Phase Locking of the Boreal Summer Upper-Level Atmospheric Response to Dry Land Surface Anomalies in the Continental United States

Land-atmosphere coupling provides an important source of subseasonal to seasonal predictability over North America during the warm season. While atmospheric processes (e.g. precipitation deficits) play a dominant role in forcing land soil moisture anomalies, there is increasing evidence that regional dry soil moisture anomalies can enhance the probability of certain patterns of atmospheric circulation anomalies, which may in turn reinforce regional dry land surface anomalies, constituting a positive land-atmosphere feedback loop. Our recent studies show that the summertime upper-level atmospheric circulation responses to regional dry land surface anomalies in the US interior tend to have the same spatial pattern: a high anomaly forms over west-central North America and a low anomaly forms to the east, regardless of the specific location of the dry land surface. Such phase locking of the upper-level atmospheric circulation anomalies is found to occur in complex Atmospheric General Circulation Model (AGCM) simulations forced by regional dry land surface as well as simple stationary wave model (SWM) experiments forced with idealized diabatic heating anomalies that mimic those forced by local dry land surface anomalies, both of which agree with observational composites in general. The phase locking is further found to be induced by the summertime climatological high over central North America, which serves as an effective anchor to the Rossby wave source term so that the response tends to be collocated with it. The climatological North American high itself is maintained primarily by the nonlinear interaction between North American orography and atmospheric flow forced by tropical diabatic heating, and secondarily by local land-sea thermal contrast. These results suggest that the phase locking is fundamentally determined by the configurations of the North American continent, primarily the Rockies and secondarily the land-sea distribution of the continent. The phase locking behavior is unique to North America, and is not present in any other land regions in the Northern Hemisphere during boreal summer.