High-latitude control of nutrient distributions

The classical explanation for the observed nutrient distribution of the ocean in the low latitudes is that the downward flux of biogenic material from the surface of the ocean is balanced by upwelling of dissolved inorganic nutrients driven by vertical mixing in the main thermocline. However, estimates of the magnitude of vertical mixing in the main thermocline are about an order of magnitude too small to explain the vertical profiles of tracers within this feature, suggesting that a more likely return path for the deep water to the surface is in the Southern Ocean.

To assess the importance of nutrient export from the Southern Ocean for global ocean productivity, Jorge Sarmiento and colleagues carried out a simulation in which nutrients in the surface ocean were depleted south of 30ºS. They find that nutrient export from the Southern Ocean via Subantarctic Mode Water (SAMW) is responsible for about three quarters of biological productivity north of 30ºS, giving the processes that control the properties and export of SAMW a central role in controlling low-latitude productivity.

An exception to this SAMW nutrient domination is the North Pacific Ocean. The team’s results show that enhanced vertical mixing in the northwest corner of the Pacific Ocean, perhaps driven by tides, is crucial to low-latitude productivity in the north and equatorial Pacific. Since models consistently underpredict productivity in the North Pacific, the findings suggest that circulation in the North Pacific deserves closer scrutiny.


Model Intercomparison

Three-dimensional ocean general circulation models (OGCMs) of the carbon cycle are the primary tools used to characterize the ocean’s response to increasing atmospheric CO2 concentration, but how well they perform relative to each other has been difficult to assess. A primary reason is the absence of quantitative measures of model “skill” and common simulation protocols.

As part of the Ocean Carbon Cycle Intercomparison Project, Jorge Sarmiento, Katsumi Matsumoto and colleagues used radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment to assess a suite of 19 ocean carbon cycle models run with a common protocol. The researchers found that only a quarter of the suite adequately reproduced the global radiocarbon distribution and CFC inventories inside and outside the Southern Ocean. The study highlights the need for standard data-based metrics for testing ocean carbon cycle models, and for sensitivity testing to determine the reason for the discrepancies among simulations.