2 Function Laboratory for Ocean Dynamics and Climate, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266000, China;
3 Qingdao Collaborative Innovation Center of Marine Science and Technology, Qingdao 266000, China;
4 State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China;
5 University of the Chinese Academy of Sciences, Beijing 100049, China
Water masses from north and south Pacific arrived at the far western equatorial region, so-called 'water mass crossroads' (Fine et al., 1994), make property (e.g., salinity) fronts here. The Mindanao Current (MC) (Nitani, 1972) and the New Guinea Coastal Current/Undercurrent (NGCC/NGCUC) (Lindstrom et al., 1987), which are the low-latitude western boundary currents (LLWBCs), play important roles in water mass redistribution in this area.
Originated in the north Pacific, the high-salinity North Pacific Tropic Water (NPTW) (Cannon, 1966; Tsuchiya, 1968) and the low-salinity North Pacific Intermediate Water (NPIW) (Hasunuma, 1978; Talley, 1993) spread southward into subtropical gyre, and being carried into the tropics by the westward North Equatorial Current (NEC) and the equatorward MC. These waters carried by the MC enter the Sulawesi Sea (Fine et al., 1994) and flow either into the Indonesian Throughflow (ITF) (Ffield and Gordon, 1992) or back to the North Pacific (Lukas et al., 1991; Bingham and Lukas, 1994; Kashino et al., 1999, 2001). From the south Pacific, the NGCUC carries the high-salinity South Pacific Tropical Water (SPTW) and the low-salinity Antarctic Intermediate Water (AAIW) to the western Pacific across equator (Lindstrom et al., 1987; Tsuchiya et al., 1989; Tsuchiya, 1991; Fine et al., 1994; Qu and Lindstrom, 2004). The shallow part of the SPTW is prevented by the MC retroflection between Morotai Island and Taluad Islands, feeding to the NECC (Kashino et al., 1996; Fig. 1b). Fine et al. (1994) and Kashino et al. (1996) suggested the AAIW may continuing northward carried by the subthermocline eddy (Kashino et al., 2015) or the northward flow (Hu et al., 1991) underneath the MC.
Between the NPTW and the SPTW, Wang et al. (2013) detected a water featuring a lateral salinity minimum in the western Pacific Ocean, and named it as North Pacific tropical subsurface water (TSSW). They suggested the TSSW is of NPTW origin, which is then modified via diapycnal mixing. This fresh pycnocline water naturally mirrors in the low sea surface salinity (Delcroix and McPhaden, 2002), located between 4°–8°N where exists maximum of precipitation minus evaporation (P-E) dominated by intertropical convergence zone (ITCZ) (Delcroix and Hénin, 1991). Most importantly, near-surface salinity and its variability in the western Pacific are potentially important in better understanding the El Niño/La Niña-Southern Oscillation (ENSO) ocean-atmosphere coupled mechanisms (Delcroix and Picaut, 1998; Lukas and Lindstrom, 1991; Qu and Yu, 2014). Previous studies mainly focused on the ENSO-related variations of the eastern edge of the fresh pool of the western Pacific Ocean (e.g., Delcroix and Hénin, 1991; Delcroix and Picaut, 1998; Delcroix and McPhaden, 2002; Maes et al., 2006; Qu et al., 2014; Qu and Yu, 2014 and references therein). The observations of the water masses in the far western equatorial Pacific west of 130°E and their interannual variations in different layers are still lack at present.
The western equatorial Pacific has an entrance of the ITF (Kashino et al., 1999), through which water masses from Pacific Ocean can be transported into Indian Ocean and involved in the global conveyer belt (Gordon, 1986; Broecker, 1991), representing a large reservoir of warm and relatively fresh water in the world ocean. Kashino et al.(1996, 1999) investigated the water masses between Mindanao and New Guinea during World Ocean Circulation Experiment (WOCE) cruises, showing significant dilution of the Pacific waters by the low-salinity water from east Indonesian seas. Unfortunately, the pycnocline water connection between the western Pacific and the east Indonesian seas was not detected well in their studies. Recently, two oceanographic surveys contributed to the Northwestern Pacific Ocean Circulation and Climate Experiment (NPOCE) (Wang and Hu, 2010), were organized by Institute of Oceanology, Chinese Academy of Sciences in the western Pacific Ocean in boreal winters of 2010 La Niña and 2012 normal year (Fig. 1a). So the comparative cruises can give us an overview of the water mass distributions during different ENSO phases in the far western equatorial Pacific. The paper is organized as follows. The expeditions and data are described in Section 2. The general characteristics of water masses and their variations are displayed in Sections 3 to 4. In Section 5 we discussed the water mass between the Mindanao and the Papua New Guinea. Main conclusions are summarized at last.2 EXPEDITION, DATA, AND METHOD 2.1 Expeditions and hydrographic data
The expeditions are relied on R/V Kexue-1. The observations in the south of 12°N this study considering were accomplished in December during both cruises. Figure 1b–c show topography and distributions of stations for conductivity-temperaturedepth (CTD) profile. The zonal sections along 8°N off Mindanao east and 2°N in the Maluku Channel, and the meridional sections along 124.5°E/124.8°E in the east Sulawesi Sea and 130°E were carried out in both cruises. During cruise 2010, a section from Mindanao Island to New Guinea Island, called 'MN', was conducted. During cruise 2012, the CTD observations along the southeast Mindanao inshore and to the Morotai north were also executed. Table 1 summarized the time windows of above hydrographic sections.
The vessel is equipped with SeaBird 911/917 plus CTD sensors manufactured by SeaBird Electronics (SBE), Inc. With the pre/post-cruise calibration, the initial accuracy of the CTD sensors was about 0.001 (PSS-78) for salinity, 0.001℃ for temperature and 0.015% of full scale for pressure, and the resolutions were 0.000 2, 0.000 2℃ and 0.001% of full scale, respectively. With intensive observations at some CTD stations or nearby, totally 58 profiles in 2010 and 42 profiles in 2012 are covered in this study.2.2 Argo data, drifter data and altimetry data
The Argo salinity and temperature profiles generally reaching 2 000 decibar (dbar) is from China Argo Real-time Data Center (ftp://ftp.argo.org.cn/pub/ARGO/global/). We select those profiles gained in the concurrent periods on Decembers as the two cruises conducted in the region of 120°–134°E, 0°– 12°N. In our study, ascending records with a quality flag of '1' or '2' which indicates 'good' or 'probably good' quality, are selected. After further stringency quality control, the final Argo dataset contain 78 and 76 profiles in Decembers 2010 and 2012, respectively.
Satellite-tracked surface drifters manufactured by the Chinese Xiaolong Instruments Co. Ltd. were deployed during the cruise 2010. The design of the drifter with a drogue locating at 15 m depth follows the WOCE standards, ensuring that the drifter follows the ocean surface currents instead of the winds. The data of drifter trajectories in the MC, in the Sulawesi Sea, in the Maluku Channel, and in the NGCC are used in this study.
The delayed-time, merged gridded sea surface height (SSH) (from AVISO, http://www.aviso.altimetry.fr) are adopted in this study. The data have a 1/4°×1/4° Mercator spatial resolution and a 1-day temporal resolution. The daily data were monthly averaged for further analyses.2.3 Method
With the sectional CTD profiles, the geostrophic current are calculated. In order to eliminate the effects of small-scale processes and resolve the closely spaced stations (Qu et al., 1998), the CTD profiles were interpolated onto 25 km zonally and 100 km meridionally using Akima interpolation (Akima, 1970). An initial level of no motion is chosen at 2 000 dbar, a common depth of the most observations reached. The circulation diagram above the pycnocline is insensitive to vertical displacements of the reference levels ranging from 1 500 to 2 000 dbar (Qu et al., 1998; Li and Wang, 2012).
The CTD and Argo salinity data interpolated onto isopycnal layer vertically are mapped onto 0.5°×0.5° grids horizontally using the inversed distance weighting (IDW) interpolation method. For each grid point, measurements within the influencing radius R are assigned to the weight value
Figure 2 shows relationship curves between the potential temperature (θ) and salinity (S) from two cruises. We see the similar reversed 'S' shape of θ-S diagram. At least five water masses are illustrated in these relations. One is in the surface layer, two are in the pycnocline, and the rest are in the intermediate layer.
At the surface, the fresh water is formed locally in the vicinity of the ITCZ (Qu et al., 1999). There are two typical pycnocline water masses, the NPTW and the SPTW, exist in this region. The NPTW, formed near the central of the North Pacific subtropical base (Cannon, 1966; Tsuchiya, 1968) resulting from excessive evaporation over precipitation, is characterized by high salinity (S>34.9) on density surfaces around 24.0σθ. The SPTW, identified by a salinity maximum (S>35.1) around 25.0σθ (Fine et al., 1994; Qu et al., 1999), is originated in the subtropical south Pacific (Wyrtki, 1961), being carried northward across equator by the NGCUC (Tsuchiya, 1991; Fine et al., 1994). The NPTW and the SPTW are of lower densities (about 23.5σθ and 24.5σθ, respectively) at the boundary area of the western equatorial Pacific than in the main part of the subtropical base (about 24.0σθ and 25.0σθ, respectively).
The most representative intermediate water masses, characterized by low salinity, are the NPIW and the AAIW. The NPIW is thought to be originated in the subarctic northwest Pacific with subsidence between the Kuroshio and the Oyashio (Reid, 1965; Talley, 1993). Salinity minimum (Smin) of the NPIW is about 33.8 near the subarctic Pacific, and 34.2–34.4 in NEC region near the boundary. The AAIW, as the southern hemisphere counterpart of the NPIW (Bingham and Lukas, 1994), is formed in Antarctic convergence zone, reaching equatorial Pacific from Coral Sea (Rochford, 1960; Reid, 1965; Fine et al., 1994). It is reported that the AAIW can be traced to about 10– 15°N in the northwest Pacific (Lukas et al., 1991; Kashino et al., 1996; Qu et al., 1999; Qu and Lindstrom, 2004; Xie et al., 2009). Like the NPIW, the AAIW is also highly variable depending on the latitude (Zenk et al., 2005). In the equatorial western Pacific the Smin of AAIW is 34.55 at 27.2σθ (Bingham and Lukas, 1995; Kashino et al., 1996).
In the pycnocline, the high-salinity water (S>34.9), considered as NPTW, is between 23.5σθ and 24.5σθ. Along 8°N section, the salty water (S>34.9) spreads from the Mindanao coast to 127.2°E in 2010 and to 128°E in 2012. The cruise 2012 shows the southeastern Mindanao inshore is also surrounded by the salty NPTW advected by the MC into the Sulawesi Sea. The much higher salinity (S>35.2) of the SPTW around 24.5σθ, occurs at the south stations of 130°E section, and extends to 2°N in 2010 while 4°N in 2012, may suggesting less salty water transported by the NGCUC in La Niña winter. The θ-S diagrams in the north inshore of Morotai in 2012 illustrates that the SPTW does not spread westward to the north of the Maluku Channel. The result agrees with the observation in 1994 of Kashino et al. (1996). Along MN section in 2010, the salty pycnocline water seems diluted, and similar appearance occurs at 3°–5°E along 130°E, which will be discussed later.
In the intermediate layer, the NPIW with lowsalinity (S < 34.4) around 26.6–26.8σθ distributes in the western station of 8°N section and north stations of 130°E section. Smin of the NPIW of 34.4 is found in the South of MC and is on the higher end of its characteristics, possibly resulting from intense mixing along the western boundary. Salty water around 26.7σθ in south stations along 130°E is found in both cruises. It is thought to be the transition water from pycnocline to intermediate layer, or the lower part of the SPTW (Kashino et al., 1996, 1999; Wang and Hu, 1998; Xie et al., 2009). At the south stations of 130°E section, the MN section, and the east stations of 8°N section, the Smin (< 34.55) around 27.2σθ of the AAIW is found. The AAIW transported across equator by the NGCUC is divided into several parts (Kashino et al., 1996). The θ-S diagrams off the Morotai north in 2012 suggests that the AAIW exists in 128°E, 3°N, spreading westward further than the SPTW.3.2 Salinity distribution on isopycnal surfaces
Four isopycnal surfaces of 21.5σθ, 24.0σθ, 26.6σθ, and 27.2σθ are chosen to be reasonable representations of the near-surface water, the pycnocline water, the NPIW, and the AAIW, respectively. Figure 3a illustrates the horizontal distributions of salinity in December 2010. The fresher water on 21.5σθ exists in the Sulawesi Sea and Maluku Channel; the fresh tough of 34.4 penetrates northwestward across the MN section, supplying western Pacific with lowsalinity water by the eastward advection into the NECC. On pycnocline of 24.0σθ, the high-salinity front (S>35.0) of the NPTW is seen along Mindanao inshore. The less salty tongue of 34.9 spreads to the northeast of Sulawesi Sea, carrying north Pacific waters as west root of the ITF (Ffield and Gordon, 1992; Gordon and Fine, 1996). Northeastward fresh water appears at Maluku Channel, resisting the spreading of salty water from Pacific. The SPTW with much higher salinity (S>35.1) reaches 3°N along 130°E with its 35.0-front penetrating northwestward to Morotai north.
Salinity distribution on 26.6σθ of the typical isopycnal depth of the NPIW, shows that the lowsalinity front of 34.4 from offshore at 8°N spreads to inshore of Mindanao southeast, continuously entering the Sulawesi Sea and the Maluku Channel. The lower part of the SPTW (S>34.6) is seen in the east Halmahera inshore, being diluted by low-salinity North Pacific water. On the 27.2σθ surface of the core depth of the AAIW, the region is dominated by low salinity. Contrary to the distributing charteristics on 21.5σθ and 24.0σθ, the salinity on this surface is more salty in the east Indonesian seas than in the western Pacific.
Horizontal salinity distributions on isopycnal surfaces during cruise 2012 (Fig. 3b) show evident variations comparing with those in 2010. The nearsurface water on 21.5σθ in 2012 is significantly fresher than that in 2010, which is consistent with the shrinkage of the western Pacific fresh water during La Nina, indicating the potential importance of surface salinity in ENSO dynamics (e.g., Delcroix, 1998; Delcroix and Picaut, 1998; Delcroix and McPhaden, 2002; Qu et al., 2014). On the 24.0σθ, the 35.0-front from the North Pacific spears to the offshore of the Mindanao southeast in 2012, while being constrained inshore in 2010. In both Decembers, the relatively fresh tongue stretches northeastward from Maluku Channel on the 24.0σθ and disappears in deeper layers. The SPTW (S>35.1) is carried by the NGCUC across equator from the southern hemisphere, reaching 4°– 5°N, further north than that in 2010. The northwest intrusion of the salinity front of 35.0 disappears in this year. Instead, much of this salty South Pacific water turns back into the NECC through the Halmahera Eddy (Kashino et al., 1996, 1999; Qu et al., 1999).
In both years, the salinity in east Indonesian seas is fresher in and above pycnocline and more salty below the intermediate layer, comparing with that in the western Pacific. With the existing of the 'barrier layer', the upper ocean in the western Pacific is not equally well-mixed in temperature and salinity (Lukas and Lindstrom, 1991). In addition, a high vertical diffusivity of 1×10-4 m2/s greater than those in the interior oceanic thermocline is conducive to vertical mixing in the Indonesian Sea (Ffield and Gordon, 1992).4 DISTRIBUTION AND VARIATION ALONG VERTICAL SECTION 4.1 North Pacific Water
The ITCZ creates a potential vorticity barrier that inhibit the direct flow of north Pacific water from subtropics to equator in the interior ocean (Lu and McCreary, 1995). The NPTW and the NPIW must first flow to the western boundary north of ITCZ and only then can they move equatorward in the MC. Along 8°N section, the NPTW (S>34.9), around 23.5σθ, spreads from Mindanao coast to 127.5°E in 2010 (Fig. 4a, red contours), and extends further to 130°E in 2012 with the high salinity core staying inshore (Fig. 4b, red contours). The NPIW around 26.6σθ also largely distributes inshore. A fresh tongue of the NPIW spreads to 128.5°E in 2010 with depth ascending and thickness decreasing (Fig. 4a, blue contours). In 2012, a separated low-salinity core exists in 129°E around 26.4σθ (Fig. 4b, blue contours).
The MC primarily determines how much salty water advects equatorward (Bingham and Lukas, 1994; Li and Wang, 2012). The geostrophic current from hydrologic observation show a strong southward MC with its principal part confined in the 128°E west inshore (Fig. 5).
Assuming a non-slip boundary condition, the volume transport above 1 000 dbar from Mindanao coast to 128°E of the southward MC are 28.1 (1 Sv=106 m3/s) Sv in 2010 and 18.1 Sv in 2012, suggesting a strengthened MC in La Niña winter. We conduct the sensitivity test with a level of no motion as 1 000 dbar, and the result shows a permanently strengthened MC during 2010 cruise (figure omit), consistent with the mooring observation of Zhang et al. (2014).
Actually, several works strongly suggested that the MC is weaker during La Niña (e.g., Kim et al., 2004; Kashino et al., 2009, 2015). Although the Niño 3.4 index can be utilized as a general indicator for an interannual-variable WBCs, the exact WBCs containing multi-scale variability not fully representable by the commonly used ENSO indices (Lukas, 1998). The inconsistence between this study and existing lectures is further discussed later. In this study, with geostrophic velocities, the southward transports of the NPTW across 8°N are estimated as 7.0 Sv in 2010 and 6.0 Sv in 2012. During 2010, the northward flow at 127.5°E (Fig. 5a) seems related to an eddy, which is visible in the satellite sea level data (figure omit). The shoreward shrink of the higher salinity (Fig. 4) accompanied by a strengthened MC may affected by eddy perturbations on the WBC, the dynamic of which will have to be reported in a further exploration.4.2 Equatorward intrusion of the north and south Pacific waters
Along 130°E section in Fig. 6, the distributions of the fresh water above 21.5σθ show significant variation between two cruises. Note that the high P-E values roughly correspond to low surface salinity in the ITCZ (Delcroix and Hénin, 1991; Delcroix and Picaut, 1998; Delcroix and McPhaden, 2002), therefore the northward retreat of the surface fresh water in 2010 may result from the shift of the ITCZ which moves polarward during La Niña (Trenberth, 1976; Chung et al., 2014; Choi et al., 2015).
In the pycnocline, the NPTW (S>34.9) stretches to about 9°–10°N in 2010 (Fig. 6a, red contour) whereas about 7°–8°N in 2012 (Fig. 6b, red contour), and the core of the NPTW sinks from 23.5σθ in 2010 to 24.0σθ in 2012. The deepening of the NPTW core is likely to be associated with the diapycnal mixing with surface water (Wang et al., 2013). The SPTW (S>35.1) extends northward to 2°–3°N along 130°E in 2010 (Fig. 6a, yellow contour) and further north to 4°–5°N in 2012 (Fig. 6b, yellow contour). With a larger current velocity and depth extent (Fig. 7), NGCUC appears to be stronger in 2010. However, the meridionnal extent of SPTW is limited, contrary to what should be expected for a strong NGCUC (Ueki and Ando, 2013). It is suggested that the significant southward MC and its retroflection contribute to the NECC, barring the northward intrusion of salty water from the southern hemisphere (Lukas et al., 1991; Kashino et al., 1996). The northward extension of the SPTW during 2010 might be blocked by an enhanced NECC at that time (Fig. 7).
Between the high-salinity fronts of the NPTW and the SPTW, there is relatively lower salinity water (S < 34.9) around 23.5σθ in 5°–8°N, featuring a lateral salinity minimum and vertical salinity maximum. It has been named as TSSW by Wang et al. (2013), although it might not be a water mass by traditional definition. The TSSW is further discussed later.
In the intermediate layers, the low-salinity water NPIW (S < 34.4) stretches southward to about 10°N in 2010 (Fig. 6a, blue contours), while about 7°N in 2012 with a decreased thickness (Fig. 6b, blue contours). From south hemisphere, the salty water (S>34.6, around 26.8σθ), of lower part of the SPTW, also show a southward retreat in 2010 (Fig. 6, cyan contours). The relatively salty tongue (34.4 < S < 34.5) of the NPIW spears further south, shoaling with depth, and intrudes into the SPTW, diluting its lower part. It is difficult to track the low-salinity fronts of the AAIW in vertical sections. More chemical tracers, such as oxygen, are necessary to explore its feature.5 WATER MASS BETWEEN MINDANAO AND NEW GUINEA
Along the MN section between the Mindanao and the Papua New Guinea in 2010 winter, the pycnocline waters (S>34.9, red contour) from north and south Pacific are to merge, which is partly suggested by the former observations (Fine et al., 1994; Kashino et al., 1996, 1999) (Fig. 8). The fresher water bridging the NPTW and the SPTW at 130°E (Fig. 6) is no longer preserve along the MN section due to the strong colliding of the LLWBCs, the MC and the NGCUC. Extreme salinity (S>35.1, yellow contour) front of the SPTW appears between 128.3°E, 4.9°N and 130°E, 3°N on the layer of 24.5σθ. Being apart from the main body eastward to interior Pacific, the remanent SPTW spreads northwestward along MN section, rather than extending westward to the Maluku Channel north (Fig. 2b, down-right panel). The observation evidences the west branch of the SPTW inferred by Kashino et al. (1996).
The pycnocline between the MN section is dominated by northeastward flow (Fig. 8, black solid contours) carrying the Indonesian seas water into the NECC. Although the significant dilution from the fresher water is evident, especially between 127.5°E, 5.5°N and 128.3°E, 4.9°N, the salty Pacific pycnocline water persists in the high salinity, obstructing the immediate connection of the low-salinity Indonesian seas water and the TSSW at 130°E. However, the mergence of the salty pycnocline waters is not a permanent phenomenon. In the investigation of the water masses between Mindanao and New Guinea in February 1994 (Kashino et al., 1996, 1999), the fresh water of 34.7 pierced into and separated the salty NPTW and SPTW on the pycnocline, showing significant dilution stronger than 2010 winter. Meanwhile, the northwestward intrusion of the SPTW in 2010 is disappeared in 2012 in horizontal distributions on 24.0σθ (Fig. 3), indicating a stronger dilution from east Indonesian seas or the weakening of the northwestward branch of the SPTW in later cruise. The mixing of the entrained low-salinity Indonesian seas water between the Mindanao and New Guinea is an important source of the low salinity of the TSSW in the western Pacific Ocean.
The trajectories of surface drifters launched in December 2010 provide us a schematic of the currents in the region (Fig. 9a). The drifters illustrate the colliding LLWBCs and their retroflections to the NECC; the eastward flows from the Sulawesi Sea and the east Maluku Channel are also displayed, indicating the surface pathway of the low-salinity Indonesian Sea water (blue and pink curves in Fig. 9b) contributing to the fresh pool of western Pacific Ocean. Ocean dynamics plays a significant role in contributing the sea surface salinity (e.g., Johnson et al., 2002; Qu et al., 2014). Around 23.0–25.0σθ in Fig. 9b, the two dotted curves represent the relatively low-salinity Pacific pycnocline water, the TSSW. The waters in the Sulawesi Sea (blue curve) and the Maluku Channel (pink curve) are fresher than the NPTW (red curve) and the SPTW (cyan curve). As trajectories indicating, the water from Sulawesi Sea is mainly flow northeastward to the 5°N north, while that from the Maluku Channel is in the south. At 6.5°N (black dotted), the TSSW is generated by the NPTW, being diluted by the Sulawesi Sea water. The TSSW at 5°N, 130°E (gray dotted) is mainly from the isopycnal mixing between the STPW and the water from Maluku Channel. Along the downstream of the NGCUC and its retroflection to the NECC, the salinity decreases with relatively stable shape of the θ-S diagram (Fig. 9c), suggesting the salinity maximum of the water mass at 5°N, 130°E is from the SPTW. As complementary to the inference that the TSSW is mainly of North Pacific water origin (Wang et al., 2013), the observation demonstrates that TSSW should be regarded as the diluted pycnocline water from both hemispheres.
Nevertheless, it is worth noting that the lowsalinity pycnocline water from east Indonesian seas cannot immediately break through the salty barrier of the colliding LLWBCs in the MN section at least in 2010 winter (Fig. 8). The salinity maximum of this subsurface fresh water is found to decrease eastward, suggesting the strong diapycnal mixing reducing its salinity. With the mixing with surface water or the upwelling (Li and Wang, 2012; Wang et al., 2013), the salty pycnocline water in the western Pacific may be further diluted, resulting a lateral salinity minimum of the TSSW. The diapycnal mixing suggested in previous studies and the isopycnal mixing of temporally entrained Indonesian sea water both contribute to the low salinity of the TSSW.6 DISCUSSION AND SUMMARY 6.1 Discussion
Over the past few years, many papers (Kashino et al., 2005, 2015; Zhang et al., 2014; Wang et al., 2016) have studied closely how important the intra-seasonal variability can have non-overlooked impact on the circulation and water mass redistributions. The intraseasonal signals in the cruising snapshot have been neglected in above analysis. We need more detection to connect the distribution variation of the circulation to the ENSO dynamics. SSH during the ENSO mature phase from November to next January (Wang, 1995) is illustrated in Fig. 10. As in Fig. 10a and b, SSH in the tropical western Pacific during cold phase is significantly higher than during normal phase. In view of the zonal gradient along 8°N and meridional gradient along 130°E, assuming geostrophy, we will have a weakened MC and NECC for La Niña phase, which is consistent with Kashino et al. (2015) and Zhao et al. (2013).
This general principle, however, is not applicable all the time for particular events. The SSH in winters 2010 and 2012 (Fig. 10c–d) is significantly higher than general condition of cold phase and normal phase (Fig. 10a–b), respectively, suggesting that 2010 winter comes at a strong La Niña event while 2012 winter is not a typical normal phase. Although Niño 3.4 index anomaly of 2012 winter is small, the magnitude of the SSH in the tropical western Pacific during 2012 (Fig. 10d) is close to cold phase (Fig. 10a). Comparing the SSH between two winters studied in this paper, SSH gradient along 8°N and 130°E are both larger during 2010 winter (Fig. 10c) than during 2012 winter (Fig. 10d), respectively. Therefore, the MC and the NECC seems stronger during 2010 winter compared with 2012 winter. The circulation inferred from the 3-month averaged SSH pattern suggests a strengthened tropical gyre in 2010 winter, which is consistent with the geostrophic current from in situ observation. Besides the ENSO process impacting on the observed difference between two cruises, multiple mechanisms related to the intra-seasonal variability, like Kelvin wave, or eddy activity, are also need to be investigated in further study.6.2 Summary
Using CTD data from two hydrographic cruises of 2010 La Niña winter and 2012 normal winter, and homochronous profiles from Argo floats, this study addresses the charteristics of water mass distributions during a normal year and the peak phase of a La Niña year, hence their interannual variations near the boundary area of the western equatorial Pacific Ocean. The effect of the eastward low-salinity water from east Indonesian seas on the pycnocline water of western Pacific Ocean is highlighted.
On the near-surface layer around 21.5σθ, the water is significant saltier during 2010 La Niña winter than 2012 normal phase. On the pycnocline, the NPTW spreads southward and enters Sulawesi Sea as west root of the ITF; the relatively fresh tongue stretches northeastward from Maluku Channel. The SPTW crosses the equator in the northwest direction along the section between the Papua New Guinea and the Mindanao and retroflects back to the interior Pacific Ocean, rather than extending westward to the north of the Maluku Channel. In both winters, the salinity in the east Indonesian seas is lower in and above the pycnocline and higher below the intermediate layer, comparing with that in the western Pacific Ocean.
The transport of the NPTW into the tropics is primarily advected by the MC. Along 8°N off Mindanao east, the NPTW (S>34.9) around 23.5σθ, spreads from the coast to 127.5°E in 2010, and further to 130°E in 2012 with the high salinity core staying inshore. However, the equatorward transport of the NPTW across 8°N is 7.0 Sv in 2010, larger than 6.0 Sv in 2012, resulting from the strengthened advection of the MC during 2010 La Niña.
Along 130°E, the NPTW stretches to about 9°– 10°N in 2010, while about 7°–8°N in 2012. From the south hemisphere, the SPTW (S>35.1) reaches 2°– 3°N along 130°E in 2010 and farther north 4°–5°N in 2012. The relatively fresh pycnocline water TSSW (S < 34.9) between the NPTW and the SPTW, is seen in 5°–8°N during both cruises. The TSSW should be regarded as diluted portion of the pycnocline waters from both hemispheres.
The relatively fresh waters from the east Indonesian seas which moves eastward into the NECC, play an important role in contributing the fresh pool of the Pacific Ocean and reducing the salinity of Pacific pycnocline waters via isopycnol mixing. However, the salinity maximum of this subsurface fresh water is found to decrease eastward, suggesting the salinity maximum is generated either by strong diapycnal mixing or by isopycnal mixing of temporally entrained Indonesian sea water into the area.
Noted that, in present paper, the comparison between different phases of ENSO is based on punctual survey data. We suggest the readers pay attention to the intra-seasonal variability in cruising snapshot in later study. Additionally, the detailed mixing process of the pycnocline waters in this area has not been well detected partly due to the relatively low horizontal resolution. The very-high resolution CTD data from seaglider or Seasoar, and more parameter, such as spiciness, will have good help to assess the dynamic process.7 ACKNOWLEDGEMENT
Thanks are extended to the crews and scientists of R/V Kexue-1. Data acquisition and sample collections were supported by the NSFC Open Research Cruise (Cruise Nos. NORC2010-09 and NORC2012-09), funded by the Shiptime Sharing Project of NSFC. This cruise was conducted onboard R/V Kexue-1 by the Institute of Oceanology, Chinese Academy of Sciences.
Akima H. 1970. A new method of interpolation and smooth curve fitting based on local procedures. Journal of the ACM, 17(4): 589-602. DOI:10.1145/321607.321609
Bingham F M, Lukas R. 1994. The southward intrusion of North Pacific Intermediate Water along the Mindanao coast. J. Phys. Oceanogr., 24(1): 141-154. DOI:10.1175/1520-0485(1994)024<0141:TSIONP>2.0.CO;2
Bingham F M, Lukas R. 1995. The distribution of intermediate water in the western equatorial pacific during JanuaryFebruary 1986. Deep Sea Research Part Ⅰ:Oceanographic Research Papers, 42(9): 1545-1573. DOI:10.1016/0967-0637(95)00064-D
Broecker W S. 1991. The great ocean conveyor. Oceanography, 4(2): 79-89. DOI:10.5670/oceanog
Cannon G A. 1966. Tropical waters in the western Pacific Ocean, August-September 1957. Deep Sea Research and Oceanographic Abstracts, 13(6): 1139-1148. DOI:10.1016/0011-7471(66)90705-4
Choi K Y, Vecchi G A, Wittenberg A T. 2015. Nonlinear zonal wind response to ENSO in the CMIP5 models:roles of the zonal and meridional shift of the ITCZ/SPCZ and the simulated climatological precipitation. J. Climate, 28(21): 8556-8573. DOI:10.1175/JCLI-D-15-0211.1
Chung C T Y, Power S B, Arblaster J M, Rashid H A, Roff G L. 2014. Nonlinear precipitation response to El Niño and global warming in the Indo-Pacific. Climate Dyn., 42(7-8): 1837-1856. DOI:10.1007/s00382-013-1892-8
Delcroix T, Hénin C. 1991. Seasonal and interannual variations of sea surface salinity in the tropical Pacific Ocean. J. Geophys. Res., 96(C12): 22135-22150. DOI:10.1029/91JC02124
Delcroix T, McPhaden M J. 2002. Interannual sea surface salinity and temperature changes in the western Pacific warm pool during 1992-2000. J. Geophys. Res., 107(C12): 8002.
Delcroix T, Picaut J. 1998. Zonal displacement of the western equatorial Pacific "fresh pool". J. Geophys. Res., 103(C1): 1087-1098. DOI:10.1029/97JC01912
Delcroix T. 1998. Observed surface oceanic and atmospheric variability in the tropical Pacific at seasonal and ENSO timescales:a tentative overview. J. Geophys. Res., 103(C9): 18611-18633. DOI:10.1029/98JC00814
Ffield A, Gordon A L. 1992. Vertical mixing in the Indonesian thermocline. J. Phys. Oceanogr., 22: 184-195. DOI:10.1175/1520-0485(1992)022<0184:VMITIT>2.0.CO;2
Fine R A, Lukas R, Bingham F M, Warner M J, Gammon R H. 1994. The western equatorial Pacific:A water mass crossroads. J. Geophs. Res., 99(C12): 25063-25080. DOI:10.1029/94JC02277
Gordon A L, Fine R A. 1996. Pathways of water between the Pacific and Indian oceans in the Indonesian seas. Nature, 379(6561): 146-149. DOI:10.1038/379146a0
Gordon A L. 1986. Interocean exchange of thermocline water. J. Geophys. Res., 91(C4): 5037-5046. DOI:10.1029/JC091iC04p05037
Hasunuma K. 1978. Formation of the intermediate salinity minimum in the northwestern Pacific Ocean. Bull. Ocean Res. Inst. Univ. Tokyo, 9: 1-47.
Hu D X, Cui M C, Qu T D, Li Y X. 1991. A subsurface northward current off Mindanao identified by dynamic calculation. In: Takanao K ed. Oceanography of Asian Marginal Seas. Elsevier Oceanography Series, 54. Elsevier, New York. p.359-365.
Kashino Y, Aoyama M, Kawano T, Hendiarti N, Anantasena Y, Muneyama K, Watanabe H. 1996. The water masses between Mindanao and New Guinea. J. Geophys. Res., 101(C5): 12391-12400. DOI:10.1029/95JC03797
Kashino Y, España N, Syamsudin F, Richards K J, Jensen T, Dutrieux P, Ishida A. 2009. Observations of the North Equatorial current, Mindanao current, and Kuroshio current system during the 2006/07 El Niño and 2007/08 La Niña. J. Oceanogr., 65(3): 325-333. DOI:10.1007/s10872-009-0030-z
Kashino Y, Firing E, Hacker P, Sulaiman A, Lukiyanto. 2001. Currents in the Celebes and Maluku Seas, February 1999. Geophys. Res. Lett., 28(7): 1263-1266. DOI:10.1029/2000GL011630
Kashino Y, Ueki I, Sasaki H. 2015. Ocean variability east of Mindanao:mooring observations at 7°N, revisited. J. Geophys. Res., 120(4): 2540-2554. DOI:10.1002/2015JC010703
Kashino Y, Watanabe H, Herunadi B, Aoyama M, Hartoyo D. 1999. Current variability at the Pacific entrance of the Indonesian Throughflow. J. Geophys. Res., 104(C5): 11021-11035. DOI:10.1029/1999JC900033
Kim Y Y, Qu T, Jensen T, Miyama T, Mitsudera H, Kang H W, Ishida A. 2004. Seasonal and interannual variations of the North Equatorial Current bifurcation in a high-resolution OGCM. J. Geophys. Res., 109: C03040.
Li Y L, Wang F. 2012. Spreading and salinity change of North Pacific tropical water in the Philippine Sea. J. Oceanogr., 68(3): 439-452. DOI:10.1007/s10872-012-0110-3
Lindstrom E, Lukas R, Fine R, Firing E, Godfrey S, Meyers G, Tsuchiya M. 1987. The western equatorial Pacific Ocean circulation study. Nature, 330(6148): 533-537. DOI:10.1038/330533a0
Lu P, McCreary J P Jr. 1995. Influence of the ITCZ on the flow of thermocline water from the subtropical to the equatorial Pacific Ocean. J. Phys. Oceanogr., 25(12): 3076-3088.
Lukas R, Firing E, Hacker P, Richardson P L, Collins C A, Fine R, Gammon R. 1991. Observations of the Mindanao current during the Western Equatorial Pacific Ocean circulation study. J. Geophys. Res., 96(C4): 7089-7104. DOI:10.1029/91JC00062
Lukas R, Lindstrom E. 1991. The mixed layer of the western equatorial Pacific Ocean. J. Geophys. Res., 96(S01): 3343-3357. DOI:10.1029/90JC01951
Lukas R. 1988. Interannual fluctuations of the Mindanao current inferred from sea level. J. Geophys. Res., 93(C6): 6744-6748. DOI:10.1029/JC093iC06p06744
Nitani H. 1972. Beginning of the Kuroshio. In: Stommel H, Yoshida K eds. Kuroshio: Physical Aspects of Japan Current. University of Washington Press, Washington. p.129-163.
Qu T D, Lindstrom E J. 2004. Northward intrusion of Antarctic Intermediate Water in the western Pacific. J. Phys. Oceanogr., 34: 2104-2118. DOI:10.1175/1520-0485(2004)034<2104:NIOAIW>2.0.CO;2
Qu T D, Mitsudera H, Yamagata T. 1998. On the western boundary currents in the Philippine Sea. J. Geophys. Res., 103(C4): 7537-7548. DOI:10.1029/98JC00263
Qu T D, Mitsudera H, Yamagata T. 1999. A climatology of the circulation and water mass distribution near the Philippine Coast. J. Phys. Oceanogr., 29(7): 1488-1505. DOI:10.1175/1520-0485(1999)029<1488:ACOTCA>2.0.CO;2
Qu T D, Song Y T, Maes C. 2014. Sea surface salinity and barrier layer variability in the equatorial Pacific as seen from Aquarius and Argo. J. Geophys. Res., 119(1): 15-29. DOI:10.1002/2013JC009375
Qu T D, Yu J Y. 2014. ENSO indices from sea surface salinity observed by Aquarius and Argo. J. Oceanogr., 70(4): 367-375. DOI:10.1007/s10872-014-0238-4
Reid J L Jr. 1965. Intermediate waters of the Pacific Ocean. In: Johns Hopkins Oceanographic Studies, No. 2. Johns Hopkins Press, Baltimore, Maryland.
Rochford D J. 1960. The intermediate depth waters of the Tasman and Coral Sea. I. The 27. 20σt surface. Australian Journal of Marine and Freshwater Research, 11(2): 127-147. DOI:10.1071/MF9600127
Talley L D. 1993. Distribution and formation of North Pacific Intermediate Water. J. Phys. Oceanog., 23: 517-537. DOI:10.1175/1520-0485(1993)023<0517:DAFONP>2.0.CO;2
Trenberth K E. 1976. Spatial and temporal variations of the Southern Oscillation. Quart. J. Roy. Meteor. Soc., 102(433): 639-653. DOI:10.1002/qj.49710243310
Tsuchiya M, Lukas R, Fine R A, Firing E, Lindstrom E. 1989. Source waters of the Pacific equatorial undercurrent. Progress in Oceanography, 23(2): 101-147. DOI:10.1016/0079-6611(89)90012-8
Tsuchiya M. 1968. Upper waters of the intertropical Pacific Ocean. In: Johns Hopkins Oceanographic Studies. No. 4. The Johns Hopkins Press, Baltimore.
Tsuchiya M. 1991. Flow path of the Antarctic Intermediate Water in the western equatorial South Pacific Ocean. Deep Sea Research Part A. Oceanographic Research Papers, 38(Suppl.): S273-S279.
Ueki I, Ando K. 2013. Detection of Pacific tropical water variability by taut-line moorings in the western equatorial Pacific. J. Oceanogr., 69(4): 429-441. DOI:10.1007/s10872-013-0184-6
Wang B. 1995. Interdecadal changes in El Niño Onset in the last four decades. J. Climate, 8: 267-285. DOI:10.1175/1520-0442(1995)008<0267:ICIENO>2.0.CO;2
Wang F, Hu D X. 1998. Dynamic and thermohaline properties of the Mindanao Undercurrent, Part Ⅱ:thermohaline structure. Chinese Journal of Oceanology and Limnology, 16(3): 206-213. DOI:10.1007/BF02848724
Wang F, Hu D X. 2010. Introduction to international NPOCE program. Chinese Journal of Oceanology and Limnology, 28(4): 953. DOI:10.1007/s00343-010-0953-7
Wang F, Li Y L, Wang J N. 2016. Intraseasonal variability of the surface zonal currents in the western tropical pacific ocean:characteristics and mechanisms. J. Phys.Oceanogr., 46(12): 3639-3660. DOI:10.1175/JPO-D-16-0033.1
Wang F, Li Y L, Zhang Y H, Hu D X. 2013. The subsurface water in the North Pacific tropical gyre. Deep Sea Research Part Ⅰ:Oceanographic Research Papers, 75: 78-92. DOI:10.1016/j.dsr.2013.01.002
Wyrtki K. 1961. Physical oceanography of the Southeast Asian Waters. In: NAGA Report vol. 2, Scientific Results of Marine Investigations of the South China Sea and the Gulf of Thailand, Scripps Institution of Oceanography.University of California, San Diego, La Joola, CA. 195p.
Xie L L, Tian J W, Hu D X, Wang F. 2009. A quasi-synoptic interpretation of water mass distribution and circulation in the western North Pacific: Ⅰ. Water mass distribution. Chinese Journal of Oceanology and Limnology, 27(3): 630-639. DOI:10.1007/s00343-009-9161-8
Zenk W, Siedler G, Ishida A, Holfort J, Kashino Y, Kuroda Y, Miyama T, Müller T J. 2005. Pathways and variability of the Antarctic Intermediate Water in the western equatorial Pacific Ocean. Progress in Oceanography, 67(1-2): 245-281. DOI:10.1016/j.pocean.2005.05.003
Zhang L L, Hu D X, Hu S J, Wang F, Wang F J, Yuan D L. 2014. Mindanao Current/Undercurrent measured by a subsurface mooring. J. Geophys. Res., 119(6): 3617-3628. DOI:10.1002/2013JC009693
Zhao J, Li Y L, Wang F. 2013. Dynamical responses of the west Pacific North Equatorial Countercurrent (NECC) system to El Niño events. J. Geophys. Res., 118(6): 2828-2844. DOI:10.1002/jgrc.20196