The Influence of Subtropical Mode Water in Nutrients in the South Atlantic

The South Atlantic subtropical mode water (STMW) is characterized by a volume of water confined between the seasonal and permanent thermoclines. It is formed during the months of winter to early spring, July to October, near the Brazil-Malvinas Confluence region and the Brazil Current recirculation gyre. The STMW presents a homogeneous temperature (T) and salinity distribution in the horizontal and vertical, on average reaching up to 170 m of layer thickness. We investigate the impact of STMW in the nutrient's distribution and the nutricline depth in the western South Atlantic. Three data sets were used to identify the mode water and determine nutrients concentration:i) My Ocean biogeochemical model reanalysis; ii) World Ocean Atlas 2013 (WOA13) monthly climatology; and iii) the In Situ Analysis System (ISAS) data. Based on WOA13, during the summer (January, February and March) the STMW is found at subsurface from 100 m to 225 m deep; it has a mean layer thickness of 112 ± 5.0 m, reaching a maximum of 145 m. During the winter, the maximum layer thickness is 250 m while the mean is 155 ± 20.3 m. In the summer, STMW is in subsurface from 100 m to 225 m deep. As expected, the formation of mode water occured mostly in the winter to mid-spring. Using the model and ISAS data, we found a correlation between a deepening of the surface layer of minimum nitrate concentration and the mode water formation period (from June to October) in vertical profiles. From 2002 to 2012, greater nutrients concentration were found during the STMW formation period, from surface to 200 m depth, than in other periods with STMW absence. We also found a peak chlorophyll-a concentration during STMW formation period, suggesting that STMW presence in the southwest South Atlantic is important for biological processes in the upper ocean.