1. Introduction
The Indonesian Throughflow (ITF) is the major connection of two ocean basins at low latitudes. It flows through the Indonesian seas, transporting the warm waters from the tropical Pacific Ocean to the Indian Ocean (Gordon 1986; Godfrey 1996; Gordon 2001; Lee et al. 2002; Gordon 2005; Qiu and Chen 2010; Gordon et al. 2010, 2012; Sprintall et al. 2013; Sprintall and Révelard 2014; Hu et al. 2015). The Pacific entrance of the Indonesian Throughflow refers to the area north of New Guinea and south of Mindanao in the eastern Sulawesi Sea (Fig. 1), where two western boundary currents, the Mindanao Current (MC) and the New Guinea Coastal Current/Undercurrent (NGCC/UC) meet and retroflect eastward to form the origin of the Northern Equatorial Countercurrent (NECC). Under the NECC is the North Equatorial Subsurface Current (NESC), flowing westward at the depth of the subthermocline (Yuan et al. 2014; Li et al. 2020; Yang et al. 2020). The confluence of the strong ocean currents generates complex ocean circulation in the area, the variations of which have profound impact on the heat and salt budgets of the equatorial Pacific Ocean and on global climate.
Bathymetry map and upper-ocean circulation schematic of the Pacific entrance of the Indonesian Throughflow and its surrounding waters. The blue triangles are three moorings TH1, TH2, and TH3 used in this study. MC: Mindanao Current; NECC: North Equatorial Countercurrent; NGCC/UC: New Guinea Coastal Current/Undercurrent; ME: Mindanao Eddy; HE: Halmahera Eddy; ITF: Indonesian Throughflow.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
The Northern Equatorial Current (NEC) of the Pacific Ocean flows westward and splits into the southward MC and the northward Kuroshio off the east Philippine coasts (Toole et al. 1990; Lukas et al. 1996; Qiu and Lukas 1996; Wang and Hu 2006). The MC is a strong western boundary current flowing equatorward along the east coast of the Mindanao Island, with the maximum velocity exceeding 1 m s−1 and an offshore width of about 100–200 km (Lukas 1988; Lukas et al. 1991; Hu et al. 1991; Wijffels et al. 1995; Schönau et al. 2015; Ren et al. 2018). As the southward MC flowing into the Pacific entrance of ITF, it separates into two parts: one part flows into the Sulawesi Sea to supply for the Indonesian Throughflow flowing into the Makassar Strait; the other part retroflects eastward to feed the NECC (Lukas et al. 1991; Kashino et al. 2001, 2013; Gordon 2005; Li et al. 2018). The southward MC, with a transport of more than 30 Sv (1 Sv ≡ 106 m3 s−1), is suggested to be important for the formation of the western Pacific warm pool (Lukas et al. 1991, 1996; Qu et al. 1999; Li et al. 2012; Duan et al. 2019). Mooring observations and Geostrophic calculations suggest that the transport of the MC is determined by the splitting location of the NEC (Qiu and Lukas 1996; Qu and Lukas 2003; Kim et al. 2004). The MC is observed to be weaker in fall and during La Niña, while stronger in spring and during El Niño (Qu et al. 1998; Yaremchuk and Qu 2004; Kashino et al. 2005; Zhang et al. 2014; Ren et al. 2020). Kashino et al. (2011) observed strong seasonal-to-interannual variation of the MC using mooring observations and suggested that the MC is strongly influenced by ENSO.
In these previous studies, the focus is on the MC at or north of 7°N (Kashino et al. 2005, 2009, 2013, 2015). The seasonal and interannual variations of the MC at the entrance of the Indonesia seas are influenced by multiple strong currents and have not been studied adequately due to lack of observations. The collision between the MC and the NGCC/UC was investigated from the perspective of nonlinear dynamic systems by several studies. Arruda and Nof (2003) suggested that the nonlinear collision of the MC and the NGCC/UC produces the ITF and NECC on the β plane. Wang and Yuan (2012, 2014) suggested that the nonlinear collision at a discontinuous boundary has multiple equilibrium states (penetration, choke, and eddy shedding), with bifurcation and hysteresis. The eddy shedding of the MC was found to induce the variations in the Sulawesi Sea at the intraseasonal time scales (Qiu et al. 1999; Masumoto et al. 2001; Wang and Yuan 2012; Chen et al. 2018). Using the time series of nearly 5-yr moorings in the Maluku Channel from 2012 to 2016, Yuan et al. (2018) found that the transport of the Maluku Channel has strong seasonal variations in the upper layer of 300 m or so, which is not forced by local winds. It is suggested that the Maluku Channel transport is induced by the movement of the MC retroflection, based on historical surface drifter trajectories: drifters were easier to penetrate the Maluku Channel in fall and winter, but stayed outside in spring and summer. Using the time series of nearly 2-yr moorings in the strait between the Talaud Islands and Halmahera (TH Channel) during December 2016–September 2018, Li et al. (2021) provided a piece of solid evidence of the seasonal variations of MC retroflection: retreats to north of the Talaud Islands in boreal summer and penetrates into the northern Maluku Sea (the area south of the line between the Talaud and Morotai Islands) in winter.
The MC and the NGCC\UC feed into the NECC, an eastward current between 3° and 8°N, which is playing an important role in modulating the heat budget of the western Pacific warm pool. Existing studies suggest that the transport of NECC is larger in boreal summer and fall, and smaller in boreal winter and spring (Wyrtki and Kendall 1967; Wyrtki 1974; McPhaden 1996; Coles and Rienecker 2001; Heron et al. 2006). The axis position of the NECC moves northward in fall and southward in spring. The transport of NECC increases in El Niño years, with the axis moving equatorward, whereas the opposite happens in La Niña years (Wyrtki 1979; Meyers and Donguy 1984; Kessler and Taft 1987; Qiu and Joyce 1992; Zhao et al. 2013). The interannual path and variations of the NECC and the MC at the Pacific entrance of the ITF have not been studied before.
In this study, using surface geostrophic currents of satellite altimeters covering January 1993 through December 2019, we investigate the seasonal and interannual variations of the MC retroflection at the Pacific entrance of the ITF. The rest of this paper are organized as follows. Section 2 describes the data and methods used in this study. In section 3, the seasonal variations of MC retroflection and their connection with the NECC, and the interannual variations of MC retroflection and their association with ENSO are presented. The conclusions are summarized in section 4.
2. Data and method
a. Data
The gridded surface geostrophic currents used in this study are provided by Arching, Validation, and Interpolation of Satellite Oceanographic (AVISO), which is collected by Ocean Topography Experiment (TOPEX)/Poseidon, Cryosat-2, GFO, Jason-1, Envisat, ERS-1, and ERS-2. The data are on daily frames, with a spatial resolution of 0.25° longitude × 0.25° latitude, covering the period from October 1992 to the present. In this study, the data during January 1993–December 2019 are used to study the seasonal-to-interannual movement of the MC retroflection.
The absolute geostrophic currents (AGCs) used in this study are calculated from the Argo monthly gridded temperature and salinity data of the Scripps Institution of Oceanography (Roemmich and Gilson 2009), with a 1° × 1° horizontal resolution and on 58 vertical levels from 2.5 to 1975 m. The AGCs are calculated based on the gridded Argo temperature and salinity data using the P-vector method (Chu 1995), which assumes conservation of potential density and potential vorticity and is equivalent to the β-spiral method under the Boussinesq and geostrophic approximations (Zhang et al. 2013; Yuan et al. 2014). To avoid the surface mixed layer, the AGCs used in this study are calculated between 800 and 1975 m, with the geostrophic currents above 800 m determined by the dynamic height calculation using the AGC at 800 m as the reference velocity (Yuan et al. 2014).
The moored current meter data used are from three moorings (TH1, TH2, TH3) deployed in the TH channel between the Talaud Islands and Halmahera during December 2016–September 2018 at the positions of 2°24.1′N, 127°30.1′E, 2°57.6′N, 127°16.0′E, and 3°30.0′N, 127°2.3′E, respectively (Li et al. 2021). Each mooring is instrumented with an upward-looking RD Instruments Long Ranger Workhorse 75-kHz ADCP, set at the nominal depth of 450 m.
The gridded sea surface chlorophyll-a concentration (chl-a) data used in this study are obtained from the Copernicus Marine Service (https://resources.marine.copernicus.eu/). The product name is OCEANCOLOUR_GLO_CHL_L4_REP_OBSERVATIONS_009_082, which is merged measurements of multisensors including the SeaWiFS, MODIS, VIIRS-SNPP, JPSS-1, OLCI-Sentinel-3A, and Sentinel-3B. The chlorophyll-a concentrations are daily data with a spatial resolution of 4 km, covering the globe from January 2000 through December 2019.
b. Methods
The seasonal variations of MC retroflection are studied using the climatological monthly mean sea surface geostrophic currents. The path of the MC retroflection at the entrance of the Indonesian seas is identified based on the maximum zonal/meridional velocity. We first identified the maximum zonal velocity in the southernmost part of the retroflection within 2.5°–6°N, 126.5°–127.2°E (the red dashed box in Fig. 2), which is called Dot A (the red dot in Fig. 2). Based on the location of this dot, the MC retroflection could be considered as two segments, the north–south segment (the connected green dots along the direction from Dot A to Mindanao) and the east–west segment (the connected green dots along the direction from Dot A to NECC). In the north–south segment, between dot A and 6.5°N, we identified the locations of the maximum meridional velocity at each latitude and connected them in a line; In the east–west segment, between dot A and 129°E, we identified the locations of the maximum zonal velocity at each longitude and connected them in a line. The two green lines (the connected green dots in Fig. 2) mark the path of the MC retroflection at the entrance of the Indonesian seas, which is also called the MC axis.
Climatological surface geostrophic currents based on AVISO altimeter data from 1993 to 2019. The axis positions of the MC retroflection are marked by green dots. The red dashed box is the area within 2.5°–6°N, 126.5°–127.2°E, the black triangles are the locations of moorings, and the black solid lines mark the area south of 6.5°N and west of 129°E.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
The ADCP velocity data are adopted from Li et al. (2021), with the raw data quality controlled using the standard procedure of Cowley et al. (2008). Due to the contamination by the reflection from the sea surface, the velocity data in the upper 50 m or so were abandoned. A fourth-order low-pass Butterworth filter was applied on the time series with the cutoff period at 120 days, and 67° clockwise from due north was defined as the along-channel velocity (ACV). Details of the data processing and analysis are contained in Li et al. (2021).
3. Results
The seasonal variations of MC retroflection and their connection with the NECC, the interannual variations of MC retroflection and their association with ENSO are studied in this section using the surface geostrophic currents from 1993 to 2019.
a. Validation of AVISO surface geostrophic currents
The surface geostrophic currents are compared with the mooring data in the Talaud–Halmahera Channel during December 2016–September 2018 (Fig. 3). The ACV of the ADCP measurements averaged in the upper 100 m shows similar variations at the three mooring sites (Figs. 3a–c). During this period, the ACV averaged in the upper 100 m at these three moorings, with stronger eastward currents in the north than in the south, (cf. TH3 and TH1, red lines in Figs. 3a–c), peaks in boreal winter, relaxes or even reverses during spring, then gets a second weaker peak in August (boreal summer), and returns to the winter values to form a complete seasonal cycle.
Comparison of the mean ACV in the upper ocean above 100 m (red lines) at the mooring sites of (a) TH1, (b) TH2, and (c) TH3 with the surface geostrophic currents interpolated onto the mooring site (black lines).
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
The seasonal variations of the ACV at all three mooring sites track well with the surface geostrophic currents interpolated onto the three mooring sites. The correlation coefficients between the geostrophic ACV and that of the moorings are 0.857, 0.925, and 0.849 at TH1, TH2, and TH3 sites, respectively, above the 95% significance level. The comparison suggests that the satellite altimeter geostrophic currents can be used to study the variability of upper-ocean circulation at the entrance of the Indonesian seas.
The retroflection index is calculated as the area of the MC retroflection to represent the variations of the MC path in this study (black line in Fig. 4), which is the area surrounded by the MC axis, south of 6.5°N, and west of 129°E (the area surrounded by the connected green dots and the black lines in Fig. 2). Throughout the year, the retroflection index decreases from January to June and increases from July to December, suggesting that the MC largely stays outside the northern Maluku Sea in summer and starts to intrude into the Maluku Sea in the latter half of the year. The result is in good agreement with the ACV variability in the upper 100 m at the TH2 site (red line in Fig. 4). The spatial pattern of the MC retroflection movement is thus uncovered by our analysis for the first time in history. The correlation coefficient between the retroflection index and the ACV at TH2 is 0.936, above the 95% significance level, suggesting that the retroflection index can be used to represent the variations of the MC retroflection.
The retroflection index from December 2016 through September 2018 (the area surrounded by the connected green dots and the black lines in Fig. 2, black line) and the mean ACV in upper 100 m at TH2 during December 2016–September 2018 (red line).
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
b. Seasonal variation of the MC retroflection
The seasonal variability of the MC retroflection is illustrated by the climatologic monthly maps of surface geostrophic currents at the entrance of the ITF (Fig. 5). A clear seasonal cycle of the MC retroflection is identified: The MC intrudes into the northern Maluku Sea during late fall through early spring, with the mainstream flowing through TH Channel. The MC path moves gradually northward in boreal spring and essentially retroflects eastward north of the Talaud Islands in boreal summer without intruding into the Maluku Sea. The MC intrusion and retroflection at the entrance of the Indonesian seas are governed by the nonlinear collision of the two western boundary currents—the MC and the NGCC/UC, which are subject to bifurcation and hysteresis. The stronger tends to intrude more deeply than the weaker when two WBCs with unequal transports should collide at a wide gap (Wang and Yuan 2014). The MC retroflection has multiple equilibria and is subject to regime shift at the perturbations of mesoscale eddies or Rossby waves (Yuan and Wang 2011; Wang and Yuan 2012, 2014). In boreal winter, the transport of the NGCC/UC is much smaller than that of the MC as the NGCC reverses to flow to the south, so that the MC penetrates more deeply into the Maluku Sea. As the transport of the NGCC/UC increases while the MC transport decreases in late spring (Kashino et al. 2013), the MC retroflection starts to move out of the northern Maluku Sea until the MC path changes from the penetration to the choke states in summer. These are in agreement with the nonlinear collision theory of Wang and Yuan (2014).
Climatologic monthly maps of surface geostrophic currents. The green pots are the axis positions of the MC retroflection.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
The Mindanao Current, which comes from the open ocean, carries low chlorophyll concentration waters. In contrast, the Indonesian seas, which receive large amounts of nutrients from runoffs and horizontal mixing, contain high chlorophyll concentration waters. The meeting of the two water masses generates a chlorophyll front, based on which the boundary of the WBCs can be identified. The climatologic monthly maps of the sea surface chlorophyll-a concentration (SSC) show a front at the Pacific entrance of the ITF produced by the MC retroflection (Fig. 6). Here, we choose the 0.1 mg m−3 chlorophyll-a concentration isoline to represent the boundary between the mesotrophic and oligotrophic waters as suggested by existing publications (e.g., Messié and Radenac 2006). Satellite observations show that the SSC front lies in TH Channel in boreal winter and spring. The 0.1 mg m−3 chlorophyll concentration contour is seen to touch the Talaud Islands outside the mooring coverage in the TH Channel in June–August, suggesting the northward movement of the MC retroflection as suggested by the altimeter geostrophic currents and the mooring observations.
Climatological monthly maps of sea surface chlorophyll-a concentration (SSC). The black solid lines are the 0.1 mg m−3 chlorophyll isoline, and the black dashed box in (a) marks the area within 2.3°–6°N, 125.5°–128°E.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
The seasonal variations of the retroflection index and the southmost latitude of the MC path are opposite in phase associated with the movement of MC retroflection (Fig. 7a). The seasonal variations of the low SSC area at the entrance of ITF (within the black dashed box in Fig. 6a) and the southmost latitude of the SSC front are also out of phase from each other (Fig. 7b), suggesting that the SSC front is produced by the MC retroflection. The correlation between the retroflection index and the area of the low SSC concentration with the black box is 0.65, above the 95% significance level.
(a) Comparison of the climatologic monthly retroflection index (black line) with the southmost latitude of the MC retroflection (red line). (b) Comparison of the climatologic monthly area of low-SSC front (black line) with the southmost latitude of the low-SSC front (red dashed line). The red solid line in (b) is the averaged latitude of the low SSC front within 2.3°–6°N, 125.5°–128°E.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
c. Connection with NECC
The NECC path at the entrance of the ITF is subject to large meandering. We estimate the seasonal variations of the NECC by using zonally averaged transports of the NECC in the longitude intervals of 140°–150°E, 160°–170°E, 180°–170°W, and 160°–150°W, respectively. The NECC transports are the smallest in boreal spring and the largest in boreal fall or winter, whereas the axis latitudes are the lowest in boreal spring or summer and the highest in boreal fall or winter (in Figs. 8a,c). The seasonal variations of the transports and axis positions lag to west, consistent with Rossby wave propagation.
(a) The climatologic monthly transports of NECC between 140° and 150°E, 160° and 170°E, 180° and 170°W, and 160° and 150°W and (b) their lag correlations with the retroflection index. (c) The climatologic monthly latitude positions of the NECC axis between 140° and 150°E, 160° and 170°E, 180° and 170°W, and 160° and 150°W and (d) their lag correlations with the retroflection index. The positive (negative) months in (b) and (d) represent the retroflection index leading (lagging) the transport of NECC and leading (lagging) the latitude positions of the NECC axis. The dashed red lines in (b) and (d) represent the 95% significance level.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
The MC retroflection is found to be highly correlated with the NECC seasonal variations. The maximum correlations between the retroflection index and the NECC seasonal transports are 0.88, 0.79, 0.80, and 0.84, respectively, above the 95% significance level, with the retroflection index lagging by 1–4 months as the longitudes of the four domains increase (Fig. 8b). The NECC axis position has similar lag correlations with the retroflection index lagging by zero to 3 months as the longitudes of the four domains increase (Fig. 8d), with the maximum lag correlation coefficients of 0.81, 0.88, 0.85, and 0.83, respectively, above the 95% significance level for the four intervals. The westward propagation speed of the NECC transports and axis latitudes are estimated as 0.81 m s−1, which are in agreements with the first baroclinic mode Rossby wave speed (Fig. 9). The Hovmöller plots of seasonal NECC transport changes (subtracting the eastward mean transport) and seasonal NECC axis latitudes suggest that the seasonal Rossby waves are generated in the central-eastern equatorial Pacific at around 150°W, which take around three months to arrive at the area of 140°–150°E. The lag correlations between the retroflection index and the NECC transports suggest that the seasonal westward Rossby waves continue to propagate into the Pacific entrance of the ITF and induce the variations of the MC retroflection.
Hovmöller plot of (a) the seasonal NECC transport changes (subtracting the eastward mean transport) and (b) the seasonal NECC axis latitudes.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
d. Interannual variability of the MC retroflection
The interannual monthly anomalies of the retroflection index are not only highly correlated with the area of low SSC concentrations from 2000 to 2019, but also show interannual variations associated with the Niño-3.4 index from 1993 to 2019 (Fig. 10). The lag correlation of the MC retroflection index and the area of low SSC concentration is 0.65, above the 95% significance level, suggesting that the retroflection index and SSC front move synchronously on interannual time scales at the Pacific entrance of the Indonesian seas. The highest lag correlation coefficient between the MC retroflection index and the Niño-3.4 index is 0.53, above the 95% significance level, with the Niño-3.4 index lagging by 2 months, suggesting the interannual movements of MC retroflection are influenced by ENSO.
Black lines: (a) interannual anomalies of the retroflection index, (b) the low-SSC area, and (c) the Niño-3.4 index. Red lines in (a) and (b) are 3-month running means. The blue and red shadings in (a) and (c) are La Niña and El Niño events, respectively.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
Analysis shows that ΔS is positive and negative maximum in the boreal winter and summer, respectively, suggesting that the MC takes a penetrating path in winter and a leap path in summer (Fig. 11a), consistent with the mooring observations in between the Talaud Islands and Halmahera (Li et al. 2021). The P1 shows significant interannual variations, but with the positive peaks not in direct associated with the El Niño events during 1993 through 2019 (Fig. 11b). This is expected, since the MC retroflection during El Niño should be in a similar intrusion path to that in the climatological winter. The highest lag correlation coefficient between P1 and the Niño-3.4 index is 0.57, above the 95% significance level, with the Niño-3.4 index lagging by 3 months. The association is more clearly illustrated in the time series of P2, which are used to compare the interannual anomalies to the seasonal changes of the retroflection index, showing that the interannual variations are stronger than the seasonal changes during La Niña events. P2 shows negative peaks smaller than −1 in the winters of 1998/99, 1999/2000, 2000/01, 2007/08, 2010/11, 2011/12, etc., and positive peaks greater than 1 in 1993, 1998, 2002, 2012, 2014, and 2015 (Fig. 11c). The negative peaks are typical during La Niña events. The time series of P2 suggest that the MC seasonal intrusion into the northern Maluku Sea has changed into a leaping path in the La Niña winter, whereas it is staying in a penetrating path during El Niño winters. The positive peaks of P2 during El Niño events are not seen, since the MC retroflection is in a similar intrusion path as in the climatological winter.
(a) Seasonal cycle of ΔS and time series of (b) P1, (c) P2, and (d) Niño-3.4 index from 1993 to 2019. The black dashed lines in (c) and (d) are ±1 and ±0.5, respectively. The blue and red shadings in (c) mark the periods when P2 is below −1 and above 1, respectively. The blue and red shadings in (d) are the periods when Niño-3.4 index is below −0.5 and above 0.5, respectively.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
Sometimes, in the summer after the El Niño events, large positive peaks of P2 occurred because the MC retroflection is in a straighter leaping path across the opening of the Sulawesi Sea than the climatological seasonal choke path does. Sometimes, during El Niño summers, the leaping path of the MC has changed into a penetrating path, potentially due to the reduced transport forced by circum-Philippine coastal Kelvin waves induced by the upwelling Rossby waves (Li et al. 2023, manuscript submitted to J. Phys. Oceanogr.), hence the positive values of P1. However, the strong nonlinear dynamics of the MC retroflection suggest that the interannual variations of the retroflection index are not in close association with El Niño events. It is the large changes from penetration to choke states during La Niña events that make the P2 have large negative values. Therefore, the interannual variations of the retroflection index are more sensitive to the La Niña events than to the El Niño events. The analysis suggests that there was a tendency for the MC to stay in the leaping path during La Niña whereas in the penetrating path during El Niño. It is the nonlinear hysteresis of the western boundary current at the wide gap that determines the response of the MC retroflection (Yuan et al. 2019; Li et al. 2021). When the changes of the equilibrium state should take place, the interannual variations would be much larger than the seasonal changes.
The P2 usually peaks in spring and fall, as ΔS is the minimum (Fig. 11a). The negative peaks smaller than −1 are basically typical during La Niña events, while the positive peaks greater than 1 in El Niño events are rare, suggesting that the interannual variations of the retroflection index are determined by the strong nonlinear dynamics discussed above. Immediate after the 1997/98 El Niño, P2 has a large negative value (Fig. 11c). An examination of the retroflection index suggests that this negative peak is generated due to an earlier retreat from the intrusion path in early 1998 than the climatological seasonal changes. This large negative peak of P2 was not present during other El Niño events. The investigation of the complicated nonlinear dynamics is beyond the scope of this study and is postponed to later researches.
During 1993–2019, the typical El Niño years are 1994/95, 1997/98, 2002/03, 2004/05, 2009/10, 2015/16, and 2018/19, and the La Niña years are 1995/96, 1998/99/2000, 2007/08, 2010/11, 2011/12 and 2017/18. The MC retroflection path suggests a deeper intrusion of the MC during the El Niño year into the northern Maluku Sea than that in the climatology, especially from June to August (Fig. 12). During the La Niña year, the MC retroflection tends to stay outside the Sulawesi and Maluku Seas, especially in November and December (Fig. 13).
Composite monthly maps of surface geostrophic currents from June of the El Niño year to May of the following year. The green line is the climatological MC retroflection, and the blue line is the MC retroflection in a composite El Niño event.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
Composite monthly maps of surface geostrophic currents from June of the La Niña year to May of the following year. The green line is the climatological MC retroflection, and the blue line is the MC retroflection in a composite La Niña event.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
The composite P1 is generally out of phase during El Niño and La Niña (Figs. 14a,b), with P1 positive during the composite El Niño year while negative during the composite La Niña year, due to the movement of MC retroflection (Figs. 12 and 13). The composite P2 shows opposite signs during the ENSO developing and mature phases (Figs. 14a,b). During the developing phase of ENSO (May–November), the seasonal changes of the retroflection index are negative, P2 has a negative or a positive peak in the El Niño or La Niña event, respectively. P1 has a higher value in La Niña than in El Niño, suggesting the asymmetric interannual variations. During the mature phase of ENSO (November–February), P2 turns into the opposite sign because of the change in sign of ΔS. During this period, ΔS reaches its positive peak. Nonetheless, P2 has a positive or a negative peak in the El Niño or La Niña winter, respectively, which has opposite variations with that in the developing phase of ENSO. During the decaying phase of ENSO (February–May of the following year), the composite P1 and P2 decrease gradually to zero in the El Niño, while still have relatively high amplitudes in the La Niña, suggesting the longer effect of the latter. The higher values and slower weakening of the composite P1 and P2 in La Niña events suggest the stronger and longer La Niña–induced variations than those induced by the El Niño.
P1 and P2 during a composite (a) El Niño year and (b) La Niña year and the following year. Year 0 and year +1 refer to the ENSO and its following years, respectively.
Citation: Journal of Physical Oceanography 54, 4; 10.1175/JPO-D-23-0125.1
4. Conclusions
In this paper, the seasonal and interannual variability of MC retroflection is studied using surface geostrophic currents of AVISO from January 1993 to December 2019. The geostrophic currents have been compared with the mooring observations in the TH Channel during December 2016–September 2018. The ACV of the moored ADCP measurements averaged in the upper 100 m are in good agreement with the surface geostrophic currents interpolated onto the three mooring sites, suggesting that the satellite altimeter geostrophic currents can be used to study the variability of upper-ocean circulation at the Pacific entrance of the ITF. The area of MC retroflection is calculated and is defined as the retroflection index. The correlation coefficients between the retroflection index and the ACV at TH2 suggest that the retroflection index can be used to represent the variation of MC retroflection.
The MC retroflection has a clear seasonal cycle: the MC intrudes into the northern Maluku Sea during late fall through early spring, moves gradually northward in boreal spring, and essentially retroflects eastward north of the Talaud Islands in boreal summer without intruding into the Maluku Sea. The MC then intrudes into the Maluku Sea again in fall to repeat the annual cycle. The variation of MC retroflection is explained with the nonlinear theories of Yuan and Wang (2011), Wang and Yuan (2012, 2014), and Yuan et al. (2019). The MC retroflection produces an SSC front that lies in between Halmahera and the Talaud Islands in boreal winter and spring.
The MC retroflection variations is found to be highly correlated with the NECC seasonal variations. The westward propagation speed of the NECC transport changes and its axis latitudes are estimated at 0.81 m s−1, which is in agreements with the first baroclinic mode of the Rossby waves. The Hovmöller plots of the NECC transport changes and its axis latitude variations suggest that the seasonal Rossby waves are generated in the central-eastern equatorial Pacific at around 150°W, which take around three months to arrive at the area of 140°–150°E. The lag correlations between the retroflection index and the NECC transports suggest that the seasonal westward Rossby wave continues to propagate into the entrance of the ITF and induce the variations of MC retroflection.
On interannual time scales, the MC retroflection and SSC front are found to move synchronously, with the Niño-3.4 index lagging by 2 months, above the 95% significance level. The P1 and P2, as the ratios of the interannual anomalies to the seasonal retroflection index and its changes, respectively, are used to study the effect of interannual variability of MC retroflection. The interannual anomalies of the retroflection index tend to be positive and the MC retroflection intrudes into the northern Maluku Sea deep during the El Niño year, while the opposite in the La Niña year. The comparison of composite P1 in different ENSO events suggests that the interannual variations of the retroflection index are more sensitive to La Niña events than to El Niño events. During La Niña winters, the equilibrium states of the MC retroflection change from the penetration to choke states, causing stronger and longer La Niña–induced variations than those induced by the El Niño. It is the nonlinear hysteresis of the western boundary current at the wide gap that determines the response of the MC retroflection (Yuan et al. 2019; Li et al. 2021). When the changes of the equilibrium state should take place, the interannual variations would be much larger than the seasonal changes. The MC seasonal intrusion into the Northern Maluku Sea has changed into a leaping path in La Niña winters, whereas it is staying in an anomalously deeper penetrating path during El Niño winters than in the climatological seasonal penetrating path. During El Niño summers, the seasonal leaping path of the MC has changed into an anomalous penetrating path sometimes, because of the anomalous intrusion superposed onto the climatological path.
Acknowledgments.
This study is supported by the National Key R&D Program of China (2020YFA0608800), and the Natural Science Foundation of China (NSFC) (92258301, 91858204, 41720104008). This work is also supported by the Oceanographic Data Center, IOCAS. D. Yuan is supported by the “Taishan Scholar Project” of the Shandong province and by the “Kunpeng Outstanding Scholar Program” of the FIO/MNR of China. Affiliation 1 and Affiliation 2 share the first position.
Data availability statement.
The mooring data used in this study are available at the web page http://itf.adio.ac.cn/xzlxz. The Argo data used to calculate the AGCs are available at http://www.argo.ucsd.edu. The surface geostrophic currents were obtained by AVISO (https://www.AVISO.altimetry.fr/en/data/data-access.html). The gridded sea surface chlorophyll-a concentration data used in this study are obtained from the Copernicus Marine Service (https://resources.marine.copernicus.eu/).
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