Sack Lunch Seminar (SLS)

Constraining ocean ventilation time scales with tracer observations: implications for anthropogenic CO2 uptake and radiocarbon ages


Ocean ventilation is the process that transports water and climatically important trace gases such as carbon dioxide from the surface mixed layer into the ocean interior. Quantifying the dominant source regions and time scales remains a major challenge in oceanography. A mathematically rigorous approach, that accounts for the multiplicity of transport pathways and transit times characteristic of an eddy-diffusive flow such as the ocean, is to quantify ventilation in terms of a Green's function, or probability distribution, that partitions fluid parcels according to their time and place of last surface contact.

In this talk, I will describe the first observationally-constrained, globally gridded estimate of this joint distribution of the age and surface origin of ocean waters. These estimates suggest that approximately 40% and 26% of the world ocean was last in contact with the Southern Ocean and North Atlantic, respectively. Some 80% of the deep ocean is ventilated from these high latitude areas supporting the classical description of the deep ocean as a mixture between a ``northern'' and ``southern'' source.

The mean transit time from the surface to the deep North Pacific is estimated at 1360+/-350y, intermediate between two widely used radiocarbon-based estimates.

A wide spectrum of ages contributes to this average, providing evidence for the fundamentally eddy-diffusive nature of the large scale general circulation of the ocean. These results have important implications for our understanding of the ocean sink of anthropogenic CO2 (Cant). The high latitude regions, for example, account for only 30% of the global ocean uptake of Cant, significantly less than expected from their volumetric contribution. In contrast, the tropics contribute only 4.5% to the water mass, but are responsible for 30% of the Cant uptake. To understand these results, I analyze the ocean's ventilation-rate distribution or volume-integrated Green's function (G) to show that the temporal structure of G is crucial for determining which surface regions have contributed most to the ocean's Cant uptake and for distinguishing their contributions to the Cant inventory from their volumetric contributions. These results indicate that CO2 uptake is not related in a simple manner to the aggregate amount of deep water formed in a particular region, but is sensitive to the shape of the ventilation-rate distribution.