Surface nutrients in the North Pacific play a crucial role in regulating marine primary productivity and ecosystem structure. Their spatiotemporal variability reflects the coupling between physical processes and biogeochemical cycles, and is therefore essential for understanding variations in marine ecosystems and primary productivity. However, due to limitations in the spatial and temporal coverage of observational data, the mechanisms driving interannual variability of nutrients in the North Pacific remain insufficiently understood.
In this study, we used the results of a physical–biogeochemical coupled model to investigate the interannual variability of surface dissolved inorganic nitrogen (DIN) in the North Pacific during a period of 1981–2020. The surface DIN from model exhibits pronounced interannual variability with significant regional differences. In the subarctic region north of 40°N, the standard deviation exceeds 1 mmol m⁻³; after removing the linear trend, variability in the eastern subarctic Pacific is higher than that in the western part. In the mid-latitude region (30°–40°N), the standard deviation is generally above 0.1 mmol m⁻³, whereas in the low-latitude region south of 30°N, it is below 0.01 mmol m⁻³.
Empirical Orthogonal Function (EOF) analysis is performed on the surface DIN time series after removing the seasonal cycle. The time series of the first two modes (PC1 and PC2) are found to be significantly correlated with the PDO and NPGO index, indicating a strong influence of climate modes on DIN variability. EOF1 exhibits a pronounced meridional dipole pattern: during the positive phase of PC1, DIN concentrations decrease in the subarctic region but increase in the mid- and low-latitude regions, primarily driven by changes in mixed layer depth. EOF2 shows a zonal dipole pattern: during the positive phase of PC2, DIN increases in the eastern Pacific and decreases in the western Pacific, mainly associated with horizontal advection. In addition, upwelling, Kuroshio transport, and local biogeochemical processes also contribute to the interannual variability of DIN.