The subtropical ridge is a belt of high pressure at a latitude of approximately 30° in both the northern and southern hemispheres. The subtropical ridge represents the subsiding component of both the Hadley and Ferrel cell circulations and is characterized by dry air, clear skies, settled weather, and very little wind. The subsiding air of the subtropical ridge is the main driver of many of the world’s arid and desert regions. Over the ocean, the easterly trade winds can be found equatorward of the subtropical ridge, and the mid latitude westerlies are located poleward of the ridge.
Like all elements of the global circulation, the subtropical ridge moves with the seasons; poleward during the summer and equatorward during the winter. In the North Atlantic, the subtropical ridge produces a semi-permanent high pressure system known as the Azores High, while in the South Atlantic it creates the South Atlantic High. Likewise, the Pacific Ocean has the North and South Pacific Highs and the Indian Ocean has the Mascarene High.
In the northern hemisphere, the position of the subtropical ridge is influenced by the El Ninõ southern oscillation (ENSO). When ENSO is in a positive phase, the position of the subtropical ridge is more southerly and the ridge is flatter. Conversely, when ENSO is in a negative phase, the subtropical ridge is more northerly. These subtle changes can impact hurricanes tracks in the north Atlantic.
Further reading:
Mascarene High: https://www.nature.com/articles/s41598-020-59964-7
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