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

• Published in: Global biogeochemical cycles 2022-07, Vol.36 (7), p.n/a
• Main Authors: Prend, Channing J., Keerthi, M. G., Lévy, Marina, Aumont, Olivier, Gille, Sarah T., Talley, Lynne D.
• Format: Article
• Language: English
• Subjects: Annual; Antarctic Circumpolar Current; Antarctic Oscillation; Biogeochemical cycle; Biogeochemical cycles; Biomass; Carbon cycle; Chlorophyll; Chlorophylls; Climate change; Climate variability; Correlation; Eddies; El Nino; El Nino phenomena; Environmental Sciences; Fluctuations; Food chains; Food webs; Global climate; Marine ecosystems; Mean; Nutrient cycles; Nutrients; Ocean circulation; Ocean currents; Oceans; Oscillations; Phytoplankton; Phytoplankton production; Plankton; Primary production; Productivity; Satellites; Seasonal variability; Seasonal variation; Seasonal variations; Seasonality; Storms; Swaths; Variation; Vortices; Water circulation
• ISSN: 0886-6236; 1944-9224; 1944-8224
• DOI: 10.1029/2022GB007329

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. Plain Language Summary Microalgae called phytoplankton are the foundation of marine food webs and play a large role in the carbon cycle. Therefore, understanding the mechanisms that drive phytoplankton variability is of critical importance to marine ecosystems and global climate. Phytoplankton growth is known to exhibit a strong seasonal cycle. In addition to this, phytoplankton biomass also varies between years. This variability is often linked to multi‐year climate oscillations like El Niño. On short timescales, phytoplankton are also influenced by processes such as storms and eddies, which alter the supply of nutrients and light that they need to grow. In this study, we use satellite measurements to untangle the different timescales of phytoplankton variability in the Southern Ocean, which surrounds Antarctica. We find that year‐to‐year fluctuations in phytoplankton biomass are driven by intermittent events associated with storms and eddies, rather than multi‐year climate oscillations. Therefore, processes occurr