Deconstructing the soil moisture-latent heat flux relationship: the range of coupling regimes experienced and the presence of nonlinearity within the sensitive regime

Abstract

Abstract The control of latent heat flux (LE) by soil moisture (SM) is a key process affecting the moisture and energy budgets at the land-atmosphere interface. SM:LE coupling relationships are conventionally examined using metrics involving temporal correlation. However, such a traditional linear approach, which fits a straight line across the full SM:LE space to evaluate the dependency, leaves out certain critical information: nonlinear SM:LE relationships and the long-recognized thresholds that lead to dramatically different behavior in different ranges of soil moisture, delineating a dry regime, a transitional regime of high sensitivity, and a wet (energy-limited) regime. Using data from climate models, reanalyses, and observationally-constrained datasets, global patterns of SM:LE regimes are determined by segmented regression. Mutual information analysis is applied only for days when SM is in the transitional regime between critical points defining high sensitivity of LE to SM variations. Sensitivity is further decomposed into linear and nonlinear components. Results show discrepancies in the global patterns of existing SM regimes, but general consistencies among the linear and nonlinear components of SM:LE coupling. This implies that although models simulate differing surface hydroclimates, once SM is in the transitional regime, the locations where LE closely interacts with SM are well-captured and resemble the conventional distribution of “hot spots” of land-atmosphere interactions. This indicates that only the transitional SM regime determines the strength of coupling, and attention should focused on when this regime occurs. This framework can also be applied to investigate extremes and the shifting surface hydroclimatology in a warming climate.