An Analysis of Extreme Evaporation Events, and the Role of Extreme Evaporation in the Climate System

摘要

The evaporation of water from Earth's surface to Earth's atmosphere plays a key role in the hydrologic cycle, influencing temperature, humidity, precipitation, and ultimately water availability across the globe. Evaporation rates are controlled by the availability of water at the surface to evaporate, and the atmospheric evaporative demand which is influenced by temperature, humidity, solar radiation, and wind speed. Large evaporation rates can cause or intensify various atmospheric and climate phenomena, including the onset of flash drought, the intensification of extratropical cyclones, and heavy precipitation. Despite the importance of extreme evaporation in influencing these phenomena, there has been a relative lack of work done on instances of extreme evaporation. Using the ERA5 reanalysis database with data from 1979-2019, we define an "Extreme Evaporation Event " (EEE) as an instance where the daily evaporation at a given location exceeds the climatological 95th percentile of evaporation at that location. This definition provides flexibility in the areas in which an EEE can occur, as opposed to different definitions where a threshold value must be met, often limiting "Extreme Evaporation" to the oceans. Using this method, we find that extreme evaporation has increased over much of the globe, particularly over the oceans, where it has increased by as much as 2 cm over the time period, however this is not uniform. We also find that EEEs are more common over the oceans during the winter months, and more common over land surfaces during the summer months, in far inland regions, nearly all EEEs occur during the summer months. When investigating the causes of EEEs, we find that EEEs over land surfaces tend to occur when there is anomalously high water available to evaporate, and anomalously high evaporative demand. Over the oceans, while there is always a supply of moisture to be evaporated, EEEs also tend to happen with anomalously high evaporative demand, almost always associated with drier than average air. Additionally, the atmosphere downwind of EEEs in the Gulf of Mexico (GoM) is investigated, and we find that there is likely an extratropical cyclone downstream of the GoM when an EEE occurs there. The findings of this study help to better predict the occurrence of extreme evaporation fluxes, and better understand the role of extreme evaporation in our climate system as the planet warms.