A data assimilating model for estimating Southern Ocean biogeochemistry

Abstract

A Biogeochemical Southern Ocean State Estimate (B‐SOSE) is introduced that includes carbon and oxygen fields as well as nutrient cycles. The state estimate is constrained with observations while maintaining closed budgets and obeying dynamical and thermodynamic balances. Observations from profiling floats, shipboard data, underway measurements, and satellites are used for assimilation. The years 2008–2012 are chosen due to the relative abundance of oxygen observations from Argo floats during this time. The skill of the state estimate at fitting the data is assessed. The agreement is best for fields that are constrained with the most observations, such as surface pCO2 in Drake Passage (44% of the variance captured) and oxygen profiles (over 60% of the variance captured at 200 and 1000 m). The validity of adjoint method optimization for coupled physical‐biogeochemical state estimation is demonstrated with a series of gradient check experiments. The method is shown to be mature and ready to synthesize in situ biogeochemical observations as they become more available. Documenting the B‐SOSE configuration and diagnosing the strengths and weaknesses of the solution informs usage of this product as both a climate baseline and as a way to test hypotheses. Transport of Intermediate Waters across 32°S supplies significant amounts of nitrate to the Atlantic Ocean (5.57 ± 2.94 Tmol yr−1) and Indian Ocean (5.09 ± 3.06 Tmol yr−1), but much less nitrate reaches the Pacific Ocean (1.78 ± 1.91 Tmol yr−1). Estimates of air‐sea carbon dioxide fluxes south of 50°S suggest a mean uptake of 0.18 Pg C/yr for the time period analyzed.

Correlation Lengths for Estimating the Large‐Scale Carbon and Heat Content of the Southern Ocean

“…The ECCO state estimation machinery have been extensively described in the literature (e.g., Wunsch & Heimbach, ). B‐SOSE is introduced by Verdy and Mazloff (), and thus in this section we give only a brief summary.…”

“…We use 3 day average output from the 2008 to 2012 B‐SOSE solution, which is available at http://sose.ucsd.edu/BSOSE_iter105_solution.html. Validation of the solution is presented by Verdy and Mazloff () and at http://sose.ucsd.edu/BSOSE_iter105_valid.html. The mean circulation in B‐SOSE is illustrated by the vertically integrated transport streamfunction (Figure ).…”

“…Here we use a different derived product, a Biogeochemical Southern Ocean State Estimate (B‐SOSE, Verdy & Mazloff, ), to estimate spatial correlation scales. This is the first attempt, we are aware of to use a numerical model solution to determine these scales.…”

Metrics for the Evaluation of the Southern Ocean in Coupled Climate Models and Earth System Models

“… This figure shows the zonal velocity (in cm/s, positive is eastward—out of the page) through Drake Passage (at 69°W) from the observations and the model simulations. The standard‐resolution B‐SOSE reanalysis (1/3°, Verdy & Mazloff, ) shows the clear frontal structures that characterize the complex surface flow in the ACC. The total transport by the ACC is noted in black in the lower left portion of each panel.…”

“…The Southern Ocean State Estimate (SOSE, Mazloff et al, , http://sose.ucsd.edu/) provides a least‐square fit to all of the available ocean observations within an ocean general circulation model. The latest generation of this state estimate called Biogeochemical‐SOSE (B‐SOSE, Verdy & Mazloff, ), includes both physical and biogeochemical properties of the ocean (between January 2008 and December 2012) to determine the “best‐fit” ocean state. B‐SOSE, while currently available at a lower 1/3° resolution than SOSE, continues to improve, by incorporating a rapidly growing quantity of observational data and incorporating biogeochemical processes (Galbraith et al, ).…”