Modeling upper ocean dynamics in the Southern Ocean : interaction of physics and biogeochemistry

A one-dimensional biophysical model was developed to simulate upper ocean dynamics and seasonal nutrient (N, P, Si) export from the euphotic zone to the depths in the Southern Ocean. Simulations were made in the Subantarctic Zone (SAZ) and the Polar Frontal Zone (PFZ). The physical part of model was forced with the heat fluxes, freshwater fluxes and wind stresses provided by the National Centers for Environmental Prediction. In both the SAZ and PFZ, the model was capable of reproducing the amplitude of the seasonal sea surface temperature (SST) and the seasonality of the mixed layer depth (MLD). The MLD was deepest in August-October (600 m in the SAZ, 160 m in the PFZ) and shallowest in January-Februry (20m in the SAZ, 35m in the PFZ). The shallower summer MLD in the SAZ was due to lower wind stress. However, the shallower winter MLD in the PFZ was due to strong stratification in the water below the mixed layer. The biological component of the model used incident light, temperature, nutrient availability and estimates of phytoplankton biomass from satellite data recorded by the Sea-viewing Wide Field-of-view Sensor to determine production. The model was tuned to reproduce the observed seasonal cycle of nutrients. A series of sensitivity studies, taking into account uncertainties in both physical fields and biological formulations, led to several robust conclusions. The simulated annual export production was significantly higher in the PFZ (~65 mol P m\\(^{-2}\\)) than in the SAZ (~55 mol P m\\(^{-2}\\)) despite the PFZ having lower seasonal nutrient depletion in the euphotic zone. The higher export production in the PFZ was accomplished by having larger resupply of phosphate to the upper ocean during the September to March period (27 ‚ÄövÑvÆ 37 mol P m\\(^{-2}\\)) than in the SAZ (8 ‚ÄövÑvÆ 15 mol P m\\(^{-2}\\)). In the PFZ, it was assumed that nutrient utilization ratio followed the Redfield ratio in the non-diatoms and the NIP/Si utilization ratios were determined in the diatoms in a steady state ocean.-The estimated annual export production is ~65 mmol P m\\(^{-2}\\), ~820 mmol N m\\(^{-2}\\) and 1826 mmol Si M\\(^{-2}\\) in the euphotic zone for phosphate, nitrate and silicate respectively. The diatoms contribute 85% and 80% of the annual phosphate and nitrate export production. In the euphotic zone, the annual N/P utilization ratio is 11.5 ‚ÄövÑvÆ 12.9 in the diatoms and 12.1 ‚ÄövÑvÆ 13.6 in the community. The seasonal Si/N utilization ratio is 3 ‚ÄövÑvÆ 6 in the diatoms and 1 ‚ÄövÑvÆ 4 in the community, with the lowest found in the late summer. The annual Si/N utilization ratio is 2.5 ‚ÄövÑvÆ 3 in the diatoms and 2 ‚ÄövÑvÆ 2.5 in the community. The low NIP depletion ratio is associated with the preferential recycling of phosphate below the euphotic zone, the low N/P ratio in the labile dissolved organic material in the euphotic zone and the low N/P utilization ratio in the diatoms. The low N/P and high Si/N utilization ratio reflect the low iron availability in the PFZ waters.