Response of terrestrial carbon exchange and atmospheric CO2 concentrations to El Nino SST forcing

摘要

Version 3 of the National Center for Atmospheric Research Community Climate Model is used to investigate the response of terrestrial carbon exchange and atmospheric CO2 concentrations to sea surface temperature (SST) anomalies associated with the El Nino phenomenon. During El Nino episodes, atmospheric CO2 concentrations are observed to rise anomalously even though CO2 outgassing is reduced in the eastern equatorial Pacific due to the cessation of upwelling. Atmospheric carbon isotope data point to a larger terrestrial carbon release as being responsible. The reasons for such a terrestrial response are examined by comparing a control run with prescribed, seasonally varying, climatological SSTs to an ensemble of integrations employing observed SST fields from the strong El Nino event of 1982-83. The model captures the main features of the El Nino induced meteorological anomalies, including the shifts in tropical rainfall patterns that are of particular importance in driving the carbon cycle changes. It is argued that the release of CO2 from terrestrial systems is fundamentally related to the overall shift of precipitation from land areas to the oceans caused by the El Nino SST forcing. Since the SST forcing and the cessation of oceanic upwelling are also intimately related, the opposing oceanic and terrestrial fluxes derived from isotope data have a sound physical explanation. The El Nino induced CO2 anomaly is tracked through space and time highlighting the important role the atmospheric circulation plays in controlling the evolution of the anomaly at any given location. The difficulty in ascribing observed fluctuations at the CO2 monitoring stations to specific regions and causes without considering atmospheric transport is demonstrated in this context. 91 refs, 12 figs, 2 tabs, 2 plates