North Pacific Ocean Wave Train Influence on Midwestern United States Summer Rainfall

The Midwest (MW) United States (US) is a highly variable region of summertime precipitation. This precipitation is categorized into light, moderate, and heavy rainfall categories using a Bayesian Statistical Model, which reflects the local climate better than using standard meteorological definitions of rainfall intensity. Then, using a backwards moisture tracking algorithm, the moisture sources for the MW US are quantified and compared against each other for the differing rainfall categories. Results indicate that the main source of MW precipitation is land followed by ocean; however, the importance of remote ocean moisture increases disproportionately with rainfall intensity. During heavy rainfall events, Atlantic Ocean moisture is conveyed along the North Atlantic Subtropical High (NASH) Western Ridge (WR), which extends northwestward in the days leading up to the event due to the presence of an anomalous high-pressure system over the eastern US. The anomalous high coincides with an anomalous low over the western US, strengthening the southerly Great Plains Low-Level Jet (GPLLJ), which allows for increased ocean moisture transport to the MW US. Additionally, the anomalous dipole pressure system is a result of an upper-level atmospheric wave train that originates over the Pacific Ocean. This wave train helps transport increased Pacific Ocean moisture to the MW US, thus contributing to the increased oceanic moisture during heavy precipitation events. Light precipitation events occur because of an opposite wave train pattern that results in an anomalous high over western US and an anomalous low over eastern US. This creates a weakened GPLLJ and southeastward recession of the NASH WR, thus reducing transport of oceanic moisture to the MW US. Therefore, light precipitation events in the MW US must rely on local moisture recycling and land sources to sustain rainfall. Given the large lead time in upper-level atmospheric circulations between the Pacific Ocean and MW US precipitation events, this increased understanding could be used to increase forecast skill in the future.