The Southern Oscillation is a weather pattern found in the tropical Pacific Ocean. It goes through two phases. One of these is a warm phase and is known as El Nino, which is Spanish for the little boy. The other is a cold phase known as La Nina, which is Spanish for the little girl.
The oscillation in this weather pattern occurs very roughly every five years or so, but could appear a couple of years earlier or later than that. The adverse and peculiar weather that the system brings with it are a source of misery and possibly danger to many people in the Americas. The Southern Oscillation is particularly associated with the pressure difference in the atmosphere changing, specifically in the region between Tahiti and Darwin.
El Nino is the phase where the surface temperature of the water is warmer than the average for that area. The La Nina phase is where the surface temperature of the water is colder than the average for the area.
The damage that El Nino can cause comes in various forms. One of these is flood, with many people losing their lives over the years as a result of both drowning and being buried in mudslides, for example.
Another possibility is drought, which can be very damaging to the fishing and agriculture of the affected places. But pretty much every disturbance in the weather will see El Nino blamed by someone. The cold phase of the Southern Oscillation,La Nina, on the other hand, is not so much associated with bad weather. The main characteristic of this phase is that the water is colder.
So whereas the heat of the water related to El Nino can cause thunderstorms and increase rainfall, there is no such problem with La Nina. Indeed, the converse weather may be observed, with colder winters and more snow. The Southern Oscillation is still not properly understood. In particular, researchers have not got a good understanding of the underlying mechanism that sustains the cycle.
They do know that it begins with the faltering of the trade winds. These are the easterly winds that are a part of what is known as the Walker circulation, which is the lower atmosphere air flow found in the tropics. They also know that so-called Kelvin waves, which are small, warm waves below the water surface, warm the water near the South America. But there is much debate over the best model of how the cycle is generated.