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

Abstract

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 Niño?Southern Oscillation (ENSO) cycle. This behavior of the natural carbon cycle provides a valuable mechanism 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 CO2 response of the climate to the ENSO cycle. Climatic changes over land during El Niño events lead to decreased gross primary productivity and increased plant and soil respiration, and hence the terrestrial biosphere becomes a source of CO2 to the atmosphere. Conversely, during El Niño events, the ocean becomes a sink of CO2 because of reduction of equatorial Pacific outgassing as a result of decreased upwelling of carbon-rich deep water. During La Niña 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?1 for an El Niño with a Niño-3 index of magnitude 1 °C, and the oceans take up about 0.5 GtC yr?1. (1 GtC = 1015 g of carbon). The net global response is thus an increase in atmospheric CO2 of about 0.6 ppmv yr?1. This is in close agreement with the sensitivity of the observed CO2 record to ENSO events.