El Nino and La Nina are terms used to describe the state of the tropical Pacific Ocean. These changes are associated with different weather conditions in Australia: El Nino leading to below average rainfall and La Nina to above average rainfall.
But what do these terms actually mean?
Consider the tropical Pacific Ocean as a basin of water. As the sun beats down on this basin of water, the surface of the water heats up. Water is not a very good at transferring heat, so the heat from the sun stays in the top part of the water. The bottom layer of water, which has not been in contact with the sun, stays cool. What we now have is a ‘thermocline’. The word thermocline describes the surface between the upper warm layer of water, and the lower cooler layer of water. So above the thermocline is warm water, and below the thermocline is cool water.
Winds normally flow from South America, across the tropical Pacific to Australia. These winds are known as the ‘trade winds’. Going back to our basin of water, the winds essentially act to tilt the basin down to the left. As the winds pass over the ocean, friction causes the surface waters to travel in the same direction as the wind.
During an El Nino event, the direction of the trade winds reverse, instead moving from Australia towards South America. What this does is tilt the thermocline (or our basin of water), so that warm water begins to pile up against South America. Rain will follow the location of the warmest water, because warm water means more evaporation, and hence more rain. What this means is that El Nino conditions lead to above average rainfall in South America, but below average rainfall in Australia. This is why we commonly associate the term El Nino with drought, in Australia.
La Nina is essentially the opposite of El Nino. During a La Nina, the trade winds strengthen, which causes the thermocline to tilt the other way, causing warm water to pool against Australia. Again, the rain follows the location of the warm water, so Australia experiences above average rainfall, while South America experiences below average rainfall.
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