What causes wind to deflect toward left in the Southern hemisphere ?

Winds are deflected to the left in the Southern Hemisphere because of the Coriolis effect, an apparent deflection that arises from the Earth’s rotation. The pressure gradient force initiates horizontal motion, but the rotating frame of reference causes moving air to deviate from a straight path; in the Northern Hemisphere this deflection is to the right and in the Southern Hemisphere to the left [1]. Authoritative treatments (e.g., NASA) also state that this apparent leftward deflection in the Southern Hemisphere results from planetary rotation rather than magnetic fields or temperature directly, though temperature differences create the pressure gradients that drive the wind initially [2]. Therefore “Rotation of the earth” is the correct cause.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 23: Pressure Systems and Wind System > UPSC Prelims 2011] Which one of the following weather conditions is indicated by a sudden fall in barometer reading? > p. 308
2. [2] https://www.nasa.gov/wp-content/uploads/2011/05/544859main_coriolis_effect.pdf