Comparison of elevation-dependent warming and its drivers in the tropical and subtropical Andes

Osmar Toledo,Iván Mauricio Cely Toro,Luca Mortarini,Elisa Palazzi

doi:10.1007/s00382-021-06081-4

Abstract

Mountain regions have been recognized to be more sensitive to climate and environmental changes, and in particular to global warming. Several studies report on elevation-dependent warming (EDW), i.e., when warming rates are different in different altitude ranges, particularly focusing on the enhancement of warming rates with elevation. The Andean chain proved to be a relevant climate change hot-spot with positive temperature trends and a widespread glacier retreat over the recent decades. To assess and to better understand elevation dependent warming in this mountain region and to identify its possible dependence on latitude, the Andean Cordillera was split into five domains, three pertaining to the tropical zone and two pertaining to the Subtropics. Further, for each area the eastern and western faces of the mountain range were separately analyzed. An ensemble of regional climate model (RCM) simulations participating in the Coordinated Regional Climate Downscaling Experiment (CORDEX), consisting of one RCM nested into eight different global climate models from the CMIP5 ensemble was considered in this study. EDW was assessed by calculating the temperature difference between the end of the century (2071–2100) and the period 1976–2005 and relating it to the elevation. Future projections refer to the RCP 8.5 high-emission scenario. Possible differences in EDW mechanisms were identified using correlation analyses between temperature changes and all the variables identified as possible EDW drivers. For the maximum temperatures, a positive EDW signal (i.e. enhancement of warming rates with elevation) was identified in each side of both the tropical and subtropical Andes and in all seasons. For the minimum temperatures, on the contrary, while a positive EDW was identified in the Subtropics (particularly evident in the western side of the chain), the Tropics are characterized by a negative EDW throughout the year. Therefore, the tropical boundary marks a transition between discordant EDW behaviours in the minimum temperature. In the Tropics and particularly in the inner Tropics, different EDW drivers were identified for the minimum temperature, whose changes are mostly associated with changes in downward longwave radiation, and for the maximum temperature, whose changes are mainly driven by changes in downward shortwave radiation. This might explain the opposite EDW signal found in the tropical Andes during daytime and nighttime. Changes in albedo are an ubiquitous driver for positive EDW in the Subtropics, for both the minimum and the maximum temperature. Changes in longwave radiation and humidity are also EDW drivers in the Subtropics but with different relevance throughout the seasons and during daytime and nighttime. Also, the western and eastern sides of the Cordillera might be influenced by different EDW drivers.

Highlights

Covering about 25% of the world’s land surface and hosting about 12% of the world population, mountains provide a multitude of goods and services to both high-altitude environments and to downstream regions
The present paper focuses on future projections of warming rates in the southern hemisphere tropical and subtropical Andes, and in particular on the existence and driving mechanisms of elevation-dependent warming (EDW), highlighting possible differences as a function of the latitude, the season, the considered variable and the side of the Andean Cordillera, i.e. considering its western and eastern slopes separately
ERA5 data are shown in red while the model results are in black and in grey

Summary

Introduction

Covering about 25% of the world’s land surface and hosting about 12% of the world population, mountains provide a multitude of goods and services to both high-altitude environments and to downstream regions. In the arid and semiarid regions of the Tropics and Subtropics, in particular, mountain regions play a very crucial role since they contain more than 80% of the freshwater made available to the countries around, especially when precipitation is scarce or absent (Viviroli et al 2007, 2011). Mountain regions have been recognized to exhibit enhanced sensitivity to climate and environmental changes, and in particular to global warming, overall showing larger temperature trends ( 0.3 ◦C/decade ) compared to the globally-averaged trend ( 0.2 ◦C/decade , IPCC 2019, and references therein). The widespread glacier retreat observed in most mountain areas of the globe over the last decades is one of the most striking evidences of the temperature increase, though other changes, such as those in precipitation, are at play (Bradley et al 2006; Tennant et al 2014)

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Conclusion