The Westerlies have their origin in the :

The westerlies are the prevailing winds of the mid-latitudes, blowing between approximately 30° and 60° latitude in both hemispheres [1]. These winds originate from the subtropical high-pressure belts, also known as the horse latitudes, where air descends and diverges [1]. From these high-pressure systems, air flows poleward toward the subpolar low-pressure belts [1]. Due to the Coriolis force, this poleward-moving air is deflected to the right in the Northern Hemisphere (becoming south-westerlies) and to the left in the Southern Hemisphere (becoming north-westerlies) [4]. The westerlies are part of the Ferrel cell circulation, which involves sinking cold air from the poles and rising warm air from the subtropics [2]. They are particularly strong and persistent in the Southern Hemisphere due to the lack of landmasses to provide frictional resistance [1].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 23: Pressure Systems and Wind System > The Westerlies > p. 319
2. [4] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 23: Pressure Systems and Wind System > Horse Latitudes > p. 312
3. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 23: Pressure Systems and Wind System > Ferrel Cell > p. 317