The response of ENSO frequency to the increasing CO2 concentration and associated mechanism are examined with outputs of four coupled climate models (GFDL/CM2.0,CNRM/CM3,IPSL/CM4 and INM/CM3.0) submitted to the IPCC Fourth Assessment Report (IPCC AR4).Results reveal a significant change of ENSO frequency as response to the increasing CO2 concentration.However,such a change exhibits an evident model dependence.The ENSO frequency tends to increase in GFDL/CM2.0 and CNRM/CM3 models and decreases in IPSL/CM4 and INM/CM3.0 models.The model dependence is found to be determined by how the model climatological background state of the tropical Pacific responds to the increasing CO 2 concentration.It is demonstrated that the change of zonally-and vertically-averaged climatological background upper ocean temperature gradient between the equator and off-equator is crucially responsible for the ENSO frequency change.As response to the increasing CO 2 concentration,the climatological background temperature gradient is increased in GFDL/CM2.0 and CNRM/CM3 models and decreased in IPSL/CM4 and INM/CM3.0 models.In terms of the recharge-discharge oscillator theory for ENSO,the increased (decreased) climatological background temperature gradient between the equator and off-equator induces a faster (slower) exchange of oceanic heat content between the equator and off-equator,thus giving rise to a shorter (longer) ENSO timescale and a higher (lower) ENSO frequency.
