The Andes influence the tropical winds and the ENSO variations

Climate changes over the tropical trade wind area have an important global impact. Climate variations over this region, also known as the El Nino Southern Oscillation (ENSO) events, alter the global atmospheric circulation, causing unusual floods or droughts occur in a lot of regions , creating threats to our society in many aspects including agriculture, fisheries , public safety, economy and productivity. However, Coupled Global Climate Models (CGCMs) are having a lot of difficulties in the simulation of ENSO. For example, in the CESM1 atmosphere-ocean coupled model, ENSO variations can not be correctly simulated. The modeled ENSO has a much larger amplitude, too symmetric El Nino and La Nina pattern, and overly regular period compared with the observation. Recognizing these problems, we hypothesize that it is related to the failure of representing the effects of the Andes with the smoothing of the global climate models. When we adjusted the height of the Andes, CESM could produce a more accurate climate mean state as well as ENSO variations. The Higher Andes case has a larger North-South SST gradient, and the precipitation over the South Pacific gets inhibited. In the meanwhile, the Higher Andes case increases the strength of the tropical zonal easterly wind and cross-equator southerly wind. The changes of the surface wind reduce the ocean stratification in the eastern Pacific and decrease the ocean currents sensitivity to the wind anomalies. As a result, the SST anomaly variations have a weaker amplitude and there are less extreme El Nino and La Nina events. The skewness also becomes larger and the ENSO cycle becomes more irregular, which is closer to the observations. Our experiment proves that by adjusting the height of the Andes in a fully-coupled climate model, we can have a better simulation of the tropical Pacific climate mean state as well as ENSO variations, and these changes are closely related to the changes of the surface wind.