Utilizing the Drake Passage Time-series to understand variability and change in subpolar Southern Ocean <i>p</i>CO<sub>2</sub>

Abstract

Abstract. The Southern Ocean is highly under-sampled for the purpose of assessing total carbon uptake and its variability. Since this region dominates the mean global ocean sink for anthropogenic carbon, understanding temporal change is critical. Underway measurements of pCO2 collected as part of the Drake Passage Time-series (DPT) program that began in 2002 inform our understanding of seasonally changing air–sea gradients in pCO2, and by inference the carbon flux in this region. Here, we utilize available pCO2 observations to evaluate how the seasonal cycle, interannual variability, and long-term trends in surface ocean pCO2 in the Drake Passage region compare to that of the broader subpolar Southern Ocean. Our results indicate that the Drake Passage is representative of the broader region in both seasonality and long-term pCO2 trends, as evident through the agreement of timing and amplitude of seasonal cycles as well as trend magnitudes both seasonally and annually. The high temporal density of sampling by the DPT is critical to constraining estimates of the seasonal cycle of surface pCO2 in this region, as winter data remain sparse in areas outside of the Drake Passage. An increase in winter data would aid in reduction of uncertainty levels. On average over the period 2002–2016, data show that carbon uptake has strengthened with annual surface ocean pCO2 trends in the Drake Passage and the broader subpolar Southern Ocean less than the global atmospheric trend. Analysis of spatial correlation shows Drake Passage pCO2 to be representative of pCO2 and its variability up to several hundred kilometers away from the region. We also compare DPT data from 2016 and 2017 to contemporaneous pCO2 estimates from autonomous biogeochemical floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) so as to highlight the opportunity for evaluating data collected on autonomous observational platforms. Though SOCCOM floats sparsely sample the Drake Passage region for 2016–2017 compared to the Drake Passage Time-series, their pCO2 estimates fall within the range of underway observations given the uncertainty on the estimates. Going forward, continuation of the Drake Passage Time-series will reduce uncertainties in Southern Ocean carbon uptake seasonality, variability, and trends, and provide an invaluable independent dataset for post-deployment assessment of sensors on autonomous floats. Together, these datasets will vastly increase our ability to monitor change in the ocean carbon sink.

Autonomous Biogeochemical Floats Detect Significant Carbon Dioxide Outgassing in the High‐Latitude Southern Ocean

“…The other likely cause of the persistent difference in the ASZ is that the ship-based estimates of CO 2 flux do not adequately capture the outgassing in this region due to a lack of wintertime data. Except for the Drake Passage region (Munro et al, 2015), over the past decade there has been, on average, less than one cruise per year that measured pCO ocn 2 south of 50 ∘ S (Bakker et al, 2016) in the austral winter, which is inadequate to properly resolve the seasonal cycle (Fay et al, 2018). Surface drifters measuring pCO 2 have previously been deployed in the Southern Ocean (Barbero et al, 2011;Boutin et al, 2008), but only a small fraction of those data was located in the ASZ.…”

The Observed Seasonal Cycle of Macronutrients in Drake Passage: Relationship to Fronts and Utility as a Model Metric

“…The length (>13 years) and frequency of sampling (5-8 times per year) of the DPT is unmatched by other Southern Ocean observing networks (Figure 2 and section 4.1.2), representing the largest repository of repeat wintertime biogeochemical observations in the ACC, particularly concurrent silicate, nitrate, and phosphate observations. While the DPT has been extensively utilized for Southern Ocean carbon flux studies in recent years (e.g., Fay et al, 2018;Landschützer et al, 2015;, this is the first study to use the DPT to fully characterize macronutrients, particularly silicate and nitrate, on seasonal time scales and in the context of the physical and biogeochemical fronts of the ACC. We also demonstrate the value and utility of DPT nutrients to inform and improve the representation of the physical and biological processes driving Southern Ocean carbon variability in biogeochemical models (sections 3.4 and 4.2).…”

“…Our current understanding of the drivers of CO 2 flux in the Southern Ocean is limited to a few observation-based studies which are heavily weighted toward the Drake Passage Time-series (DPT; Gregor et al, 2018;Fay et al, 2018;Landschützer et al, 2015;. The variability in the strength of the Southern Ocean carbon sink on seasonal time scales is largely driven by compensatory variations in temperature, biology, and vertical exchange with the enriched deep waters, which in turn, drive variability on interannual time scales (Gregor et al, 2018;Takahashi et al, 1993).…”

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

“…The array of biogeochemical floats in the Southern Ocean is providing novel insights into air-sea CO 2 fluxes and biogeochemical processes. For example, large discrepancies in winter surface pCO 2 between float observations and climatologies based on shipboard observations [25] are consistently observed [98][99][100][101]. This is not particularly surprising because wintertime shipboard data are sparse.…”