In West African meteorology, what is the Inter-Tropical Discontinuity (ITD)?

1. The surface boundary between the dry northeasterly Harmattan air and the moist southwesterly monsoon air
2. A zone of high-pressure sinking air that stays stationary over the Sahara Desert
3. A localized microburst that occurs over coastal lagoons near Accra
4. The boundary where the jet stream meets the equatorial troposphere