An analysis of anomalous southwest United States summer monsoon precipitation.

摘要

The North American Monsoon System (NAMS) is an important feature of the atmospheric circulation, and its effects are distinguishable over a large area of the western United States and northwest Mexico. Seasonally warm land surfaces in lowlands and elevated areas, combined with atmospheric moisture supplied by nearby maritime sources, are conducive to monsoon-like systems. The western North American summer precipitation regime plays an important role in the hydrological cycle of the arid southwest U.S. Knowledge of intraseasonal, annual and decadal variations in this system is crucial for forecasting moisture potential availability for management of water resources. This thesis seeks to improve forecasts by evaluating the hypothesis that large-scale variations in atmosphere-ocean-land surface interactions modulate monsoon precipitation in the Southwest.; The research examines and quantifies the hypothesized relationships between atmospheric and land-surface processes and anomalous summer rainfall in the Southwest; the primary study area is Arizona and New Mexico. Observed climate data are analyzed to identify variability in precipitation, winds, temperature, humidity, and geopotential height fields of the wettest and driest monsoon seasons of during 1948-2005. Various diagnostic tools have been applied to observed climate data. Preferred modes of atmospheric circulation for anomalous monsoon seasons are identified through the use of climatology, composite and regression analyses of the 57-year data set. Through analysis of monsoon-related climate variables, potential predictors are identified that can be used to develop a model that incorporates such information for monsoon season forecasts, one to three months ahead.; Four hypotheses are tested to establish whether relationships exist between antecedent ocean/atmosphere or land surface conditions and monsoon season precipitation. Two hypotheses test the relationship between antecedent sea surface temperatures and antecedent soil moisture conditions and monsoon precipitation. Two other hypotheses test the relationship between early spring ENSO and PDO indices and subsequent monsoon season precipitation. Based on these findings, and the published literature on characteristics of the Southwest summer precipitation regime, a conceptual scheme for a seasonal monsoon forecast has been developed. A discussion follows of implications for southwest monsoon research and prospects for future work.