Surface ocean

Nevison et al (1995) calculated a global mean N2O surface saturation of 103.5 per cent, which indicates that the ocean, on a global scale, is supersaturated with N2O and acts as a net source of N2O to the atmosphere. N2O saturations in the ocean surface layer are not uniform and can show considerable seasonal variability (Nevison et al, 1995). However, the current data coverage does not make it possible to decipher the seasonality in most parts of the ocean. Global maps of ApN2O in the upper 10m of the world's oceans have been computed by Nevison et al (1995) and Suntharalingam and Sarmiento (2000). Common features of both maps (see Plate 3.1) are: (1) enhanced N2O anomalies in the equatorial upwelling regions of the eastern Pacific and Atlantic Oceans, enhanced N2O anomalies along coastal upwelling regions such as along the west coasts of North and Central America, off Peru, off Northwest Africa and in the northwestern Indian Ocean (Arabian Sea); and (2) N2O anomalies close to zero (i.e. near equilibrium) in the North and South Atlantic Ocean, the South Indian Ocean and the central gyres of the North and South Pacific Oceans.

Differences in the two maps result mainly from different computation methods. Additionally, both maps are biased by insufficient data coverage in some parts of the ocean (for example in the Indian and western Pacific Oceans).

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