Nonlinearity and multivariate dependencies in land-atmosphere coupling

Abstract:

Global pattern of strength in land-atmosphere coupling has been assessed by a variety of measurement under linear dependency framework. However, more complex dependencies exist, including the long recognized nonlinear relationship between components of land-atmosphere coupling and the recent realization of the transmutability of the relationship between soil moisture and surface heat fluxes under different environmental conditions. To address both nonlinear and multidimensional aspects of land-atmosphere coupling, we use technique called multivariate mutual information to quantify how surface heat flux depends on the both energy and wetness conditions of the land surface, i.e. net radiation and soil moisture. A newly integrated partitioning method proposed in this study decomposed such dependency quantity as the linear and nonlinear dependency, which are further decomposed as different components explainable as the unique contribution from peculiar land conditions, redundant contribution from both land conditions, and the synergistic contribution from the cooperation of net radiation and soil moisture. The results shows that the linear dependency contributed from soil moisture bears a similar pattern as the past-identified hot spots. The linear unique contributions of net radiation and soil moisture are spatially nonoverlapping which suggests the two dominated regimes governed by either energy or water. These patterns persist when the nonlinearity is superimposed, thus reinforcing the validity of the land-atmospheric coupling hot spot paradigm and the spatial division of energy-limited as well as water-limited regions. Nevertheless, strong nonlinear relationships are detected in arid regions and across midlatitudes and subtropical areas in the winter hemisphere. Synergistic components are found over the whole globe , implying widespread multidimensional physical relationships among net radiation, soil moisture, and surface heat fluxes that previously had only been inferred locally.