Role of Southern Ocean stratification in glacial atmospheric CO₂ reduction evaluated by a three-dimensional ocean general circulation model

Abstract

Atmospheric carbon dioxide (CO 2 ) concentration during glacial periods is known to be considerably lower than during interglacial periods. However, previous studies using an ocean general circulation model (OGCM) fail to reproduce this. Paleoclimate proxy data of the Last Glacial Maximum indicate high salinity (>37.0 practical salinity unit) and long water mass residence time (>3000 years) in the Southern Ocean, suggesting that salinity stratification was enhanced and more carbon was stored there. Reproducibility of salinity and water mass age is considered insufficient in previous OGCMs, which might affect the reproducibility of atmospheric CO 2 concentration. This study investigated the role of increased stratification of the Southern Ocean in the glacial CO 2 variation using an OGCM. We found that deep water formation in East Antarctica is required to explain high salinity in South Atlantic. Saltier deep Southern Ocean resulted in increased atmospheric CO 2 concentration against previous estimates. This is partly due to increased volume transport of Antarctic Bottom Water and associated decrease in the water mass age of the deep Pacific Ocean. On the other hand, weakening of vertical mixing contributed to increase of the vertical gradient of dissolved inorganic carbon and decrease of atmospheric CO 2 concentration. However, we show that it is unable to explain all of the glacial CO 2 variations by the contribution of the Southern Ocean. Our findings indicate that detailed understanding of the impact of enhanced stratification in the Southern Ocean on the Pacific Ocean might be crucial to understanding the mechanisms behind the glacial-interglacial ocean carbon cycle variations.

Influence of glacial ice sheets on the Atlantic meridional overturning circulation through surface wind change

“…Although, reconstructions show both a weaker and stronger AMOC during the LGM, the fact that all of the PMIP3 models show a strong AMOC suggests that more investigations are needed for a future study, for example the processes around the Southern Ocean (e.g. Roche et al 2012;Kobayashi et al 2015). Previous studies have suggested the importance of a Southern Ocean process for simulating a weaker AMOC in the LGM simulation, which is related to the production of sea ice and bottom water formation in this region (Stouffer and Manabe 2003;Schmittner 2003;Shin et al 2003;Liu et al 2005).…”

Response of Atmospheric pCO2 to Glacial Changes in the Southern Ocean Amplified by Carbonate Compensation

“… where C is the concentration of each ocean biogeochemical tracer, ν is current velocity, K H and K V are horizontal and vertical diffusivity, and S C is a source/sink term of tracers related to ocean biogeochemical processes. The ocean biogeochemical model used in this study is based on that of Parekh et al () and the same as that used in Oka et al () and Kobayashi et al (). The prognostic variables are phosphate, DIC, alkalinity, dissolved organic phosphate, dissolved oxygen, iron, and silicate.…”

“…Ocean circulation and the carbon cycle in the Southern Ocean may have changed greatly associated with glacial changes in winds (Menviel et al, ; Toggweiler et al, ; Tschumi et al, ; Völker & Köhler, ), sea ice (Stephens & Keeling, ; Sun & Matsumoto, ; Kurahashi‐Nakamura et al, ), brine rejection (Bouttes et al, ), sea surface buoyancy (Sun et al, ; Watson et al, ), and stratification of water columns (Francois et al, ; Kobayashi et al, ; Schmittner & Galbraith, ). Kobayashi et al () point out that the insufficient reproducibility of the Southern Ocean stratification might be a factor in the underestimation of the glacial decline in atmospheric p CO 2 in past OGCM studies. Due to its small spatial scale, coarse‐resolution OGCMs have difficulty in simulating realistic deep water formation process in the Southern Ocean, which leads to model biases around the Southern Ocean in the modern simulations (Heuzé et al, ).…”

Last Glacial Maximum and deglacial abyssal seawater oxygen isotopic ratios

“…OttoBliesner et al (2006), Intergovernmental Panel on Climate Change (2013), and Kobayashi et al (2015). Recently Miller (2014) and Miller et al (2015) have challenged this interpretation showing, using a Monte Carlo method, that the uncertainties of the inferences were too great to assert that the LGM abyssal stratification could be determined with useful accuracy.…”