On the Differential Conditioning of Sub‐Antarctic Mode Water and Antarctic Intermediate Water Over Decadal Time Scales

Abstract We report results from a meridional transect performed along 150°W, 30°S to 60°S in the Southern Ocean (SO). This study aimed to document the rates of conditioning of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW), which are formed during deep mixing events in the austral winter. The water masses are subducted as they flow toward the northern hemisphere. Chloroflourocarbons (CFCs) were sampled along the section in order to estimate the time since atmospheric equilibration of SAMW and AAIW. Combining this information with standing stock information of other biogeochemical variables along isopycnals allowed rates of change of these variables to be deduced. Results demonstrated that temperature and salinity varied with water mass age, possibly reflecting changes in source waters over decadal time scales. Results also showed remineralization rates of nitrate and phosphate in northbound SAMW were faster than remineralization rates in AAIW. In contrast, silicate remineralization rates were faster in AAIW than in SAMW. These trends are expected to affect macronutrient availability, phytoplankton biogeography and productivity in the north Pacific Ocean, especially when the waters are subsequently mixed upwards into the euphotic zone decades later. Vertical mixing of nitrogen‐rich, SAMW into surface waters would support the growth of more non‐silicious algae (such as nannoplankton, coccolithophores and dinoflagellates) whereas more energetic vertical mixing of deeper AAIW into surface waters will encourage the growth of silicious algae like diatoms.