As warm water flows northwards in the Atlantic, it cools, while evaporation increases its salt content.
Low temperature and a high salt content raise the density of the water, causing it to sink deep into the ocean. The cold, dense water deep below slowly spreads southward eventually, it gets pulled back to the surface and warms again, and the circulation is complete.
This continual mixing of the oceans, and distribution of heat and energy around the planet, contribute to global climate.
El Nino-Southern Oscillation (ENS0)
This involves temperature changes of 1 °-3°C in the central and eastern tropical Pacific Ocean, over periods between three and seven years.
El Nino refers to warming of the ocean surface and La Nina to cooling, while "Neutral" is between these extremes. This alternating pattern affects rainfall distribution in the tropics and can haw a strong influence on weather in other parts of the world.
As the Indian Ocean warms faster and faster, it creates additional precipitation. This draws more air from other parts of the world to the Indian Ocean, including the Atlantic. With so much precipitation in the Indian Ocean, there will be less precipitation in the Atlantic Ocean. Less precipitation will lead to higher salinity in the waters of the tropical portion of the Atlantic — because there won't be as much rainwater to dilute it.