In particular, the mid-latitudes support some of the most productive agricultural lands, and also span large gradients in seasonal land-atmosphere coupling and evaporative regime.5,9,10,11 Previous studies show that land cover change cools mid-latitude climates during the summer via latent heat increases and can also increase rainfall locally and remotely.2,4,12,13,14,15 Land cover change can also decrease snow-masking effects, resulting in late winter/early spring cooling responses.16 Furthermore, land cover change studies for North American growing regions suggest that pre-planting and post-harvest temperature responses differ substantially from those during the prime growing season.12,14 It is thus important to understand how ALCM can modulate regional land-atmosphere coupling to potentially impact seasonal and background climate conditions.
We present new work to understand how ALCM modulates the background climate conditions and seasonal land-atmosphere coupling across mid-latitude growing regions (35˚N-65˚N) through impacts on soil moisture, evaporative regime, and synoptic circulation patterns. We conduct a set of novel climate model sensitivity experiments comparing the background climate under natural vegetation conditions to a global representation of agriculture, inclusive of cropping calendars (planting and harvest dates) and regionally representative seasonal growth cycles for maize, rice, wheat, and soybean, as well as crop rotations. We then evaluate the impacts of this agricultural representation on key climate variables, such radiative and turbulent fluxes, temperature, precipitation, and circulation features, and on the land-atmosphere coupling via changes to soil moisture and the correlation between latent heat and surface temperature (e.g., Figure 1a, shown for the “natural vegetation” experiment). In doing so, we place particular emphasis on the simulated responses along mid-latitude transition areas between moisture and energy limited conditions, namely in the North American Great Plains, the USA Midwest, and in Central Asian growing regions (boxed areas in Figure 1). Our initial results indicate that ALCM weakens the prevailing land-atmospere coupling in these regions (Figure 1b and 1c). In some cases, like the Great Plains, this is due to enhanced precipitation which buffers soil moisture stores. In Central Asia and the far easter portion of the USA Midwest, there are resulting shifts from water-limited conditions to energy-limited conditions. In the USA Midwest, this shift in land-atmosphere coupling occurs despite year-round reductions in soil moisture. We further present a detailed examination of the surface climate and synoptic-circulation mechanisms associated with these changes in land-atmosphere coupling. We also suggest avenues and methods for more detailed assessments and discuss the implications for agricultural suitability and interactions with irrigation and anthropogenic GHG forcing.
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