Human-induced land use changes and the resulting alterations in vegetation features are major but poorly recognized drivers of regional climatic patterns.In order to investigate the impacts of anthropogenically-induced seasonal vegetation cover changes on regional climate in China,harmonic analysis is applied to 1982-2000 National Oceanic and Atmospheric Administration（NOAA） Advanced Very High Resolution Radiometer（AVVHRR）-derived normalized difference vegetation index（NDVI） time series（ten day interval data）.For two climatic divisions of South China,it is shown that the first harmonic term is in phase with air temperature,while the second and third harmonics are in phase with agricultural cultivation.The Penman-Monteith Equation and the Complementary Relationship Areal Evapotranspiration（CRAE） model suggest that monthly mean evapotranspiration is out of phase with temperature and precipitation in regions with signiffcant second or third harmonics.Finally,seasonal vegetation cover changes associated with agricultural cultivation are identiffed：for cropped areas,the temperature and precipitation time series have a single maximum value,while the monthly evapotranspiration time series has a bimodal distribution.It is hypothesized that multi-cropping causes the land surface albedo to sharply increase during harvesting,thereby altering the energy distribution ratio and contributing to observed seasonal vegetation cover changes.
Correlation analysis and the complementary relationship model are used to indirectly measure and characterize the dynamic and thermodynamic properties at the land-atmosphere interface in China.Results show that thermodynamic forces driving the exchange fluxes of energy,water and carbon between the land surface and the overlying atmosphere are significantly enhanced along the mid-upper reaches of the Yangtze River basin around Sichuan Basin and the northern region of Heilongjiang.The thermodynamic forces exhibit a decreasing trend in southern Xinjiang and some regions of Yunnan.Low cloud cover was shown to be a significant factor leading to changes in thermodynamic properties at the land-atmosphere interface.Western Pacific Subtropical High (WPSH) and storm data provide strong evidence that a changing synoptic system is likely a principle factor causing the abnormal spatiotemporal distribution of thermodynamic forces over the land surface in China.This data also suggests that cyclical and quasi-cyclical fluctuations of low cloud cover in most regions of China are responsible for annual rainfall patterns.At the same time,NOAA-AVHRR satellite remote sensing data illustrates that changes to land surface characteristics may cause atypical spatiotemporal distributions of thermodynamic forces over the land surface.We show that thermodynamic forces are weakening in regions with increasing vegetation.However,where urbanization has decreased the quantity of vegetation,thermodynamic forces are rising.Finally,an increase in large-scale thermodynamic forces is associated with a reduction in low cloud cover due to changes in the synoptic system.
