Bange, Hermann W.

Naik, H.

Naqvi, S.W.A.

OceanRep

www.so las - in t .o rg / /09The world‘s oceans (including the coastalzones) account for about one-third of theglobal nitrous oxide (N2O) emissions to theatmosphere (IPCC, 2007). In the tropicalocean containing pronounced oxygenminimum zones (OMZs) – such as thosefound in the Arabian Sea and the easterntropical North Pacific (ETNP) – maximumaccumulation of up to 80 nmol L–1 ofdissolved N2O generally occurs at waterdepths of 50-150 m (Bange, 2008) Forcomparison: typical N2O concentrations inthe surface mixed-layer are 5-10 nmol L–1(Bange, 2008). However, N2O from thisupper-ocean maximum is not ventilated tothe atmosphere because of stratification.Weather disturbances such as storms can,nevertheless, deepen the mixed layerconsiderably, thereby entraining N2Ofrom the subsurface maximum to thesurface layer, where it easily escapes tothe atmosphere. This simple scenario ofenhanced N2O emissions from the oceancaused by strong wind events (such ascyclones in the North Indian Ocean andhurricanes in the East Pacific Ocean) hasnever been demonstrated because scientificship campaigns usually avoid storms.In a recent paper, Naik et al. (2008) providethe first evidence for this phenomenon.These authors could obtain data on verticaldistribution of N2O in the central ArabianSea just before and after a cyclone crossedthe region in December 1998, and founda significant enhancement of N2Oconcentration in the surface layer alongwith a decrease in temperature and anenrichment of macronutrients (nitrate andphosphate), all testifying to intense verticalmixing caused by the cyclone. On the basisof their data they calculated that the N2Oinventory in the upper 50 meters increasedby about 2.3 Gg N2O following the cyclone.Assuming that an equal amount wouldalready have escaped to the atmosphere(because of the high wind speedsfacilitating vigorous air-sea gas exchange)during and shortly after the cyclone(4-8 days) before the observations weremade, we estimate that this single event,which lasted only for 7 days, contributedan extra 1% to the annual Arabian SeaN2O emissions of about 500 Gg N2O (Bangeet al., 2001). The central Arabian Sea on anaverage experiences 2 cyclones every year(mean calculated from the 1945-2007 datagiven at http://weather.unisys.com/hurricane/n_indian/index.html). Thus, theannual contribution of cyclone-triggeredN2O should be at least 4.6 Gg yr–1. Theaverage hurricane frequency in the ETNP ismuch higher (14 per year, calculated fromthe 1949-2007 data given athttp://weather.unisys.com/hurricane/e_pacific/index.html). In the ETNP comparableN2O accumulation occurs in the shallowsubsurface layer. Assuming a similar effluxof N2O caused by each cyclone or hurricane,we estimate that a total of about 80 GgN2O is released from the Arabian Sea andthe ETNP annually as a result of tropicalstorms. This amounts to about 1.3% ofthe global open ocean N2O emissions of6000 Gg yr–1 (IPCC, 2007). This is a veryconservative estimate, however, becausethe effect of storm events on N2O emissionsfrom adjacent ocean areas such as thecoastal upwelling in the Arabian Sea hasnot been taken into consideration, andCyclone 08A-98, the data for which havebeen used here, was of a very moderateintensity (maximum wind speed ~120 km h-1as compared to, for example, 270 km h-1 forthe super-cyclone Gonu that hit exactly thesame area during 27 May - 5 June 2007).Moreover, Patra et al. (2004) used a modelto simulate N2O emissions from the ArabianSea triggered by a cyclone (03A-98) inJune 1998: They estimated that as much as40 Gg N2O was released to the atmosphereduring this single 7-day event (that wasintermediate of 08A-98 and Gonu inintensity – maximum wind 185 km h-1).This corresponds to about 8% of the annualN2O emissions from the Arabian Sea alone,and about 0.7 % of the annual global openocean emissions. The global contribution ofcyclone-triggered N2O emissions from theNorth Indian Ocean should be even largerwhen including regions such as the Bay ofBengal (where there is also a large build-upof N2O beneath the strongly stratifiedsurface layer (Naqvi et al., 1994) andcyclones are far more frequent and intensethan in the Arabian Sea). Therefore, weconclude that N2O emissions triggered bystrong wind events – cyclones in the NorthIndian Ocean and hurricanes in the ETNP –contribute significantly to both the regionaland global oceanic N2O fluxes. Thiscontribution must be included in futuremodel scenarios of oceanic N2O emissions.ReferencesBange, H.W., et al., 2001. Nitrous oxideemissions from the Arabian Sea:A synthesis. Atmospheric 80 Chemistryand Physics 1, 61–71.Bange, H.W., (2008). Gaseous nitrogencompounds (NO, N2O, N2, NH3) in theocean, in Nitrogen in the marineenvironment, edited by D.G. Capone et al.,in press, Elsevier.IPCC (2007). Climate Change 2007:The Physical Science Basis. Contribution ofWorking Group I to the Fourth AssessmentReport of the Intergovernmental Panel onClimate Change, Cambridge 85 UniversityPress, Cambridge, UK and New York, NY, USA.Naik, H., et al., (2008). Impact of a tropicalcyclone on biogeochemistry of the centralArabian Sea. Global Biogeochemical Cycles,22, GB3020, doi:10.1029/2007GB003028.Naqvi, S.W.A., et al., (1994). Nitrous oxidein the western Bay of Bengal. MarineChemistry 47, 269-278. 90Patra, P.K., et al., (2004). Severe weatherconditions in the Arabian Sea and theirimpact on atmospheric N2O budget.Indian Journal of Marine Science 33, 84–94.Enhancement of oceanic nitrous oxide emissions by stormsHermann W Bange1, H Naik2 and SWA Naqvi2,3 , 1IFM-GEOMAR, Kiel, Germany, 2National Institute of Oceanography, Goa, India, 3Max-Planck-Institut fürMarine Mikrobiologie, Bremen, Germany Contact: hbange@ifm-geomar.deSince 2001 Hermann Bange has been working as a chemical oceanographer in the MarineBiogeochemistry Research Division of IFM-GEOMAR in Kiel. Currently he is coordinating theGerman SOPRAN project. His research interests include the oceanic emissions and pathwaysof trace gases such as nitrous oxide, methane and dimethyl sulfide. He is interested in thedistributions of short-lived intermediates of the marine nitrogen cycle such as hydroxylamineand hydrazine. Hermann participated in several cruises to the North and Baltic Seas, the AegeanSea, the Arabian Sea and the tropical North Atlantic Ocean.

Enhancement of oceanic nitrous oxide emissions by storms

Abstract

Similar works

Full text

Available Versions

OceanRep