Sub‐Seasonal Forcing Drives Year‐To‐Year Variations of Southern Ocean Primary Productivity

Abstract Primary productivity in the Southern Ocean plays a key role in global biogeochemical cycles. While much focus has been placed on phytoplankton production seasonality, non‐seasonal fluctuations exceed the amplitude of the seasonal cycle across large swaths of the Antarctic Circumpolar Current. This non‐seasonal variability comprises a broad range of timescales from sub‐seasonal (1 year), all of which can project onto the annual mean value. However, year‐to‐year variations of surface chlorophyll (SChl), a proxy for phytoplankton biomass, are typically attributed to ocean circulation changes associated with the Southern Annular Mode (SAM), which implicitly assumes that sub‐seasonal variability averages to near‐zero over long timescales. Here, we test this assumption by applying a timeseries decomposition method to satellite‐derived SChl in order to separate the low‐frequency and high‐frequency contributions to the non‐seasonal variability. We find that throughout most of the Southern Ocean, year‐to‐year SChl variations are dominated by the sub‐seasonal component, which is not strongly correlated with the SAM. The multi‐annual component, while correlated with the SAM, only accounts for about 10% of the total SChl variance. This suggests that changes in annual mean SChl are related to intermittent forcing at small scales, rather than low‐frequency climate variability, and thus do not remain correlated over large regions.