The purpose of this study was to identify the climatological variability in onset dates for the North American Monsoon in order to gain a better understanding of the North American Monsoon. The NARR reanalysis data set from 1979-2010 was used to supply daily rainfall totals in northwestern Mexico, one of the three major areas impacted yearly by the monsoon (Barlow 1998). From this data, monsoon rainfall onset was defined as when daily precipitation surpassed 2mm (Barlow 1998). This study strove to prove that the average monsoon onset was on or about the 1st of July each year over the 30 year period, using the aforementioned NARR data and the established monsoon onset parameters. At the same time, monsoon season total rainfall data was collected, to use for later comparison. Over the course of the study, a large variability was found in the monsoon onset date, and thus, the study continued by attempting to find a atmospheric trigger that would indicate when monsoon onset was going to occur each year.

In order to compare the monsoon onset to various tele-connection patterns, the period of monsoon variability was broken up into five different periods, with the middle period representing 50% of the monsoon onset dates—the most common onset dates—and the periods on either side representing significant deviations from the mean. Each of the five periods of monsoon onset were then compared to various different indices including: variation in sea surface temperatures, the El Nià ±o-Southern Oscillation (ENSO), the Pacific-North American Tele-connection Pattern (PNA), and the Pacific Decadal Oscillation (PDO). These indices were chosen because they had been identified in the literature as possibly contributing to the climatology of the North American Monsoon (Gao 2007, Adams 1997). Monsoon onset was also compared to Iowa summer rainfall totals, for the purpose of comparison with a mid-latitude system. The goal of these comparisons was to determine if a particular phase of a teleconnection pattern or spike in sea surface temperatures (SST) or mid-latitude rainfall could be used to indicate when the monsoon onset would take place in Northwestern Mexico.

Following these lines of questioning, the study was able to determine that the average onset date of the North American Monsoon is 25 June, with a 40 day window of variability over the 30 years which were studied. Breaking the monsoon onset window into smaller periods allowed for a comparison between monsoon onset with monsoon total rainfall, Iowa season rainfall, sea surface temperatures, and other tele-connection patterns. The most significant correlation found was between early monsoon onset and above average sea surface temperatures in the Pacific Ocean, while other correlations were found regarding above average monsoon total season rainfall and the Pacific Decadal Oscillation.