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, nonseasonal 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 (<3 months) to multi-annual (>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 nonseasonal 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.

The Importance of Spatial and Temporal Scales in Understanding Chlorophyll Variability in the Southern Ocean

“…Events on ∼weekly timescales are likely mechanisms for the short term variability observed in chlorophyll concentration in the ACC, such as mixing events (e.g., storms, eddies, and variability in submesoscale physical dynamics) or biological variability (resulting from nutrient and/or light limitation or grazing). Prend et al (2022) demonstrate that, in the ACC, variations in the annual mean chlorophyll concentration occur over small spatial scales (∼100-300 km; shown with an autocorrelation analysis).…”

“…Previous work has regressed satellite chlorophyll concentration in the Southern Ocean against climate oscillation indices, such as the Southern Annular Mode (SAM), but found limited explanatory power for chlorophyll variability (Prend et al, 2022;see, e.g., Lovenduski & Gruber, 2005). Overall, Prend et al (2022) found that the multi-annual component explained very little of the total chlorophyll variability across most of the Southern Ocean. To investigate why the SAM index has little explanatory power of chlorophyll variability, the authors correlated the SAM index with chlorophyll variability within each temporal component (sub-seasonal, seasonal, and multi-annual).…”

“…A key finding of Prend et al (2022) is that the relevant timescales over which chlorophyll concentration varies differ by region. The subtropics (generally taken to be south of 30°S and north of the Subtropical Front) are characterized by a repeating sinusoidal cycle with low overall magnitudes and little change in amplitude from year-toyear.…”

“…In order to identify climate change trends, it is necessary to disentangle scales of variability from sub-seasonal (∼0.5-3 months) to multi-annual (>12 months) and to investigate these trends regionally. Prend et al (2022) rise to this challenge by employing a statistical approach from Keerthi et al (2020) to identify the temporal scales (sub-seasonal to multi-annual) over which Southern Ocean phytoplankton production varies. To quantify the variability along various time scales, the authors employed a statistical decomposition method to separate chlorophyll variability into the following frequency bands: sub-seasonal (0.5-3 months), seasonal (3-12 months), and multi-annual (>12 months).…”

“…The subtropics are an example of a region characterized by seasonal variability.In other regions, such as the Antarctic Circumpolar Current, sub-seasonal spikes in chlorophyll determine the annual mean chlorophyll concentration, as demonstrated in the bottom panel. Note that there is also a multi-year component in chlorophyll concentration, but as it only explains ∼10% of chlorophyll variability, it is not represented here (seePrend et al (2022) for real data).…”