The "sea"saw Effect of the Pacific
Darwin, Tahiti, and Callao
By the early years of 20th century, scientists were working to understand the world's weather. Sir Gilbret Walker was a Director-General of British Observatories in India during the 1920s, and he noticed a strange phenomenon associated with the barometric pressure readings on the eastern and western sides of the pacific. When the pressure was higher on the east, it was lower in the west. The opposite was also true. This is the effect that many people simply call the 'seesaw' effect of the equatorial Pacific Ocean. Walker called this phenomenon the 'Southern Oscillation.'
The two cities that are most commonly used for measurements and example are Darwin, Australia, and Tahiti, French Plolynesia. If the air pressure is low at Darwin, then the air pressure will be high at Tahiti. And we know by now that winds blow from areas of high pressure to areas of low pressure to achieve maximum equillibrium. Therefore, in this scenario, winds will blow strongly from east to west, thus making the trade wind stronger. However, if the air pressure is low at Tahiti and it is high in Darwin, then winds will blow from Darwin to Tahiti [east to west]. In this scenario, the trade winds will be of less strength, thus propagating El Nino like conditions.
|Air Pressure Differences due to the Southern
If you take a look at the first graph, you see the mean pressure measurements taken for 12 months. These measurements are expressed in millibars. [1 mb = 102 Pa]. If you take a look at the chart from 1960 to 1980, then you can easily the validity of the Southern Oscillation. Take July, 1964 for example. The pressure in Tahiti is about 1013 mb, a relative high for Tahiti. While the pressure in Darwin is about 1007 mb, a relative low for that area. Tahiti has high pressure while Darwin has low pressure.
Now let us take October, 1976 for example. The pressure at Tahiti is about 1011.5, which is somewhat for Tahiti. The pressure at Darwin, Australia is about 1009, which is high for Australia. Again, the Southern Oscillation proves to be true.
SSTs are closely related to air pressure
If you take a look at the Sea Surface Temperatures of the twelve month running mean for sea-surface temperature off Callao in degrees Celcius below, you will easily see the relationship. As can be seen, the low points in the SST at Callao closely match up with the low pressure points at Darwin. Thus proving that Pressure in the equatorial Pacific Ocean has a very close correlation with the Sea Surface Temperature in the Pacific Ocean.
|The Southern Oscillation Index or the SOI
The pressure measurements from the Southern Oscillation is used to calculate a mathematical equation. The results of this equation is an index called the "The Southern Osciallation Index." If the index is low on negative then the air pressure is lower in the east. That means that air pressure is high in the west. This will mean that the trade winds will be weakened, this will start an El Nino phase.
If the index is high on positive then the air pressure is lower in the west. This means that air pressure is high in the east. This will cause winds to blow from east to west. This will help the trade winds and influence normal conditions. The term 'high-index' is used to refer to the state where the pressure is high in the eastern pacific, and 'low-index' when pressure is low in the east.
If you take a look at the graph on the right, you can see that when the SOI is greatly negative, El Nino events are more likely to occur than normal conditions. Also, if the SOI is positive, then one can see that La Nina conditions are most likely to occur. The SOI graphs for the 1996-97 were also greatly negative giving a good prediction that this was not going to be an average El Nino. Because El Nino and the Southern Oscillation are so closely linked, scientist refer to this entire phenomenon as ENSO, or El Nino/Southern Oscillation.