooring data

To determine variability and vertical structure within the NEC basin, moorings were deployed along 130°E. The corresponding water depth is approximately 5 400-5 900 m. Moorings were deployed at 8.0°N, 10.5°N, 13.0°N, 15.5°N, and 18.0°N in September 2014 and retrieved one year later. From 2016, the mooring locations were changed to 8.5°N, 11.0°N, 12.5°N, 15.0°N, and 17.5°N (Fig.1). It should be noted that mooring data during September 2014 to September 2015 at 8.0°N were lost, and that three ADCPs (two at 12.5°N and one at 15.0°N) failed during September 2015 to December 2016. Full details of the moorings are listed in Table 1.

Fig.1 Root mean square (m) of sea surface height anomaly in the North Pacifi c Ocean (a) and the mooring region (b) based on AVISO products from January 1993 to December 2015

Table 1 Location, water depth, number of observation days, and ADCP data period for the nine moorings

The sampling frequency of the 75-kHz ADCPs deployed on our moorings was set to 1 h. At each mooring, two ADCPs (one looking upward and one looking downward) were attached at the depth of approximately 450 m. As each ADCP was set to observe 60 (8 m/bin) bins, the observation range of the two ADCPs was around 900 m. In fact, the measurement range of the ADCPs was variable because strong currents incline the mooring system and reduce the observational range. Before use in the analysis, the ADCP data underwent a strict quality control procedure. ADCP data with pitch or roll of ＞18°, current speed of ＞2 m/s, and ＜80% good beam coverage were removed. Furthermore, the blank zone between adjacent ADCPs was fi lled by interpolation. Generally, most ADCP data were considered poor in the surface layer where beam emission echo is very strong; thus, data in the 0-50 m layer were deleted directly, except for the mooring at 13.0°N because the ADCP data were considered very good in the upper 50 m. The accuracy and resolution of the ADCPs were limited to within 1% velocity ±5 mm/s and ＜1 mm/s, respectively. The ADCP data were processed as a daily dataset to remove tidal signals.

2.2 AVISO products

AVISO data were used in this study (Boebel and Barron, 2003; Barron et al., 2009). This altimetryobserved dataset comprises merged European Remote Sensing Satellite-1 and -2, Ocean Topography Experiment Topex/Poseidon, Geosat Follow-On, and Jason-1 and -2 data (Le Traon et al., 1998; Ducet et al., 2000). The 0.25°×0.25° resolution AVISO products (January 1, 1993 to December 31, 2015) that included daily sea surface height and corresponding geostrophic currents were downloaded from https://www.aviso.altimetry.fr.

3 RESULT

3.1 Mean structures of the NEC and zonal jets

The mean zonal velocities (red lines) and standard deviations (dashed blue lines) at the nine mooring stations are shown in Fig.2. In this paper, the positive direction is defi ned as northward and eastward. It can be seen that the NEC is a surface-intensifi ed feature. Generally, the intensity of the surface NEC increases from 8.5°N to 10.5°N and then decreases slowly with increasing latitude. Vertically, the main body of the NEC deepens with latitude. At 18°N, the NEC is completely below the sea surface layer. The greatest standard deviation of zonal velocity occurs at 76 m (8.5°N), 50 m (10.5°N), 63 m (11.0°N), 60 m (12.5°N), 62 m (13.0°N), 67 m (15.0°N), 50 m (15.5°N), 63 m (17.5°N), and 59 m (18.0°N). It is interesting that all depths are focused in the upper 100 m. The mean meridional velocities at the mooring stations are much smaller than the zonal velocities but their standard deviations are comparable (fi gure not shown).

It is worth emphasizing that eastward zonal jets exist beneath the NEC at 8.5°N, 10.5°N, 12.5°N, 13.0°N, and 17.5°N. The strongest zonal jet is found at 8.5°N with a value of 0.08 m/s at 455 m depth. To depict the zonal jets intuitively, latitude-depth sections of the mean zonal velocities interpolated from the ADCP data are shown in Fig.3. Although zonal jets are found to exist at 10.5°N and 13.0°N, their intensity is smaller than at 8.5°N and 12.5°N. Furthermore, a zonal jet is evident at 17.5°N. Therefore, it is suggested that the zonal jets are concentrated at 8.5°N, 12.5°N, and 17.5°N along 130.0°E, consistent with the fi ndings of Qiu et al. (2013a, b), who reported zonal jets centered around 9°N, 13°N, and 18°N along 130°E-135°E. In addition, the zonal jets deepen with latitude. For example, the zonal jet core is around 500 m depth at 8.5°N and 700 m depth at 12.5°N. At 17.5°N, the zonal jet is very weak above 800 m depth, which suggests that its core might be below 800 m. At 18.0°N, the STCC rather than the NEC occupies the upper layer; instead, the NEC is completely beneath the STCC.

3.2 NEC stability

Although Qiu (1999) discussed NEC stability qualitatively, the relationship between the NEC growth rate and several key parameters remains unclear yet. To investigate NEC stability quantitatively, a 2½-layer linearized reduced gravity model was utilized under the quasi-geostrophic approximation as follows:

Fig.2 Mean zonal velocities (red lines) and standard deviations (dashed blue lines) at 8.5°N (a), 10.5°N (b), 11.0°N (c), 12.5°N (d), 13.0°N (e), 15.0°N (f), 15.5°N (g), 17.5°N (h), and 18.0°N (i) along 130.0°E

Fig.3 Latitude-depth zonal velocities (m/s) interpolated using data from the nine moorings at 8.5°N, 10.5°N, 11.0°N, 12.5°N, 13.0°N, 15.0°N, 15.5°N, 17.5°N, and 18.0°N along 130.0°E

where qnis the perturbation potential vorticity, Пnis the mean potential vorticity, and φnis the perturbation stream function in the n-th layer. In the following, H1/ H2and U1/ U2denote the mean layer thickness and the mean zonal velocity in the surface ( n=1) and second ( n=2) layers, respectively. The potential vorticity and the meridional gradient of the mean potential vorticity are shown in the following:

Fig.4 Growth rate (/d) dependence on latitude (a), stratifi cation ratio γ (b), mean fl ow U 1 when U 2 is constant (c), mean fl ow U 2 when U 1 is constant (d), layer depth ratio δ when H 1 is constant (e), and layer depth ratio δ when H 2 is constant (f) in the boundary layer of the NEC-zonal jet system

In fact, c is composed of the real part crand the imaginary part ci; thus, c ≡ cr+ ici, where cris the phase speed and kcirepresents the growth rate. If ci=0, the system is stable; otherwise, it is unstable. Past studies have reported that the NEC is stable (Qiu, 1999; Kobashi and Kawamura, 2002); however, the reason for the stability of the NEC remains unresolved quantitatively. It seems that the Coriolis parameter f, shear gradient of zonal velocity, layer depth ratio δ, and stratifi cation ratio γ are all closely associated with the growth rate of the NEC. Our calculations, which indicate that the growth rates at the nine mooring stations are all zero, suggest that the NEC is stable.

Fig.5 Growth rate (/d) calculated from mooring data at 8.5°N, 12.5°N, 13.0°N, and 18.0°N (a) and two idealized experiments at 12.5°N (b)

Table 2 Parameter values appropriate for the boundary layer of the NEC-zonal jet system

A series of idealized experiments was designed to discuss the relative contributions of key parameters to the NEC growth rate. First, all parameters except f were artifi cially set as constants, which tends to make the NEC unstable. The resultant relationship between latitude and growth rate is shown in Fig.4a. As f is proportional to latitude, the NEC growth rate becomes larger with increasing latitude/ f. Second, γ was similarly considered as the only variable, while all other parameters were set as constants, which also makes the NEC unstable. It is demonstrated in Fig.4b that a lower value of γ corresponds to a greater growth rate. For U1and U2, it is shown that a smaller value of U1or a greater value of U2leads to a higher growth rate of the NEC (Fig.4c & d). In other words, the NEC growth rate is proportional to velocity shear. For the layer depth ratio δ, if H1is held constant, the increase of H2meridionally corresponds to the decrease of the H1/ H2ratio, which leads to decrease of the growth rate (Fig.4e). If H2is held constant, the increase of H1meridionally corresponds to the increase of the H1/ H2ratio, which leads to decrease of the growth rate (Fig.4f). In summary, the layer depth ratio δ always generates decrease of the NEC growth rate meridionally. The calculation shows that both the ratio γ and the shear of zonal velocity decrease with latitude in the NEC region. Therefore, f and γ lead to an increase of the NEC growth rate, while U1- U2and δ generate a decrease of the NEC growth rate meridionally. In fact, as the NEC is stable, it would appear that the two inversed physical processes cancel each other, leading to zero growth rate of the NEC. Specifi cally, f and γ maintain NEC stability in low latitudes, while velocity shear and the layer depth ratio δ play an important role in NEC stability in high latitudes.

3.3 Instability in the boundary layer between the NEC and zonal jets

It is shown in Fig.2 that the zonal jets are much stronger at 8.5°N and 12.5°N, where the standard deviation is comparable with that in the surface layer, in comparison with the other stations. At stations without zonal jets, or where the zonal jets are very weak, the standard deviations generally decrease with depth. It is very likely that the growth rate in the boundary layer comprising the bottom of the NEC (the fi rst layer) and the upper zonal jets (the second layer) is no longer zero. Here, we choose 12.5°N as an example for which to calculate the growth rate in the boundary layer; the corresponding parameters are listed in Table 2. Specifi cally, ρ1=26.0 σθ, ρ2=26.5 σθ, ρ3=27.0 σθ, H1= 88 m, H2= 186 m, and U1( U2) is given by the mean zonal velocity between ρ1and ρ2( ρ2and ρ3) based on the mooring observations at 12.5°N. Here, depth ρ2is set at the point of zero zonal velocity in the NEC-zonal jet system. For simplicity, 2π l is set as the mean value in the NEC region. The result shows that it is unstable. Similar calculation indicates that the growth rate in this layer is not zero at 8.5°N or 13.0°N (Fig.5a), where both the zonal jets and the velocity shear between the NEC and the zonal jets are larger than at the other stations (Table 2). The smaller velocity shear at 13.0°N (compared with that at 8.5°N and 12.5°N) leads to the smaller growth rate. In comparison, the corresponding layers at 11.0°N, 15.0°N, 15.5°N, and 18.0°N without zonal jets are stable. It is striking that the growth rate is zero at both 10.5°N and 17.5°N, where zonal jets exist. At 17.5°N, the velocity shear is so small that the growth rate is equal to zero. At 10.5°N, although the velocity shear is comparable with that at 13.0°N, the lower latitude tends to attenuate the growth rate to zero. At 18.0°N, the layer comprising the bottom of the STCC and the upper NEC is unstable, consistent with previous research (Qiu, 1999; Kobashi and Kawamura, 2002). It is shown in Fig.1b that the high variability of sea surface height along 130.0°E is concentrated around 8.0°N, 9.0°N-10.0°N, and 12.0°N-13.0°N. Meanwhile, the mooring data shown in Fig.3 and Table 2 indicate that the zonal jets at 8.5°N, 12.5°N, and 13.0°N are stronger than at the other mooring stations, both of which support our calculations well.

Based on the above analysis, it seems that the stronger the zonal jets, the higher the growth rate. Therefore, it is interesting to investigate whether an equivalent velocity shear at diff erent velocities in the boundary layer produces the same growth rate. To clarify this point, we set an idealized experiment at 12.5°N. The growth rate was calculated with U1= -0.05 and U2=0.01 and with U1=-0.10 and U2=-0.04. It should be noted that the velocity shear ( U1- U2) and all other parameters were the same in both cases. It is shown in Fig.5b that the growth rates are diff erent in the two cases, suggesting that the growth rate is associated not only with the shear gradient of the zonal velocities but also with the velocities in two layers. It would appear that when the signs of the two zonal velocities diff er, the growth rate is more likely to become larger than when the signs of the two zonal velocities are the same.

To investigate the dynamics, a key factor D, based on Eq.3, is defi ned as follows:

3.4 Variability in the NEC-zonal jet system

The time series of zonal velocities at the nine mooring stations along 130.0°E are shown in Figs.7 & 8. Beneath the permanent NEC, eastward zonal jets and the vertically extended NEC exist alternately meridionally. At low latitudes, e.g., 8.5°N, the NEC is concentrated on the surface, and the zonal jets are centered at around 500 m. The main body of the NEC and the intermittent zonal jets deepen with latitude. In addition, the intensity of the zonal jets varies with latitude; it is strongest at 8.5°N on June 6, 2016 (i.e., 0.65 m/s) and weakest at 15.5°N. Generally, the zonal jets at both 8.5°N and 12.5°N are much stronger than at the other stations. At 11.0°N, 15.0°N, and 15.5°N, although there is no zonal jet climatologically, one does appear intermittently over time. At 18.0°N, the zonal jets disappear completely. To investigate the variability of the NEC-zonal jet system, a power spectrum analysis is shown in Figs.9 & 10. A common peak of 70-120 d is almost coherent from the sea surface to the bottom of the range of ADCP measurements at all mooring stations, which is signifi cant (insignifi cant) below (above) the thermocline at the 95% confi dence level. It should be noted that measurements for the full two years were retrieved only at 8.5°N, 11.0°N, and 17.5°N; the time series at the other stations extended for approximately one year. A semiannual signal is evident at 8.5°N, 11.0°N, and 17.5°N. Specifi cally, it is concentrated in the upper 250 m at both 8.5°N and 11.0°N and below 400 m at 17.5°N at the 95% confi dence level.

Fig.6 Growth rate (/d) dependence on U 1 and U 2 when k= 5×10 -6 /m (a), k= 1×10 -5 /m (b), k= 1.3×10 -5 /m (c), and k= 2.0×10 -5 /m (d)

To investigate the variability of the relationship between the NEC and the zonal jets, the vertical mean NEC averaged from the sea surface to the depth of zero velocity (blue line), and the mean zonal jets (red line) averaged from the depth of zero velocity to the bottom of the range of the ADCP measurement are shown in Figs.11 & 12. It should be noted that the depth of zero velocity at 11.0°N, 15.0°N, and 15.5°N is artifi cially defi ned at 300, 500, and 500 m, respectively. It is striking that the blue and red lines are well consistent with each other, suggesting that the intensity of the NEC is inversely proportional to the strength of the zonal jets. When the NEC is strong, it is not only a surface-intensifi ed feature but it also extends to deeper layers. Vertical extension of the NEC could weaken the zonal jets or even lead to occupation of the region of the zonal jets. Sometimes the NEC is suffi ciently strong that the zonal jets disappear completely. It should be noted that the blue and red lines at 18.0°N represent the STCC and the NEC, respectively. It can be seen that they are well coherent, suggesting that STCC variability is inversely proportional to the NEC at 18.0°N along 130.0°E.

Fig.7 Time series of zonal velocities (m/s) along 130.0°E collected by ADCPs from September 2014 to September 2015 under a 30-d low-pass fi lter at 10.5°N (a), 13.0°N (b), 15.5°N (c), and 18.0°N (d)

It is interesting to check the growth rate in the boundary layer of the NEC-zonal jet system on the time node, when zonal jets exist beneath the NEC. Calculation shows the boundary layer is unstable during May 15 to June 15, 2016 at 8.5°N, and a similar calculation was applied to the other mooring stations. Quantitatively, in low latitudes, e.g., 8.5°N, when U1- U2is ＞0.05 m/s, the system tends to be unstable. Around the bifurcation latitude of the NEC, e.g., 12.5°N, when U1- U2is ＞0.04 m/s, the system is unstable. In high latitudes, e.g., 17.5°N, when U1- U2is ＞0.01 m/s, the growth rate is not zero. In addition, the stronger the zonal jets, the higher the growth rate. It should be noted that the above analysis is based on the longwave assumption.

4 DISCUSSION AND CONCLUSION

This study was based on direct ADCP measurements obtained at 8.5°N, 10.5°N, 11.0°N, 12.5°N, 13.0°N, 15.0°N, 15.5°N, 17.5°N, and 18.0°N along 130.0°E. The observations extended vertically to depths of 800-1 000 m, providing a dataset convenient for investigation of the features of the NEC and its underlying zonal jets. The main body of the NEC is concentrated in the upper layer at 8.5°N but it deepens with increasing latitude. At 18.0°N, the NEC is completely below the sea surface layer, and the surface current is instead the eastward-fl owing STCC. Climatologically, eastward-fl owing intermittent zonal jets are concentrated around 8.5°N, 12.5°N, and 17.5°N, consistent with previous research (Qiu et al., 2013a, b) that found three eastward zonal jets centered around 9°N, 13°N, and 18°N within the 26.9-27.3 σθsurfaces along 130°E-135°E. Similar to the NEC, the main body of the zonal jets also deepens with latitude. Generally, the standard deviation decreases with depth. However, in regions where the zonal jets are strong, the standard deviation is comparable with that in the surface layer. During the observation period, zonal jets were found to exist intermittently. Calculation showed that the variability of the NEC is inversely proportional to the strength of the zonal jets. The power spectral density analysis demonstrated that intraseasonal signals of 70-120 d are common below the thermocline at the nine mooring stations along 130.0°E.

Fig.8 Time series of zonal velocities (m/s) along 130.0°E collected by ADCPs under a 30-d low-pass fi lter at 8.5°N (a), 11.0°N (b), 17.5°N (c), 12.5°N (d), and 15.0°N (e)

A series of idealized experiments was designed to investigate the growth rate of the NEC. As the Coriolis parameter, velocity shear, layer depth ratio, and stratifi cation ratio are all closely related to the NEC growth rate, in the experiments, one parameter was set as the only variable, while all other parameters were held constant to maintain the NEC instability. It was found that the Coriolis parameter and stratifi cation ratio play important roles in the stability of NEC at low latitudes, and that the velocity shear and the layer depth ratio maintain NEC stability at high latitudes.

In the boundary layer comprising the bottom NEC and upper zonal jets, the growth rate was found not zero at 8.5°N, 12.5°N, and 13.0°N, whereas it was zero at the other stations. Although zonal jets exist at 10.5°N and 17.5°N, the low latitude of 10.5°N and the weak velocity shear at 17.5°N lead to NEC stability at the two stations. At 18.0°N, the layer comprising the bottom STCC and upper NEC was found unstable, consistent with previous research (Qiu, 1999; Kobashi and Kawamura, 2002).

On the assumption of longwaves, the growth rate is mainly proportional to the velocity shear. In the low latitudes of the NEC basin, the boundary layer is unstable when the velocity shear is ＞0.05 m/s. Near the NEC bifurcation latitude, the boundary layer is unstable when the velocity shear is ＞0.04 m/s. The corresponding value is 0.01 m/s at high latitudes. On the assumption of shortwaves, the growth rate is unrelated to velocity shear, but it is well proportional to the velocity of the zonal jets. This could be attributed to the asymmetry of the velocities of diff erent layers in П1yand П2y.

5 DATA AVAILABILITY STATEMENT

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Fig.9 Power spectral density (PSD) of zonal velocities from ADCPs along 130.0°E at 10.5°N (a), 13.0°N (b), 15.5°N (c), and 18.0°N (d)

6 ACKNOWLEDGMENT

We thank all the scientists, technicians, and the crew of R/V Science for the deployment and retrieval of the moorings. The daily sea surface height and geostrophic currents from AVISO products can be downloaded from https://www.aviso.altimetry.fr/.

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