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    First EOF mode of sea surface salinity (psu) from (a) Argo and (b) ECCO and (c) their time series compared with the SSS ENSO index, Niño-S34.8, and SEPSI from ECCO for the period 2005–14. The white lines in (a) and (b) indicate the equator. Values in (c) are normalized by their standard deviations.

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    Long-term (1993–2014) mean meridional overturning streamfunction (Sv; contour) superimposed with zonally averaged ocean salinity (psu; shading) from ECCO.

  • View in gallery

    Long-term (1993–2014) mean sea surface salinity (psu) from ECCO. The white rectangle shows the sea surface salinity maximum in the subtropical South Pacific, where a passive tracer is released in the surface mixed layer.

  • View in gallery

    (top) Long-term (1993–2011) mean passive tracer (atu m−2 month−1) that enters the surface mixed layer per unit horizontal area per month, and vertical distribution of (middle) salinity (psu) and (bottom) temperature (°C) averaged in the equatorial band (3°S–3°N). The white solid and dashed lines denote the top of the thermocline and the base of the mixed layer, respectively. The depth of the thermocline is defined using a temperature difference of 0.5°C from the surface value, and the depth of the mixed layer is defined using a density difference that is equivalent to a temperature decrease of 0.5°C.

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    (a) Normalized volume transport (1013 atu month−1) and (b) longitudinal location (degree longitude) of barycenter of the passive tracer that enters the surface mixed layer in the equatorial Pacific (3°S–3°N) each month compared with Niño-3.4 and Niño-S34.8 from ECCO for the period 1993–2011. The long-term mean values of 7.06 × 1012 atu month−1 and 151°W have been removed from the time series before plotting.

  • View in gallery

    (top) Passive tracer (atu m−2 month−1) that enters the surface mixed layer per unit horizontal area per month superimposed with sea surface salinity (psu) and vertical distribution of (middle) salinity (psu) and (bottom) temperature (°C) averaged in the equatorial band (3°S–3°N) in September–November during the composite (left) El Niño and (right) La Niña events. The white solid and dashed lines denote the top of the thermocline and the base of the mixed layer, respectively.

  • View in gallery

    (top) Anomalous passive tracer (atu m−2 month−1) that enters the surface mixed layer per unit horizontal area per month averaged in September–November and (middle) anomalous sea surface salinity (psu) and (bottom) temperature (°C) averaged in December–February during the composite (left) El Niño and (right) La Niña events.

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    Long-term (1993–2011) mean difference (psu) between the tracer-tagged SPTW salinity averaged in the equatorial Pacific (10°S–10°N) below the surface mixed layer and the climatological SSS in September–November.

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    (top) Anomalous vertical entrainment equivalent to surface freshwater flux (mm month−1) converted from the resurfacing passive tracer and (bottom) anomalous E − P from ECCO averaged in September–November during the composite (left) El Niño and (right) La Niña events. For convenience the equivalent freshwater flux in the top panels is multiplied by −1 before plotting.

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Resurfacing of South Pacific Tropical Water in the Equatorial Pacific and Its Variability Associated with ENSO

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  • 1 Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, Los Angeles, California, and International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawaii
  • 2 Institute of Oceanology, Chinese Academy of Sciences, and Function Laboratory for Ocean Dynamics and Climate, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China
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Abstract

Analysis of results from a simulated passive tracer confirms the resurfacing of South Pacific Tropical Water in the equatorial Pacific. Over the period of integration (1993–2011), both the volume and barycenter of the South Pacific Tropical Water that resurfaces in the equatorial Pacific are tightly linked to El Niño–Southern Oscillation (ENSO), with their correlation with the Niño-3.4 index reaching −0.79 and 0.84, respectively. Their correlation (−0.75 and 0.85) with the sea surface salinity index, Niño-S34.8, is also high. Of particular interest is that both the volume and barycenter of the resurfacing South Pacific Tropical Water peak earlier than the ENSO indices by about 3 months. On the interannual time scale, the resurfacing of South Pacific Tropical Water may modulate the sea surface salinity in the equatorial Pacific at a rate equivalent to as much as 25% of the surface freshwater flux. The results suggest that the resurfacing of South Pacific Tropical Water directly contributes to the sea surface salinity variability in the equatorial Pacific and potentially plays a role in ENSO evolution.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Dr. Shan Gao, gaoshan@qdio.ac.cn

Abstract

Analysis of results from a simulated passive tracer confirms the resurfacing of South Pacific Tropical Water in the equatorial Pacific. Over the period of integration (1993–2011), both the volume and barycenter of the South Pacific Tropical Water that resurfaces in the equatorial Pacific are tightly linked to El Niño–Southern Oscillation (ENSO), with their correlation with the Niño-3.4 index reaching −0.79 and 0.84, respectively. Their correlation (−0.75 and 0.85) with the sea surface salinity index, Niño-S34.8, is also high. Of particular interest is that both the volume and barycenter of the resurfacing South Pacific Tropical Water peak earlier than the ENSO indices by about 3 months. On the interannual time scale, the resurfacing of South Pacific Tropical Water may modulate the sea surface salinity in the equatorial Pacific at a rate equivalent to as much as 25% of the surface freshwater flux. The results suggest that the resurfacing of South Pacific Tropical Water directly contributes to the sea surface salinity variability in the equatorial Pacific and potentially plays a role in ENSO evolution.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Dr. Shan Gao, gaoshan@qdio.ac.cn

1. Introduction

The El Niño–Southern Oscillation (ENSO) is one of the most prominent phenomena in the world’s climate system. Studies of ENSO have been focused on the oscillatory nature of the ocean–atmosphere coupled system in the equatorial Pacific, in which different negative feedbacks are involved (e.g., Suarez and Schopf 1988; Graham and White 1988; Battisti and Hirst 1989; Cane et al. 1990; Weisberg and Wang 1997; Jin 1997; Picaut et al. 1997). To characterize this oscillatory nature of ENSO, a number of indices, including the atmospheric Southern Oscillation index (SOI) and the oceanic Niño-3, Niño-3.4, and Niño-4 sea surface temperature (SST) indices (e.g., Rasmusson and Carpenter 1982; Trenberth 1997), have been introduced and widely used over the past decades. As more sea surface salinity (SSS) observations become available, SSS indices have also been introduced in recent years, giving different perspectives to characterize ENSO. Among others, the Niño-S34.8 index is defined as the longitudinal location of the 34.8-psu isohaline along the equator (e.g., Delcroix 1998; Delcroix and Picaut 1998; Picaut et al. 2001; Maes et al. 2004; Bosc et al. 2009; Singh et al. 2011; Qu et al. 2014), the SSS ENSO index is defined as the difference of normalized SSS anomalies between regions 25°–10°S, 160°E–160°W and 2°S–2°N, 150°E–170°W (Singh et al. 2011), and the southeastern Pacific SSS index (SEPSI) is defined as a regional average of SSS anomalies at 0°–10°S, 150°–90°W (Qu and Yu 2014). These newly defined SSS indices complement the existing atmospheric and SST indices of El Niño. Investigating the variability of these SSS indices may help reveal new characteristics of ENSO.

To further demonstrate the relationship between SSS variability and ENSO represented by the SSS indices described above, Fig. 1a shows the first empirical orthogonal function (EOF) mode of the SSS variability in the equatorial Pacific, which explains about 36% of the total variance, based on the gridded Argo data product during 2005–14 from the Asian Pacific Data Research Center of the International Pacific Research Center, University of Hawaii. Despite some quantitative discrepancies, the spatial pattern of this EOF mode is similar to that shown by Singh et al. (2011), who based their study on relatively sparse but longer (1977–2008) SSS observations. The time series of this EOF mode corresponds quite well with ENSO, and its correlation with Niño-S34.8 reaches as high as 0.93 (Fig. 1c), reflecting its strong connection with the zonal displacement of the SSS front (e.g., Delcroix 1998; Singh et al. 2011; Qu et al. 2014). During the mature phase of El Niño, strong negative SSS anomalies near the date line shift the SSS front eastward, and the situation during the mature phase of La Niña is reversed.

Fig. 1.
Fig. 1.

First EOF mode of sea surface salinity (psu) from (a) Argo and (b) ECCO and (c) their time series compared with the SSS ENSO index, Niño-S34.8, and SEPSI from ECCO for the period 2005–14. The white lines in (a) and (b) indicate the equator. Values in (c) are normalized by their standard deviations.

Citation: Journal of Physical Oceanography 47, 5; 10.1175/JPO-D-16-0078.1

The time series of this first EOF mode is also highly (R = 0.86) correlated with the SSS ENSO index (Fig. 1c) introduced by Singh et al. (2011). As the SSS signature in the South Pacific convergence zone (25°–10°S, 160°E–160°W) is different during El Niño and La Niña, the SSS ENSO index reflects different characteristics between the two events. Singh et al. (2011) found a high (R = −0.88) correlation between the SSS ENSO index and SOI, using the gridded SSS product of Delcroix et al. (2011) for the tropical Pacific. This high correlation is confirmed with the newly available Argo data, reaching −0.86 for the period from 2005 to 2014.

Another important feature of SSS variability is the positive SSS anomalies in the eastern equatorial Pacific, with a core lying slightly south of the equator (Fig. 1a). As a regional average of these positive SSS anomalies, the SEPSI reflects strong influence of surface wind anomalies along the equator and has been shown to be effective for identifying the type of El Niño (e.g., Qu and Yu 2014). During the Argo period, the SEPSI peaked in 2009/10, when central Pacific El Niño or El Niño Modoki took place (e.g., Yu and Kao 2007; Ashok et al. 2007; Kug et al. 2009), while its anomaly during the 2006/07 eastern Pacific El Niño is relatively weak. The 2004/05 El Niño was also identified as a central Pacific El Niño (e.g., Ashok et al. 2007). The 13-month low-pass filter used in preparing Fig. 1c apparently reduced the value of SEPSI during this period (Qu and Yu 2014). The correlation of SEPSI with the first EOF mode time series reaches 0.88 (Fig. 1c), suggesting that the basinwide SSS variability in the equatorial Pacific is part of the coupled ocean–atmosphere system.

A number of previous studies have examined the processes that govern the SSS variability in the equatorial Pacific (e.g., Vialard et al. 2002; Maes et al. 2005; Maes 2008; Bosc et al. 2009; Singh et al. 2011; Hasson et al. 2013; Qu et al. 2014; Gao et al. 2014). Among others, Singh et al. (2011) attributed the SSS variability in the equatorial Pacific to changes in zonal currents and precipitation. Besides zonal current and precipitation, the role of subsurface processes was also recognized by these previous studies. For example, Maes (2008) noted that changes in ocean salinity stratification around the thermocline play an important role in the zonal displacement of the SSS front along the equator. Using results from an ocean general circulation model, Gao et al. (2014) also noted that the vertical entrainment and mixing of high-salinity water from the subsurface are of equal importance as surface freshwater flux and horizontal advection in generating the SSS variability in the equatorial Pacific.

Here, we hypothesize that the SSS variability in the equatorial Pacific is partially forced by the resurfacing of subtropical water. Water of subtropical origin may reach the equatorial Pacific and modulate the SSS through a shallow meridional circulation that is often referred to as the subtropical cell (McCreary and Lu 1994). The role of the subtropical cell in modulating the equatorial thermocline and SST has been extensively studied in the past decades, which among other things has been linked to the decadal variability of ENSO (e.g., Gu and Philander 1997; Luo et al. 2003; Zhang and McPhaden 2006). Less is known about its impacts on the equatorial halocline and SSS. This study focuses on the resurfacing of South Pacific Tropical Water (SPTW; e.g., Tsuchiya and Talley 1996; O’Connor et al. 2005) in the equatorial Pacific and its role in the SSS variability associated with ENSO, using results from a simulated passive tracer.

The rest of the paper is organized as follows: In section 2, we describe the model configuration and passive tracer experiments. In section 3, we describe the meridional circulation in the South Pacific, and in section 4, we examine the resurfacing of SPTW in the equatorial Pacific and its variability associated with ENSO. The impacts of resurfacing SPTW on the SSS and its associated SSS indices of El Niño are discussed in section 5, and the results are summarized in section 6.

2. Model configuration

Used for this study is a model of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO), based on the Massachusetts Institute of Technology general circulation model (Marshall et al. 1997). The model is nearly global, extending from 80°S to 80°N, with a horizontal resolution of 1° globally, except within 20° of the equator, where meridional grid spacing is gradually reduced to 0.3° within 10° of the equator. Vertical resolution varies from 10 m within 150 m of the surface to 400 m near the bottom of the ocean. The K-profile parameterization (KPP) vertical mixing scheme of Large et al. (1994) is employed for realistic simulation of near-surface mixing processes. Mixing effects of mesoscale eddies are represented using the Redi (1982) isoneutral mixing scheme and the Gent and McWilliams (1990) parameterization.

The model is initially at rest with climatological temperature and salinity and spun up for 10 yr by time-mean seasonal wind stress and heat flux based on the Comprehensive Ocean–Atmosphere Dataset (COADS). Following spin up, the model is forced from 1980 to 2014 by wind stress, heat flux, and evaporation minus precipitation (E − P) estimates of the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalyses (Kalnay et al. 1996). SST is relaxed to observe temperature (Reynolds and Smith 1994) with a spatially varying relaxation coefficient computed from the NCEP–NCAR products using the method employed by Barnier et al. (1995). The equivalent of a freshwater flux is implemented by relaxing SSS to climatological values with a 60-day relaxation coefficient. See Fukumori et al. (2004) for more details about this model configuration.

The ECCO model has been used by many previous studies (e.g., Fukumori et al. 2004; Qu et al. 2013; and references therein), suggesting that it simulates the ocean’s large-scale circulation and water property distribution well. In particular, the model reproduces most observed SSS features in the equatorial Pacific (Fig. 1b). For the period 2005–14, the first EOF mode from ECCO, which explains about 40% of the total variance, shows a similar pattern to that from Argo (Fig. 1a), except for a slightly northward shift of the positive SSS anomalies around the SEPSI region at 0°–10°S, 150°–90°W. The time series of this simulated EOF mode is strongly linked to ENSO. Its correlation with the SSS ENSO index and Niño-S34.8 reaches 0.97 and 0.96, respectively, showing a good agreement with the results from Argo (Fig. 1c). For comparison, the EOF analysis is also conducted for the period 1993–2011, during which the “offline” integration of a simulated passive tracer will be conducted (discussed in section 4). The results (not shown) from this EOF analysis are essentially the same as those shown in Figs. 1b and 1c. It seems that the simulated circulation and SSS variability in the equatorial Pacific have a good representation of the real ocean and thus the ECCO model is well suited for the present study.

3. Shallow meridional overturning circulation

The long-term mean meridional streamfunction simulated by ECCO clearly demonstrates the existence of a shallow meridional overturning circulation (Fig. 2), corresponding to the subtropical cell in the South Pacific (McCreary and Lu 1994). This meridional circulation consists of the subduction of subtropical water, equatorward geostrophic flow in the upper thermocline, upwelling along the equator, and poleward Ekman current near the sea surface. As part of this meridional circulation, roughly as much as 15 Sv (1 Sv ≡ 106 m3 s−1) of subtropical water subducts to the upper thermocline at latitudes between 10° and 30°S (Fig. 2), and from there the water enters the equatorial Pacific either through its interior or western boundary pathway (e.g., Fine et al. 1994; Johnson and McPhaden 1999; Qu et al. 2013). Up reaching the equatorial region, some of this subtropical water reenters the surface mixed layer and returns to the subtropics as part of the Ekman current to close the overturning circulation. With its high-salinity signature, the resurfacing of subtropical water may alter the SSS in the equatorial Pacific by vertical entrainment and mixing (e.g., Gao et al. 2014). The details are further investigated below.

Fig. 2.
Fig. 2.

Long-term (1993–2014) mean meridional overturning streamfunction (Sv; contour) superimposed with zonally averaged ocean salinity (psu; shading) from ECCO.

Citation: Journal of Physical Oceanography 47, 5; 10.1175/JPO-D-16-0078.1

4. Resurfacing of SPTW

The resurfacing of SPTW is investigated using a simulated passive tracer from ECCO (Fukumori et al. 2004). In the context of a general circulation model, the temporal evolution of a passive tracer is dictated by the same advection–diffusion equation as temperature and salinity. If a particular patch of water is uniformly initialized by a passive tracer with no other sources or sinks, the subsequent movement of this tracer indicates the pathway of the initial patch of water. In particular, the relative magnitude of the tracer at a given location to the value of the initial patch describes the concentration of this initial water mass at the location in question.

The offline circulation from ECCO for the period from January 1993 to December 2011 is available for the present study. The passive tracer is initialized each month with a unit value (arbitrary tracer units per volume, atu m−3) in the mixed layer of the SSS maximum region (marked by the white box in Fig. 3) and integrated forward in time using the velocity and mixing tensors of the model’s offline circulation averaged at 10-day intervals from January 1993 to December 2011. To minimize the effect of the initial condition, a spinup is conducted by the recurrent use of the offline circulation in 1993 for 10 yr. When a tracer-tagged water parcel reenters the surface mixed layer in the equatorial Pacific, it is considered to be resurfaced, and no tracking of this water parcel will be further conducted. Computationally, the tracer in the surface mixed layer is tabulated and reset to zero after each model time step. The monthly integrated tabulated tracer content represents the amount of initially tagged SPTW that has resurfaced in the equatorial Pacific until the end of each month.

Fig. 3.
Fig. 3.

Long-term (1993–2014) mean sea surface salinity (psu) from ECCO. The white rectangle shows the sea surface salinity maximum in the subtropical South Pacific, where a passive tracer is released in the surface mixed layer.

Citation: Journal of Physical Oceanography 47, 5; 10.1175/JPO-D-16-0078.1

On the long-term (1993–2011) average, the high-salinity SPTW is seen to enter the surface mixed layer in the central equatorial Pacific, mostly between 180° and 120°W, with a mean barycenter lying at about 151°W (Fig. 4a). This spatial pattern of passive tracer is consistent with the vertical distribution of salinity (Fig. 4b) and temperature (Fig. 4c). West of the date line, water is well stratified, and a layer of fresh surface water covers the salty subsurface water, forming a barrier layer between the base of the mixed layer and the top of the thermocline (Lukas and Lindstrom 1991). The thickest barrier layer is present near the eastern edge of the warm pool (e.g., Delcroix 1998; Maes et al. 2004; Bosc et al. 2009; Singh et al. 2011; Qu et al. 2014). It gets thinner eastward and gradually disappears in the central equatorial Pacific, corresponding well with the resurfacing of SPTW (Fig. 4a).

Fig. 4.
Fig. 4.

(top) Long-term (1993–2011) mean passive tracer (atu m−2 month−1) that enters the surface mixed layer per unit horizontal area per month, and vertical distribution of (middle) salinity (psu) and (bottom) temperature (°C) averaged in the equatorial band (3°S–3°N). The white solid and dashed lines denote the top of the thermocline and the base of the mixed layer, respectively. The depth of the thermocline is defined using a temperature difference of 0.5°C from the surface value, and the depth of the mixed layer is defined using a density difference that is equivalent to a temperature decrease of 0.5°C.

Citation: Journal of Physical Oceanography 47, 5; 10.1175/JPO-D-16-0078.1

The resurfacing of SPTW in the central equatorial Pacific forms a sharp contrast in salinity with the freshwater in the western equatorial Pacific (Fig. 4b), which is apparently linked to the SSS front near the date line. The longitudinal location of the SSS front has been used as an index of El Niño, termed Niño-S34.8, to characterize ENSO evolution (e.g., Delcroix 1998; Maes et al. 2004; Singh et al. 2011; Qu and Yu 2014). On interannual time scale, both the volume and barycenter of the SPTW-tagged passive tracer that reenters the surface mixed layer in the equatorial Pacific are highly correlated with Niño-S34.8, with their correlation reaching −0.75 and 0.85, respectively (Fig. 5). High correlations with the SSS ENSO index (−0.69 and 0.78) and Niño-3.4 (−0.79 and 0.84) are also found, suggesting their potential importance in modulating the SSS in the equatorial Pacific. Of particular interest is that both the volume and barycenter lead these ENSO indices by about 3 months, which is consistent with the fluctuations of vertical entrainment and mixing at the base of the mixed layer (Gao et al. 2014). In other words, the resurfacing of SPTW reaches its minimum volume and easternmost position about 3 months prior to the mature phase of El Niño. The situation during La Niña events is reversed (Fig. 5).

Fig. 5.
Fig. 5.

(a) Normalized volume transport (1013 atu month−1) and (b) longitudinal location (degree longitude) of barycenter of the passive tracer that enters the surface mixed layer in the equatorial Pacific (3°S–3°N) each month compared with Niño-3.4 and Niño-S34.8 from ECCO for the period 1993–2011. The long-term mean values of 7.06 × 1012 atu month−1 and 151°W have been removed from the time series before plotting.

Citation: Journal of Physical Oceanography 47, 5; 10.1175/JPO-D-16-0078.1

To further illustrate the relationship described above, we conduct a composite analysis of four (1997/98, 2002/03, 2006/07, and 2009/10) El Niño and three (1998/99, 1999/2000, and 2007/08) La Niña events during the period of integration. Given the phase difference between the resurfacing SPTW and ENSO, Fig. 6 shows the horizontal distribution of the SPTW-tagged passive tracer that resurfaces in the equatorial Pacific per unit horizontal area compared with the vertical distribution of salinity and temperature along the equator averaged in September–November of the composite events, about 3 months prior to the mature phase of ENSO. During this period of the El Niño event, less than normal (5.7 × 1012 vs 6.7 × 1012 atu) SPTW-tagged passive tracer resurfaces in the equatorial Pacific, and the location of its barycenter shifts eastward by about 20° longitudes from its mean location (Fig. 6a). During this period of the La Niña event, the resurfacing of the SPTW-tagged passive tracer reaches its maximum volume (7.4 × 1012 atu) and westernmost location (170°W; Fig. 6b).

Fig. 6.
Fig. 6.

(top) Passive tracer (atu m−2 month−1) that enters the surface mixed layer per unit horizontal area per month superimposed with sea surface salinity (psu) and vertical distribution of (middle) salinity (psu) and (bottom) temperature (°C) averaged in the equatorial band (3°S–3°N) in September–November during the composite (left) El Niño and (right) La Niña events. The white solid and dashed lines denote the top of the thermocline and the base of the mixed layer, respectively.

Citation: Journal of Physical Oceanography 47, 5; 10.1175/JPO-D-16-0078.1

The zonal displacement of the barycenter corresponds well with the vertical distribution of salinity and temperature along the equator. During the composite El Niño event, relatively less high-salinity water approaches the sea surface in the central equatorial Pacific (Fig. 6c). Most of this water extends farther eastward in the depth range of the thermocline. During the composite La Niña event, more high-salinity water reaches the sea surface at longitudes between 160°E and 140°W, farther westward by about 40° longitude than during the El Niño event (Fig. 6d), directly contributing to the westward shift of the SSS front. Closely related to the zonal displacement of the SSS front, the eastern edge of the warm pool moves eastward during the El Niño event (Fig. 6e) and westward during the La Niña event (Fig. 6f). This alters the thermocline structure and surface stratification and consequently shifts the thick barrier layer back and forth along the equator (e.g., Bosc et al. 2009; Qu et al. 2014), which is believed to play a role in ENSO evolution (e.g., Maes et al. 2005).

5. Impacts on sea surface salinity

Figure 7 shows the anomalies of passive tracer that reenters the surface mixed layer per unit horizontal area averaged in September–November compared with the anomalies of sea surface salinity and temperature during the mature phase (December–February) of El Niño and La Niña. Strong negative anomalies are present in the western equatorial Pacific, roughly west of 140°W, prior to the mature phase of El Niño, while the positive anomalies in the eastern equatorial Pacific are relatively weak (Fig. 7a). The situation prior to the mature phase of La Niña (September–November) is the opposite (Fig. 7b). These passive tracer anomalies in the equatorial Pacific correspond well with the SSS anomalies (Figs. 7c,d). During the mature phase of El Niño, when the equatorial Pacific gets warmer (Fig. 7e), the center of precipitation moves eastward, and as a result of the combined effects of precipitation with horizontal advection and vertical entrainment and mixing, the western Pacific freshwater pool also moves eastward, which largely explains the negative SSS anomalies near the date line (e.g., Maes et al. 2006; Bosc et al. 2009; Singh et al. 2011; Hasson et al. 2013; Gao et al. 2014). During the mature phase of La Niña, when the western Pacific warm pool retreats westward (Fig. 7f), positive SSS anomalies take place west of the date line, directly contributing to the zonal displacement of the SSS front and consequently the variability of Niño-S34.8 (e.g., Qu and Yu 2014).

Fig. 7.
Fig. 7.

(top) Anomalous passive tracer (atu m−2 month−1) that enters the surface mixed layer per unit horizontal area per month averaged in September–November and (middle) anomalous sea surface salinity (psu) and (bottom) temperature (°C) averaged in December–February during the composite (left) El Niño and (right) La Niña events.

Citation: Journal of Physical Oceanography 47, 5; 10.1175/JPO-D-16-0078.1

Now we focus on the impacts of resurfacing SPTW, a question raised at the beginning of the paper (section 1). Recently, using results from the same model, Gao et al. (2014) noted that the vertical entrainment and mixing of high-salinity water from the subsurface are of equal importance as surface freshwater flux and horizontal advection in generating the SSS variability in the equatorial Pacific. As one can see from Fig. 6 (middle panels), a sharp zonal salinity front exists near the date line in the upper 50 m or deeper. East of this front, high-salinity water of subtropical origin extends all the way to the sea surface, in contrast to the freshwater west of the front. This subsurface front moves consistently with the SSS front, eastward during El Niño events and westward during La Niña events. Local processes (e.g., precipitation, horizontal advection) alone do not appear to explain the zonal displacement of this salinity front. Here, we emphasize the importance of resurfacing SPTW. The details are discussed below.

As is well known, the vertical entrainment of high-salinity water from the subsurface depends on the vertical salinity gradient or, as used by many previous studies, the salinity jump across the base of the mixed layer. The resurfacing of SPTW may immediately alter the surface stratification and reduce the salinity jump. But its role in SSS variability cannot be easily estimated by salinity budget analyses. The problem is that once SPTW reaches the sea surface, the salinity jump across the base of the mixed layer usually becomes too small (<0.2 psu in Figs. 6c,d), and as a consequence, the vertical entrainment of this high-salinity water cannot account for a significant part of the SSS anomalies. We argue that, in order to precisely estimate the impacts of resurfacing SPTW, a salinity jump before the water reaches the sea surface, which can be as large as 0.8 psu in the equatorial Pacific, should be used.

Based on the idea described above, Fig. 8 shows the long-term (1993–2011) mean differences between the tracer-tagged SPTW salinity (35.4 psu) under the mixed layer in the equatorial Pacific (10°S–10°N) and the climatological-mean SSS averaged in September–November. These salinity differences are mostly positive in the equatorial Pacific, except for slightly negative values in the central part of the basin. These negative values represent strong influence of the SSS maximum water in the subtropical South Pacific through an interior pathway (e.g., Qu et al. 2013). With the salinity differences shown in Fig. 8, the resurfacing of SPTW may raise SSS in the western and eastern equatorial Pacific but slightly reduce it in the central part of the basin. As both the volume and barycenter of the resurfacing SPTW vary with time (Fig. 5), this water of subtropical origin may contribute to the SSS variability in the equatorial Pacific.

Fig. 8.
Fig. 8.

Long-term (1993–2011) mean difference (psu) between the tracer-tagged SPTW salinity averaged in the equatorial Pacific (10°S–10°N) below the surface mixed layer and the climatological SSS in September–November.

Citation: Journal of Physical Oceanography 47, 5; 10.1175/JPO-D-16-0078.1

To quantify the contribution from the resurfacing SPTW, we calculate the vertical entrainment term equivalent to surface freshwater flux WeΔS/S converted from the passive tracer integration (e.g., Foltz et al. 2004; Ren and Riser 2009; Qu et al. 2011; Katsura et al. 2013). Here, We is the rate of the resurfacing of SPTW, representing the integral effects of mean circulation, eddies, and various small-scale processes. The quantity ΔS is the salinity jump across the base of the mixed layer. The mean salinity of the region is S. Using the rate of the resurfacing of SPTW derived from the passive tracer integration (Figs. 6a,b, 7a,b) and the salinity jump shown in Fig. 8, we calculate the vertical entrainment of the resurfacing SPTW equivalent to surface freshwater flux in the equatorial Pacific (Fig. 9). Prior to the mature phase (September–November) of El Niño, the negative anomalies of vertical entrainment act to freshen the surface water in a large part of the equatorial Pacific, mostly west of 140°W (Fig. 9a). Its maximum, equivalent to a surface freshwater flux of 26 mm month−1, occurs at 160°E–180°, coinciding with the strong negative SSS anomalies in December–February (Fig. 7c). The resurfacing of SPTW during this period of time is slightly enhanced in the eastern equatorial Pacific (Fig. 7a), generating positive anomalies of vertical entrainment with their maximum equivalent to a surface freshwater flux of about −13 mm month−1 (Fig. 9a). These positive anomalies of vertical entrainment add to processes that increase SSS in the SEPSI region (e.g., Qu and Yu 2014). Prior to the mature phase (September–November) of La Niña, the opposite takes place, and the vertical entrainment of SPTW (Fig. 9b) directly contributes to the positive SSS anomalies in the western equatorial Pacific and negative SSS anomalies in the eastern equatorial Pacific (Fig. 7d).

Fig. 9.
Fig. 9.

(top) Anomalous vertical entrainment equivalent to surface freshwater flux (mm month−1) converted from the resurfacing passive tracer and (bottom) anomalous E − P from ECCO averaged in September–November during the composite (left) El Niño and (right) La Niña events. For convenience the equivalent freshwater flux in the top panels is multiplied by −1 before plotting.

Citation: Journal of Physical Oceanography 47, 5; 10.1175/JPO-D-16-0078.1

Now it seems clear that the vertical entrainment of SPTW has a notable impact on SSS in the equatorial Pacific. On the interannual time scale, this impact can reach up to 25% of the surface freshwater flux before ENSO is fully developed (Figs. 9c,d). In space, the vertical entrainment of SPTW (Figs. 9a,b) shows a similar pattern to the first EOF mode of SSS variability (Figs. 1a,b), with negative anomalies near the date line and positive anomalies in the eastern part of the basin during El Niño events. The negative anomalies of vertical entrainment near the date line correspond well with the eastward displacement of the SSS front or Niño-S34.8 (e.g., Singh et al. 2011; Qu et al. 2014), while the positive anomalies in the eastern part of the basin are related to SEPSI (e.g., Qu and Yu 2014). As a result of weak salinity stratification (Figs. 7c,d, 8), the anomalies of vertical entrainment caused by the resurfacing of SPTW are minor in the central equatorial Pacific (Figs. 9a,b). With a maximum salinity jump of up to 0.8 psu, it seems that the resurfacing of SPTW (Fig. 3) can contribute to the SSS variability and its associated SSS indices in the equatorial Pacific and potentially play a role in ENSO evolution. A detailed salinity budget analysis based on this idea will be conducted in a separate study.

6. Summary and discussion

The present study focuses on the resurfacing of SPTW in the equatorial Pacific and its variability associated with ENSO. The results demonstrate that both the volume and barycenter of the resurfacing SPTW are highly correlated with ENSO indices. The resurfacing SPTW reaches its minimum volume and easternmost position during El Niño years and maximum volume and westernmost position during La Niña years. Of particular interest is that both the volume and barycenter of the resurfacing SPTW lead ENSO by about 3 months, which is consistent with the fluctuation of vertical entrainment and mixing at the base of the mixed layer (e.g., Gao et al. 2014).

The impacts of resurfacing SPTW on the SSS in the equatorial Pacific cannot be easily estimated by salinity budget analyses because once the water reaches the sea surface, the salinity jump across the base of the mixed layer will become too small, leading to an underestimate of the vertical entrainment of SPTW. Here, we suggest that to better estimate the impacts of resurfacing SPTW, a salinity jump before rather than after the water reaches the sea surface should be used. To do so, we first average the salinity of tracer-tagged SPTW under the surface mixed layer in the equatorial Pacific (10°S–10°N) and then use its difference from the climatological-mean SSS in September–November as a salinity jump for the budget analysis. With this salinity jump, we are able to quantify the vertical entrainment of SPTW and its impacts on SSS in the equatorial Pacific. On interannual time scale, the anomalies of this vertical entrainment can reach up to 26 mm month−1 in magnitude or about 25% of the anomalies of surface freshwater flux. Since both its volume and barycenter lead the mature phase of ENSO by about 3 months, we speculate that the resurfacing of SPTW may directly contribute to the sea surface salinity variability in the equatorial Pacific and play some active role in ENSO evolution.

Finally, we note that, in addition to the SPTW, there are several other subtropical waters, including the North Pacific Tropical Water (NPTW) and subtropical mode waters (STMW) in both hemispheres (e.g., Hanawa and Talley 2001). The NPTW is identified as a subsurface salinity maximum formed in the center of the North Pacific Subtropical Gyre, where evaporation is excess over precipitation (e.g., Tsuchiya 1968; Fine et al. 1994; Suga et al. 2000). According to a recent study by Nie et al. (2016), up to 18% of the NPTW enters the surface mixed layer in the equatorial Pacific. The resurfacing of this relatively cold and salty subtropical water is believed to play a role in modulating the sea surface salinity in the equatorial Pacific. The STMWs are identified as vertically homogeneous layers in the thermocline of the ocean and consist of the eastern, western, and central STMWs in the Pacific [see Hanawa and Talley (2001) and references therein]. The STMWs have been implicated as an important agent for subsurface decadal variability as well as the uptake of climatologically important gases (e.g., Yasuda and Hanawa 1997; Fine et al. 2011). Their role in sea surface salinity variability and ENSO evolution in the equatorial Pacific needs to be investigated further by research.

Acknowledgments

This study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences through Grant XDA11010102 and the National Natural Science Foundation of China through Grants 61233013 and 41676009. T. Qu was supported by the National Science Foundation through Grant OCE11-30050. (The gridded Argo data products were obtained from http://apdrc.soest.hawaii.edu/, and the ECCO model outputs were obtained from http://ecco.jpl.nasa.gov/.) The authors are grateful to R. Fine and X. Lu for frequent communication and discussion on the topic and to two anonymous reviewers for valuable comments and thoughtful suggestions.

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