Space Observations of Water and Carbon Balance at Ocean Surface

We have produced daily maps of the carbon dioxide partial pressure and pH over global ocean, using satellite data. Ocean surface carbon dioxide partial pressure governs major variability of the ocean uptake of atmospheric carbon dioxide. The up-take mitigates greenhouse warming and increases ocean acidification. Acidification is harmful to marine life and affects primary production and carbon sequestration in the ocean. pH is a logarithm scale of the concentration of hydrogen ion in a solution; more acidic solutions have lower value and neutral is 7. These ocean carbon data sets, together with space-based measurement of salinity and rainfall open up a new perspective on the linkage of water and carbon cycles over oceans. Our data reveal that addition of water freshens up the ocean (lowers surface salinity), dilutes the carbon in the ocean, causes lower partial pressure and higher pH (lower acidity). The relation was observed in region of major water input; under the intertropical convergence zones (ITCZ), at major river discharges, and from polar ice melt, at synoptic to intra-seasonal time scales. At the mouth of the Amazon River, space-based salinity measurements give a good indication of water river discharge by the river; lower salinity corresponds to lower partial pressure and higher pH all year round. Such relation also hold for the fresh water from the Amazon carried by the North Brazilian Current Retroflection along 8°N from July to October. The relation is also observed under the ITCZ in the Pacific. There is a sharp boundary between the region influenced by rainfall under the ITCZ and the region where ocean chemistry is driven by ocean upwelling along the equator to the south. If the carbon dioxide partial pressure in the atmosphere and the prevailing wind do not change over temporal and spatial scales of partial pressure in the ocean, a reduction of ocean partial pressure with water input implies potential increase in uptake of atmospheric carbon dioxide and potential for increase in ocean acidification. The increase in carbon dioxide flux into the ocean counters the reduction of acidification by water input, and should bring the ocean back towards an equilibrium state. The time scales and locations of such processes remains to be better characterize and understand. The study opens up a new perspective of using space observation of salinity, not only as an indicator of hydrologic coupling between ocean and the atmosphere but also a linkage to carbon cycle.