Quantifying the strength of soil moisture-precipitation coupling and its sensitivity to changes in surface water budget

This paper presents a new index to quantify the strength of soil moisture-precipitation coupling in AGCMs and explores how the soil moisture-precipitation coupling in Community Atmosphere Model version 3 (CAM3)-Community Land Model version 3 (CAM3-CLM3) responds to parameterization-induced surface water budget changes. Specifically, this study (a) compares the regions of strong coupling identified by the newly proposed index and the index currently used in the Global Land-Atmosphere Coupling Experiment (GLACE); (b) examines how the surface water budget changes influence the strength of soil moisture-precipitation coupling as measured by the two indexes, respectively; and (c) examines how these changes influence the memory of the coupled land-atmosphere system as measured by the correlation between soil moisture and subsequent precipitation. The new index and the GLACE index are consistent in identifying central North America and West Africa as major regions of strong coupling during June August (JJA). However, in some areas of western Europe and of subtropical South America where the GLACE index is low, the new index suggests a modest significant coupling during JJA. In response to the surface water budget changes that presumably favor a stronger soil moisture-precipitation coupling, the new index increases, but the GLACE index decreases in a majority of the regions of modest-to-strong coupling, although both show some mixed response. Changes in the land-atmosphere system memory suggest an increase of coupling strength, consistent with results from the new index. The strong dependence of the GLACE index on the relative importance of atmospheric internal variability is identified as a potential cause for the differences between the two indexes. The two indexes emphasize different aspects of soil moisture - precipitation coupling, and one might be more suitable than the other depending on the purpose of individual studies.