Diem, J.E., D.P. Brown, and J. McCann. 2013. Multi-decadal changes in the North American monsoon anticyclone. International Journal of Climatology 33:2274–2279.
The purpose of this study was to assess trends in the intensity of the North American monsoon anticyclone over multiple decades from 1948 to 2010 during July and August, with a focus on the Lower Colorado River Basin (LCRB). The methodology included a 500 hPa geopotential-height regionalization of the monsoon-anticyclone domain (i.e. a large portion of the western United States and northern Mexico), a typing of 500 hPa circulation patterns over the LCRB, and an examination of multi-decadal trends as well as inter-epochal differences in geopotential heights and frequencies of synoptic types. Three regions (i.e. Northwest, Northeast, and South) were revealed that differed based on inter-annual variations in 500 hPa geopotential heights. The Northwest and South regions had significant increases in geopotential heights from 1948–1978 to 1980–2010. The synoptic types reflected the location of the monsoon anticyclone over the LCRB. The monsoon anticyclone intensified primarily over the northwestern region, which includes the LCRB, since the mid- to late 1970s. The anticyclone has thus been expanding over the LCRB. The anticyclone has been in a northern position, specifically a north-central position, over the basin more frequently over the past 30 years; conversely, the anticyclone has been in southern and eastern positions over the basin less frequently.
Diem, J.E. and D.P. Brown. 2009. Relationships among monsoon-season circulation patterns, gulf surges, and rainfall within the Lower Colorado River Basin, USA. Theoretical and Applied Climatology 97:373–383.
The purpose of this study is to assess the connections between the monsoon anticyclone, gulf surges, and rainfall within the Lower Colorado River Basin (LCRB) during North American monsoon seasons from 1988-2006. The methods involved calculating rainfall characteristics and near-surface humidity for 500-hPa circulation patterns, creating circulation and near-surface humidity composites for rainfall days, and creating near-surface humidity composites for rainfall days occurring under each circulation pattern. The circulation was dominated by the monsoon anticyclone being over or to the immediate east of the basin. The anticyclone was shifted to the northwest (east) of its seasonal mean position on rainfall days in the central portion of the basin (far eastern portion of the basin). Rainfall influenced by gulf surges was most likely when the monsoon anticyclone was shifted westward, especially northwestward, of its typical position. The central portion of the basin received substantially more surge-influenced rainfall than did the far eastern portion of the basin.
Diem, J.E., and D.P. Brown. 2006. Tropospheric moisture and monsoonal rainfall over the southwestern United States. Journal of Geophysical Research 111, D16112, doi:10.1029/2005JD006836.
This study investigates the role of increased atmospheric humidity in occurrences of wet days in the southwestern United States during the monsoon seasons of 1996–2002, using spatiotemporal analyses of ground-measured precipitation, spatial analyses of surface and 700-hPa humidity, and air-parcel trajectory analyses. A precipitation regionalization indicates that the Gila River basin in southern Arizona/southwest New Mexico should be divided into a western region and an eastern region. A rainfall peak occurred in late August/early September for the western region; however, similar to that of the core monsoon zone in northwestern Mexico, the eastern region of the basin had a rainfall peak in late July/early August. Wet days in the western (eastern) region were associated with a large (moderate) peak in dew point temperature in the southwestern (south central) portion of the basin. The middle troposphere was more humid than normal on both sets of days, with the anomalies for western region wet days being larger and located more over the Gila River basin than anomalies for the eastern region wet days. The Sierra Madre Occidental was the most likely source of middle troposphere moisture for both regions; however, the Gulf of Mexico may have been a significant contributor to rainfall in parts of the eastern region. The Gulf of California probably was the dominant source of low-level moisture for western region wet days, with gulf surges likely causing the late August/early September peak.
Diem, J. E. 2006. Anomalous monsoonal activity in central Arizona, USA. Geophysical Research Letters 33, L16706, doi:10.1029/2006GL027259.
Published research has suggested that urban and agricultural activities in central Arizona may be enhancing monsoonal precipitation in the region; therefore, this study employed cloud-to-ground lightning data and topographic data to reveal spatially anomalous zones of lightning activity in central Arizona. A multiple linear regression model with topographic variables as predictors explained 85% of the variance in gridded lightning-flash counts. Clustering of large positive residuals of lightning flashes existed between 40 km and 100 km north/northeast of urbanized Phoenix. Observed lightning flashes in this zone were ∼40% more frequent than lightning flashes predicted by the model. Two plausible causes of the enhanced lightning activity are intensified convective storms due to Phoenix-derived water vapor and altered microphysical processes in storm clouds due to Phoenix-derived atmospheric pollution. It is possible that the positive-anomaly zone also had enhanced rainfall.
Diem, J.E. 2005. Northward extension of intense monsoonal activity into the southwestern United States. Geophysical Research Letters 32:Art.No.L14702.
Although it is centered in northwestern Mexico, the Mexican monsoon also has been shown to impact portions of the southwestern United States. To provide more information about the spatial distribution of monsoonal impacts in the Southwest, this study employed multiple linear regression modeling to reduce local topographic impacts on monsoonal precipitation to reveal intense monsoonal activity within the Gila River basin. The precipitation data were daily precipitation totals from 115 stations from June 16–September 15 of 1996–2002. An intense monsoonal zone was found in the south-central portion of the basin. Therefore, intense monsoonal activity associated with the Sierra Madre Occidental in northwestern Mexico extended into south-central and southeastern Arizona but not into New Mexico.