DOI: 10.57760/sciencedb.07646
title: Data set for “Upper Ocean Biogeochemistry of the Oligotrophic North Pacific Subtropical Gyres: from Nutrient Sources to Carbon Export”
title: Data set for “Upper Ocean Biogeochemistry of the Oligotrophic North Pacific Subtropical Gyres: from Nutrient Sources to Carbon Export”

description: This dataset is based on figures and tables in a Review entitled "Upper Ocean Biogeochemistry of the Oligotrophic North Pacific Subtropical Gyres: from Nutrient Sources to Carbon Export", which includes:Figure 2. Ranges of the subtropical gyres based on (a) climatological sea surface absolute dynamic topography (ADT) of &gt; 0.76 m for the NPSG, SPSG, and IOSG, and &gt; 0.35 m for the NASG and SASG, and (b) using surface Chl-a concentrations of &lt; 0.1 mg m−3 (black line). ADT is a satellite-based product. Chlorophyll-a concentrations are an ensemble product from Ocean-Colour Climate Change Initiative (OC-CCI) using multiple satellite sensors, including MERIS, SeaWiFS, MODIS-Aqua, and VIIRS from 1997–2019. Figure 3. Climatological (a, b) NOx (nitrate+nitrite), (c, d) phosphate, (e, f) silicate, and (g, h) dissolved iron (Fe) concentrations (μM) at 10 m and 200 m. NOx, phosphate, and silicate data are from World Ocean Atlas 2018. Dissolved Fe data are from a machine-learning product based on field measurements. The solid black lines represent the boundaries of the subtropical gyres using the criterion of Chl-a concentration ≤ 0.1 mg m−3.Figure 4. Depth profiles for temperature (a, h), salinity (b, i), concentrations of NOx (nitrate+nitrite) (c, j), phosphate (d, k), silicate (f, m), and dissolved iron (Fe) (g, n), and NOx-to-phosphate (N/P) ratios (e, l) in subtropical gyres. Except for dissolved Fe, measurements at Station ALOHA are used for the plots of the NPSG; data from GLODAP version 2.2022, which is a merged and adjusted data product, are used for the other subtropical gyres. The lines and error bars represent the mean and one standard error in each subtropical gyre. The dashed lines in (e) and (f) represent the Redfield ratio.Figure 5. Climatological excess NOx (N* = [NOx] – 16 × [phosphate]) (a) and silicate (Si* = [silicate] – 16 × [phosphate]) (b) relative to phosphate concentration at a depth of 200 m using data from the World Ocean Atlas 2018. The solid black lines represent the boundaries of the subtropical gyres using the criterion of Chl-a concentration ≤ 0.1 mg m−3.Figure 6. Profiles of chlorophyll-a concentrations (a–d) and net primary production (NPP) (e–h) at Stations ALOHA and BATS. The data were measured during 1988–2020 at Station ALOHA and from 1988–2016 at BATS. Chlorophyll-a concentrations were measured by high-performance liquid chromatography (HPLC). Original data in each profile were first interpolated to standard depth intervals and then averaged over spring (March–May), summer (June–August), autumn (September–November), and winter (December–February). The error bars represent one standard error. Figure 7. Net primary productivity (NPP) from 1988 estimated via two remote sensing algorithms (a) VGPM and (b) CbPM, and (c) an Earth system model (CESM-BEC)-based results. The solid black lines represent the boundaries of the subtropical gyres based on the criterion of Chl-a concentration ≤ 0.1 mg m−3.Figure 8. In situ measurements of surface NOx (a, d, g), phosphate (b, e, h) and silicate (c, f, i) concentrations in the oligotrophic North Pacific Ocean. The results are averaged in (a, b, c) all seasons, (d, e, f) summer (June–August) and (g, h, i) winter (December–February). The solid black lines represent the boundaries of the NPSG using the criterion of Chl-a concentration ≤ 0.1 mg m−3. Nutrient measurements were extracted from the World Ocean Database 2018, a gridded nutrient dataset in the North Pacific version 2 and the Global Ocean Data Analysis Project (GLODAP) v2.2022. Data below the detection limits (0.1 μM for NOx, 0.02 μM for PO43−, and 0.6 μM for silicate) are not shown. Figure 9. In situ measured dissolved Fe concentrations (nM) at (a) 0–50 m, (b) 50–100 m and (b) 100–150 m. The solid black lines represent the boundaries of the NPSG using the criterion of Chl-a concentration ≤ 0.1 mg m−3.Figure 10. Monthly mean climatology of biogeochemical properties in the surface NPSG. The values include the averages of the entire NPSG (red circles), along 137°E (blue squares) and at Station ALOHA (black triangles) (if available). The error bars represent one standard error. (a–d) Physical properties including sea surface temperature (SST) and salinity (SSS), mixed layer depth (MLD) and thermocline stratification defined by bin-averaging (150 m below the MLD) the squared buoyancy frequency (N2). Global ocean Argo gridded data were used for the NPSG and 137°E averages. Note that lack of data along 137°E prevented deriving the monthly mean climatology for these physical properties. (e–i) Concentrations of NOx (nitrate+nitrite), phosphate, silicate, dissolved iron (dFe) and chlorophyll a (Chl-a) at the sea surface (at a depth of 5 m at Station ALOHA). At Station ALOHA, NOx and phosphate concentrations were analyzed using highly sensitive methods, and Chl-a concentrations were measured by high-performance liquid chromatography (HPLC). Silicate concentrations in the NPSG and along 137°E were measured using conventional analyses and the same data sources as Figure 8. Simulations of dissolved Fe concentrations from a machine-learning model and satellite-derived Chl-a concentrations were used to calculate their climatology for the entire NPSG and along 137°E, whereas in situ measurements of dissolved Fe and Chl-a were used for Station ALOHA. (j) Area of the NPSG based on a threshold of Chl-a ≤ 0.1 mg m−3 using satellite-derived Chl-a concentrations. (k–l) Calculated NPP climatology in the entire NPSG, along 137°E and at Station ALOHA from satellite-based NPP using VGPM and CbPM models. The monthly climatology of the in situ measured NPP at Station ALOHA (black dashed line and stars) is compared to the CbPM-NPP.Figure 11. Subsurface isopycnal monthly mean climatology of biogeochemical properties along 137°E and at Station ALOHA in the NPSG. Isopycnal layers with potential densities (σθ) of 23.0 and 24.5 kg m−3 were selected for 137°E and Station ALOHA, respectively, to represent the deep chlorophyll maximum layer. The data at 137°E are from the Global Ocean Data Analysis Project (GLODAP) v2.2022 product in the NPSG center (10°N–20°N). The data at Station ALOHA are from HOT, in which the low-level concentrations of NOx and phosphate measured by highly sensitive methods are used. Note that the concentrations of NOx (a) along 137°E and at Station ALOHA are plotted using different scales. Chlorophyll-a (Chl-a) concentrations at Station ALOHA were measured by high-performance liquid chromatography (HPLC).Figure 12. N2 fixation rates in the NPSG. (a) In situ N2 fixation measurements (gridded on 1°×1°) in the upper 50 m (μmol m−3 d−1) and (b) integrated through the water column (μmol m−2 d−1). (c–f) Latitudinal and meridional averages of measured N2 fixation in the NPSG for (a) and (b) using the samples inside a climatological surface chlorophyll-a concentration of 0.1 mg m−3 (marked by the black contours). Indirect estimates of depth-integrated N2 fixation rates (μmol N m−2 d−1) by (g) observation-based regression, (h) P* convergence, (i) an inverse model and (j) a mechanistic model (CESM-BEC).Figure 13. Nutrient (Fe and/or P) limitation patterns of N2 fixation across subtropical North Pacific. These results are from previously reported nutrient amendment bioassay experiments. Red, Fe limitation; blue, P limitation; split red/blue, Fe-P colimitation; white, Fe and P replete.Figure 14. Spatial and seasonal (summer, left panels; winter, right panels) distributions of critical vertical depths in the North Pacific Subtropical Gyre (NPSG): the bottom depths (m) of the surface mixed layer (SML) (a–b), nutrient-depleted layer (NDL) (c–d) and euphotic zone (EZ) (e–f), and comparison of the NDL and EZ (bottom depth of EZ minus bottom depth of NDL) (g–h). The bottom of the SML is defined as the depth where potential density differs from that at 10 m by 0.125 kg m−3. The bottom of the NDL is defined at the depth where the vertical gradient of NOx concentration exceeds 8 nmol L−1 m-1 (the white area denotes NOx concentrations higher than 1 μmol L−1 throughout the whole water column). The depth of EZ is defined as where usable solar radiation is 1% of the surface. Density and NOx data are from the World Ocean Atlas.The solid black lines represent the boundaries of the NPSG using the criterion of Chl-a concentration ≤ 0.1 mg m−3. See Box 1 for the definition of the NDL and EZ depths.Figure 16. Depth of the surface mixed layer (SML), nutrient-depleted layer (NDL), deep chlorophyll maximum (DCM), nutrient-replete layer (NRL) and euphotic zone (EZ) at Station ALOHA. The depth of the NDL is defined as where the NOx concentration equals 0.1 μmol L−1 and the depth of the euphotic zone is calculated using 1% of the surface usable solar radiation (USR; see Box 1). The background shows the monthly climatology of NOx concentrations.Figure 17. Vertical profiles of the total 234Th：238U ratio at Station ALOHA in (a) spring, (b) summer, (c) autumn, and (d) winter. Profiles of the concentrations of nitrate+nitrite (NOx) and fluorescence-based chlorophyll-a (Chl-a) are also shown (spring: May 1999; summer: July 1999; autumn: September 2014; winter: February 2000). The arrows represent export hotspots in the euphotic zone. The Chl-a and NOx data were downloaded from HOT. Figure 18. One-dimensional steady-state 234Th and particulate organic carbon (POC) fluxes at the base of the nutrient-depleted layer (NDL) and euphotic zone (EZ) along 155°W in the North Pacific Subtropical Gyre. Note that the data at 23°N are from Station ALOHA located at 158°W. POC fluxes are calculated by multiplying 234Th flux with POC:234Th ratios that are measured in trap-collected sinking particles at 150 m.Figure 19. Long-term variability in (a–c) sea surface temperature (SST), (d–f) sea surface salinity (SSS), (g–i) mixed layer depth (MLD), (j–l) thermocline stratification, (m–o) surface silicate concentrations and (p–r) surface chlorophyll-a concentration in the NPSG. Depicted are the anomalies (monthly mean value subtracted) of each property. The three columns of the panels, from left to right, are the results of the averages in the NPSG, of the averages along the 137°E transect, and at Station ALOHA. Only significant trendlines (p &lt; 0.05) are plotted (solid red or blue lines), with the rates of the trends marked on top of each panel (*: p &lt; 0.05; **: p &lt; 0.01). All the data for Station ALOHA are from the HOT program. (a, d, g, j) Black dots and their trends (red lines) are from satellite remote sensing data, and blue dots and their trends (blue lines) are from the Argo data. (m–n) Silicate concentrations are from the same data sources as in Figure 8. (o) Silicate concentrations at Station ALOHA are near-surface (5 m) values. (j–l) Thermocline stratification is defined by bin-averaging (150 m below the mixed layer depth) squared buoyancy frequency (N2). (r) Chlorophyll-a concentrations at Station ALOHA were measured by high-performance liquid chromatography (HPLC).Figure 20. Long-term changes in the estimated surface area of the NPSG and climate indices. (a) The estimated area of NPSG is based on a threshold of satellite-derived Chl-a concentration of less than 0.1 mg m−3. The trend over the whole period (solid red line, with the rate marked on top of the panel, p &lt; 0.01) and during three subperiods (1998–2002, 2003–2013, 2014–2020) (dashed blue lines) are overlaid. (b) Pacific Decadal Oscillation (PDO) index and (c) Niño 3.4 index.Figure 21. Long-term variability in (a) NOx and (b) phosphate concentrations at a depth of ~5 m at Station ALOHA (measured using highly sensitive methods). Depicted are the anomalies (monthly mean value subtracted) of each property.Figure 22. Interannual variations in concentrations and export fluxes of suspended particulate carbon (PC) at Station ALOHA. Depicted are the anomalies (monthly mean value subtracted) of each property. (a) Surface (0–10 m) suspended PC concentration, (b) subsurface (100 m, near the depth of the deep chlorophyll maximum) suspended PC concentration, and (c) PC flux at a depth of 150 m. Figure 23. Subsurface isopycnal long-term variations in biogeochemical properties along 137°E in the NPSG (panels in the left column) and at Station ALOHA (panels in the right column). Isopycnal layers with potential densities (σθ) of 23.0 and 24.5 kg m−3 were selected for 137°E and Station ALOHA, respectively, to represent the deep chlorophyll maximum layer. Depicted are the anomalies (monthly mean value subtracted) of each property. Only significant trendlines (p &lt; 0.05) are plotted (solid red or blue lines), with the rates of the trends marked on top of each panel (*: p &lt; 0.05; **: p &lt; 0.01). The data along 137°E are from the Global Ocean Data Analysis Project (GLODAP) v2.2022 product between 4°N and 26°N. The data at Station ALOHA are from HOT, in which the low-level concentrations of NOx and phosphate were measured using highly sensitive methods. Chlorophyll-a (Chl-a) concentrations at Station ALOHA were measured by high-performance liquid chromatography (HPLC).Table 1. Ranges of subtropical gyres based on different criteria.Table 2. Total inventories and concentrations of nutrients in upper subtropical gyres. The concentrations are shown as the mean ± s.d. NOx (nitrate + nitrite), phosphate, and silicate data are from the World Ocean Atlas 2018 and dissolved iron data are from a machine-learning product based on field measurements.Table 3. Net primary production (NPP) estimated using different satellite models.Table 4. Average sea-air CO2 flux (mol m−2 yr−1) in 1990–2020 in subtropical gyres and the global ocean (mean ± standard deviation). Data are from Japan Meteorological Agency. Positive values represent that the ocean is a CO2 source, and negative values represent that the ocean is a CO2 sink.Table 5. Depth-integrated areal N2 fixation (NF) rates and total N2 fixation in the NPSG by 4 different methods, including observation-based regression, P* convergence, an inverse model and a mechanistic model (CESM-BEC).Table 6. The correlation coefficients between the properties of the NPSG.Table 7. Correlation coefficients describing relationships between various properties of the NPSG and the climate indices.
description: This dataset is based on figures and tables in a Review entitled "Upper Ocean Biogeochemistry of the Oligotrophic North Pacific Subtropical Gyres: from Nutrient Sources to Carbon Export", which includes:Figure 2. Ranges of the subtropical gyres based on (a) climatological sea surface absolute dynamic topography (ADT) of &gt; 0.76 m for the NPSG, SPSG, and IOSG, and &gt; 0.35 m for the NASG and SASG, and (b) using surface Chl-a concentrations of &lt; 0.1 mg m−3 (black line). ADT is a satellite-based product. Chlorophyll-a concentrations are an ensemble product from Ocean-Colour Climate Change Initiative (OC-CCI) using multiple satellite sensors, including MERIS, SeaWiFS, MODIS-Aqua, and VIIRS from 1997–2019. Figure 3. Climatological (a, b) NOx (nitrate+nitrite), (c, d) phosphate, (e, f) silicate, and (g, h) dissolved iron (Fe) concentrations (μM) at 10 m and 200 m. NOx, phosphate, and silicate data are from World Ocean Atlas 2018. Dissolved Fe data are from a machine-learning product based on field measurements. The solid black lines represent the boundaries of the subtropical gyres using the criterion of Chl-a concentration ≤ 0.1 mg m−3.Figure 4. Depth profiles for temperature (a, h), salinity (b, i), concentrations of NOx (nitrate+nitrite) (c, j), phosphate (d, k), silicate (f, m), and dissolved iron (Fe) (g, n), and NOx-to-phosphate (N/P) ratios (e, l) in subtropical gyres. Except for dissolved Fe, measurements at Station ALOHA are used for the plots of the NPSG; data from GLODAP version 2.2022, which is a merged and adjusted data product, are used for the other subtropical gyres. The lines and error bars represent the mean and one standard error in each subtropical gyre. The dashed lines in (e) and (f) represent the Redfield ratio.Figure 5. Climatological excess NOx (N* = [NOx] – 16 × [phosphate]) (a) and silicate (Si* = [silicate] – 16 × [phosphate]) (b) relative to phosphate concentration at a depth of 200 m using data from the World Ocean Atlas 2018. The solid black lines represent the boundaries of the subtropical gyres using the criterion of Chl-a concentration ≤ 0.1 mg m−3.Figure 6. Profiles of chlorophyll-a concentrations (a–d) and net primary production (NPP) (e–h) at Stations ALOHA and BATS. The data were measured during 1988–2020 at Station ALOHA and from 1988–2016 at BATS. Chlorophyll-a concentrations were measured by high-performance liquid chromatography (HPLC). Original data in each profile were first interpolated to standard depth intervals and then averaged over spring (March–May), summer (June–August), autumn (September–November), and winter (December–February). The error bars represent one standard error. Figure 7. Net primary productivity (NPP) from 1988 estimated via two remote sensing algorithms (a) VGPM and (b) CbPM, and (c) an Earth system model (CESM-BEC)-based results. The solid black lines represent the boundaries of the subtropical gyres based on the criterion of Chl-a concentration ≤ 0.1 mg m−3.Figure 8. In situ measurements of surface NOx (a, d, g), phosphate (b, e, h) and silicate (c, f, i) concentrations in the oligotrophic North Pacific Ocean. The results are averaged in (a, b, c) all seasons, (d, e, f) summer (June–August) and (g, h, i) winter (December–February). The solid black lines represent the boundaries of the NPSG using the criterion of Chl-a concentration ≤ 0.1 mg m−3. Nutrient measurements were extracted from the World Ocean Database 2018, a gridded nutrient dataset in the North Pacific version 2 and the Global Ocean Data Analysis Project (GLODAP) v2.2022. Data below the detection limits (0.1 μM for NOx, 0.02 μM for PO43−, and 0.6 μM for silicate) are not shown. Figure 9. In situ measured dissolved Fe concentrations (nM) at (a) 0–50 m, (b) 50–100 m and (b) 100–150 m. The solid black lines represent the boundaries of the NPSG using the criterion of Chl-a concentration ≤ 0.1 mg m−3.Figure 10. Monthly mean climatology of biogeochemical properties in the surface NPSG. The values include the averages of the entire NPSG (red circles), along 137°E (blue squares) and at Station ALOHA (black triangles) (if available). The error bars represent one standard error. (a–d) Physical properties including sea surface temperature (SST) and salinity (SSS), mixed layer depth (MLD) and thermocline stratification defined by bin-averaging (150 m below the MLD) the squared buoyancy frequency (N2). Global ocean Argo gridded data were used for the NPSG and 137°E averages. Note that lack of data along 137°E prevented deriving the monthly mean climatology for these physical properties. (e–i) Concentrations of NOx (nitrate+nitrite), phosphate, silicate, dissolved iron (dFe) and chlorophyll a (Chl-a) at the sea surface (at a depth of 5 m at Station ALOHA). At Station ALOHA, NOx and phosphate concentrations were analyzed using highly sensitive methods, and Chl-a concentrations were measured by high-performance liquid chromatography (HPLC). Silicate concentrations in the NPSG and along 137°E were measured using conventional analyses and the same data sources as Figure 8. Simulations of dissolved Fe concentrations from a machine-learning model and satellite-derived Chl-a concentrations were used to calculate their climatology for the entire NPSG and along 137°E, whereas in situ measurements of dissolved Fe and Chl-a were used for Station ALOHA. (j) Area of the NPSG based on a threshold of Chl-a ≤ 0.1 mg m−3 using satellite-derived Chl-a concentrations. (k–l) Calculated NPP climatology in the entire NPSG, along 137°E and at Station ALOHA from satellite-based NPP using VGPM and CbPM models. The monthly climatology of the in situ measured NPP at Station ALOHA (black dashed line and stars) is compared to the CbPM-NPP.Figure 11. Subsurface isopycnal monthly mean climatology of biogeochemical properties along 137°E and at Station ALOHA in the NPSG. Isopycnal layers with potential densities (σθ) of 23.0 and 24.5 kg m−3 were selected for 137°E and Station ALOHA, respectively, to represent the deep chlorophyll maximum layer. The data at 137°E are from the Global Ocean Data Analysis Project (GLODAP) v2.2022 product in the NPSG center (10°N–20°N). The data at Station ALOHA are from HOT, in which the low-level concentrations of NOx and phosphate measured by highly sensitive methods are used. Note that the concentrations of NOx (a) along 137°E and at Station ALOHA are plotted using different scales. Chlorophyll-a (Chl-a) concentrations at Station ALOHA were measured by high-performance liquid chromatography (HPLC).Figure 12. N2 fixation rates in the NPSG. (a) In situ N2 fixation measurements (gridded on 1°×1°) in the upper 50 m (μmol m−3 d−1) and (b) integrated through the water column (μmol m−2 d−1). (c–f) Latitudinal and meridional averages of measured N2 fixation in the NPSG for (a) and (b) using the samples inside a climatological surface chlorophyll-a concentration of 0.1 mg m−3 (marked by the black contours). Indirect estimates of depth-integrated N2 fixation rates (μmol N m−2 d−1) by (g) observation-based regression, (h) P* convergence, (i) an inverse model and (j) a mechanistic model (CESM-BEC).Figure 13. Nutrient (Fe and/or P) limitation patterns of N2 fixation across subtropical North Pacific. These results are from previously reported nutrient amendment bioassay experiments. Red, Fe limitation; blue, P limitation; split red/blue, Fe-P colimitation; white, Fe and P replete.Figure 14. Spatial and seasonal (summer, left panels; winter, right panels) distributions of critical vertical depths in the North Pacific Subtropical Gyre (NPSG): the bottom depths (m) of the surface mixed layer (SML) (a–b), nutrient-depleted layer (NDL) (c–d) and euphotic zone (EZ) (e–f), and comparison of the NDL and EZ (bottom depth of EZ minus bottom depth of NDL) (g–h). The bottom of the SML is defined as the depth where potential density differs from that at 10 m by 0.125 kg m−3. The bottom of the NDL is defined at the depth where the vertical gradient of NOx concentration exceeds 8 nmol L−1 m-1 (the white area denotes NOx concentrations higher than 1 μmol L−1 throughout the whole water column). The depth of EZ is defined as where usable solar radiation is 1% of the surface. Density and NOx data are from the World Ocean Atlas.The solid black lines represent the boundaries of the NPSG using the criterion of Chl-a concentration ≤ 0.1 mg m−3. See Box 1 for the definition of the NDL and EZ depths.Figure 16. Depth of the surface mixed layer (SML), nutrient-depleted layer (NDL), deep chlorophyll maximum (DCM), nutrient-replete layer (NRL) and euphotic zone (EZ) at Station ALOHA. The depth of the NDL is defined as where the NOx concentration equals 0.1 μmol L−1 and the depth of the euphotic zone is calculated using 1% of the surface usable solar radiation (USR; see Box 1). The background shows the monthly climatology of NOx concentrations.Figure 17. Vertical profiles of the total 234Th：238U ratio at Station ALOHA in (a) spring, (b) summer, (c) autumn, and (d) winter. Profiles of the concentrations of nitrate+nitrite (NOx) and fluorescence-based chlorophyll-a (Chl-a) are also shown (spring: May 1999; summer: July 1999; autumn: September 2014; winter: February 2000). The arrows represent export hotspots in the euphotic zone. The Chl-a and NOx data were downloaded from HOT. Figure 18. One-dimensional steady-state 234Th and particulate organic carbon (POC) fluxes at the base of the nutrient-depleted layer (NDL) and euphotic zone (EZ) along 155°W in the North Pacific Subtropical Gyre. Note that the data at 23°N are from Station ALOHA located at 158°W. POC fluxes are calculated by multiplying 234Th flux with POC:234Th ratios that are measured in trap-collected sinking particles at 150 m.Figure 19. Long-term variability in (a–c) sea surface temperature (SST), (d–f) sea surface salinity (SSS), (g–i) mixed layer depth (MLD), (j–l) thermocline stratification, (m–o) surface silicate concentrations and (p–r) surface chlorophyll-a concentration in the NPSG. Depicted are the anomalies (monthly mean value subtracted) of each property. The three columns of the panels, from left to right, are the results of the averages in the NPSG, of the averages along the 137°E transect, and at Station ALOHA. Only significant trendlines (p &lt; 0.05) are plotted (solid red or blue lines), with the rates of the trends marked on top of each panel (*: p &lt; 0.05; **: p &lt; 0.01). All the data for Station ALOHA are from the HOT program. (a, d, g, j) Black dots and their trends (red lines) are from satellite remote sensing data, and blue dots and their trends (blue lines) are from the Argo data. (m–n) Silicate concentrations are from the same data sources as in Figure 8. (o) Silicate concentrations at Station ALOHA are near-surface (5 m) values. (j–l) Thermocline stratification is defined by bin-averaging (150 m below the mixed layer depth) squared buoyancy frequency (N2). (r) Chlorophyll-a concentrations at Station ALOHA were measured by high-performance liquid chromatography (HPLC).Figure 20. Long-term changes in the estimated surface area of the NPSG and climate indices. (a) The estimated area of NPSG is based on a threshold of satellite-derived Chl-a concentration of less than 0.1 mg m−3. The trend over the whole period (solid red line, with the rate marked on top of the panel, p &lt; 0.01) and during three subperiods (1998–2002, 2003–2013, 2014–2020) (dashed blue lines) are overlaid. (b) Pacific Decadal Oscillation (PDO) index and (c) Niño 3.4 index.Figure 21. Long-term variability in (a) NOx and (b) phosphate concentrations at a depth of ~5 m at Station ALOHA (measured using highly sensitive methods). Depicted are the anomalies (monthly mean value subtracted) of each property.Figure 22. Interannual variations in concentrations and export fluxes of suspended particulate carbon (PC) at Station ALOHA. Depicted are the anomalies (monthly mean value subtracted) of each property. (a) Surface (0–10 m) suspended PC concentration, (b) subsurface (100 m, near the depth of the deep chlorophyll maximum) suspended PC concentration, and (c) PC flux at a depth of 150 m. Figure 23. Subsurface isopycnal long-term variations in biogeochemical properties along 137°E in the NPSG (panels in the left column) and at Station ALOHA (panels in the right column). Isopycnal layers with potential densities (σθ) of 23.0 and 24.5 kg m−3 were selected for 137°E and Station ALOHA, respectively, to represent the deep chlorophyll maximum layer. Depicted are the anomalies (monthly mean value subtracted) of each property. Only significant trendlines (p &lt; 0.05) are plotted (solid red or blue lines), with the rates of the trends marked on top of each panel (*: p &lt; 0.05; **: p &lt; 0.01). The data along 137°E are from the Global Ocean Data Analysis Project (GLODAP) v2.2022 product between 4°N and 26°N. The data at Station ALOHA are from HOT, in which the low-level concentrations of NOx and phosphate were measured using highly sensitive methods. Chlorophyll-a (Chl-a) concentrations at Station ALOHA were measured by high-performance liquid chromatography (HPLC).Table 1. Ranges of subtropical gyres based on different criteria.Table 2. Total inventories and concentrations of nutrients in upper subtropical gyres. The concentrations are shown as the mean ± s.d. NOx (nitrate + nitrite), phosphate, and silicate data are from the World Ocean Atlas 2018 and dissolved iron data are from a machine-learning product based on field measurements.Table 3. Net primary production (NPP) estimated using different satellite models.Table 4. Average sea-air CO2 flux (mol m−2 yr−1) in 1990–2020 in subtropical gyres and the global ocean (mean ± standard deviation). Data are from Japan Meteorological Agency. Positive values represent that the ocean is a CO2 source, and negative values represent that the ocean is a CO2 sink.Table 5. Depth-integrated areal N2 fixation (NF) rates and total N2 fixation in the NPSG by 4 different methods, including observation-based regression, P* convergence, an inverse model and a mechanistic model (CESM-BEC).Table 6. The correlation coefficients between the properties of the NPSG.Table 7. Correlation coefficients describing relationships between various properties of the NPSG and the climate indices.