Chinese Journal of Oceanology and Limnology   2016, Vol. 34 issue(6): 1332-1346     PDF       
http://dx.doi.org/10.1007/s00343-016-5119-9
Institute of Oceanology, Chinese Academy of Sciences
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Article Information

ZHAO Jun(赵君), LI Yuanlong(李元龙), WANG Fan(王凡)
Seasonal variation of the surface North Equatorial Countercurrent (NECC) in the western Pacific Ocean
Chinese Journal of Oceanology and Limnology, 34(6): 1332-1346
http://dx.doi.org/10.1007/s00343-016-5119-9

Article History

Received Apr. 12, 2015
accepted for publication Jun. 24, 2015
accepted in principle Aug. 18, 2015
Seasonal variation of the surface North Equatorial Countercurrent (NECC) in the western Pacific Ocean
ZHAO Jun(赵君)1, LI Yuanlong(李元龙)1,2, WANG Fan(王凡)1,3        
1 Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;
2 Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado, USA;
3 Function Laboratory for Ocean Dynamics and Climate, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266000, China
ABSTRACT: The North Equatorial Countercurrent (NECC) is an important zonal flow in the upper circulation of the tropical Pacific Ocean, which plays a vital role in the heat budget of the western Pacific warm pool. Using satellite-derived data of ocean surface currents and sea surface heights (SSHs) from 1992 to 2011, the seasonal variation of the surface NECC in the western tropical Pacific Ocean was investigated. It was found that the intensity (INT) and axis position (YCM) of the surface NECC exhibit strikingly different seasonal fluctuations in the upstream (128°-136°E) and downstream (145°-160°E) regions. Of the two regions, the seasonal cycle of the upstream NECC shows the greater interannual variability. Its INT and YCM are greatly influenced by variations of the Mindanao Eddy, Mindanao Dome (MD), and equatorial Rossby waves to its south. Both INT and YCM also show semiannual signals induced by the combined effects of equatorial Rossby waves from the Central Pacific and local wind forcing in the western Pacific Ocean. In the downstream region, the variability of the NECC is affected by SSH anomalies in the MD and the central equatorial Pacific Ocean. Those in the MD region are especially important in modulating the YCM of the downstream NECC. In addition to the SSH-related geostrophic flow, zonal Ekman flow driven by meridional wind stress also plays a role, having considerable impact on INT variability of the surface NECC. The contrasting features of the variability of the NECC in the upstream and downstream regions reflect the high complexity of regional ocean dynamics.
Key words: North Equatorial Countercurrent (NECC)     seasonal variation     western Pacific Ocean     wind stress    
1 INTRODUCTION

The North Equatorial Countercurrent (NECC) is a major upper-ocean zonal flow in the tropical Pacific Ocean, which flows eastward across the Pacific Ocean basin between 2° and 10°N. It is also the boundary between the tropical gyres of the North and South Pacific (Wyrtki and Kendall, 1967 ; Philander et al., 1987 ; Donguy and Meyers, 1996). Estimations of the mean NECC transport in the western Pacific based on historical in situ observations range widely from 0-90 Sv (1 Sv=106 m3 /s)(Wyrtki and Kendall, 1967 ; Delcroix et al., 1987, 1992 ; Qiu and Joyce, 1992 ; Donguy and Meyers, 1996 ; Johnson et al., 2002). On average, the NECC transports 10-30 Sv of surface warm water annually from the West Pacific warm pool to the relatively cold eastern Pacific region (Wyrtki and Kendall, 1967 ; Philander et al., 1987 ; Gouriou and Toole, 1993 ; Donguy and Meyers, 1996 ; Johnson et al., 2002). The NECC is centered at ~5°N in the western Pacific and as it proceeds eastward, it moves poleward to ~7°N in the Central Pacific (Donguy and Meyers, 1996). Its main body is shallower in the west and deeper in the east (Johnson et al., 2002 ; Sprintall et al., 2009). The warm water carried by the NECC has been suggested to be one of the factors that cause the equatorial asymmetry of the Intertropical Convergence Zone (Richards et al., 2009 ; Masunaga and L'Ecuyer, 2011). The NECC transport is essential to the modulation of tropical Pacific sea-level changes (Wyrtki, 1979 ; Qiu and Chen, 2012) and to the distribution of nutrient concentrations in the western Pacific (Christian et al., 2004).

The upper-ocean circulation pattern near the western boundary of the tropical Pacific Ocean is rather complex (Fig. 1), because it includes energetic zonal equatorial currents, western boundary currents, and quasi-permanent recirculation structures (Qu et al., 1999). For comprehensive reviews of these currents' structures, variabilities, and roles in the climatic system, see Lukas et al.(1996) and Hu et al.(2015). In the Northern Hemisphere, the Mindanao Current (MC), which is the southward component of the North Equatorial Current (NEC) bifurcation at 13°-14°N (Toole et al., 1990 ; Qiu and Lukas, 1996), flows southward along the east coast of Mindanao Island. In the Southern Hemisphere, the New Guinea Coastal current flows northwestward from New Guinea to Halmahera Island (Lindstrom et al., 1987 ; Kuroda, 2000). These two equatorward western boundary currents meet at the entrance of the Indonesian seas. Some of their water spreads westward into the Celebes Sea, feeding the Indonesian throughflow (Fine et al., 1994 ; Kashino et al., 1996); however, the main body of the converged water turns eastward, forming the narrow energetic NECC at ~5°N. The turning of these strong currents also generates several quasi-permanent recirculation structures in this region. A large cyclonic recirculation gyre is formed between the flows of the NEC, MC, and NECC, which is centered at 6°-9°N, 130°-140°E (Fig. 1). This recirculation is named the Mindanao Dome (MD), reflecting its shallow thermocline and low sea surface height (SSH)(Masumoto and Yamagata, 1991 ; Suzuki et al., 2005 ; Zhao et al., 2013a). In the area of origin of the NECC, there are many strong mesoscale eddies related to the dynamic instabilities of the NECC (Heron et al., 2006 ; Zhao et al., 2013b). Of these, the cyclonic Mindanao Eddy (ME) to the north of the NECC and the anticyclonic Halmahera Eddy (HE) to the south of the NECC are recognized as quasi-permanent features of the local circulation (Kashino et al., 1999, 2013 ; Arruda and Nof, 2003). However, the MD and ME are somewhat confusing because they represent completely different oceanic phenomena. The MD is a large-scale manifestation of the westward intensification of the North Pacific tropical gyre, which is formed by basinscale wind forcing. Conversely, the ME is a quasipermanent mesoscale eddy formed by the eastward retroflection of the MC, which arises largely from the dynamical instability of the regional circulation.

Figure 1 Schematic of the upper-ocean circulation in the western tropical Pacific Ocean Major currents and quasi-permanent recirculations marked include the NEC, NECC, SEC, Kuroshio, MC, NGCC, Mindanao Dome, ME, and HE.

The circulation of the western tropical Pacific exhibits prominent temporal variations at various timescales. Controlled by the East Asian monsoon system, seasonal variations of the major currents are especially strong. The principal features observed include the strengthening of the NEC in boreal winter (Yaremchuk and Qu, 2004), seasonal reversal of the NGCC northeast of New Guinea (Lindstrom et al., 1987 ; Kuroda, 2000), enhancement of the MD in boreal winter (Masumoto and Yamagata, 1991), and enhancement/weakening and positional shifting of the HE (Heron et al., 2006 ; Kashino et al., 2013). Unless otherwise stated, the seasons in this paper denote those in the Northern Hemisphere (boreal). Regarding the NECC, its observed transport is smaller in boreal winter-spring and larger in summer-fall (Wyrtki and Kendall, 1967 ; Delcroix et al., 1987 ; Gouriou and Toole, 1993 ; Reverdin et al., 1994 ; McPhaden, 1996). Two controlling factors have been identified by previous studies. The first is the annual meridional shift of the Intertropical Convergence Zone, which reaches its northernmost latitude during summer-fall and involves a northward shift of positive Ekman pumping velocity (Philander et al., 1987 ; Donguy and Meyers, 1996 ; Qiu and Lukas, 1996 ; Johnson et al., 2002). The second is the westwardpropagating annual Rossby waves from the central and eastern tropical Pacific (Qiu and Lukas, 1996 ; Yu and McPhaden, 1999 ; Ando and Hasegawa, 2009). Recent studies based on satellite-derived current estimates have shown that the NECC axis in the western (eastern) Pacific shifts northward during the first (second) half of the year (Hsin and Qiu, 2012).

The above-mentioned studies focused mainly on the basin-scale features of the NECC variability. However, the NECC variability in its area of origin, i.e., the far western Pacific Ocean, has not been investigated comprehensively. Because of the strong local monsoonal wind forcing and accumulation of Rossby wave signals near the western boundary, the current variability in the far western Pacific is much stronger and more complex than in the Pacific interior (Masumoto and Yamagata, 1991 ; Qiu and Lukas, 1996 ; Li et al., 2012 ; Zhao et al., 2013b). Previous studies have reported strong seasonal variations based on repeated in situ measurements (Wyrtki and Kendall, 1967 ; Wyrtki, 1975 ; Delcroix et al., 1987 ; Toole et al., 1990). However, these observations have been confined mainly to several sections and are far from adequate for clarifying the seasonal dynamics of the circulation of the western tropical Pacific Ocean. In particular, the dynamical relations between the surface flows and local recirculations/eddies remain unclear. Such interactions might have considerable influence on the intensity and structure of the NECC (Heron et al., 2006 ; Zhao et al., 2013b). As the MD, ME, and HE exhibit strong and unique signals of seasonal variability, they could induce large fluctuations in the NECC that might be distinctively different from those induced by large-scale wind forcing.

The recent availability of high-resolution, highaccuracy satellite-derived observational data of SSH and sea surface winds, and the development of related products for the estimation of ocean currents, enable the investigation of the dynamics of regional ocean circulations. By analyzing these data, recent research has revealed many new characteristics of circulation variability in the western tropical Pacific (Heron et al., 2006 ; Shinoda et al., 2011 ; Hsin and Qiu, 2012 ; Qiu and Chen, 2012 ; Zhao et al., 2012, 2013b ; Li et al., 2014), which have improved our understanding of regional ocean dynamics. The current study aimed to provide a comprehensive description of the seasonal variation of the NECC in the western tropical Pacific based on satellite observational data. The remainder of this paper is organized as follows. Data and methods are introduced in Section 2. The primary features of the mean structure and seasonal variation of the NECC are described in Section 3. Unique features of the NECC variability in the upstream and downstream regions are discussed in Sections 4 and 5, respectively. A summary and discussion of the main findings are presented in Section 6.

2 DATA AND METHOD

Satellite altimeter SSH data from the multisatellite merged Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) data (http://www.aviso.oceanobs.com/) project from October 1992 through July 2011 were used in this study. The SSH product merges SSH measurements from the Ocean Topography Experiment (TOPEX)/Poseidon, European Remote Sensing Satellite-1(ERS-1) and ERS-2, Geosat Follow-On, and Jason-1 and Jason-2 along-track satellite altimeters (Le Traon et al., 1998 ; Ducet et al., 2000). The version of the SSH product used in this study has Mercator spatial resolution and 7-day temporal resolution. To correct for the aliasing of tides and barotropic variability, the altimeter data were updated with a tidal model (GOT2000) and barotropic hydrodynamic model (MOG2D-G)(Dibarboure et al., 2008). The SSH value is the sum of the sea-level anomaly and mean dynamic topography based on GRACE data, altimetry measurements, and in situ observations (Rio et al., 2011). The surface geostrophic velocities, UG and VG, were estimated using horizontal gradients of SSH:

    (1)

where g is the gravitational acceleration and f is the Coriolis parameter.

The ocean surface current data used the multisatellite OSCAR product (Bonjean and Lagerloef, 2002 ; Johnson et al., 2007), which is available from the Physical Oceanography Distributed Active Archive Center at the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration (NASA) through the website http://podaac.jpl.nasa.gov/. Based on the combination of the altimeter-based SSH anomaly, sea surface scatterometer wind, sea surface temperature (SST), and mean dynamic topography, the near-surface flows were estimated based on the geostrophic, Ekman, and Stommel shear dynamics. OSCAR U and V parameters both contain geostrophic and ageostrophic velocity components averaged over the upper 30 m of the ocean (Bonjean and Lagerloef, 2002). Comparison of the OSCAR velocities and in situ observations from the Tropical Atmosphere Ocean/Triangle Trans-Ocean Buoy Network (TAO/TRITON) mooring arrays (McPhaden, 1995) suggested good consistency in zonal velocity U but largely degraded quality in OSCAR V (Johnson et al., 2007 ; Shinoda et al., 2011 ; Hsin and Qiu, 2012 ; Zhao et al., 2013b). Therefore, for further study of the zonal currents, we only used the OSCAR U component here. The version of the OSCAR product used in this study has spatial resolution, 5-day temporal interval, and a 19-year time span ranging from October 1992 to July 2011. In the western tropical Pacific Ocean, OSCAR Ugenerally performs well in comparison with the in situ measurements of the TAO/ TRITON buoys (see Fig. 2 of Zhao et al.(2013b)). The correlation for the annual cycle was 0.65-0.85. There were some large discrepancies at buoy sites near the equator, but these had little impact on our estimation of the properties of the NECC because the NECC exists mainly to the north of 2°N and the current axis is at ~5°N (Zhao et al., 2013b).

Figure 2 Annual-mean maps of (a) SSH (cm) based on AVISO altimeter data from 1992-2011, (c) zonal surface current U (m/s) from OSCAR data from 1992-2011, and (e) wind stress (black vectors; N/m2) and WSC (shading; 10-8 N/m3) from the ECMWF ORA-S3 data from 1992-2009 (b), (d), and (f) are the corresponding seasonal standard deviation (STD) of SSH, U, and WSC calculated with monthly climatological data.

We also used the 1°×1° monthly wind stress data of the European Center for Medium-Range Weather Forecasts (ECMWF) Ocean Reanalysis System 3(ORA-S3)(Balmaseda et al., 2008) between January 1992 and December 2009, which are available from the ECMWF website http://data-portal.ecmwf.int/. Ekman pumping velocity (EPV) w E induced by wind forcing, which is the primary cause of low-frequency tropical ocean current variability, was calculated as w E = curl (τ /f- 1, where τ is the wind stress and ρ =1 021 kg/m3 is the mean density of the Ekman layer.

The NECC has large variations in both strength and meridional position. Following Hsin and Qiu (2012), the axis position YCM of the NECC jet was calculated using the following equation:

    (2)

where YN and YS are the northern and southern limits of integration, and u is the zonal velocity (which can be the near-surface velocity or altimeter-based geostrophic velocity), which was set to zero for negative u, because NECC is considered an eastward flow. Here, we chose YN =8°N and YS =2°N based on the distribution of surface U (Hsin and Qiu, 2012 ; Zhao et al., 2013b). The intensity (INT) of the surface NECC was calculated using the following equation (Hsin and Qiu, 2012):

    (3)

where YCM was derived from Eq.2 and U was also set to zero if negative. The integral width of ± 4° was adopted because the NECC is a narrow jet between 2° and 10°N. In addition, because the geostrophic current is calculable only for the area north of 2°N, the southern limit for the INT integration was set to the north of YCM -4° and 2°N. Similarly, the intensity of the geostrophic current INTG and axis of the geostrophic NECC YG could be calculated using Eqs.3 and 2 with AVISO-based UG data.

3 GENERAL FEATURES

Visible in the annual-mean SSH map of the western Pacific Ocean (Fig. 2a), sandwiched between high- SSH areas and a low-SSH area (<100 cm) near the Mindanao coast between 5° and 10°N, is a manifestation of the MD. The large meridional SSH gradient at ~5°N indicates the strong geostrophic flow of the NECC. The seasonal standard deviation (STD) of SSH, calculated using the monthly climatological SSH field, quantifies the distribution of seasonal variability amplitude (Fig. 2b). In the basin interior, there are two zonal high-STD bands at 5°N and 12°N, reflecting the waveguides of the 1 st and 2 nd meridional mode (l =1 and l =2) Rossby waves. Along the 5°N waveguide (1 st meridional mode), the SSH signals weaken with westward propagation but rebound in the MD region, probably because of local wind forcing (Matsumoto and Yamagata, 1991). The SSH STD reaches a local maximum at 6°N, 129°E, reflecting the strong variability of the ME. The pronounced SSH variations of the MD and ME can lead to large fluctuations of the NECC through the geostrophic relationship. Based on OSCAR U data, the annual-mean NECC has its maximum velocity at its origin (5°N, 130°E), and it moves northward slightly as it proceeds eastward into the Pacific basin interior (Fig. 2c). Meander-type structures can be seen in the main body of the NECC jet. Seasonal variability of U is relatively weak along 5°N but strong at 6°- 9°N and at 0°-4°N (Fig. 2d), implying large fluctuations in the meridional position of the NECC.

In the tropical oceans, large-scale SSH variations are driven mainly by wind forcing (Meyers, 1979 ; Kessler, 1990). The study region shows a meridional transition from the southeast trade winds to the northeast trade winds. Mean wind stress curl (WSC) from the ECMWF ORA-S3 data is generally positive and it has a maximum in the latitudinal band of the MD (Fig. 2e). The STD distribution of WSC (Fig. 2f) somewhat mimics that of SSH, suggesting the dominance of wind forcing in driving the seasonal variations of SSH and geostrophic current.

The climatological mean NECC based on OSCAR U has a meridional width of ~8° near the western boundary and it narrows slightly east of 160°E (Fig. 3). The beginning of the NECC is characterized by an eastward velocity of >0.5 m/s at ~5.5°N. The NECC jet is accompanied by the cyclonic ME and anticyclonic HE, which appear as a low-SSH center (85 cm; 7°N, 128°E) to the northwest and a high-SSH center (105 cm; 3°N, 134°E) to the southeast. These two quasi-permanent eddies characterize the retroflection of the North Pacific water from the MC and South Pacific water from the NGCC into the NECC (Kashino et al., 1996 ; Qu et al., 1999 ; Li and Wang, 2012). The jet body shows strong mesoscale meanders that fluctuate between 4° and 6°N, although the jet axis YCM is generally tilted slightly northward as it flows eastward into the Central Pacific. The eastward U drops rapidly to 0.2-0.3 m/s to the east of 140°E. YCM exhibits pronounced seasonal variability, as quantified by the range of the seasonal STD in Fig. 3. The seasonal variability of YCM is weak at its origin but is enhanced gradually as it flows eastward. The STD value is ~0.3° near the western boundary and it grows up to ~1° east of 140°E. The large difference of YCM variability between the upstream and downstream areas is worthy of in-depth investigation.

Figure 3 Mean AVISO SSH (black contours; cm) and OSCAR U (color shading; m/s) from October 1992 through July 2011 Red dots denote the mean North Equatorial Countercurrent (NECC) axis position (YCM), and the black bars denote the seasonal standard deviation (STD) range of YCM.

To examine the seasonal relationship between the NECC and SSH in the surrounding areas, the monthly maps of SSH anomaly and YCM are displayed in Fig. 4. Consistent with Fig. 3, the monthly variability of YCM is larger in the downstream region east of 140°E. During January-March, the NECC jet tilts northward from the upstream to the downstream regions, and the tilting is especially evident between 135° and 145°E (as shown by the shading in Fig. 4). During this season, the NECC jet is accompanied by positive- SSH anomalies to the south (3°-5°N) and negative SSH anomalies to the north (the MD region). In contrast, the NECC jet during late spring and summer (May-September) tilts southward into the downstream region. In spring, as the MD begins to weaken and the SSH gradually elevates in its latitudinal band (6°- 9°N), the NECC jet shifts gradually southward. By summer, the entire NECC jet body has moved to ~4°N and it is accompanied by negative SSH anomalies between 3° and 5°N. Compared with the downstream region, the change of YCM in the upstream region is much weaker. One notable phenomenon is that the beginning of the NECC is always situated at the position of the large SSH gradient between the low-SSH ME and the high-SSH HE. As the positions and strengths of these two eddies undergo seasonal change, the YCM also exhibits some seasonal fluctuation. These direct comparisons suggest tight association between the NECC and local SSH variations.

Figure 4 Monthly climatologic SSH anomaly (color shading; cm) and total SSH (black contours; cm) Pink curve denotes the NECC axis YCM. Area between 135° and 145°E is highlighted with gray shading.

For better visualization of the seasonal differences in the NECC structure, the U anomaly maps for January and July are presented in Fig. 5. It can be seen that in January, the eastward U anomalies are concentrated along a tilted band that extends from 3°-4°N at the NECC origin to 5°-6°N at 160°E (downstream). The NECC axis generally follows the path of this eastward U anomaly belt. However, it is also sandwiched between westward U anomalies that give rise to the cyclonic anomalous gyre north of the NECC and the anticyclonic gyre to the south. The distribution of the U anomaly along the western boundary also indicates a strengthening of the ME. Under such conditions, the YCM is shifted southward in the upstream region and northward in the downstream region. In July, there is an anticyclonic anomalous recirculation near the western boundary between 2° and 8°N and a large-scale cyclonic gyre east of 135°E (corresponding to the negative SSH anomalies in Fig. 4). Such an anomalous circulation pattern shifts the NECC axis to the north in the upstream region and to the south in the downstream region. Given the contrasting upstream/downstream features of variability of the NECC and the local circulation, the following analysis is conducted separately for the two regions to achieve a more indepth understanding.

Figure 5 Mean OSCAR U anomaly (m/s) and North Equatorial Countercurrent axis position (YCM)(dots) in January (top) and July (bottom)

The monthly properties of the NECC for each year between 1993 and 2010 are plotted in Fig. 6. Both INT and YCM are averaged in the upstream (128°-136°E) and downstream (145°-160°E) regions. It is clear that the upstream YCM seasonal variability is much weaker than downstream. YCM is higher (at ~5.8°N) during March-May and lower (at ~4.6°N) during August- January. In the downstream region, a prominent seasonal cycle can be seen, with the northernmost YCM at 6°-7°N in February and the southernmost YCM at 3°-4°N in July. It is interesting to note that the YCM seasonal cycles show smaller interannual differences in the downstream region, with an approximately similar shape each year. The INT seasonal cycle of the NECC is radically different from YCM, irrespective of region. In the upstream region, INT shows a steady decrease from January to December throughout the year, although in some years its reaches a maximum in June and July. The interannual variability of the yearly mean INT is much stronger than the seasonal variation. For example, INT stays at ~0.8×105 m2 /s in 1998 but is as large as 2.3×105 m2 /s in 2004. The interannual variability of the NECC and its relationship with El Niño events have been discussed in Zhao et al.(2013b). In the downstream region, INT is smaller in the first half of the year and larger in the second half. Similar to YCM, the seasonal cycle of INT is visibly more robust and significant in the downstream region compared with the interannual fluctuations. This is likely related to differences in the controlling mechanism. In the downstream area, the NECC variation is primarily caused by deterministic large-scale wind forcing, while in the upstream region, local dynamical instabilities near the western boundary, such as those associated with mesoscale eddies and eddy-mean flow interactions, lead to a less predictable seasonal cycle of INT and YCM.

Figure 6 Monthly seasonal cycle of NECC axis position (YCM) averaged in (a) upstream (128°-136°E) and (b) downstream (145°-160°E) areas for each year from 1993-2010 (c) and (d) are the same as (a) and (b) but for NECC intensity (INT). Both YCM and INT are calculated based on OSCAR U data.
4 UPSTREAM REGION (128°-136°E)

This section focuses on the seasonal variability of the upstream NECC. To quantify the contributions of the different processes, we use semimonthly OSCAR U and AVISO Ug data to calculate the INT and INTG of the NECC, respectively (Fig. 7a). The total INT has a mean value of ~1.4×105 m2 /s. Its peak-to-peak amplitude is slightly smaller than INTG. Furthermore, there are visible semiannual signals in INT, which are absent in INTG. Annually, INT achieves two maxima in January and July and two minima in late March and September. In contrast, INTG shows mainly annual timescale signals, i.e., it is larger in winter-spring and smaller in fall. Both INT and INTG show minima in September. This means that the September minimum in INT is mainly induced by SSH-related geostrophic flow, while the other fluctuations contain a larger contribution from the wind-driven Ekman flow.

Figure 7 Semimonthly seasonal cycles of NECC (a) intensity (INT), (b) axis position (YCM), and (c) sea surface height (SSH) difference (δh) across the axis of the NECC averaged in the upstream region (128°-136°E) Open (black) dots in (a) and (b) denote the variables calculated using OSCAR U (AVISO UG) data.

Different from the cases of INT and INTG, YCM is very consistent with YG (Fig. 7b). Both have a peakto- peak amplitude of 1.2° and some degree of semiannual signal. The first and second northernmost (southernmost) positions occur in April-May (August-September) and October-November (December-January), respectively. The SSH difference across the NECC jet (δh) is a critical factor in determining the strength of the NECC's geostrophic flow (Fig. 7c). Here, δh is evaluated at each longitude by first averaging the SSH values over a 1.5° band centered at 1° north and south of YCM (hN and hS) and then by taking the difference δh = hS - hN. Consistent with INTG, δh is strong from January to April and shows a decreasing trend from August to October. There are also no semiannual signals in δh, which further confirms that the INT's semiannual signals are not derived from the geostrophic component but are from the combined effects of the geostrophic and Ekman flows.

The above analysis has indicated the significant impact of SSH variations on NECC variability. To better resolve this relationship, we calculated the linear correlation between the seasonal cycle of SSH and that of the upstream-mean INTG (Fig. 8a). The use of INTG instead of the total INT is based on the consideration that SSH variation is linked dynamically with geostrophic flow but not with the Ekman component. A region of negative correlation with r < -0.80 covers the western part of the MD and the ME area; the correlation is close to -1.00 at the center. The positive (negative) SSH anomaly in this area leads to smaller (larger) values of INTG upstream. However, positive correlations (r >+0.80) can be seen southeast of the NECC between 3° and 5°N. This highcorrelation band corresponds approximately to the waveguide of the equatorial Rossby waves (also called 1 st meridional mode Rossby waves). Positive (negative) SSH anomalies at this latitudinal band will strengthen (weaken) the eastward flow of the NECC. Based on these relationships, we selected two regions of high correlation, i.e., region N (7°-9°N, 128°- 133°E) and region S (3°-5°N, 140°-145°E). The MD is a large-scale recirculation gyre that covers an area west of 150°E, and the ME is considered located at the retroflection points of the MC and mainly to the west of 132°E (Heron et al., 2006 ; Kashino et al., 2013). Thus, Region N approximately covers the western portion of the MD and the entire ME. The prevailing negative correlations in this area indicate that at the seasonal timescale, the SSH variabilities of the ME and MD are in phase. This relationship is to be expected because a stronger MD, with a background current that is more energetic, is favorable for the development of the ME. Therefore, the seasonal SSH variations of the HE, ME, and equatorial Rossby waveguide are all important in causing the upstream INT changes. In Fig. 8b, it is clear that the upstream YCM is generally in phase with the SSH of Region S and out of phase with Region N. The correlation coefficient is 0.81 between the SSH and YCM of Region S and -0.42 between the SSH and YCM of Region N. Therefore, it can be inferred that the variability in Region S plays a greater role in shifting YCM, especially the semiannual signals, which are likely mainly induced by the SSH signals of Region S.

Figure 8 Linear correlation between the seasonal cycles of SSH and NECC intensity (INTG) averaged between 128° and 136°E (a); SSH seasonal cycle averaged in region N (blue) and region S (red)(b); green bars denote the upstream NECC axis position (YCM)

It is necessary to explore further the mechanisms that control SSH variability at the latitudes of Regions N and S. Hence, we plotted the semimonthly longitude map of SSH anomaly along 4° and 8°N (Fig. 9). At 4°N, SSH exhibits both annual and semiannual signals (Fig. 9a). Swift westward propagation of the wave signals is clearly discernible. An estimation based on Fig. 9 suggests that the propagation speed of these signals is ~0.32 m/s at 4°N and ~0.21 m/s at 8°N, which is close to the 1 st baroclinic mode Rossby waves at these latitudes. In addition to the Rossby wave signals from the Central Pacific, local wind forcing is also important. For example, the negative SSH anomaly west of 140°E during December- March is caused by a local positive EPV anomaly (Fig. 9c). The arrival of a strong positive-SSH Rossby wave during March-July gives rise to the positive peak of SSH in Region S. The second positive SSH peak in September-November is likely induced by a negative local EPV anomaly. These results suggest that SSH variability in Region S is caused by both equatorial Rossby waves from the Central Pacific Ocean and local wind forcing, which also give rise to the semiannual fluctuations in the upstream NECC.

The propagation of SSH signals at 8°N (Fig. 9b) is slower than at 4°N because of the higher latitude. Again, SSH variations in the MD are likely contributed by both remotely forced Rossby wave signals and local wind forcing, which is consistent with the wellestablished knowledge of many previous studies (Masumoto and Yamagata, 1991 ; Suzuki et al., 2005 ; Qu et al., 2008 ; Zhao et al., 2012). Rossby wave signals are enhanced by local wind forcing when arriving at the western boundary, which leads to strengthened SSH variability in the MD region.

Figure 9 Time-longitude plots of SSH anomaly (cm) at (a)4°N and (b)8°N;(c) and (d) are the same as (a) and (b) but for Ekman pumping velocity (EPV) anomaly (10-6 m/s)

In fact, the analysis of SSH and geostrophic flow is not sufficient to explain the upstream INT completely. There are evident differences between INT and INTG during February-April and July-September (Fig. 7). Zonal Ekman flow should also play a role in INT variability. Here, we calculated the zonal Ekman transport through T E = τ y /(ρf), where τ y is the meridional wind stress and ρ = 1 021 kg/m3 is the mean density of the Ekman layer. As shown in Fig. 10, the southward (northward) anomalous τ y induces westward (eastward) TE, which weakens (intensifies) the NECC from January (June) to April (October) in the upstream region. This means that the Ekman flow variation induced by meridional wind changes (monsoonal winds) also contributes to the seasonal variability of the upstream NECC.

Figure 10 Zonal Ekman transport (dashed) and meridional wind stress (solid) in the upstream region (2°-8°N, 128°-136°E; red) and downstream region (2°- 8°N, 145°-160°E; blue) of the North Equatorial Countercurrent
5 DOWNSTREAM REGION (145°-160°E)

Seasonal variability of the downstream NECC INT is greater than in the upstream region. INT is ~0.4×105 m2 /s and it grows to ~1.3×105 m2 /s in August and November (Fig. 11a). The large difference between INT and INTG indicates that the role of Ekman flow must be considered in explaining the total seasonal variation of INT. The YCM seasonal amplitude is also larger in the downstream area with peak-to-peak amplitude of ~2.5°(Fig. 11b). The northernmost and southernmost positions are at 6°N in February-March and 3.5°N in July-August, respectively. Semiannual signals, which can be seen in the upstream region, are hardly detectable here. The SSH difference δh across the NECC jet (Fig. 11c) is again in phase with INTG.

Figure 11 Semimonthly seasonal cycle of NECC (a) intensity (INT), (b) axis position (YCM), and (c) SSH difference (δh) across the axis of the NECC in the downstream region (145°-160°E) Variables are presented with semimonthly resolution. Open (black) dots in (a) and (b) denote the variables calculated with OSCAR U (AVISO Ug).

We also calculated the linear correlation between INTG and SSH (Fig. 12a). Again, the correlation shows negative values north of the NECC jet (partly covering the MD region) and positive values near the equator. The negative (positive) SSH anomalies in the MD region induce cyclonic (anticyclonic) anomalous circulation, which strengthens (weakens) the eastward flow of the NECC in winter (summer), as shown, in the downstream seasonal cycle of INT. Similarly, positive (negative) SSH anomalies near the equator enhance (reduce) the NECC intensity through the geostrophic relationship.

We again selected two regions to examine the relationship between SSH and the downstream YCM. Note that the areas were chosen based on the distribution of correlation and that they differ from those in section 4. Region N has larger seasonal amplitude than Region S (Fig. 12b) and higher correlation with downstream YCM. The correlation coefficient between SSH and YCM of Region N is -0.43, while that between SSH and YCM of Region S is <0.10. This means that the MD plays a role in modulating the YCM but that the central equatorial Pacific variability has little impact. The downstream NECC is likely more affected by the MD.

Figure 12 Linear correlation between the seasonal cycles of SSH and NECC intensity (INTG) averaged between 145° and 160°E (a); SSH seasonal cycle averaged in Region N (blue) and Region S (red)(b); green bars denote the downstream axis position (YCM) of the NECC

The downstream NECC has a larger difference between INT and INTG than the upstream. INT is obviously stronger (weaker) than INTG during July- September (January-March). In fact, as shown in Fig. 10, the zonal Ekman transport is large and westward in January-March and eastward in July- September, caused by the anomalous southward and northward τ y, respectively. The downstream seasonal variations of the NECC are generally governed by the seasonal changes of the MD and the Ekman flow induced by the meridional wind stress. Regarding the MD seasonality, previous findings have suggested it is controlled both by local wind forcing of the East Asian monsoon and by Rossby waves originating from the Central Pacific (Masumoto and Yamagata, 1991 ; Suzuki et al., 2005 ; Qu et al., 2008 ; Zhao et al., 2012).

6 SUMMARY AND DISCUSSION

The NECC is an important zonal current in the tropical Pacific circulation system and it plays a potentially important role in climatic variability. It transports warm water from the western Pacific warm pool to the cold eastern Pacific region along ~5°N, which is an essential process in the adjustment of the heat and mass distribution of the tropical Pacific Ocean. The dynamics of the NECC have been explored primarily from a basin-scale viewpoint in previous studies (Wyrtki and Kendall, 1967 ; Philander et al., 1987 ; Gouriou and Toole, 1993 ; Donguy and Meyers, 1996 ; Hsin and Qiu, 2012). However, its variability is larger in amplitude and more complex in terms of behavior and mechanism in the far western Pacific Ocean, which is also its region of origin. In this study, the seasonal dynamics of the NECC in the western tropical Pacific Ocean were investigated using satellite-based surface current and SSH data from 1992-2011 in conjunction with reanalysis wind data. The primary characteristics of the seasonal cycle and underlying mechanisms of the NECC were described and discussed in detail. In particular, its relationship with the regional recirculation and permanent eddies (i.e., the ME, MD, and HE) was discussed. The main findings are summarized as follows.

Analysis of satellite-derived surface current data suggests that in the western Pacific, the mean NECC weakens rapidly and shifts northward as it proceeds from the upstream (128°-136°E) to downstream (145°-160°E) regions. Its INT and YCM show distinctively different seasonal characteristics in the upstream and downstream regions. The upstream NECC has a larger mean INT (~1.4×105 m2 /s) and weaker seasonal variation, while the downstream NECC has a smaller mean INT (0.8×105 m2 /s) and stronger seasonal variation. The peak-to-peak difference of the YCM seasonal cycle is 1.2° and 2.5° in the upstream and downstream regions, respectively. The seasonal cycle of the upstream NECC exhibits strong interannual variability, while that of the downstream NECC is more robust. In the upstream region, NECC INT and YCM are influenced considerably by variations of the ME, MD, and equatorial Rossby waves in the south. The associated SSH anomalies can adjust the INT through the geostrophic relation and shift YCM by changing the flow structure. The upstream NECC also shows evident semiannual signals, which are induced by the combined effects of equatorial Rossby waves from the Central Pacific between 3° and 5°N and local wind forcing in the western Pacific. In the downstream region, NECC variability is affected by SSH anomalies in the MD and the central equatorial Pacific Ocean. Those in the MD region are especially important in modulating the YCM of the downstream NECC. In addition to the SSH-related geostrophic flow, zonal Ekman flow, driven by meridional wind stress, also plays a role and has considerable impact on the INT of the surface NECC.

The surface characteristics of the NECC described here are based on satellite observations. However, the driving mechanisms, and especially the roles of the MD and ME, require further examination to achieve an in-depth understanding. It is possible that the observed NECC variability includes the interaction between the current jet and the HE and ME, but such eddy-mean-flow interaction is highly nonlinear and complex, and beyond the scope of the present study. In this analysis, we stressed the effect of tropical wind forcing. Chen and Wu (2011) proposed a possible extratropical wind forcing effect through coastally trapped Kelvin waves propagating southward along the East Asian continent. Such a process could influence NECC variability, especially in the upstream region. The barotropic and baroclinic instabilities of the background current give rise to active mesoscale eddies and filaments, and the ME and HE are two of the quasi-permanent mesoscale structures. The variation of the mesoscale eddy field in response to changes of the NECC and the feedback mechanism from the eddies to the NECC are interesting subjects. It has been shown that the HE shifts northward with increasing depth (Qu et al., 1999 ; Kashino et al., 2013), and that the seasonal variability of the thermocline depth of the MD is rather prominent. Consequently, it would be interesting to explore the NECC in the subsurface layer and to consider its impact on local thermocline variability (Li et al., 2012).

In this study, we considered the seasonal cycle of the NECC from a forced ocean viewpoint. The seasonal variations are believed to be mainly responses to tropical wind forcing and local eddy signals. However, recent studies have proposed that the observed tropical circulation variability on an annual scale might be, in part, a response to the resonance of wind forcing in the Pacific basin (Pinault, 2013, 2014). A detailed description of this theory and observational facts can be found on the following website: http://climatorealist.neowordpress.fr/2015/06/11/climate-change/. This theory is rather new and it remains debatable. The extent to which such a process could affect the structure and variability of the tropical Pacific Ocean circulation is worthy of further examination.

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