GLOBAL OCEANIC EMISSIONS OF NITROUS-OXIDE

The global N2O flux from the ocean to the atmosphere is calculated bas ed on more than 60,000 expedition measurements of the N2O anomaly in s urface water. The expedition data are extrapolated globally and couple d to daily air-sea gas transfer coefficients modeled at 2.8 degrees x 2.8 degrees 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 estim ate arises mainly from uncertainties in the air-sea gas transfer coeff icients and in the global extrapolation of the summertime-biased surfa ce N2O data set. The strongest source is predicted from the 40-60 degr ees S latitude band. Strong emissions also are predicted from the nort hern Pacific Ocean, the equatorial upwelling zone, and coastal upwelli ng 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 N2O production at depth a nd to the surface N2O 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 subs et are extrapolated to produce two latitude-by-month matrices in which monthly surface N2O is expressed as the deviation from the annual mea n. Both matrices contain large uncertainties. The low-AOU matrix, whic h mainly includes surface N2O data from the North Atlantic and the sub tropical gyres, suggests many regions with positive summer deviations and negative winter deviations, consistent with a seasonal cycle predo minantly driven by seasonal heating and cooling of the surface ocean. The high-AOU subset, which includes the regions most important to the global N2O ocean source, suggests some regions with positive winter de viations and negative summer deviations, consistent with a seasonal cy cle predominantly driven by wintertime mixing of surface water with N2 O-rich deep water. Coupled seasonal changes in gas transfer coefficien ts and surface N2O in these important source regions could strongly in fluence the global ocean source.