Potential mechanism for response of El Niño–Southern Oscillation variability to change in land surface energy budget

Abstract

El Niño–Southern Oscillation (ENSO) variability was found to be sensitive to the land surface energy budget from a comparison of two integrations of the coupled general circulation model of Center for Ocean‐Land‐Atmosphere Studies, a control simulation in which global soil wetness in the three layers is predicted, and a sensitivity experiment in which deep soil moisture is specified. In contrast to the control experiment, in which the net land surface energy flux is zero, the sensitivity experiment leads to land becoming a unphysical and unexpected net energy sink. However, the comparison points toward a physically realizable mechanism by which ENSO can be influenced by changes in land surface properties. The net energy sink causes cooling tropical land surface. The cooling over tropical land is connected with the mean state changes of the coupled system, including a shift in the land/sea partitioning of precipitation toward the oceans, a more westerly wind stress over the tropical Pacific, and a more El Niño‐like mean state of the tropical Pacific with a weaker east‐west temperature contrast. Meanwhile, sea surface temperature (SST) variance decreases in the central and eastern tropical Pacific, and the ENSO becomes less energetic. A series of diagnostic simulations using an intermediate coupled model tests the impact of the simulated mean state and atmospheric noise changes on the ENSO variability. It is demonstrated that the mean state change plays a key role in determining the ENSO variance change. The mean state change in the sensitivity experiment causes a reduction in the sensitivity of ENSO SST variability to surface wind stress, and is consistent with a decrease in ENSO SST variance.