Abstract
AbstractLand–atmosphere interactions are often interpreted as local effects, whereby the soil state drives local atmospheric conditions and feedbacks originate. However, nonlocal mechanisms can significantly modulate land–atmosphere exchanges and coupling. We make use of GCMs at different resolutions (low ~1° and high ~0.25°) to separate the two contributions to coupling: better represented local processes versus the influence of improved large-scale circulation. We use a two-legged metric, complemented by a process-based assessment of four CMIP6 GCMs. Our results show that weakening, strengthening, and relocation of coupling hot spots occur at high resolution globally. The northward expansion of the Sahel hot spot, driven by nonlocal mechanisms, is the most notable change. The African easterly jet’s horizontal wind shear is enhanced in JJA due to better resolved orography at high resolution. This effect, combined with enhanced easterly moisture flux, favors the development of African easterly waves over the Sahel. More precipitation and soil moisture recharge produce strengthening of the coupling, where evapotranspiration remains controlled by soil moisture, and weakening where evapotranspiration depends on atmospheric demand. In SON, the atmospheric influence is weaker, but soil memory helps to maintain the coupling between soil moisture and evapotranspiration and the relocation of the hot spot at high resolution. The multimodel agreement provides robust evidence that atmospheric dynamics determines the onset of land–atmosphere interactions, while the soil state modulates their duration. Comparison of precipitation, soil moisture, and evapotranspiration against satellite data reveals that the enhanced moistening at high resolution significantly reduces model biases, supporting the realism of the hot-spot relocation.
Highlights
The land surface is recognized as a key driver of climate variability and predictability at different time scales (Koster et al 2000; Guo et al 2011, and references therein)
The comparison between low- and high-resolution global climate models (GCMs) simulations enables the investigation of both sources of land–atmosphere coupling changes: local effects originated by increased variability in land surface parameters and nonlocal effects caused by changes in atmospheric moisture fluxes, precipitation patterns, and/or the dynamics of the easterly flow
In this study we have evaluated the sensitivity of land– atmosphere coupling to GCM horizontal resolution in a set of 14 AMIP-type and COUPLED simulations submitted to CMIP6
Summary
Land–Atmosphere Coupling Sensitivity to GCMs Resolution: A Multimodel Assessment of Local and Remote Processes in the Sahel Hot Spot. MÜLLER,a,b,f PIER LUIGI VIDALE,a,b BENOIT VANNIÈRE,a,b REINHARD SCHIEMANN,a,b RETISH SENAN,c REINDERT J. JUNGCLAUSe a National Centre for Atmospheric Science, Reading, United Kingdom b Department of Meteorology, University of Reading, Reading, United Kingdom c European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom d Royal Netherlands Meteorological Institute (KNMI), De Bilt, Netherlands e The Ocean in the Earth System, Max Planck Institute for Meteorology, Hamburg, Germany (Manuscript received 26 April 2020, in final form 19 October 2020)
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