How the co-evolution of major mountain ranges affects global climate

The topographic formation of large mountains and plateaus significantly impacts regional and global climate. Previous studies demonstrated that major mountain ranges can explain important aspects of synoptic scale climate dynamics and notable features of the climate system, such as the position of the intertropical convergence zone. Quantifying the synergistic climatic effects of the coeval evolution of major mountain ranges fosters a deeper understanding of climate and Earth system dynamics. Furthermore, it helps estimate where (and by how much) a regional climate signal recorded in a geological archive is affected by topographic changes in distant, off-site orogens. In this study, we use ECHAM5-wiso General Circulation Model (GCM) simulations to explore the synergistic global effects of systematically co-varying the height of the Andean and Himalaya-Tibet Plateaus. The simulations are conducted with different topographic evolution scenarios for these orogens, while environmental boundary conditions, such as global ice cover and greenhouse gas concentrations, are kept constant. More specifically, the topographies of the orogens are incrementally reduced by 25% of their current height. This results in 5 topographic scenarios for the Himalaya-Tibet by setting its elevation to 100%, 75%, 50%, 25% and 0% of current values. These are nested in the analogous 5 topographic scenarios for the Andes, resulting in a total of 25 scenarios and GCM simulations. We then conduct an empirical orthogonal functions (EOF) analysis on the pressure fields produced by each of the simulations to track changes in quasi stable pressure systems. Furthermore, we track changes in cross-equatorial atmospheric transport and synoptic scale atmospheric flow. While most of the regional impacts of evolving topographies can be explained by atmospheric lapse rates and physical air flow disruption, global impacts can be explained by changes in surface heat distribution and pressure centres affecting synoptic scale atmospheric flow. We also find that the height of Himalaya-Tibet modifies the impact of Andean topography on northern hemisphere climate, highlighting interhemisphere climate teleconnections between the two orogens. Our results suggest that robust interpretations of climate signals recorded in geological archives in many regions on Earth are only possible when the global climatic effects of the topography of distant, off-site orogens are considered.