Convective control of ENSO simulated in MIROC5

A high sensitivity of the El Niño-Southern Oscillation (ENSO) property on cumulus convection is examined by means of a series of climate simulations using an updated version of the Model for Interdisciplinary Research on Climate (MIROC), called MIROC5. Given that the pre-industrial control run by MIROC5 shows realistic ENSO, we repeated the integration with four different values of a parameter, l, which affects efficiency of entrainment rate in cumuli. The ENSO amplitude is found to be proportional to l and varies from 0.6 to 1.6 K.

A comparison of the four experiments reveals the mechanisms by which the cumulus convections control ENSO behavior in MIROC as follows. The efficient entrainment due to large l increases congestus over the intertropical convergence zone (ITCZ), which acts to set wetter ITCZ and drier cold tongue via accelerated meridional circulation. The dry cold tongue then shifts the atmospheric responses to El Niño/La Niña westward, bringing a reduction in the effective Bjerknes feedback. The first half of these processes is identifiable in a companion set of the atmosphere model experiments, but the difference of the mean precipitation contrast is quite small. It is thus indicated that the mean meridional precipitation contrast over the eastern Pacific is a relevant indicator of the ENSO amplitude in MIROC. At the same time, the change in mean state should not be regarded as cause of different ENSO amplitude because of the presence of strong interaction between the mean state and ENSO.