What is a westerly trough? What does groove mean?

In the middle and high latitudes on the north side of the subtropical high in the northern hemisphere, westerly airflow prevails at an altitude of more than 3 kilometers (700 hectopascals), which is called westerly belt. The westerly airflow often fluctuates, forming troughs (low pressure) and ridges (high pressure). The trough line in the westerly belt is called the westerly trough. The westerly trough is mostly northeast-southwest, and the warm and humid southwest airflow prevails in the east of the westerly trough (in front of the trough), with rainy weather; West of the westerly trough (behind the trough), dry and cold northwest downward airflow prevails, corresponding to the ground where cold and high pressure are active, and the weather is fine. The westerly trough that affects the weather in China mainly includes the northwest trough and the Qinghai-Tibet trough that pass through the Qinghai-Tibet Plateau from the northwest and affect the northern region, and the south branch trough (Indo-Myanmar trough) that moves eastward from India and Myanmar and affects the southern region. There are frequent activities in the westerly trough, and each activity will bring a certain intensity of cold air, resulting in a wide range of precipitation or rainy weather.

When the trough line appears in the cold low altitude of the westerly belt, it is the westerly trough. The westerly trough is cold baroclinic, and there is a strong positive vortex in the trough. Before the trough, it often develops with the temperate cyclone, and after the trough, an anticyclone is formed. When the westerly trough passes through the Qinghai-Tibet Plateau, it will separate the north branch trough from the south branch trough. If the south branch trough moves eastward away from the plateau, a low pressure area will be formed near Yunnan, which is the southwest vortex. The southwest vortex usually extends out of the trough of low pressure, bringing rainy weather to South China. The westerly trough also caused the subtropical high to move eastward and the tropical cyclone to move northeast.

In an isobar whose two ends are high and the middle is concave, the concave part is called a groove: