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    Map for standard deviation of QD-scale temperature anomaly (°C) at 10-m depth from MOAA GPV. The standard deviation is calculated for the period from January 2001 to December 2010.

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    (a) Time series of QD index from January 1980 to December 2010. QD index is defined as QD-scale (37-month running mean) SST anomalies averaged over 5°S–5°N, 160°E–130°W. The QD index is calculated from extended reconstructed SST, version 3b, provided by NOAA (Smith et al. 2008). (b) As in (a), but for the QD index from January 2000 to December 2010. Warm (cold) QD phase is labeled for the positive (negative) QD index period for January 2002–December 2005 (January 2007–December 2009).

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    (a) A map of averaged no-time-filtered monthly salinity anomalies for the warm QD phase at 10-m depth from MOAA GPV. (b) As in (a), but for 50-m depth. (c) As in (a), but for 100-m depth. Location of the TRITON buoy at 0°, 156°E is indicated by a green square in (a).

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    As in Fig. 3, but for the cold QD phase.

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    As in Fig. 3, but for temperature anomaly (°C).

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    (a) Depth–longitude diagram of salinity anomalies (color shading) and salinity (contour interval is 0.2) along the equator from 130°E to 100°W averaged during the warm QD phase, using MOAA GPV. (b) As in (a), but for temperature (contour interval is 2.0°C).

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    Map of correlation coefficients between salinity and temperature anomalies at 10-m depth on the QD scale using MOAA GPV. Values less than −0.8, which satisfy a significance level of 5% (within a confidence limit of 95%) by the Student's t test, are shown. Contour interval is 0.1.

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    (a) (left) Time series of QD-scale salinity anomalies from the sea surface to 150-m depth from TRITON mooring buoy observation at 0°, 156°E. (right) Salinity profiles averaged for salinity anomalies for the warm QD phase (dotted line) and salinity averaged for total analysis period (i.e., from January 2000 to December 2010; solid line). (b) As in (a), but for temperature anomalies (°C). (c) Time series of zonal current velocity anomalies at 10-m depth (cm s−1). The location of the TRITON buoy is shown by a green square in Fig. 3a.

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    (a) Time–longitude diagram of surface salinity advection averaged on the equatorial band 5°S–5°N on the QD scale. (b) As in (a), but for interannual scale.

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    Time series of QD index (black thick solid line) and Niño-3.4 index (black thin dotted line). El Niño Modoki years are shown by red labels (2002/03, 2004/05, 2006/07, and 2009/10), and La Niña years are shown by blue labels (2005/06 and 2007/08). We defined El Niño Modoki based on Singh et al. (2011).

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    Map of averaged monthly no-time-filtered precipitation anomalies from TRMM 3B-43 during the warm QD phase (color shading; mm h−1). Averaged monthly salinity anomalies at 10-m depth from MOAA GPV during the warm QD phase are also shown (contour interval is 0.05, and negative values are indicated by broken lines).

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    Potential density (kg m−3) profiles averaged over 5°S–5°N, 170°E–170°W for the warm QD phase (red line) and cold QD phase (blue line).

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Upper-Ocean Salinity Variability in the Tropical Pacific: Case Study for Quasi-Decadal Shift during the 2000s Using TRITON Buoys and Argo Floats

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  • 1 Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa, Japan
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Abstract

Upper-ocean salinity variation in the tropical Pacific is investigated during the 2000s, when Triangle Trans-Ocean Buoy Network (TRITON) buoys and Argo floats were deployed and more salinity data were observed than in previous periods. This study focuses on upper-ocean salinity variability during the warming period of El Niño–Southern Oscillation (ENSO)-like quasi-decadal (QD)-scale sea surface temperature anomalies over the central equatorial Pacific (January 2002–December 2005; hereafter “warm QD phase”). It is shown that strong negative salinity anomalies occur in the western tropical Pacific and the off-equatorial Pacific in the upper ocean at depths less than 80 m, showing a horseshoe-like pattern centered at the western tropical Pacific during the warm QD phase. TRITON mooring buoy data in the western equatorial Pacific show that low-salinity and high-temperature water could be transported eastward from the western equatorial Pacific to the central equatorial Pacific during the warm QD phase. Similar patterns, but with the opposite sign of salinity anomalies, appear in the cold QD phase during January 2007–December 2009 with negative sea surface temperature anomalies over the central equatorial Pacific. It is suggested that effects from zonal salinity advection and precipitation could contribute to the generation of the salinity variations in the western equatorial Pacific for QD phases during the 2000s. On the other hand, the contribution of meridional salinity advection is much less than that of zonal salinity advection. In addition, El Niño Modoki and La Niña events could affect salinity changes for warm and cold QD phases via interannual-scale zonal salinity advection variations in the western equatorial Pacific during the 2000s.

Corresponding author address: Takuya Hasegawa, Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-Cho, Yokosuka, Kanagawa 237-0061, Japan. E-mail: takuyah@jamstec.go.jp

Abstract

Upper-ocean salinity variation in the tropical Pacific is investigated during the 2000s, when Triangle Trans-Ocean Buoy Network (TRITON) buoys and Argo floats were deployed and more salinity data were observed than in previous periods. This study focuses on upper-ocean salinity variability during the warming period of El Niño–Southern Oscillation (ENSO)-like quasi-decadal (QD)-scale sea surface temperature anomalies over the central equatorial Pacific (January 2002–December 2005; hereafter “warm QD phase”). It is shown that strong negative salinity anomalies occur in the western tropical Pacific and the off-equatorial Pacific in the upper ocean at depths less than 80 m, showing a horseshoe-like pattern centered at the western tropical Pacific during the warm QD phase. TRITON mooring buoy data in the western equatorial Pacific show that low-salinity and high-temperature water could be transported eastward from the western equatorial Pacific to the central equatorial Pacific during the warm QD phase. Similar patterns, but with the opposite sign of salinity anomalies, appear in the cold QD phase during January 2007–December 2009 with negative sea surface temperature anomalies over the central equatorial Pacific. It is suggested that effects from zonal salinity advection and precipitation could contribute to the generation of the salinity variations in the western equatorial Pacific for QD phases during the 2000s. On the other hand, the contribution of meridional salinity advection is much less than that of zonal salinity advection. In addition, El Niño Modoki and La Niña events could affect salinity changes for warm and cold QD phases via interannual-scale zonal salinity advection variations in the western equatorial Pacific during the 2000s.

Corresponding author address: Takuya Hasegawa, Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-Cho, Yokosuka, Kanagawa 237-0061, Japan. E-mail: takuyah@jamstec.go.jp

1. Introduction

El Niño–Southern Oscillation (ENSO) is the most dominant air–sea interaction phenomenon in the tropical Pacific on an interannual (2–7-yr periodicity) time scale (e.g., Trenberth 1997; McPhaden et al. 2006). In addition to the interannual ENSO signal, previous studies have pointed out that quasi-decadal (QD)-scale variability (dominant cycle of 8–12 years) exists in the tropical Pacific (e.g., Tourre et al. 2001; Luo and Yamagata 2001; Hasegawa and Hanawa 2003; White et al. 2003; Hasegawa et al. 2007). It has been shown that the QD-scale sea surface temperature (SST) and sea level pressure (SLP) show similar spatial patterns to those of ENSO (e.g., Tourre et al. 2001; White et al. 2003; Hasegawa et al. 2007). Furthermore, previous studies used historical data to show that QD-scale upper-ocean heat content (OHC) propagates from the western tropical Pacific toward the eastern tropical Pacific along the equator, similar to that of ENSO (e.g., Luo and Yamagata 2001; White et al. 2003; Luo et al. 2003; Hasegawa and Hanawa 2003; Hasegawa et al. 2007). Such OHC propagation of the QD scale was also found in a 200-yr simulation of the ocean–atmosphere coupled model (Tourre et al. 2005).

The nature of the QD-scale SST anomaly variability differs from that of interdecadal time scale variations found in the middle to high North Pacific, which is widely called the Pacific decadal oscillation (PDO; Mantua et al. 1997). Although both QD variation and PDO show ENSO-like horseshoe-type SST anomaly spatial patterns in the Pacific Ocean, the time scale of PDO (nearly 20–80 years) is longer than that of the QD scale; it displays a large signal in the mid- to high latitudes of the North Pacific rather than the tropical Pacific in contrast with the QD scale (Mantua et al. 1997; Zhang et al. 1997; Minobe 1997, 1999; Tourre et al. 2001; Hasegawa et al. 2007).

As described above, numerous studies on QD-scale SST and OHC anomalies in the tropical Pacific as well as those for ENSO- and PDO-scale SST and OHC anomalies have been conducted using historical oceanic thermal data. In contrast to SST and OHC, upper-ocean salinity variability on the QD scale in the tropical Pacific has not been adequately investigated using observational data, mainly due to a lack of upper-ocean salinity observations. Some previous studies of sea surface salinity variations related to PDO and linear trend in the tropical Pacific used observational data (Delcroix et al. 2007; Cravatte at al. 2009). Cravatte et al. (2009) reported that the western tropical Pacific showed a warming and freshening trend during 1955–2003. Hosoda et al. (2009) used salinity data from historical data and Argo floats data to explore a trend of salinity and its relationship to freshwater flux variation during the last 30 years. They showed a trend of higher surface salinity in the subtropics and lower surface salinity in the subpolar and tropical regions. Recently, Singh and Delcroix (2011) estimated the effects of ENSO upon the surface freshening trends of the western tropical Pacific using historical sea surface salinity data. However, on the QD scale the spatial pattern of upper-ocean salinity anomalies and the relationship between salinity and temperature variations in the tropical Pacific remain unclear.

Exploration of QD-scale salinity change is important for a better understanding of QD-scale climate variability because salinity variations influence oceanic density related to the subduction process from the midlatitudes to the tropics (e.g., Schneider 2000, 2004; Sasaki et al. 2010) and also by generating a salinity barrier layer that affects the thermal condition of the upper ocean (e.g., Lukas and Lindstrom 1991; Ando and McPhaden 1997; Maes et al. 2002, 2005, 2006). Recently, Bosc et al. (2009) showed that the thick salinity barrier layer robustly occurs in the west of the eastern edge of the warm pool. They also showed that the position of the salinity barrier layer moves eastward during El Niño events, which is important to the air–sea interaction similar to the advective–reflective oscillator ENSO scheme (Picaut et al. 1997).

Prior to the year 2000, the number of observations of upper-ocean salinity is much fewer than that of temperature. At that time, it was not possible to analyze basin-scale temporal variation for QD-scale upper-ocean salinity. Upper-ocean salinity has been observed more frequently and more extensively since the early 2000s, when the Triangle Trans-Ocean Buoy Network (TRITON) mooring buoys (e.g., Ando et al. 2005) and Argo float (e.g., Hosoda et al. 2008) observations began. The purpose of the present study is to examine salinity variability in the tropical Pacific during the 2000s. In this study, the spatial salinity pattern is explored and compared to that of SST. Furthermore, the temporal relationship between salinity and temperature is explored.

During the 2000s, several weak warm events called El Niño Modoki (Ashok et al. 2007), central Pacific El Niño (Kao and Yu 2009), warm pool El Niño (Kug et al. 2009), or dateline El Niño (Larkin and Harrison 2005) frequently occurred (hereafter referred to as El Niño Modoki). Singh et al. (2011) showed that El Niño Modoki is accompanied by smaller eastward displacement of the eastern sector of the low-salinity warm pool waters in the equatorial Pacific than in the eastern Pacific El Niño. The data length of this study (~10 years) is not enough to reveal QD-scale upper-ocean salinity variability with satisfying statistical significance. Therefore, in this study we analyze the upper-ocean salinity data as a case study for the 2000s in association with a phase shift of the QD-scale equatorial SST variations. We also discuss a relationship between the QD-scale salinity variation and zonal salinity advection related to El Niño Modoki and La Niña events during the 2000s.

This paper is organized as follows: section 2 presents the data and methods used in this study. Section 3 presents the results. Section 4 provides the summary and discussion.

2. Data

This study uses oceanic data observed by the TRITON mooring buoys, deployed in the Pacific warm pool region by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) after late 1999 (e.g., Ando et al. 2005; Kashino et al. 2007; Hasegawa et al. 2009). The TRITON buoys are the Japanese counterpart to the Tropical Atmosphere Ocean (TAO) buoys, which have been sustained by the U.S. National Oceanic and Atmospheric Administration (NOAA) since the 1980s. Both buoys consist of a TAO/TRITON array from January 2000 to monitor tropical Pacific climate variations such as ENSO (e.g., McPhaden et al. 2009).

The unique point of TRITON buoys is to observe salinity variations in the upper ocean in addition to temperature variations. The TRITON buoys measure salinity and temperature near the surface (at a depth of 1.5 m) and at 11 other subsurface depths ranging from a 25- to 750-m depth and surface current velocity at a depth of 10 m as well as surface meteorological data. Temperature and salinity are observed by 12 type Sea-Bird Electronics (SBE) 37IM conductivity and temperature (CT) sensors. Ando et al. (2005) reported that the drift of the temperature sensors of SBE37IM CT installed on TRITON buoys was very small, within 0.003 K of the postdeployment calibration data. On the other hand, conductivity drift observed in the shallow layer (from 1.5 to 100 m) was more significant, showing +0.010 S m−1 (1 S = 1 Ω−1), which is equivalent to 0.065 on the Practical Salinity Scale of 1978 (PSS-78) at 30°C and 6 S m−1. Drift observed in the deeper layer (125–200 m) was also positive and relatively smaller (0.0053 S m−1). The drift of conductivity in the deepest layer (250–700 m) was much smaller (0.000 02 S m−1). Ando et al. (2005) also reported that in situ comparison of the TRITON salinity data with the shipboard CTD system (SBE 9/11plus) showed a positive conductivity drift with time. They attempted to correct the TRITON buoys' conductivity data from postdeployment calibration data, assuming a linear trend of conductivity drift with time and showed that the salinity data quality was greatly improved after their correction method. The mean and standard deviations between salinity measured by CT sensors on TRITON buoys and the shipboard CTD salinity in shallow layers above 100-m depth became −0.001 and 0.033 (PSS-78), respectively. In this study, we use salinity data calculated from the conductivity data corrected by the method of Ando et al. (2005).

In addition, the acoustic Doppler current meter (ADCM; SonTek Argonaut-MD ADCM) installed on each TRITON buoy observes the surface current velocity. The resolution of the ADCM is 0.1 cm s−1 in speed and 0.1° in direction. The nominal accuracy is 0.5 cm s−1 in velocity and 2° in direction. Hase and Kuroda (2002) confirmed that the amplitude and direction of the horizontal current velocity show good agreement between the ADCM installed on the TRITON buoy and the separately moored upward-looking acoustic Doppler current profile (ADCP) within a distance of approximately 10 km. They showed that the correlation coefficient and root-mean-square error for the amplitude were 0.89 and 3.3 cm s−1 and for direction were 0.99° and 3.1°, respectively, for 25-h running-mean data. The mean differences (ADCM minus ADCP) in current speed and direction were small: −0.1 cm s−1 and −1.4°, respectively. The fitting slopes for linear regressions for the amplitude and direction were 1.00 and 1.01, respectively. The mean differences are within the manufacturer's nominal accuracy for the ADCM. Therefore, the ADCM performance is adequate to measure the equatorial currents.

We analyze upper-ocean salinity and temperature data and surface current data obtained by 13 TRITON buoys in the western equatorial Pacific, ranging from 5°S to 8°N and 137° to 156°E. All oceanic data are sampled every 10 min. We calculate daily mean data after conducting a 49-h running mean to filter the effects of short-term fluctuations such as internal tides; we subsequently calculate monthly mean data.

We also use gridded (1° × 1°) salinity and temperature data derived from Argo float and TRITON buoy data [monthly objective analysis using Argo data (MOAA) gridpoint value (GPV); Hosoda et al. 2008, 2009]. MOAA GPV provides monthly data at 25 levels, from 10 to 2000 dbar in the quasi-global range (60°S–60°N) for the period beginning in January 2001. After 2001, there was a large increase in the number of Argo float observations in the western tropical Pacific. Thus, this study focuses on the salinity variations after 2001 when we use MOAA GPV data. The optimal interpolation method is applied when the gridded data are made from the original data of Argo floats and TRITON buoys. Therefore, any spatial scale smaller than the decorrelation scale of the optimal interpolation (~2000 km in zonal direction and 1000 km in meridional direction in the upper ocean of the tropical Pacific) is smoothed out in MOAA GPV. Salinity is defined as PSS-78 for both MOAA GPV and TRITON buoy data. The accuracy of the profile data observed by Argo floats is 2.4 dbar for pressure, 0.005°C for temperature, and 0.01 for salinity (Argo Science Team 2000).

In this study, we first explore the basin-scale spatial pattern of QD-scale upper-ocean salinity anomalies in the tropical Pacific (30°S–30°N, 110°E–70°W) using MOAA GPV. Then, we observe the salinity variability in the key region that is pointed out by MOAA GPV analysis, using TRITON buoy data that are not affected by the spatial smoothing technique, such as the optimal interpolation used in MOAA GPV.

Surface current data from the Ocean Surface Current Analysis–Real Time (OSCAR; Bonjean and Lagerloef 2002; Johnson et al. 2007) are used to calculate zonal salinity advection in the equatorial Pacific for both the QD and interannual scales to explore the effect of El Niño Modoki on QD-scale salinity change during the 2000s. In addition, we use gridded (0.25° × 0.25°) precipitation data of the Tropical Rainfall Measuring Mission (TRMM) and other satellite products (TRMM product 3B-43) provided by the National Aeronautics and Space Administration (NASA; Huffman et al. 2007) to compare the spatial pattern of QD-scale anomalies in precipitation and salinity.

The analysis period is January 2000–December 2010 for TRITON buoy data and January 2001–December 2010 for other data. For each variable, monthly climatologies are produced by averaging the raw data for January 2001–December 2010, and then monthly anomalies are calculated by subtracting the monthly climatologies from the individual values. When the QD-scale component is highlighted, we use a 37-month running-mean filter. We use no-time-filtered data for composite analysis during the 2000s.

3. Results

a. Detection of the QD phase

Previous studies using historical thermal data (e.g., Tourre et al. 2001; White et al. 2003; Hasegawa et al. 2007) reported that QD-scale SST anomalies have large signals around the central equatorial Pacific near the international date line and several warm and cold phases occurred after the 1980s (e.g., Tourre et al. 2001; White et al. 2003; Hasegawa et al. 2007). To check if a similar QD-scale temperature pattern appears during the analysis period for the present study, a standard deviation map of the QD-scale temperature anomalies is made (Fig. 1). Figure 1 shows that strong signals appear in the equatorial Pacific around the international date line (5°S–5°N, 160°E–130°W), which is similar to the QD-scale SST pattern pointed out by previous studies (e.g., Tourre et al. 2001; White et al. 2003; Hasegawa et al. 2007). Based on previous studies and the result of Fig. 1, QD-scale (37-month running mean) SST anomalies averaged over the central equatorial Pacific ranging from 5°S to 5°N and 160°E to 130°W (hereafter QD index) are used to detect warm and cold phases of QD-scale variability in this study. In the present study, the warm (cold) QD phase corresponds to the periods for which the QD index shows positive (negative) anomalies.

Fig. 1.
Fig. 1.

Map for standard deviation of QD-scale temperature anomaly (°C) at 10-m depth from MOAA GPV. The standard deviation is calculated for the period from January 2001 to December 2010.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

Figure 2a shows the QD index for January 1980–December 2010. We can see in Fig. 2a that QD-scale variation shows several warm and cold phases after the 1980s, which supports the previous studies (e.g., Tourre et al. 2001; White et al. 2003; Hasegawa et al. 2007). To focus the QD phase shifts for the present analysis period, we also make time series of the QD index during the 2000s (Fig. 2b). Figure 2b shows that a warm QD phase occurred from 2002 to 2005 and a cold QD phase occurred prior to 2001 and then from 2007 to 2009. This study focuses on the warm QD phase during January 2002–December 2005 and the cold QD phase during January 2007–December 2009; the cold QD phase prior to 2001 is excluded because of the relative lack of Argo float observations in the tropical Pacific during this period.

Fig. 2.
Fig. 2.

(a) Time series of QD index from January 1980 to December 2010. QD index is defined as QD-scale (37-month running mean) SST anomalies averaged over 5°S–5°N, 160°E–130°W. The QD index is calculated from extended reconstructed SST, version 3b, provided by NOAA (Smith et al. 2008). (b) As in (a), but for the QD index from January 2000 to December 2010. Warm (cold) QD phase is labeled for the positive (negative) QD index period for January 2002–December 2005 (January 2007–December 2009).

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

b. Spatial patterns of QD-scale salinity and temperature anomalies

Figures 3a–c show maps of salinity anomalies averaged during the warm QD phase at depths of 10, 50, and 100 m, respectively, using no-time-filtered monthly salinity anomalies from MOAA GPV. Large negative salinity anomalies of almost −0.2 occur at 10-m depth in the western tropical Pacific, centered at 0°, 160°E (Fig. 3a). The negative anomalies appear to be related to relatively large negative anomaly values (nearly −0.1) in the off-equatorial South Pacific, from around 0°, 170°E to around 20°S, 130°W. In the North Pacific, negative anomalies of almost −0.1 also occur around 12°N, 170°E. The negative salinity anomalies show a horseshoe-like pattern, centered on the western tropical Pacific. A very similar pattern is seen at 50-m depth (Fig. 3b), but the amplitudes of salinity anomalies at 100-m depth are smaller than those at shallower depths (Fig. 3c). The amplitudes are further reduced at a 150-m depth (~0.05) and become very small (near zero) at 200- and 300-m depths (not shown). Therefore, the present result shows that the horseshoe-like pattern of salinity is unique to the upper ocean at depths less than 100 m. For the cold QD phase, similar patterns at 10-, 50-, and 100-m (Figs. 4a–c) depths are obtained for the cold QD phase, but the sign is reversed.

Fig. 3.
Fig. 3.

(a) A map of averaged no-time-filtered monthly salinity anomalies for the warm QD phase at 10-m depth from MOAA GPV. (b) As in (a), but for 50-m depth. (c) As in (a), but for 100-m depth. Location of the TRITON buoy at 0°, 156°E is indicated by a green square in (a).

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

Fig. 4.
Fig. 4.

As in Fig. 3, but for the cold QD phase.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

On the other hand, upper-ocean QD-scale temperature variation is dominant in the central equatorial Pacific (Figs. 5a–c). Figure 5a shows that positive temperature anomalies appear east of 150°E centered at the international date line during the warm QD phase. At a depth of 50 m, a similar pattern is found but the signal of the northwestern tropical Pacific is enhanced (Fig. 5b). At a depth of 100 m, the signal of the northwestern tropical Pacific is further enhanced (Fig. 5c). The pattern found in Fig. 5a corresponds to the ENSO-like QD-scale SST pattern, which was pointed out in previous studies (e.g., Tourre et al. 2001; White et al. 2003; Hasegawa et al. 2007). Such a pattern is very similar to that of El Niño Modoki (Ashok et al. 2007). It is suggested that the occurrence of El Niño Modoki events may be related to the QD-scale variations during the 2000s. We will discuss the relationship between the QD-scale variations and El Niño Modoki in section 3d. A similar temperature pattern is found for the cold QD phase, but the sign is reversed (not shown).

Fig. 5.
Fig. 5.

As in Fig. 3, but for temperature anomaly (°C).

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

Figures 3 and 4 indicate that the horseshoe-like pattern of upper-ocean QD-scale salinity anomalies differs from a gross feature of the spatial pattern of upper-ocean QD-scale temperature anomalies that displays a strong signal in the central Pacific. Furthermore, it is shown that negative QD-scale salinity anomalies of the upper ocean appear in the horseshoe-like pattern during the warm QD phase, while positive QD-scale temperatures of the upper ocean appear in the central equatorial Pacific at that time.

Figure 6a shows a depth–longitude diagram of salinity anomalies (color shading) and salinity (contour) along the equator from the surface to 300-m depth, averaged during the warm QD phase. Consistent with the results shown in Fig. 3, large salinity anomalies of roughly −0.3 occur at depths less than 80 m in the freshwater area (ranging from 34.2 to 35.1) of the western Pacific, centered at 160°E (Fig. 6a). On the other hand, relatively positive temperature anomalies appear in the upper ocean at depths less than 80 m, east of 150°E and centered at the international date line (Fig. 6b), which is consistent with Fig. 5. During the cold QD phase, similar anomaly patterns of salinity and temperature (but with reversed sign) occur (not shown). In Fig. 6b, we can see that negative and positive temperature anomalies appear in the western and eastern parts of the thermocline layer centered around 22°C, respectively, during the warm QD phase. This implies that the cooling and warming of the western and eastern equatorial Pacific is due to the reduction of the basin-scale zonal thermocline slope during the warm QD phase. During the warm QD phase from 2002 to 2005, two El Niño Modoki events occurred in 2002/03 and 2004/05 (Singh et al. 2011). It is suggested that the reduction of the basin-scale zonal thermocline slope due to the two events contributed to shoaling of the thermocline in the western equatorial Pacific and the resultant negative temperature anomalies around the thermocline during the warm QD phase shown in Fig. 6b.

Fig. 6.
Fig. 6.

(a) Depth–longitude diagram of salinity anomalies (color shading) and salinity (contour interval is 0.2) along the equator from 130°E to 100°W averaged during the warm QD phase, using MOAA GPV. (b) As in (a), but for temperature (contour interval is 2.0°C).

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

Figures 35 show that negative (positive) anomalies of QD-scale upper-ocean salinity in the horseshoe pattern including the western-to-central equatorial Pacific accompany positive (negative) QD-scale upper-ocean temperature anomalies centered at the international date line, east of 150°E, during the warm (cold) QD phase. This can suggest that such negative correlation between salinity and temperature can contribute to the change of seawater density of the upper ocean around the international date line; both negative salinity anomalies and positive temperature anomalies can induce a decrease in the density of seawater. To explore further a negative correlation relationship between upper-ocean salinity and temperature, we make a map of the correlation analysis between QD-scale salinity and temperature anomalies at 10-m depth (Fig. 7). Figure 7 shows that salinity and temperature anomalies negatively correlate with each other in the western equatorial Pacific (east of 150°E) within the horseshoe-like area. Negative correlation coefficients are also found in the western to central South Pacific around 5°S, 180° and in the central North Pacific centered at 12°N, 170°W, which are also within the horseshoe-like area. Strong negative correlation between salinity and temperature is not found in a wide range outside of the horseshoe-like area.

Fig. 7.
Fig. 7.

Map of correlation coefficients between salinity and temperature anomalies at 10-m depth on the QD scale using MOAA GPV. Values less than −0.8, which satisfy a significance level of 5% (within a confidence limit of 95%) by the Student's t test, are shown. Contour interval is 0.1.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

c. Upper-ocean salinity and temperature and surface zonal current in the western tropical Pacific

In this subsection, we investigate the relationship among QD-scale upper-ocean salinity and temperature and surface zonal current changes. We use in situ data from the TRITON mooring buoys in the Pacific warm pool region within the horseshoe-like area. In this area, QD-scale upper-ocean salinity anomalies show very strong signals and negatively correlate with temperature anomalies as mentioned in the previous subsection. In this subsection, we focus on data from the TRITON buoy at 0°, 156°E.

Figures 8a–c show time series of QD-scale upper-ocean salinity and temperature anomalies and surface zonal current anomalies at 10-m depth observed by TRITON buoys at 0°, 156°E. During the warm QD phase from 2002 to 2005, relatively strong negative salinity anomalies (nearly −0.4) appear at depths less than 80 m and weaken below 80-m depth (Fig. 8a), which supports the results shown in the previous subsection. Salinity anomaly profiles averaged for the warm QD phase (Fig. 8a, right) also show larger negative anomalies (around −0.3) above 80-m depth than in deeper layers, causing an enhancement of low-salinity property (around 34.5) above 80-m depth of the mean salinity profile (Fig. 8a, right). During the cold QD phase (2006–09), strong positive salinity anomalies (nearly 0.2) appear above 80-m depth, as in the warm QD phase but with reversed sign.

Fig. 8.
Fig. 8.

(a) (left) Time series of QD-scale salinity anomalies from the sea surface to 150-m depth from TRITON mooring buoy observation at 0°, 156°E. (right) Salinity profiles averaged for salinity anomalies for the warm QD phase (dotted line) and salinity averaged for total analysis period (i.e., from January 2000 to December 2010; solid line). (b) As in (a), but for temperature anomalies (°C). (c) Time series of zonal current velocity anomalies at 10-m depth (cm s−1). The location of the TRITON buoy is shown by a green square in Fig. 3a.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

Figure 8b shows that positive and negative temperature anomalies appear above and below 80-m depth, respectively, during the warm QD phase. This supports the results of Figs. 5a and 5b. The sign of the temperature anomalies is reversed during the cold QD phase. Negative salinity anomalies below 80-m depth during the warm QD phase (Fig. 8a) correspond to negative temperature anomalies at about 90-m depth in the western region around 150°E (Fig. 8b), which would be related to the shoaling of the thermocline in the western equatorial Pacific (Fig. 6b) due to the reduction of the basin-scale thermocline slope as described in the previous subsection.

It is interesting that the surface zonal current velocity anomalies also change sign in association with the warm and cold QD phases (Fig. 8c): eastward (around 20 cm s−1) and westward (around −20 cm s−1) anomalies appear during the warm and cold QD phase, respectively.

Similar behavior of salinity and temperature is found in other buoy data at 5°S, 156°E; 2°S, 156°E; and 2°N, 156°E (not shown); while the behavior is most clearly seen at 0°, 156°E. From the results shown in this subsection, it is suggested that upper-ocean water at depths less than 80 m with low salinity and high temperature could be transported eastward during the warm QD phase, but such water cannot be transported eastward from the western equatorial Pacific during the cold QD phase.

d. Relationship with El Niño Modoki and La Niña during the 2000s

Previous studies (e.g., Picaut et al. 1996, 1997; Delcroix and Picaut 1998; Hénin et al. 1998; Picaut et al. 2001) pointed out that the zonal displacement of warm/freshwater pool (i.e., zonal advection in the western equatorial Pacific) has a large role in controlling the water variations in the western tropical Pacific on an interannual scale related to ENSO. Recently, Singh et al. (2011) showed that El Niño Modoki (Ashok et al. 2007) is accompanied by smaller eastward displacement of the eastern part of the low-salinity warm pool waters in the equatorial Pacific than in the eastern Pacific El Niño. In this subsection, we discuss the relationship between the QD-scale variation and El Niño Modoki and La Niña events, concerning salinity tendency due to zonal advection in the western equatorial Pacific during the 2000s.

To focus the role of the current variations in the zonal salinity advection on interannual and on QD scales, we calculate zonal salinity advection using surface current from OSCAR (Bonjean and Lagerloaf 2002; Johnson et al. 2007) for both the QD scale (using QD-scale zonal current anomalies and surface salinity zonal gradients averaged for January 2001–December 2010) and the interannual scale (using a zonal current filtered by a 5-month running-mean filter to remove intraseasonal variations and surface salinity zonal gradients averaged for January 2001–December 2010), respectively (Figs. 9a,b).

Fig. 9.
Fig. 9.

(a) Time–longitude diagram of surface salinity advection averaged on the equatorial band 5°S–5°N on the QD scale. (b) As in (a), but for interannual scale.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

Negative values of roughly −0.15 month−1 are found around the western equatorial Pacific (nearly 155°–170°E) during the warm QD phase on the QD scale (Fig. 9a). It suggests that the negative salinity anomalies of the western equatorial Pacific within the horseshoe-like pattern shown in the previous subsections could be related to the negative zonal advection during the warm QD phase.

The periods of the 2002/03 and 2004/05 El Niño Modoki events (see Fig. 10) are included in the warm QD phase (Fig. 10). During the two events, strong negative signals from roughly −0.1 to −0.3 month−1 distribute in the western equatorial Pacific west of the international date line (west of nearly 170°E), as pointed out by previous studies (e.g., Singh et al. 2011). Such negative advection could contribute to the continuation of the low salinity of the western equatorial Pacific during the warm QD phase. Then, 2005/06 La Niña events occurred in the termination period of the warm QD phase (Fig. 10), accompanying interannual-scale positive salinity zonal advection between 155°E and 180° (Fig. 9b). Such positive salinity advection could contribute to the weakening of the low salinity during the termination period of the QD phase.

Fig. 10.
Fig. 10.

Time series of QD index (black thick solid line) and Niño-3.4 index (black thin dotted line). El Niño Modoki years are shown by red labels (2002/03, 2004/05, 2006/07, and 2009/10), and La Niña years are shown by blue labels (2005/06 and 2007/08). We defined El Niño Modoki based on Singh et al. (2011).

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

The 2007/08 La Niña event occurring during the cold QD phase (2007–09; Fig. 10) shows positive salinity zonal advection on the interannual scale (Fig. 9b) as well as the QD scale (Fig. 9a). This suggests that the high-salinity anomalies for the cold QD phase could be generated by both QD- and interannual-scale positive zonal salinity advections. On the other hand, the 2009/10 El Niño Modoki accompanies strong negative salinity advection in the western equatorial Pacific between 155° and 170°E (Fig. 9b). Such negative tendency could contribute to the termination of the high salinity of the western equatorial Pacific during the cold QD phase.

In contrast to the zonal salinity advection, meridional salinity advection does not show a good relationship to salinity changes in the western tropical Pacific; that is, weak positive values of meridional salinity advection (roughly −0.02 month−1) appear between 150° and 165°E during the positive QD phase on both the interannual and QD scales (not shown here). Therefore, the present results indicate that the zonal advection related to El Niño Modoki and La Niña events could contribute considerably to QD-scale salinity changes in the western equatorial Pacific west of 170°E during the 2000s than meridional salinity advection.

e. Relationship between salinity and precipitation changes

In this subsection, we discuss the relationship between salinity and precipitation changes on the QD scale. Figure 11 shows the precipitation pattern averaged during the warm QD phase, using no-time-filtered monthly data of TRMM product 3B-43. It is shown that positive anomalies occur in the western tropical Pacific, centered at 0°, 160°E, and the off-equatorial South Pacific. In gross feature, the spatial pattern of the precipitation anomalies and salinity anomalies are similar. In particular, positive precipitation anomalies and negative salinity anomalies overlap in several areas of the horseshoe-like region centered at 0°, 160°E; 5°S, 180°; 20°S, 130°W; and 12°N, 170°E. During the negative QD phase, similar patterns of precipitation and salinity anomalies are found but with reversed sign (not shown). It is expected that positive precipitation anomalies may locally generate the negative salinity anomalies observed in these areas.

Fig. 11.
Fig. 11.

Map of averaged monthly no-time-filtered precipitation anomalies from TRMM 3B-43 during the warm QD phase (color shading; mm h−1). Averaged monthly salinity anomalies at 10-m depth from MOAA GPV during the warm QD phase are also shown (contour interval is 0.05, and negative values are indicated by broken lines).

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

Recently, Singh et al. (2011) pointed out the importance of precipitation in the variation of the sea surface salinity in the western equatorial Pacific as well as zonal salinity advection during El Niño Modoki periods on the interannual scale. There is a possibility that salinity variations due to precipitation change during El Niño Modoki periods within the warm QD phase could affect the salinity changes for the warm QD phase, similar to the zonal salinity advection as described in the section 3d.

Figure 11 shows that there are regions in which precipitation and salinity changes are not consistent. In particular, strong negative precipitation anomalies that can induce an increase in salinity anomalies occur on the western edge of the equatorial Pacific, around 0°, 140°E (i.e., coastal region near New Guinea), while negative salinity anomalies appear in this region (Fig. 11). Thus, precipitation changes cannot explain the QD-scale upper-ocean salinity changes in such regions. It is speculated that oceanic advection and upwelling might be more influential than the local effect of precipitation changes in such regions.

4. Summary and discussion

The present study explores QD-scale upper-ocean salinity variability in the tropical Pacific in association with QD-scale SST shift during the 2000s, when the introduction of the TRITON buoys and Argo floats greatly facilitated the collection of observational salinity data. It is shown that strong salinity anomalies appear in the western tropical Pacific centered at 0°, 160°E and spread to the off-equatorial Pacific, displaying a horseshoe-like pattern. The horseshoe-like upper-ocean salinity pattern is different from the ENSO-like QD-scale SST pattern that was pointed out in previous studies (e.g., Tourre et al. 2001; Luo and Yamagata 2001; White et al. 2003; Hasegawa et al. 2007). It is also shown that salinity anomalies negatively correlate with temperature anomalies in a wide area within the horseshoe-like region on the QD scale. Furthermore, TRITON mooring buoy data in the western tropical Pacific around 0°, 156°E within the horseshoe-like area show that anomalies of upper-ocean salinity and temperature and surface zonal currents change their sign at the same time as SST QD phase transition during the 2000s.

The results suggest that several mechanisms could contribute to the generation of upper-ocean salinity variation in the western equatorial Pacific for QD phases during the 2000s. One is an oceanic effect from zonal salinity advection in the western equatorial Pacific west of the international date line. The other is a local effect from precipitation anomalies in several areas within the horseshoe-like region. It is also suggested that the interannual-scale zonal salinity advection variations related to El Niño Modoki and La Niña events could contribute to the generation of salinity change for warm and cold QD phases in the western equatorial Pacific during the 2000s.

In contrast to zonal salinity advection, meridional salinity advection does not show a good relationship to salinity changes in the western tropical Pacific. Therefore, equatorial zonal salinity advection is much more important than meridional advection for salinity changes in the tropical western Pacific during the 2000s. There is a possibility that vertical salinity advection and vertical mixing/entrainment in the mixed layer can also affect the salinity changes as well as zonal salinity advection and precipitation. However, the effect from such vertical processes cannot be directly calculated in this study because of the lack of vertical velocity data. We would like to investigate the effect of such vertical processes on salinity changes and compare it with other factors during QD phases in future works.

The results from TRITON buoy data analysis suggest that upper-ocean water with high temperature and low salinity in the western equatorial Pacific can be transported eastward toward the central Pacific during the warm QD phase. At that time, strong positive SST anomalies appear in the central equatorial Pacific. Therefore, the eastward transportation of upper-ocean high-temperature and low-salinity water from the western equatorial Pacific might contribute to the preservation or enhancement of positive SST anomalies on the western side of the central Pacific during the warm QD phase.

In addition, it can be speculated that the eastward transportation of upper-ocean warm and low-salinity water during the warm QD phase can enhance the vertical salinity shear, which is favorable for generation of a salinity barrier layer. The occurrence of a thick barrier layer can contribute to the maintenance of positive temperature anomalies (Lukas and Lindstrom 1991; Ando and McPhaden 1997; Maes et al. 2002, 2005, 2006). Figure 12 shows that the seawater density of the upper ocean (above nearly 100-m depth) decreases by roughly 0.5 kg m−3 from the cold QD phase to the warm QD phase. This suggests that warm and low-salinity water transported from the western tropical Pacific during the warm QD phase could generate lighter water in the upper ocean than during the cold QD phase in which such eastward transportation of warm and low-salinity water does not occur. It is difficult to explore the detailed mechanism for generation of the salinity barrier layer on the QD-scale at present: exploration of this issue and its impact on SST warming in detail require higher-resolution observations of upper-ocean salinity and vertical temperature profile. Improvement of OGCMs and assimilation models with greater vertical resolution of the upper ocean is also required for better understanding of upper-ocean QD-scale salinity variability.

Fig. 12.
Fig. 12.

Potential density (kg m−3) profiles averaged over 5°S–5°N, 170°E–170°W for the warm QD phase (red line) and cold QD phase (blue line).

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00187.1

Previous studies showed that a trend of sea surface salinity shows a large signal in the western tropical Pacific and that the trend is related to freshwater flux changes in this region (e.g., Cravatte et al. 2009; Singh and Delcroix 2011). The present study shows that QD-scale upper-ocean salinity anomalies observed during the 2000s are also dominant in the western tropical Pacific as well as the trend of sea surface salinity. In this study, we conduct a case study on upper-ocean salinity variations in the tropical Pacific during the 2000s in association with QD-scale SST warm and cold phase shift. Further comparison of salinity variability on the QD scale with that on shorter time scales and trend requires long-term various in situ observations using Argo floats, mooring buoys, research vessels, and new satellite remote-sensing observations like the Soil Moisture Ocean Salinity (SMOS) mission (Kerr et al. 2001; Alory et al. 2012), which provides the first satellite remote-sensing observation data of sea surface salinity, and also the Aquarius/Satelite de Aplicaciones Cientificas-D (SAC-D) mission (Lagerloef et al. 2008). In the future, repeat observations using gliders (e.g., Davis et al. 2012) would also be useful for further analysis of salinity variability in the upper ocean.

It has been pointed out by previous studies that QD-scale SST, OHC, and SLP anomaly changes occur in the tropical Pacific (e.g., Tourre et al. 2001; Luo and Yamagata 2001; White et al. 2003; Hasegawa and Hanawa 2003; Hasegawa et al. 2007). The present study indicates that QD-scale precipitation anomalies seem to correspond to QD-scale salinity anomalies in several parts of the horseshoe-like area during the 2000s. In future works, we would like to explore a possibility of the QD-scale air–sea-coupled variations, including upper-ocean salinity change in the tropical Pacific.

Acknowledgments

This study was conducted under the Tropical Ocean Climate Study (TOCS) and TRITON projects of JAMSTEC. The TRITON mooring buoys were maintained during cruises of the R/Vs Kaiyo and Mirai, which were conducted by JAMSTEC. The authors are very grateful to the crews of these R/Vs, technicians from Marine Work Japan Ltd., Nippon Marine Enterprises Ltd., Global Ocean Development Inc., and all participants in these cruises.

We thank colleagues at the Tropical Climate Variability Research Program and Argo group of JAMSTEC for their useful advice and help with various products. The international Argo program is operated by the Argo Steering Team (AST) and the original Argo float data are freely available from the GDAC website (at http://www.coriolis.eu.org/ and http://www.usgodae.org/argo/argo.html). In addition, we thank the anonymous reviewers for their useful comments and suggestions to improve the present study. This work was partly supported by Grant-in-Aid for Scientific Research (22106006) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

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