Vertical eddy diffusivity in the subsurface pycnocline across the Pacific

Abstract

In the stratified ocean, vertical eddy diffusivity in the subsurface pycnocline plays a major role in new production through transporting nutrients upwards from the darker/nutrient-rich layer. In order to evaluate the diffusivity that was less available in the subsurface layers where indirect estimations are difficult, we conducted direct microstructure measurements in the upper 300 m of the open Pacific during summer, from 40°S to 50°N along 170°W and from 137°E to 120°W across the subtropical North Pacific (21°30′N–23°N). The subsurface pycnocline in the mid- and low-latitude regions was occupied primarily by the Subtropical Underwaters characterized by high salinity, and the Subtropical Mode Waters appeared with an increasing depth and latitude. In the North Pacific, low-salinity subarctic water with shallow seasonal pycnocline was observed in the high-latitude region. The base level of the diffusivity was 0.14–0.47 × 10–5 m2 s–1, while elevations up to 0.51–13 × 10–5 m2 s–1 were observed at the equator where the Equatorial Undercurrents formed the strong shear in the subsurface layer, and in areas high internal tide energy such as the Hawaiian Ridge. The effect of wind on the diffusivity was less clear, probably because the wind energy is generally low during summer. The shear-to-strain ratio showed dome-shaped profiles with respect to latitude and an increasing trend from west to east in the subtropical North Pacific. The geographical distributions of the diffusivity presented in this study will contribute to better understanding biogeochemical cycles in the stratified upper ocean through improving estimations of the material transport.

East-west variabilities of N2 fixation activity in the subtropical North Pacific Ocean in summer: the field evidence of iron and phosphorus co-limitation in the western area

“…The geographic pattern in the N 2 fixation found in this study is also compatible with the possible effect of islands: the frequent occurrences of hotspots of diazotrophs near the islands, which were reported near the Hawaiian Islands as well as the western South Pacific (Bonnet et al, 2018;Dutheil et al, 2018;Shiozaki et al, 2014c). This phenomenon can be explained by the relatively enriched supply of either iron or phosphate from the deep layer (Itoh et al, 2021), or from the sedimentary sources around the islands (Dutheil et al, 2018), because the N 2 fixation activity near the Hawaiian Islands could be limited by either iron or phosphorus (Grabowski et al, 2008), although the activity was considered iron-limited in the eastern area in this study.…”

East‐West Variabilities of N2 Fixation Activity in the Subtropical North Pacific Ocean in Summer: Potential Field Evidence of the Phosphorus and Iron Co‐Limitation in the Western Area

“…UCYN‐A1 was exceptionally predominant in the quantified diazotrophs at St. 10 in the western area. The isotherm depths of 22–24°C were shallower at St. 10 as well (Figure 2a), probably related to high eddy diffusivity (0.85 × 10 −5 m 2 s −1 ) attributed to active vertical mixing due to the Marcus‐Wake Seamounts (Itoh et al., 2021). Thus, UCYN‐A1 could be a major diazotroph in the western area, depending on physical condition, as it increased in response to vertical mixing by eddies (Cheung et al., 2020).…”

“…Soluble dust‐Fe flux was calculated from dust‐Fe, considering 0.45% solubility (C. Luo et al., 2008). Vertical diffusion fluxes of DFe and phosphate were calculated from the vertical gradient of DFe and phosphate using the eddy diffusivities observed during the same cruise in the subsurface pycnocline (Itoh et al., 2021). Total Fe flux = soluble dust‐Fe flux + vertical DFe diffusion flux.…”

“…Itoh et al. (2021) reported the enhancement of vertical mixing at St. 4–6 (150–170°W), where the highest N 2 fixation was observed, due to the effect of Mid‐Pacific Seamounts and Hawaiian Ridge. Furthermore, N 2 fixation activity in the eastern area correlated significantly with the eddy kinetic energy, which also supports the importance of vertically supplied DFe (Figure S3e in Supporting Information ).…”

“…In the eastern area, with replenished phosphate and lower dust‐Fe input compared to that in the western area, iron was indicated to primarily limit N 2 fixation. The iron limitation is expected to relax around 150–170°W because there is a supply of DFe from below the euphotic zone owing to the effect of Mid‐Pacific Seamounts and Hawaiian Ridge (Itoh et al., 2021) in addition to dust‐Fe input from the Asian continent. The present study suggests that the zonal variation in N 2 fixation in the subtropical North Pacific is likely controlled by different processes underlying iron and phosphorus supplies.…”