A jet stream is a current of high speed winds that occurs high up in the atmosphere. They form in regions where there is a rapid change in pressure. The most commonly known jet stream forms along the polar front, where the pressure gradient is quite large.

In certain circumstances, pressure systems will pull water away from the coast. When this happens, the water is replaced by water from beneath. Since it is coming from the bottom of the ocean, it is cold and nutrient-rich. This process is known as upwelling.

When global wind patterns are in the right place, upwelling occurs on a large scale off the equatorial west coasts (for example, Peru). However, upwelling stops when this pattern moves. This last event is known as El Niño, or a sudden warming of the equatorial waters. 

When El Niño occurs, we often see effects that happen globally - effects in weather, agriculture, economy, and many other ways.