Recent change of North American Summer Monsoon revealed by multiple monsoon indexes

The observational analysis shows that North American Monsoon (NAM) has a wetter summer season since 1979. By defining three monsoon indexes representing three different aspects of monsoon, i.e., surface precipitation, precipitable water, and zonal wind, this research finds an earlier onset, which is opposite to the delayed onset demonstrated by the CMIP5. The median onset date based on the precipitable water and zonal wind indexes is generally consistent over the central and southern Mexico (roughly south of 25N), it begins at the end of May over the southern Mexico, then migrates northwestward gradually, arrives the central Mexico at middle June, but the onset date represented by the rainfall index lags behind two weeks over these two areas. On the other side, over the northwestern Mexico and southwestern US, the onset date based on precipitable water and rainfall indexes is consistent, occurring at late June and early July respectively, but it happens a week later than the zonal wind index.

Besides, both precipitable water and zonal wind index demonstrate a consistent southward retreat date pattern. The monsoon first starts to retreat at early August over the eastern Arizona and western New Mexico within US, then over the Northwestern Mexico from middle August to middle September, lastly to the central and southern Mexico from late September to middle October. However, compared to the precipitable water and zonal wind, the retreat date represented by the rainfall index lags behind nearly one month over the southwestern US and Northwestern Mexico with a sudden withdrawal date.