engleski

Temporal variability of carbon fluxes in the subtropical North Atlantic at 24.5° N

The Atlantic meriodinal overturning circulation (AMOC) carries warm upper waters north where they cool and sink before returning as cold deep water. The associated ocean-atmosphere heat flux is responsible for northwest Europe's mild climate. A transatlantic hydrographic section including carbon measurements has been occupied at 24.5° N in 1992, 1998 and 2004, allowing us to examine decadal changes in the circulation and fluxes of heat, salt and carbon. In 1998 and 2004 the inferred AMOC is smaller than in 1992, compensated by an increase in southward thermocline circulation shallower than 1000 m. The northward transport of inorganic carbon/anthropogenic carbon in the Gulf Stream is increasing with time and is compensated by an increasing southward flux in the interior. The net flux of inorganic carbon is southward, driven by the large atmosphere to ocean flux of carbon to the north, while net flux of anthropogenic carbon is northward a result higher surface concentrations and lower deepwater concentrations. Most of the carbon flux variability in different years is determined by circulation variability. Although both inorganic and anthropogenic carbon concentrations are increasing with time so that the northward flux in the Gulf Stream is increasing as are the southward return fluxes in the Deep Western Boundary Current or thermocline the overall net carbon fluxes through 24.5° N appear constant. The mean net flux of the 1992, 1998 and 2004 inorganic carbon is -0.89 ± 0.17 Pg C/yr while the net anthropogenic carbon flux is 0.27 ± 0.04 Pg C/yr (both relative to the Bering Strait). A recent study of ten different Ocean General Circulation Models (OGCMs) suggest they underestimate the southward inorganic carbon fluxes by a factor of two in the subtropical North Atlantic region. Intra-annual changes of carbon fluxes are studied using RAPID mooring array and available carbon data in North Atlantic.

North Atlantic; carbon fluxes

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