Land and Ocean Contributions to United States Summertime Precipitation Variability
Open Access
- Author:
- Sala, Christopher
- Graduate Program:
- Meteorology and Atmospheric Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 19, 2025
- Committee Members:
- Sukyoung Lee, Major Field Member
Pengfei Zhang, Major Field Member
Shujie Wang, Outside Unit & Field Member
Laifang Li, Chair & Dissertation Advisor
Paul Markowski, Program Head/Chair - Keywords:
- Midwest United States
Southeast United States
Precipitation
North Atlantic Subtropical High
Wave Trains
Great Plains Low-Level Jet
Asia–North America Teleconnection
Summer - Abstract:
- Variability of summertime contiguous United States (US) precipitation is among the most important contributors to the environment, ecology, and economy of the country. As such, understanding the processes that govern precipitation variability is necessary to increase seasonal-to-subseasonal forecasting skill and future climate projections to make science-informed policy decisions. The dominant mode of this precipitation variability is a dipole pattern of opposing hydrological extremes between the Midwest (MW) and Southeast (SE) US. Years of increased MW precipitation coincide with years of decreased SE precipitation, and vice versa. This relationship has been well explored on interannual time scales but remains to be fully elucidated on intraseasonal time scales. Using a Bayesian statistical model, daily summertime precipitation in the MW and SE are independently classified as light, moderate, and heavy categories and then backtracked using a moisture tracking algorithm to determine the moisture origin points for the precipitation events. Results indicate that both regions are experiencing increasing trends in moderate and heavy precipitation event frequency, with light events becoming less frequent from 1970 to 2019. Light precipitation events rely on local moisture recycling and land-based moisture to fuel the events, whereas heavier precipitation events are supplemented by oceanic moisture pathways dynamically regulated by the North Atlantic Subtropical High (NASH) Western Ridge (WR). As the NASH WR shifts northwest of the climatological position, Atlantic Ocean moisture travels along the southern flank of the NASH, through the Caribbean Sea, into the Gulf of Mexico and funneled into the Great Plains Low-Level Jet (GPLLJ). The bolstered GPLLJ transports oceanic moisture to the MW US, which concurrently receives Pacific Ocean moisture through geopotential height anomalies that create favorable conditions for MW precipitation and unfavorable conditions for SE precipitation. The northwest expansion of the NASH WR reduces ocean moisture to the SE US, resulting in light precipitation events. Heavier SE US precipitation events occur as the NASH WR recedes southeast of climatology, resulting in the redirection of moisture from the GPLLJ and MW to the SE. Simultaneously, geopotential height anomalies create unfavorable conditions for MW precipitation and favorable conditions for SE precipitation events. The geopotential height anomalies that affect MW and SE US precipitation events are approximately 12-day impressions of the Asia–North America teleconnection pattern that yields anomalous barotropic wave trains from East Asian Summer Monsoon latent heating anomalies.