Atmospheric winds and oceanic currents are strongly influenced by rotation, density stratification, and their small aspect ratio. We will present emblematic examples highlighting peculiar properties of these flows, with an emphasis on phenomena taking place at the equator. A useful general reference for the lectures is the book "Atmospheric and Oceanic Fluid Dynamics", 2nd Edition 2017, Cambridge University Press, by Geoffrey Vallis. Lecture 3: Stratification Up in the stratosphere, equatorial winds change direction each 14 months. This quasi biennal oscillation is the archetype of a wave - mean flow interaction problem in geophysical fluid dynamics. It involves a two-way coupling between slowly varying winds and fast oscillating internal gravity waves that propagate in density stratified flows. We will present a dynamical system introduced in the seventies to account for this phenomenon. Keywords: buoyancy, Boussinesq equations, internal gravity waves, streaming, doppler shift, quasilinear approach, Taylor-Goldstein equation, Lindzen-Holton Plumb model. References: [1] Plumb, R. A. "The interaction of two internal waves with the mean flow: Implications for the theory of the quasi-biennial oscillation." Journal of Atmospheric Sciences 34, no. 12 (1977): 1847-1858. [2] Renaud, Antoine, and Antoine Venaille. "On the Holton–Lindzen–Plumb model for mean flow reversals in stratified fluids." Quarterly Journal of the Royal Meteorological Society 146, no. 732 (2020): 2981-2997.