A new perspective on the regional hydrologic cycle over North and South America

The GEOS-1 vertically-integrated 3-hr moisture flux reanalyses and hourly-gridded United States station precipitation plus a satellite-based, 6-hr global precipitation estimate were employed to investigate the impacts of nocturnal low-level jets (LLJs) on the regional hydrological cycle over the central United States (Part I) and the subtropical plains of South America (Part II). Research stressed the influences of upper-level synoptic-scale waves (i.e., synoptic-scale forcings) upon the regional hydrologic processes, which were explored by the impacts associated with the occurrence of LLJ. Besides the conventional budget analysis, the adopted \u27synoptic-forcing approach\u27 was proven illustrative in describing these impacts through the down-scaling process of LLJs;In Part 1, the major findings include: (1) the seasonal-averaged hydrological cycle over the Great Plains is strongly affected by the occurrence of GPLLJ, (2) the synoptic-scale forcing provided by the upper-level propagating jet (ULJ) streams is essential in generating the large-scale precipitation after the GPLLJ forms from the diurnal boundary layer process, (3) without the dynamic coupling between the ULJ and LLJ, the impact of LLJ on the hydrological cycle is demonstrated to be less important, and (4) the importance of synoptic-scale forcings in preconditioning the setting of wet/dry seasons in the interannual variability of rainfall anomaly is further illustrated by examining the changes of intensity as well as the occurrence frequency between the different types of LLJ;In Part II of this study, it was found that the occurrence of Andean LLJ represents a transient episode that detours the climatic rainfall activity along the South Atlantic Convergent Zone (SACZ) to the subtropical plains (Brazilian Nordeste) in its southwestern (northeastern) flank. The appearance of a seesaw pattern in the rainfall and flux convergence anomalies along the southeastern portion of South America, which is spatially in quadrature with the seasonal mean circulation, reflects the synoptic-scale forcing generated by the upper-level propagating transient-scale waves. In this regard, the function of the Andean LLJ in providing a scale-interaction mechanism that links the synoptic-scale setting with the localized rainfall event is the same as the GPLLJ. Due to the unique geographic background such as the narrow east-west landmass extension and the relative orientation between the Andean LLJ and the ULJ, however, the enhanced rainfall activity over the subtropical plains in response to the perturbed flux convergence is smaller than the case in the GPLLJ