Effects of synoptic events on modulating seasonal variations in the North American Monsoon System

The impacts of climatic variability and change on hydrology are of increasing concern, particularly as demands for water and hydroelectric power increase with growing human population. A better knowledge of the linkages between atmospheric circulation and surface hydroclimate would provide a firmer foundation for interpreting hydroclimatic variability, and may provide a basis for predicting water supply one-to-two seasons ahead.

This study investigates the hypothesis that the seasonal evolution of the North American Monsoon System (NAMS) is modulated by intraseasonal variations in atmospheric circulation. The monsoon phenomenon undergoes considerable intraseasonal variability with alternating periods of widespread, heavy thunderstorm activity (bursts) and drier periods (breaks). The monsoon bursts (breaks) occur on time scales of days (weeks) and appear to be modulated by synoptic patterns that help organize convectively favorable (unfavorable) environments. The role played by individual synoptic events in these bursts and breaks is not well understood, although past case studies have described some of the dynamics involved in monsoonal moisture surges. Analyses of composite synoptic pressure patterns have identified atmospheric configerations that enhance or stifle convective activity in terms of cloudiness, precipitation, and cloud-to-ground lightning.

For this investigation, we have chosen six case studies using two subregions (Arizona and New Mexico) and high/medium/low monsoon intensity. Precipitation data from stations in Arizona and New Mexico are ranked by total July-August precipitation to identify monsoon seasons by strength. Significant precipitation events are then chosen from each of the high/medium/low intensity monsoon seasons. The NCAR-NCEP Reanalysis Data are used to identify regional atmospheric features preceding and following each of the six precipitation events. We examine both the 500 mb geopotential height and the 850 mb moisture flux fields over North America. In particular, we focus on midlatitude upper level trough features, easterly flow from the Gulf of Mexico, and moisture surges from the Gulf of California. This work represents a first step in a broader investigation that will examine interactions between the land surface and atmospheric circulation as they relate to the North American Monsoon System.