Desert Amplification and Its Diurnal Asymmetry

Abstract:

Recent studies using observations, reanalysis data and climate model simulations have found that 2m surface air temperature (T2m) in mid- and low- latitudes have warmed most over the world’s hottest and driest Sahara Desert and the Arabian Peninsula (SDAP), and this large-scale warming amplification over deserts, or desert amplification (DA), has intensified with increasing greenhouse gas concentration. This study analyzes hourly surface observations, radiosonde temperature measurements, and two latest reanalysis products for the period 1979-2018 to further our understanding of the diurnal and vertical variations of DA and their connections with planetary boundary layer height (PBLH). It focuses on the Arabian Peninsula (AP), where observations are relatively abundant compared to the data scarce Sahara regions. Both observational and reanalysis data consistently show that the diurnal cycle of surface warming rate depends inversely on the magnitude of climatological PBLH, and so DA has a distinct diurnal asymmetry – the strongest at nighttime when the PBLH is shallowest and the least warming at daytime when the PBLH is deepest. Results of upper air profiles reveal that DA is a bottom-heavy warming profile limited to the lower troposphere and surface, and its diurnal asymmetry maximizes near the surface and decreases quickly with height. These results indicate that the diurnal phase and the magnitude of PBLH play an important role in modulating the diurnal warming asymmetry of DA and its vertical structure over the AP.

DA is conceptually similar to the well-known arctic amplification (AA), a subject of intensive research for several decades. In contrast, DA is an emerging new concept and its causes are largely unknown. This study suggests that the role of PBL is to amplify the diurnal surface warming stronger with shallower PBLH over the deserts from the perspective of heat redistributing via turbulent mixing. The reported findings have important implications as DA may accelerate in the context of global warming and has significant impacts on climate, environment, water resources, agriculture, biodiversity, ecosystems, and food security in arid and semi-arid countries.