New insights into heavy rainfall events
Dr. Ray Schmitt
August 26, 2022
One of the hazards of our warming climate is the intensification of the global water cycle. The water molecule is more mobile at higher temperatures; a warmer atmosphere can carry more water vapor, and a warmer ocean more readily evaporates. As the ocean is the ultimate source of all rain, its changing salinities have most clearly revealed the ongoing water cycle intensification. Salty areas dominated by evaporation have gotten saltier over the past six decades, while fresher areas have gotten even fresher. The dangerous trend for terrestrial society is for dry areas to get drier, with more prolonged and intense droughts. Wet areas will experience more intense rainfall events, as we have recently seen in the disastrous floods in Kentucky.
In past research, we have described how ocean surface salinity variations can predict rainfall on land a season ahead. The idea is simple — if some area of the ocean gets saltier due to increased evaporation, that extra moisture export guarantees that it will rain more somewhere else. Similarly, a fresher than normal ocean indicates less moisture has been exported, so less rain will be available. Source-sink relationships have been discovered for the US East coast salinities in the spring and Midwest rainfall in the following summer. Similar predictive relationships between ocean salinity and terrestrial rainfall have been identified for the Sahel of Africa and the Yangtze River Valley of China. More recently, we have discovered that additional useful salinity predictors exist for Midwest summer rainfall even further afield. Certain patterns of salinity variations in the Western Tropical Pacific have been shown to significantly improve summer rainfall predictions in the Midwest, particularly for the most intense rain events. We found that the total rain for the season was most strongly correlated with the number of heavy rain events, those days when more than 0.7 inches fell. While we find that the summer moisture source for the upper Midwest is the Gulf of Mexico, the predictability supplied by the western tropical Pacific salinity pattern is related to tropical rainfall events that both inject latent heat energy into the atmosphere and create salinity anomalies. The injection of energy into the tropical atmosphere can generate standing waves in the jet stream at mid-latitudes that affect the storm track on the continental US. That is, while the Gulf of Mexico may be supplying moisture to the north flowing Great Plains Low Level Jet, the pattern of the peaks and troughs of the jet stream is important for sending in the fronts that wring the moisture out in heavy rain events.
This figure from Li, Schmitt and Ummenhofer (2022) shows the pattern of highs and lows arising in the North Pacific that are correlated with tropical salinity anomalies. Surface salinity is a good indicator of the latent heat energy transfer from the ocean to the atmosphere. When that energy is released at height in the tropical atmosphere, it sets up a pattern of planetary waves (Rossby waves) that propagate to mid-latitudes. In the summer (bottom), fronts track along the jet stream path and wring the moisture out of the low-level flows carrying moisture northward from the Gulf of Mexico. There is a strong correlation between the tropical salinity features with the number of heavy rainfall days in the Midwest that can lead to the flash flooding observed this summer in Kentucky, Missouri, Illinois and West Virginia.
The Earth’s climate is a very complex system, and the nuggets of predictability that can be mined out of ever-expanding oceanic and atmospheric data are being put to good use by the team at Salient. Salient’s unequaled Sub-seasonal to Seasonal forecasts can provide your company with the knowledge needed to anticipate the increasingly volatile weather we are experiencing.
Reference: Li, L., Schmitt, R.W. and Ummenhofer, C.C., 2022. Skillful Long‐Lead Prediction of Summertime Heavy Rainfall in the US Midwest From Sea Surface Salinity. Geophysical Research Letters, 49(13), p.e2022GL098554.