Answer: In ocean, the less denser warm water due to the heat from the sun, rises up to the ocean surface and tends to move towards higher latitudinal areas. The wind generates the ocean currents and it slowly loses heat and becomes cold and denser as it approaches the polar region and then it sinks down, moves back towards the equatorial region. This constant motion of these water current in the ocean is known as the convection current.
In the atmosphere, similar mechanism occurs. the air gets warm due to the heating of the sun and rises up to the atmosphere and as it rises up, it slowly becomes dense and again at one point it falls back because of its higher density. This continuous motion of air represents the convection current within the atmosphere.
This two phenomenon is the same as the convection that occurs at the earth's mantle, in which the hot molten magma because of its lower density rises up and gets cooled when it reaches the upper part, becoming dense again falls back.