Krijnen, Justin (2013) Key mechanisms of surface water pCO2 variability in the North Atlantic Ocean. Doctoral thesis, University of East Anglia.
A proxy for the North Atlantic gyre circulation has been developed, using sea-surface height from altimetry. In conjunction with the winter North Atlantic Oscillation (NAO) index, statistical analysis has been applied to understand the key mechanisms of surface water partial pressure of CO2 (pCO2) variability, both on the seasonal and inter-annual timescale.
With respect to the seasonal amplitude of surface pCO2 in temperate regions (>40°N), it is found that the gyre circulation strength, in response to the winter NAO index, drives this seasonal amplitude. Under positive NAO index winters, the formation of mode waters is favoured through strong surface cooling. This deepens the mixed layer, entraining carbon and nutrient-rich subsurface waters into the surface layer and increasing the surface pCO2 in winter. This deep winter mixing, bringing up nutrients in combination with enhanced advection of nutrients from the subpolar region, may also enhance and prolong the following spring bloom, decreasing the pCO2 in both spring and early summer. Thus, the seasonal amplitude of surface pCO2 under a positive NAO phase would increase.
Under negative NAO winters, surface cooling is not as pronounced compared to a positive NAO winter and therefore the mixed layer is not as deep. Thus, both vertical and horizontal (via advection) carbon and nutrient entrainment are reduced thereby decreasing the pCO2 in winter and potentially weakening the following spring bloom. Thus the seasonal amplitude of surface pCO2 under a negative NAO phase would decrease. The subtropical regions (25 - 40°N) are also subjected to similar processes as the temperate regions, under both positive and negative NAO winters.
However, the above-mentioned lagged effect of carbon-rich sub-surface water and nutrient entrainment in winter on the intensity of the spring bloom has to be treated with caution given the lack of statistically significant correlations between the surface pCO2 in winter and the proxy for carbon-rich subsurface water in spring.
On inter-annual timescales, the phase of the winter NAO alters the ocean circulation in all regions. Under a positive NAO index, the subtropical gyre is more spun-up and with increased SST, increasing the annual mean pCO2. In the temperate zone, the interplay between carbon entrainment and biological drawdown dominates, dampening the inter-annual pCO2 variability.
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