Effect of surface sea water pCO2 variations on CO2 fluxes

Figure 3.3 shows the distribution of total net sea-air CO2 fluxes. The darker shades indicate oceanic areas where there is a net source of CO2 to the atmosphere and the lighter shades indicate regions where there is a net sink of CO2. The equatorial Pacific is a strong source of CO2 to the atmosphere throughout the year as a result of upwell-ing that brings deep, high CO2 waters to the surface in the central and eastern regions. This upwelling, and thus the CO2 flux to the atmosphere, is heavily modulated by the El Niño-southern oscillation (ENSO) cycle. During strong El Niño years the equatorial Pacific CO2 source can drop to zero. During La Niña the CO2 source to the atmosphere is enhanced. High CO2 outgassing fluxes are also observed in the tropical Atlantic and Indian oceans throughout the year. The Arabian Sea becomes a significant source of CO2 to the atmosphere in the late summer and early fall months as the south-east monsoon generates intense upwelling off the Arabian peninsula.

Strong convective mixing also brings up high CO2 values in the north-western sub-Arctic Pacific and Bering Sea during the northern winter. However, just outside this region there is a seasonal oscillation in CO2 flux. The geochemical response of the ocean to changing temperatures is to decrease the pCO2 by 4.23%/°C of sea water cooling (Takahashi et al., 1993). In some regions, decreasing temperatures in the winter can lower the ocean's pCO2 values

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Fig. 3.3. Global climatology of the annual net sea-air CO2 flux (mol CO2/m2/year) based on interpolation of sea-air pCO2 differences as in 1995. (From Takahashi et al., 2002.)

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Fig. 3.3. Global climatology of the annual net sea-air CO2 flux (mol CO2/m2/year) based on interpolation of sea-air pCO2 differences as in 1995. (From Takahashi et al., 2002.)

sufficiently to counteract the elevated CO2 brought to the surface from stronger winter time mixing (e.g. temperate North Pacific and North Atlantic oceans). The fluxes out of the ocean from elevated temperatures during summer are limited by stratification, resulting in a small net annual flux into the ocean. Similar seasonal changes are observed in the southern temperate oceans, but are out of phase by half a year.

Intense regions of CO2 uptake are seen in the high-latitude northern ocean in summer and in the high-latitude South Atlantic and southern Indian oceans in austral summer. This uptake is associated with high biological utilization of CO2 in thin mixed layers. As the seasons progress, vertical mixing of deep waters eliminates this uptake of CO2. These observations indicate that the CO2 flux in high-latitude oceans is governed primarily by deep convection in winter and biological uptake during the spring and summer months, whereas in the temperate and subtropical oceans, the flux is governed primarily by water temperature. Outside the equatorial belt, the ApCO2 (sea water pCO2

- atmospheric pCO2) is highest during winter in subpolar and polar waters, whereas it is highest during summer in the temperate regions. Thus, the seasonal variation of ApCO2 and, consequently, the shift between net uptake and release of CO2 in subpolar and polar regions are about 6 months out of phase with those in the temperate regions.

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