Observations of vertical turbulent nitrate flux across the Kuroshio

Authors


  • This article was corrected on 27 FEB 2015. See the end of the full text for details.

Abstract

[1] The vertical turbulent nitrate flux was calculated across the Kuroshio based on direct turbulence measurements and nitrate concentrations. Profile data were analyzed relative to the main axis of the Kuroshio front, the location of maximum surface velocity. Large upward nitrate flux on the northern side of the front, O(10−6) mmol N m−2 s−1, was observed from 300 m up to the base of the euphotic layer. Enhanced turbulence and a large vertical nitrate gradient contributed to this flux. On this side, high concentrations of chlorophyll-a were also observed near the surface, and the large nitrate flux is consistent with a previous estimate of the rate of new production. In contrast, south of the axis, the nitrate flux was O(10–8–10−7) mmol N m−2 s−1 because of the small vertical nitrate gradient, although vertical diffusivity at the euphotic layer base remained relatively high.

1 Introduction

[2] The Kuroshio is a western boundary current of the North Pacific and is accompanied by large horizontal gradients of temperature, density, nutrient, and other factors (i.e., a front) in its cross-current direction. The front is generally observed on the northern (dense) side of the downstream current maximum of the Kuroshio; and along the front, a band of high chlorophyll-a is frequently observed by satellites in mid-to-late spring (Figure 1). Such a high chlorophyll-a band implies nutrient supply into the euphotic layer, even after the development of the surface pycnocline.

Figure 1.

Study sites and stations for VMP and CTD observations. Monthly averaged chlorophyll-a concentration (April 2009) and absolute dynamic topography (available from AVISO; 15 April 2009) are shown as an example of the conditions. Some stations located on the northern side of the Kuroshio front are labeled as examples. Note that sections along 138°E and 34°40′N were measured twice and thus they are drawn partly overlapping.

[3] Although some previous studies suggested that nutrients might be supplied through upwelling associated with the mesoscale eddies that are frequently found around fronts [Kimura et al., 1997, 2000; McGillicuddy et al., 1998, 2007; Chu and Kuo, 2010; Sasai et al., 2010; Sumata et al., 2010], it has also been suggested that nutrients are supplied by the vertical transport on a narrower horizontal scale that occurs very close to the front and rim of such eddies [Mahadevan and Archer, 2000; Lévy et al., 2001; Mahadevan et al., 2008]. Recent studies conducted in other frontal regions have revealed that vertical transport due to turbulent mixing is important for nutrient supply on the dense side of the front (the Gulf Stream [Hales et al., 2009] and the California Current [Johnston et al., 2011]). Hales et al. [2009] reported that nitrate fluxes on the dense side of the front were sufficient to support net community productivity.

[4] Near the Kuroshio, recent direct measurements of turbulence have shown enhanced turbulent mixing on the dense side of the front [Nagai et al., 2009, 2012; D'Asaro et al., 2011; Kaneko et al., 2012]. Some of this enhancement in turbulence is thought to be triggered by frontogenesis and associated secondary ageostrophic circulation [Nagai et al., 2009], and symmetric instability [D'Asaro et al., 2011; Nagai et al., 2012]. Interaction between the Kuroshio jet and internal waves could also contribute to the enhanced turbulence on the dense side [e.g., Rainville and Pinkel, 2004]. The turbulence may affect vertical turbulent nutrient flux, because a large vertical gradient of nutrients (i.e., a nutricline) at shallow depth was observed on the northern side, associated with the Kuroshio current and density structure [Kimura et al., 1997]. On the other hand, turbulence enhancement on the opposite (southern) side of the Kuroshio Current core, where internal waves may be trapped [Kunze, 1985], has been reported by Kaneko et al. [2012] and Nagai et al. [2012].

[5] Around the Kuroshio, the cross-frontal distribution of vertical turbulent nutrient flux has not yet been fully described because of insufficient simultaneous observations of turbulence and nutrients away from the shelf edge. In the present study, which focuses on nitrate, an important nutrient, the distribution of mean turbulent nitrate flux across the Kuroshio jet was obtained by estimating the composite distributions of nitrate and turbulence as functions of a coordinate system representing the distance from the surface jet axis. We discuss the factors that control the distribution of the flux and that lead to the band of high chlorophyll-a along the frontal zone north of the Kuroshio axis.

2 Data and Methods

[6] We used data from nine crosssections of turbulence and nitrate observations. These were obtained generally at intervals of 20 km during R/V Tansei-Maru cruises that crossed the Kuroshio axis in the springs of 2008 (30 April to 5 May), 2009 (6–16 April), and 2011 (14–22 April) (Figure 1). Turbulence observations were made with vertical microstructure profilers (hereafter VMPs; VMP2000 or VMP500; Rockland Scientific International) equipped with two shear probes temperature and conductivity sensors (SBE3 and SBE4, respectively; Sea-Bird Electronics). Conductivity-Temperature-Depth (CTD) observations were also made with an SBE 911plus (Sea-Bird Electronics) in 2011, and ICTD (Falmouth Scientific, Inc.) in 2008 and 2009. A nitrate profiler (In Situ Ultraviolet Spectrophotometer, Satlantic) and fluorometer (AQUA tracka3 (Chelsea Technologies Group) in 2011, and Seapoint Chlorophyll Fluorometer (Seapoint Sensors, Inc.) in 2008 and 2009) were attached to a mounting frame on the CTD, providing vertical distributions of nitrate and chlorophyll-a, respectively, at about 1 m intervals, smoothed using a 20 m running mean. Photosynthetically active radiation (PAR) was observed from 5 m to 200 m using an Integrating Natural Fluorometer (INF-300; Biospherical Instruments). Horizontal velocity, starting at 26 m depth, was measured at 40 levels at 16 m vertical intervals, using a vessel-mounted acoustic Doppler current profiler (Ocean Surveyor 75 kHz; Teledyne RD Instruments).

[7] The turbulent energy dissipation rate ε was calculated following Kaneko et al. [2012]. Shear spectra estimated from segments of about 10 m (16 s with a fall rate of ~0.6 m s−1) were integrated over the wave number to obtain ε. The spectra were verified to be consistent with Nasmyth's universal spectrum [Oakey, 1982] (Figure 2). Data from the surface to 50 m depth, where the VMPs were suspected to be tilted, and data below the noise level (ε < 3 × 10−10 W kg−1) were not used.

Figure 2.

Examples of turbulent shear spectra and Nasmyth shear spectra. Numbers and colors of each station correspond to those in Figure 1.

[8] Along-stream and cross-stream velocities were obtained, averaged in 5 min segments, at 1 min intervals and then spatial means were calculated at a horizontal interval of 0.5 km. The along- and cross-stream coordinates were labeled x and y, respectively. We calculated a composite mean of each variable including ε, potential density σθ, nitrate concentration, and chlorophyll-a using a coordinate that was zero at the surface velocity maximum (referred to as the “axis”, as in Kaneko et al. [2012]). The mean was calculated at sub-grids with horizontal and vertical intervals of 25 km and 50 m, respectively. The composite mean is denoted by an overbar. Confidence intervals at the 95% level (hereafter 95% CI) were estimated using the bootstrap method.

[9] The mean vertical diffusivity was determined from inline image according to Osborn [1980], with Γ = 0.2. The mean squared buoyancy frequency was estimated as inline image, where g and ρ0 are the gravitational acceleration and reference potential density, respectively. The vertical turbulent flux of nitrate, inline image, was estimated using inline imageinline image, where inline image is the mean nitrate concentration, and inline image is the mean vertical gradient of nitrate (upward is positive). The mean relative vorticity was defined as inline image, assuming inline image is negligible, where u (v) is the velocity along (across) the Kuroshio. Based on downward observations of PAR, the base of the euphotic layer was defined as the depth where the PAR fell to 1% of its value at 5 m [Venrick et al., 1973].

3 Results

[10] The composite cross-frontal structures of density and velocity were consistent with those described in previous studies of the Kuroshio [e.g., Taft, 1978; Howe et al., 2009] and of the Gulf Stream [e.g., Halkin and Rossby, 1985] (Figure 3a). The composite mean potential density, inline image, showed a large horizontal density gradient at depths of 0–300 m on the northern side of the axis (y = 0 to 50 km), indicating an intense density front on this side. The horizontal density gradient was smaller on the southern side of the axis than on the northern side. A slanting pycnocline (a strong vertical density gradient) was indicated by sloping and closely spaced isopycnals. The isopycnals of 25.0–26.0 inline image were shallow on the northern side and deep on the southern side. The composite mean downstream velocity inline image decreased more sharply with increasing y on the northern side, where the intense density front and large positive relative vorticity (inline image > 1 × 10−5 s−1) were observed at depths of 0–300 m from y = 0 to 50 km. The velocity maximum shifted toward the south with increasing depth.

Figure 3.

Composite mean structure across the Kuroshio jet axis (indicated by 0 km along the horizontal axis), showing (a) downstream current velocity inline image, (b) chlorophyll-a, (c) nitrate concentration, (d) vertical nitrate gradient inline image, (e) vertical turbulent vertical nitrate flux inline image, (f) inline image at 1% light depth (g) vertical diffusivity inline image, and (h) depth-averaged inline image. Thick and thin lines in Figures 3f and 3h denote the average and the 95% confidence interval, respectively.

[11] Chlorophyll-a concentration in the euphotic layer varied across the Kuroshio axis (Figure 3b). On the northern side of the axis, chlorophyll-a concentrations were high (>1.0 mg m−3), with a horizontal maximum of chlorophyll-a (1.2 mg m–3) in the frontal zone at y = 25 to 50 km. This horizontal maximum is thought to represent the band of high chlorophyll-a along the northern edge of the Kuroshio (e.g., Figure 1, especially 144–146°E). In contrast, chlorophyll-a on the southern side was relatively low (<0.66 mg m−3).

[12] The nitrate distribution below the euphotic layer was analogous to that for density; nitrate concentration was generally similar along each isopycnal and increased with increasing density as described by Kimura et al. [1997] (Figure 3c). Since the isopycnals slant and are shallower on the northern side, relatively high nitrate concentrations were distributed just beneath the base of the euphotic layer (~100 m) on the northern side. In contrast, high nitrate concentrations of >10 mmol N m−3 were observed only at depths of >200 m on the southern side of the axis. Nitrate near the base of the euphotic layer showed a remarkable difference across the Kuroshio axis between the nutrient-rich north and nutrient-poor south. For example, at 100 m depth, which is just below the 1% light depth, nitrate concentration was >5.0 mmol N m−3 on the northern side, but <2.0 mmol N m−3 on the southern side. Large vertical nitrate gradient was located near the stratification maximum (25.0–26.6 inline image) that implies the mixed layer base on the northern side and the base during winter on the southern side (Figure 3d).

[13] A large vertical turbulent nitrate flux, O(10–6) mmol N m−2 s−1, was observed in the domain 50–300 m depth and y = 0 to 50 km, which corresponds to the intense frontal zone (Figure 3e). The flux at the 1% light depth had a maximum of 4 × 10−6 mmol N m−2 s−1 (95% CI: 2–7 × 10−6 mmol N m−2 s−1) from y = 25 to y = 50 km (Figure 3f). The location of the horizontal maximum of the flux at the base of the euphotic layer agreed with that for the horizontal maximum of chlorophyll-a in the euphotic zone (Figure 3b).

[14] The large vertical turbulent nitrate flux on the northern (dense) side coincided with large inline image (Figure 3g). In particular, from y = 25 to 50 km, where the maximum flux at the 1% light depth was attained, a large inline image value (5 × 10−5 m2 s−1 with 95% CI: 3–9 × 10−5 m2 s−1) was also observed (Figure 3h). Down to 300 m depth, below the 1% light depth, the flux in the zone from y = 25 to 50 km remained high (O(10−6) mmol N m−2 s−1; Figure 4a). This large flux from the subsurface to mid-depth results mainly from the large inline image at depths of 50–300 m (Figure 4b). The importance of the frontal zone turbulence to the vertical flux was also suggested by the fact that the flux at the 1% light depth was relatively small in the region far north of the front where currents were weak (y = 50–75 km; Figure 4a), because of the weaker vertical diffusivity (Figure 4b), even though nitrate concentration increased rapidly with depth there (Figure 4c), with a large vertical gradient (Figure 4d).

Figure 4.

Vertical profiles of (a) inline image, (b) inline image, (c) nitrate, and (d) inline image. Thick and thin lines denote the average and the 95% confidence interval, respectively.

[15] In contrast, on the southern side of the axis, the flux was O(10−8–10−7) mmol N m−2 s−1, one to two orders of magnitude smaller than that for y = 0 to 50 km (Figure 3e). The flux at the 1% light depth on this side was far smaller than that on the northern side (Figure 3f). In addition, chlorophyll-a concentration in the euphotic zone was low (Figure 3b). This small flux on the southern side resulted from the weak vertical nitrate gradient, nearly zero (0.0009–0.004 mmol N m−4 at depths of 0–150 m; Figure 3d), even with the relatively large vertical diffusivity (Figure 3g). This point is clearly shown in the vertical profiles for y = −50 to −25 km presented in Figure 4 for the southern side. The vertical gradient of nitrate around the 1% light depth was nearly zero, and the maximum gradient was observed at 300–400 m, far below the euphotic layer.

4 Discussion

[16] The calculations of turbulent nitrate flux at the base of the euphotic layer suggest that the large upward turbulent nitrate supply in the frontal zone of the Kuroshio could maintain the large phytoplankton population indicated by the high chlorophyll-a concentration (Figures 1 and 3b). The large turbulent nitrate flux (4 × 10−6 mmol N m−2 s−1, with 95% CI: 2–7 × 10−6 mmol N m−2 s−1) observed at the 1% light level in the frontal zone (y = 25 to 50 km) was comparable to the new production rate of 3.1–4.2 × 10−6 mmol N m−2 s−1 calculated from the spring-mean primary production rate (April and May) in the Kuroshio region, as reported by Yokouchi et al. [2006]. This calculation is similar to that of Hales et al. [2009] and assumes that the f ratio (the ratio of new production to primary production) is 0.1 and the Redfield ratio C:N = 106:16 (where C is carbon and N is nitrogen). The observation that the large turbulent nitrate flux on the northern side of the front could maintain the observed phytoplankton production is consistent with the findings of Hales et al. [2009] and Johnston et al. [2011]. It also emphasizes the importance of the nutrient supply that occurs on scales smaller than mesoscale, as suggested by Mahadevan et al. [2008], in that the composite section presented by this study is relative to the Kuroshio as it meanders along the rims of the mesoscale eddies (Figure 1).

[17] The enhancement of turbulence and the consequent large vertical nitrate flux on the dense side along the Kuroshio front could be ubiquitous. Turbulence enhancement has been observed in different seasons by previous studies; e.g., from April to May by Kaneko et al. [2012], and in August and October by Nagai et al. [2012]. In addition, this kind of turbulent enhancement would be triggered by inherently frontal processes, as mentioned in section 1. Thus, the northern (dense) side of the Kuroshio is suggested as the area where abundant nitrate may be pulled up toward the surface from the depths where nitrate is rich, since the enhanced flux on the northern (dense) side is driven by the large vertical diffusivity as well as the steep vertical gradient of nitrate (Figure 4).

[18] It can be presumed that the northern (dense) side of the Kuroshio is a favorable feeding ground for fishery resources such as the Japanese sardine Sardinops melanostictus, and Japanese anchovy Engraulis japonicus. Nutrient supply to the surface after the spring stratification due to the turbulent mixing could lead to the high concentration of phytoplankton around the Kuroshio front, which would have an effect on the abundance of zooplankton (e.g., through effects on egg production as suggested by Nakata et al. [2004]) and fishes, in particular larvae, which feed on zooplankton as reported previously [Nakata et al., 2000; Okazaki et al., 2002, 2003]. This may be one reason why the Kuroshio is an important nursery ground for many pelagic fishes. The influence of the vertical nitrate transport on plankton and fisheries productivity should be carefully examined by further studies that include more detailed phytoplankton processes, such as the impact of light inhibition in summer.

[19] In contrast to the northern side, the nitrate flux at the 1% light depth was small on the southern side, because the vertical nitrate gradient was almost zero. Even if the nitrate concentration of 0.8–1.1 mmol N m−3 in the euphotic layer (<100 m) was assumed to be consumed and depleted by subsequent biological activity, the gradient of nitrate at the euphotic zone base inline image would be 0.026–0.04 mmol N m−4, where inline image = 1.3–2.0 mmol N m−3 at depths of 100–150 m and Δz = 50 m. Using the vertical diffusivity of 0.7–2.0 × 10−5 m2 s−1 observed for y = −50 to −25 km (Figure 4b) and the estimated vertical nitrate gradient, the flux at the 1% light depth was estimated to be 0.2–0.8 × 10−6 mmol N m−2 s−1. This estimated flux is still less than in the frontal zone north of the axis. The small vertical gradient of nitrate at depths of 0–150 m south of the axis may result from the well-developed winter mixed layer reaching 200–300 m; this has been referred to as subtropical mode water [Hanawa and Talley, 2001; Suga et al., 2004]. Thus, the present results emphasize the importance of the vertical distribution of nutrients in subtropical mode water on their vertical fluxes.

Acknowledgments

[20] We thank the captain, officers, and crew of the R/V Tansei-Maru. We thank all onboard scientists for their help in deploying VMPs, and Dr. Ikeya for his support in determining calibration coefficients of the nitrate profilers. This study was supported by a Grant-in-Aid for Scientific Research (S) (#20221002) and Scientific Research on Priority Areas (#18067002) from the Japan Society for the Promotion of Science (JSPS), and by a fellowship from the JSPS (#090J09478). The authors also thank two anonymous reviewers for their valuable comments and suggestions.

[21] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper.

Erratum

  1. In the originally published version of this article, several instances of text appeared incorrectly. The following have since been corrected, and this version may be considered the authoritative version of record.In section 2, paragraph [6], the 4th and 5th sentences, “SBE 911plus (Sea-Bird Electronics)” should read “SBE 911plus (Sea-Bird Electronics) in 2011, and ICTD (Falmouth Scientific, Inc.) in 2008 and 2009”; and “(AQUA tracka3; Chelsea Technologies Group)” should read “(AQUA tracka3 (Chelsea Technologies Group) in 2011, and Seapoint Chlorophyll Fluorometer (Seapoint Sensors, Inc.) in 2008 and 2009)”.In the acknowledgments, paragraph [20], the sentence “We thank all onboard scientists for their help in deploying VMPs” should read “We thank all onboard scientists for their help in deploying VMPs, and Dr. Ikeya for his support in determining calibration coefficients of the nitrate profilers.”