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There is significant interannual variability in the atmospheric concentration of carbon dioxide (CO2) even when the effect of anthropogenic sources has been accounted for. This variability is well correlated with the El Nino-Southern Oscillation (ENSO) cycle. This behavior of the natural carbon cycle provides a valuable mech- anism for validating carbon cycle models. The model in turn is a valuable tool for examining the processes involved in the relationship between ENSO and the carbon cycle. A GCM coupled climate-carbon cycle model is used to study the mechanisms involved. The model simulates the observed temperature, precipitation, and CO 2 response of the climate to the ENSO cycle. Climatic changes over land during El Nino events lead to decreased gross primary productivity and increased plant and soil respiration, and hence the terrestrial biosphere becomes a source of CO 2 to the atmosphere. Conversely, during El Nino events, the ocean becomes a sink of CO 2 because of reduction of equatorial Pacific outgassing as a result of decreased upwelling of carbon-rich deep water. During La Nina events the opposite occurs; the land becomes a sink and the ocean a source of CO2. The magnitude of the model's response is such that the terrestrial biosphere releases about 1.8 GtC yr 21 for an El Nino with a Nino-3 index of magnitude 18C, and the oceans take up about 0.5 GtC yr21 .( 1 GtC5 1015 g of carbon). The net global response is thus an increase in atmospheric CO 2 of about 0.6 ppmv yr21. This is in close agreement with the sensitivity of the observed CO 2 record to ENSO events.

The Carbon Cycle Response to ENSO: A Coupled Climate–Carbon Cycle Model Study