| The role of antecedent land surface conditions including precipitation, surface skin temperature, soil moisture, and snow water equivalent anomalies on the onset and intensity of the North American monsoon system (NAMS) is explored using retrospective data analysis and the MM5 mesoscale climate model coupled with the Variable Infiltration Capacity (VIC) land surface model. The retrospective data analysis suggests that significant positive and negative relationships exist between the antecedent winter precipitation over a large area in southwestern United States (SW) and northwestern Mexico (NW Mexico) and summer monsoon onset and magnitude. It is also showed that the previous winter's precipitation anomaly signal (through soil moisture) persists until the pre-monsoon month---early June, but contributes little to the surface skin temperature anomaly, and monsoon onset and strength, over the northern part of the NAMS domain. On the other hand, in the core monsoon region, where the monsoon starts in May and June, there is some evidence that the previous winter's precipitation anomalies can affect soil moisture, and hence surface skin temperature, at the onset of the monsoon. In addition to the land surface feedback mechanism, large-scale circulation plays a strong (and possibly dominant) role in modulating monsoon onset because of its strong influence on the pre-monsoon surface thermal condition. The retrospective data analysis was augmented by MM5/VIC coupled model experiments with pre-monsoon soil moisture prescribed as both extremely wet and dry. The coupled model consistently demonstrated a positive soil moisture---precipitation feedback, contrary to what was found (albeit with somewhat less extreme anomalies) in the retrospective data analysis. Specifically, anomalously wet pre-monsoon soil moisture always lead to enhanced monsoon precipitation, and the reverse is true. The surface temperature changes induced by evaporation differences associated with pre-monsoon soil moisture anomalies changed the surface pressure and consequently the flow field in the coupled model, which in turn changed the moisture convergence and accordingly precipitation. Both the large-scale circulation change and local land-atmospheric interactions in response to pre-monsoon soil moisture anomalies play important roles in the coupled model's positive soil moisture---monsoon precipitation feedback. |
