474 The Impacts of Agricultural Land Cover and Management on Land–Atmosphere Coupling: A Midlatitude Sensitivity Assessment

Tuesday, 9 January 2018
Exhibit Hall 3 (ACC) (Austin, Texas)
Sonali McDermid, New York Univ., New York, NY; and B. Cook, C. Montes, and M. Puma

Numerous studies have shown that land cover change can impact regional temperature, precipitation, and weather patterns via modifications in surface energy partitioning and vegetation attributes (e.g. albedo and roughness).1,2,3,4 However, fewer studies have investigated how agricultural land cover and management (ALCM) impact background climate conditions, such as land-atmosphere coupling strength. This coupling is largely modulated through soil moisture, and is an important determinant of surface climate conditions.5 The strength of regional land-atmosphere coupling is highly sensitive to evaporative regime (energy versus moisture limited conditions). Additionally, vegetation characteristics, growth, and transitions also play an important role in determining soil moisture-climate feedbacks (and, thus, land-atmosphere coupling).5 Spatial and temporal vegetation changes can alter transpiration and the total latent heat flux, thus impacting regional moisture cycling and evaporative regimes.6 Prior work shows that a substantial portion of rainfall variation is forced by vegetation, and in water-limited growing regions, croplands can reduce the land-atmosphere coupling strength.7,8

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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