Export pathways of mode waters, intermediate waters, and carbon out of the Southern Ocean

Recent observational evidence suggests that the water masses that ventilate the Southern Ocean thermocline (e.g. mode water, intermediate water) are formed and subducted via narrow, intense export windows [Sallée et al., 2010]. The idea that ventilation is localized is somewhat at odds with the more traditional picture of water mass formation as a broad circumpolar process. In this work, we aim to (i.) identify the primary mode and intermediate water export pathways out of the Southern Ocean and (ii.) to better understand the factors that control the location and intensity of these pathways. We examine the three-dimensional structure of the mode and intermediate water export routes on decadal to centennial timescales using both passive tracers and Lagrangian floats in an eddy-permitting Southern Ocean model. We find windows of strong mode water export in the Indian and Pacific sectors of the Southern Ocean and identify two potential tracks along which anthropogenic carbon dioxide and other tracers may be subducted. Our results suggest that the Pacific export pathway is especially intense, i.e. a large proportion of mode waters from both the Indian and Pacific sectors of the Southern Ocean enter subtropical latitudes via the Pacific pathway.

Reference

Sallée, J., K. Speer, S. Rintoul, and S. Wijffels (2010), Southern Ocean thermocline ventilation, Journal of Physical Oceanography, 40, 509-529.