Simulated impacts of agricultural land use change on the climate systems in Asia

We find that simulated irrigation impacts the climate near the surface as well as in the upper troposphere over the irrigated regions using the Community Atmosphere Model (CAM3.0) coupled to the Community Land Model (CLM3.5). In addition, these irrigation-induced changes could affect the atmospheric circulation over Asia and, in turn, jet streams over the irrigated and surrounding regions.

The effects of irrigation on the near-surface climate are significant over parts of major river basins in the Middle East (Tigris-Euphrates river basin) and Central Asia (Amu Darya and Syr Darya river basins) during boreal summer. Decreased sensible heat and increased latent heat fluxes lead to significant cooling in surface air temperature. The irrigation cooling effect is not confined to the near-surface, but rather spreads to the lower-troposphere and reaches almost the mid-troposphere. We find that irrigation could significantly decrease temperature in the lower-troposphere, by 0.1~0.8 K over the Middle East and Central Asia in summer. The irrigation-induced cooling in the lower-troposphere causes a decrease in the layer-averaged temperature between two pressure surfaces in the troposphere (thickness temperature), and in turn it leads to a decrease in tropospheric height over the irrigated regions. In the model simulations, the differences of the averaged temperatures in 1000-850, 1000-500, and 1000-200 layers between the irrigated and control runs show significant cooling in the troposphere over the Middle East and Central Asia. Thus, a cooling in the depth-averaged temperature results in a significant decrease in tropospheric height over the irrigated regions.

These results suggest that irrigation activities can impact the atmospheric circulation over the Middle East and Central Asia through the changes in tropospheric height due to the irrigation-induced cooling in tropospheric layers. In boreal summer, cyclonic differences of upper-level winds are found around the negative differences of upper-level height (lower height) across northern Africa and Central Asian region. The cyclonic differences weaken the atmospheric circulation over the irrigated and surrounding regions, which is a strong anti-cyclonic circulation over the tropical to mid-latitude Africa-Asian regions. These changes lead to a weakening of the westerly Asian jet located over the irrigated regions. The meridional cross-section of u-wind supports a significant weakening of the Asian jet (~2.5 m/s at 200 hPa) by ~10% of its maximum wind speed (~30 m/s). The simulated impacts of irrigation on the atmospheric circulation and in turn Asian jet are supported by a comparison with observational results. The spatial patterns of the differences of the upper-level height and zonal wind between the irrigated and control simulations are significantly correlated with those from the observations between the recent- and early-irrigated periods over the irrigated and surrounding regions. The overall physical and dynamic effects of irrigation on the atmospheric circulation over Asia are shown below:

(1) Irrigation over the Middle East and Central Asia in boreal spring and summer (2) A decrease in sensible heat and an increase in latent heat fluxes in summer (3) A cooling of both the surface and the lower-troposphere (4) A decrease in the layer-averaged temperature in the troposphere (5) A decrease in the tropospheric height (6) A weakening of the upper-level anti-cyclonic circulation (7) A weakening of Asian jet in summer