Land-atmosphere coupling sensitivity to GCMs resolution: A multi-model analysis of the Sahel hot-spot

MÜLLER, OMAR V.; VIDALE, PIER LUIGI; VANNIERE, BENOIT; SCHIEMANN, REINHARD

San Francisco

Conferencia; AGU Fall Meeting 2019; 2019

AGU

The land surface is recognized as a key driver of climate. There are particular regions of the Earth with strong land-atmosphere coupling, where the land surface state has a direct effect on the overlying atmosphere. On these hot-spots, soil moisture modulates land-atmosphere feedbacks through the exchanges of latent and sensible heat fluxes. It also interacts and modifies runoff, leading to potential changes in river flows.On this research, we evaluate the effect of the increasing GCM resolution on the land-atmosphere coupling. We (a) quantify the Terrestrial Coupling Index for a set of GCMs at different resolution, and (b) analyze the resolution effects on the interplay between land and atmosphere. A set of climate models experiments produced within the framework of the HighResMIP v1.0 for CMIP6 is used to evaluate the land-atmosphere coupling. The experiments include simulations at low and high horizontal resolution (LR and HR respectively) using 4 GCMs under two different configurations: atmosphere-land (AMIP) and ocean-atmosphere-land (COUPLED) runs. The runs are organized in four multi-model ensembles: AMIP LR and HR (4 members each), COUPLED LR and HR (3 members each).The results show that it is not clear that enhanced model resolution is sufficient to cause, per se, any effect on the land-atmosphere coupling. However, the better resolved orography at HR improves the large-scale circulation that governs the precipitation location and timing leading to changes in the location land-atmosphere seasonal hot-spots. The most noticeable change is observed over the Sahel. At HR, all GCMs agree with a northward shift in this hotspot that starts in boreal summer and continues in autumn. The shift is originated by changes in atmospheric conditions during JJA. The enhanced orography favours the horizontal wind shear, increasing vertical atmospheric instability and producing more rain and soil moisture over the Sahel. It leads to a positive feedback (a) with evapotranspiration to the north of Sahel increasing the coupling, (b) with runoff to the south of Sahel reducing the coupling. In the next season the atmospheric differences between HR and LR are weak. However, the coupling shift persists due to the soil moisture memory that keeps the same conditions in the land-atmosphere interplay.