Mapping of sea surface nutrients in the North Pacific: Basin-wide distribution and seasonal to interannual variability

Monthly maps of sea surface nutrient (phosphate, nitrate, and silicate) concentrations were produced for the North Pacific (10N-60N, 120E-90W) for the years 2001-2010 using a selforganizing map trained with temperature, salinity, chlorophyll-a concentration, and mixed layer depth. Nutrient sampling was carried out mainly by ships of opportunity, providing good seasonal coverage of the surface ocean. Using the mapping results, we investigated the spatiotemporal variability of surface North Pacific nutrient and dissolved inorganic carbon (DIC) distributions on seasonal and interannual time scales. Nutrient and DIC concentrations were high in the subarctic in winter and low in the subtropics. In the summer, substantial amount of nutrients remained unutilized in subarctic and the northern part of the subarctic-subtropical boundary region while that was not the case in the southern part of the boundary region. In the subtropics, nutrients were almost entirely depleted throughout the year, while DIC concentrations showed a north-south gradient and significant seasonal change. Nutrients and DIC show a large seasonal drawdown in the western subarctic region, while the drawdown in the eastern subarctic region was weaker, especially for silica. The subarctic-subtropical boundary region also showed a large seasonal drawdown, which was most prominent for DIC and less obvious for nitrate and silicate. In the interannual time scale, the Pacific Decadal Oscillation was related to a seesaw pattern between the subarctic-subtropical boundary region and the Alaskan Gyre through the changes in horizontal advection, vertical mixing, and biological production.