| An observational diagnosis of the warm season hydroclimate of North America is undertaken. The climatological evolution is examined in terms of the North American Monsoon System, the dominant summertime structure, while the variability is examined in relation to the decadal modes of Pacific ocean variability.; A dynamic climatology is developed for the onset and mature phases of the North American Monsoon System. The striking out-of-phase linkage in precipitation between the Mexican monsoon and the central U.S. is verified as robust for different time periods and different estimates. Thermodynamic forcing, as measured by convective available potential energy, is closely associated with the Mexican monsoon component of the linkage, but not with the central U.S. The changes over the central U.S. are, however, consistent with several possible dynamic forcings: Mexican monsoon deep heating, elevated surface heating of the North American Cordillera's high terrain, and orographic forcing associated with seasonal movement of the easterlies onto the cordillera. Both NCEP/NCAR and ECMWF reanalysis products yield a tropical-type deep tropospheric heating profile in the monsoon region, but with notable differences between the vertical structures in the two reanalyses.; The relationship between the two primary decadal modes of Pacific Sea Surface Temperature (SST) variability and U.S. warm season hydrology is examined. Drought and riverflow data were included, in addition to precipitation, to provide a more comprehensive picture of the lower frequency components of the hydrologic variability. In contrast to previous analyses, a technique is presented that extracts the two decadal modes and the El Nino-Southern Oscillation from a single unfiltered analysis, allowing a direct intercomparison of the modal structures. A significant relationship is found between the decadal SST modes---the Pacific Decadal Oscillation (PDO), and the North Pacific mode---and U.S. summertime precipitation, drought, and riverflow. The PDO and the North Pacific mode represent distinct modes of decadal variability with signal in the North Pacific, but with the North Pacific mode capturing most of the local variability. The three SST modes are also related to specific long term U.S. drought episodes. In particular, the northeastern U.S. drought of 1962--66 is closely associated with the North Pacific mode. |
