The Effects of Topography on the Seasonality of Aridity and Humidity: A Case Study in the Andes from Observations and Modeling Results

The Andes form a topographic wall between one of Earth's wettest climates (the Amazon) and one of its driest (the Atacama). In the northern Andes, predominantly easterly winds from the Atlantic bring orographically focused precipitation to Andes' eastern flanks, contributing to hyperarid conditions on the western side. By contrast, westerly winds prevail in the southern Andes, where most precipitation occurs near the coast. Paleoclimatic indicators have been used to constrain the rate and timing of Andean uplift: these and climate modeling studies have shown that the Central Andes must have reached an elevation of 2500 - 3000m by the mid-Miocene. We propose that these indicators are affected by the seasonality of specific humidity in that region. Seasonal specific humidity is significant for two reasons: (1) strongly seasonal (monsoonal) precipitation erodes more, and larger, sediments than does moderate year-round precipitation; and (2) moist air cools more slowly with elevation than does dry air, implying that estimates of paleoelevation that are derived from paleothermometry require an understanding of seasonal specific humidity and lapse rate. We examine the Central Andes in 18 coupled climate models with similar atmospheric forcing but differing orographies (the maximum height varies by over 5000m) in order to examine the interplay between simulated orography and the transport of water vapor along and across the Andes. We compare surface temperatures, specific humidities and precipitation over the Central Andes for the late 20th century from observations with output available at The Program for Climate Model Diagnosis and Intercomparison (PCMDI) for the coupled climate models submitted to the Intergovernmental Panel on Climate Change’s 4th Assessment (IPCC-AR4). Our analysis demonstrates that the seasonality of temperature, humidity and precipitation are correlated with mountain volume over the Central Andes.