An Examination of Moisture and Precipitation in Tropical Easterly Waves

Abstract

Tropical easterly waves (TEWs) are important components of synoptic-scale variability in the tropical atmosphere, affecting precipitation across the tropics and sometimes serving as precursor disturbances for tropical cyclones (TCs). While their behavior can be partially explained by temperature variations and shear-related instabilities, recent work has turned to the role of moisture in TEW maintenance and propagation. One such framework for understanding the importance of moisture to tropical systems is the moisture mode, a category of systems where moisture is the main driver of thermodynamic variations. Some recent studies have determined that TEWs in some oceanic basins are moisture modes, but not all tropical basins have been studied, and TEWs over land have had very little attention. Using a global database of TEW tracks, we find that TEWs in many regions, especially over oceans, are moisture mode-like, but that temperature variations still play roles in the thermodynamics of TEWs over land, near coastlines, and in oceanic regions with lower SSTs. The differences in the relative importance of moisture translate to differences in near-TEW environmental structure, with less moisture mode-like TEWs also having weaker cyclonic circulations. The diurnal cycle of precipitation near TEWs approximately mirrors that of tropical precipitation, with precipitation peaking over the ocean in the early morning and over land in the late afternoon to early evening. TEW strength, however, does not have a strong diurnal cycle, especially for TEWs over ocean. Despite this lack of diurnal coupling between TEW strength and precipitation, we find that over the life of TEWs, the two are related, with the rainiest TEWs on average being stronger than the average TEW, while the driest TEWs are typically weaker. Pre-TC TEWs are also more likely to be stronger and rainier than the average TEW. Even though TEW-associated precipitation follows a similar phasing to the typical diurnal cycle of the tropics, the presence of TEWs enhances the diurnal range, mainly by increasing the daily maximum rain rate. TEW-associated diurnal ranges are greatest in the areas where TEWs most frequently occur, but their relative enhancement is strongest at the periphery of some peak TEW activity regions, suggesting TEWs interact with other tropical convective features like the ITCZ. TEWs are most active during the summertime, but occur year-round. The TEWs with the least and most precipitation also follow this general pattern, though the frequency of the driest TEWs increases earlier in the warm season and stays elevated longer than that of the rainiest TEWs. The rainiest TEWs occur most frequently in climatologically rainy locations, but some highly precipitating TEWs occur over regions with less rainfall, highlighting the need to study TEWs not only in areas of peak activity but across the entire tropics. Better understanding the relationship between TEWs, precipitation, and moisture helps us to better understand tropical convection and hydroclimate. These connections have implications for forecasting TEW-related impacts, including TC genesis and rainfall extremes, both wet and dry. Better understanding the relationship between TEWs, moisture, and precipitation may also be valuable in building theories of how TEWs will behave in a warmer climate.