Diapycnal fluxes of nutrients in the North Pacific Subtropical Gyre

The supply of nutrients via diapycnal processes from depth to the euphotic zone (EZ) is thought to be a main source sustaining new production in oligotrophic oceans. However, such diapycnal fluxes of nutrients remain insufficiently constrained due to limited observations and the dynamic nature of ocean turbulence. In this study, we present a comprehensive dataset of diapycnal fluxes of nutrients, including diapycnal diffusive (F_{diff_NOx}) and effective diapycnal fluxes (F_{e_NOx}, representing the net diapycnal influx to the upper water column) of NO₃⁻ + NO₂⁻ (NO_x⁻), based on measurements of turbulence microstructure and nutrients with high vertical resolutions from two cruises conducted during summer and winter in the oligotrophic western North Pacific Subtropical Gyre (NPSG). The F_{diff_NOx} (F_{e_NOx}) exhibits evident spatial variations, with higher values observed at the south boundary of the NPSG near the North Equatorial Current and at the northern NPSG influenced by the North Pacific Tropical Subtropical Mode Water. In contrast, lower values are found in the central NPSG. These spatial variations are primarily attributable to the vertical concentration gradient of NO_x⁻. At the base of the EZ, the cruise-averaged F_{diff_NOx} (F_{e_NOx}) are 11.7 ± 9.6 (11.9 ± 8.6) and 8.5 ± 6.1 (11.3 ± 9.3) μmol m⁻² d⁻¹ in summer and winter, respectively, displaying insignificant seasonal variations. Moreover, we observed significantly higher flux ratios of F_{diff_NOx} to diapycnal diffusive flux of phosphate (F_{diff_DIP}), which were 18.2 ± 2.0 and 13.9 ± 2.0, compared to the N/P concentration ratios of 10.4 ± 1.1 and 7.4 ± 1.2 at the base of the EZ during summer and winter cruises, respectively, suggesting that the diapycnal transport could relieve nitrogen limitation in the upper NPSG. Notably, we identified strong linear relationships between the logarithm of F_{diff_NOx} (F_{e_NOx}) and the NO_x⁻ gradient. Leveraging these relationships, we estimate the climatological distributions of F_{diff_NOx} (F_{e_NOx}) utilizing nutrient data from the World Ocean Atlas (WOA23). The F_{e_NOx} is estimated to be 20.2 ± 16.6 μmol m⁻² d⁻¹ and contributes to 8.5 ± 8.3 % of the nitrogen required for new production in the NPSG. These estimates are slightly lower than previous studies, but highlight that diapycnal fluxes play a less important role on nitrogen budget compared to N₂-fixation and atmospheric deposition in the oligotrophic NPSG. In contrast, the effective diapycnal diffusive flux of phosphate (F_{e_DIP}) is 1.5 ± 1.3 μmol m⁻² d⁻¹, contributing to 18.1 ± 17.9 % of the phosphorus required by new production, and is roughly ten times larger than the atmospheric phosphorus deposition in the NPSG.