From AMS Glossary
An atmospheric trough with pressure increasing toward the pole, which is opposite of the orientation of most midlatitude troughs. There are three primary types:
- Easterly waves, which are lower-tropospheric waves embedded in the tropical easterlies on the equatorward side of the subtropical high. They typically move from east to west and are sometimes identified as an "inverted V" on satellite imagery, particularly over West Africa and the subtropical North Atlantic Ocean.
Upper-tropospheric lows or troughs that form over the Gulf of Mexico or the southern United States on the equatorward side of the anticyclone in the monsoon region. Theses features move from east to west and are a common transition feature that can increase thunderstorm activity due to the associated cooling aloft and increased wind shear along their flanks during the North American monsoon.
Term updated 18 March 2019.