How South American Topography Influences Climate Simulation over the South Pacific Ocean in CESM

Current climate models significantly overestimate precipitation and temperature over the southeast Pacific because the models absorb too much in the Southern Hemisphere and underestimate the extent of the cold tongue in the southeastern Pacific Ocean. We hypothesize that the topographic smoothing in the low-resolution model causes the elevation of the Andes too low, allowing too much air flow across the mountains. When we adjusted the height of the Andes and blocked low-level air flow across the mountains using the Community Earth System Model (CESM) version 1.2.2, the anticyclonic motion in the South Pacific is enhanced, increasing the subsidence of air and the low-level cloud fraction. The radiative effect from low-level clouds decreases the sea surface temperature (SST) over the southeastern Pacific, and the enhanced descent inhibits precipitation. We also performed a sensitivity test, varying the elevation of the Andes from 2 to 6 km, and analyzed the resulting energy budget in both hemispheres. Increase of the elevation of the Andes causes the energy flux equator (EKE) to shift northwards, which decreases the energy absorption in the Southern Hemisphere, and precipitation shift from the eastern to western Pacific. Our research suggests that the warm and wet biases in the southeastern Pacific may result from the smoothed topography of the Andes in the climate models.