A global monthly climatology of phosphate, nitrate, and silicate in the upper ocean: Spring-summer export production and shallow remineralization

Abstract

We have created monthly climatologies of nutrients in the upper 500 m of the ocean using the 1998 release of the World Ocean Atlas from the Ocean Climate Laboratory at the National Oceanographic Data Center. The data processing is similar to that used by Najjar and Keeling [1997] to create an oxygen climatology. The spatial extrapolation of the nutrients exploits regional relationships between nutrients and temperature in the ocean. The annual mean horizontal and vertical distributions of the nutrients follow the large scale patterns of oceanic circulation as previously reported in the literature. Surface seasonal variations of nutrients are high in the high latitudes and some restricted upwelling areas, whereas in the subtropical oligotrophic gyres nutrients are low all year. Surface seasonal variations are characterized by high values in winter and low values in summer, consistent with the dominance of entrainment during the winter and biological production during the summer. Good agreement is found between the climatologies and the limited reports of seasonal nutrient variations in the literature. Weaker seasonal variations of opposite phasing are found below roughly 100 m and likely reflect the dominance of remineralization during the summer and ventilation during the winter. Spring-summer export production derived from the seasonal nutrient drawdown in the upper 100 m is 4.2±0.6 Tmol P, 59±8 Tmol N, and 70±15 Tmol Si. The N:P drawdown ratio is, within the error, in agreement with the traditional value of 16. Similarly, the Si:N drawdown ratio is in agreement with the value of 1 expected for diatom growth in unstressed conditions. The export of organic carbon estimated from the phosphate drawdown is 5.3±0.8 Gt C. The shallow remineralization inferred from seasonal phosphate variation between 100 and 200 m is 2.6±1.1 Gt C. The carbon and silica fluxes, considering that they are lower bounds on global export production because they do not capture the production signal in advectively dominated systems, are in reasonable agreement with other large scale estimates of organic carbon and silica export. The computed f ratio (using satellite-based estimates of primary production) and the ratio of shallow aphotic zone remineralization to new production tend to increase with increasing latitude, supporting an increase in respiration with temperature, as suggested in recent studies.