Investigation of the intra-annual variability of soil moisture-temperature coupling over North America

The land surface state plays an important role in our climate system. Soil moisture anomalies, in particular, can enhance temperature extremes. Given its potential impacts on our climate, soil moisture-temperature coupling has been well documented in recent years [Koster et al., 2004; Diro et al., 2014]. Most studies on coupling usually only focus on the summer season. However, a study by Tawfik and Steiner [2011] demonstrated that coupling could also be strong during the colder seasons. In our study, we therefore performed and compared simulations with and without coupling, using the fifth generation Canadian regional climate model (CRCM5), to investigate the intraannual variability of soil moisture-temperature coupling for the current and future climates.

In the current climate, coupling is strongest over the region extending from the southern US Great Plains to the southern Canadian Prairies. While coupling over the southern US Great Plains is more or less persistent throughout the year, coupling over the southern Canadian Prairies exhibits a migratory behavior that mimics the seasonal north-south evolution of the freezing line. Given these results, the latter region was further explored. During summer, coupling over this region is modulated by the influence of soil moisture on the partitioning of turbulent heat fluxes. During the transition seasons, on the other hand, the interactive soil moisture phase influences the snow depth and surface albedo, which in turn influences the surface energy budget, and subsequently the surface air temperature. The surface air temperature then influences the snow depth through a feedback loop.

Further analysis suggests that the influence of soil moisture on the number of hot days is strongest during the transition seasons over the southern Canadian Prairies. Similar to the coupling strength, the location of the maximum difference in the number of hot days between simulations with and without coupling follows the freezing line (see Figure below).

In the future climate, changes in soil moisture-temperature coupling are observed as a result of modifications in the interactive soil water phase, snow depth, and cloud cover. During spring, we observed that the earlier onset of snowmelt results in an earlier northward migration of the coupling strength maxima over the southern Canadian Prairies. We also observed that the influence of soil moisture on the number of hot days covers a much broader region over North America in the future climate, as opposed to the current climate.

Through this study, we provided insight on the growing importance of acquiring a better understanding of the complex coupling process and its impacts on temperature variability in both the current and future climates.

For more information, please read the full paper at: http://onlinelibrary.wiley.com/doi/10.1002/2016JD025423/full

Figure: Monthly differences in the number of hot days between simulations with and without coupling for the current climate. The strongest influence of soil moisture on the number of hot days can be observed during transition months, particularly May and October.

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