Global oceanic emissions of nitrous oxide

Abstract

The global N 2 O flux from the ocean to the atmosphere is calculated based on more than 60,000 expedition measurements of the N 2 O anomaly in surface water. The expedition data are extrapolated globally and coupled to daily air‐sea gas transfer coefficients modeled at 2.8°×2.8° resolution to estimate a global ocean source of about 4 (1.2–6.8) Tg N yr−1. The wide range of uncertainty in the source estimate arises mainly from uncertainties in the air‐sea gas transfer coefficients and in the global extrapolation of the summertime‐biased surface N 2 O data set. The strongest source is predicted from the 40–60°S latitude band. Strong emissions also are predicted from the northern Pacific Ocean, the equatorial upwelling zone, and coastal upwelling zones occurring predominantly in the tropical northern hemisphere. High apparent oxygen utilization (AOU) at 100 m below the mixed layer is found to be correlated positively both to N 2 O production at depth and to the surface N 2 O anomaly. On the basis of these correlations, the expedition data are partitioned into two subsets associated with high and low AOU at depth. The zonally averaged monthly means in each subset are extrapolated to produce two latitude‐by‐month matrices in which monthly surface N 2 O is expressed as the deviation from the annual mean. Both matrices contain large uncertainties. The low‐AOU matrix, which mainly includes surface N 2 O data from the North Atlantic and the subtropical gyres, suggests many regions with positive summer deviations and negative winter deviations, consistent with a seasonal cycle predominantly driven by seasonal heating and cooling of the surface ocean. The high‐AOU subset, which includes the regions most important to the global N 2 O ocean source, suggests some regions with positive winter deviations and negative summer deviations, consistent with a seasonal cycle predominantly driven by wintertime mixing of surface water with N 2 O‐rich deep water. Coupled seasonal changes in gas transfer coefficients and surface N 2 O in these important source regions could strongly influence the global ocean source.