A climate index is any calculated value that can describe the state of the climate as well as changes in a climate system. Of course, that isn’t a very good explanation and so an analogy with money often serves to be more informative.
In general, climate indices use air temperature, air pressure, sea surface temperature, and precipitation data in their calculations.
These are all numbers that deal with the weather but consider for a moment that we are instead dealing with a section of the stock market. In that situation, selected stocks (their values and trends) are used to describe the state of the stock market as a whole.
For instance, the Dow Jones Industrial Average and S&P 500 are both used often in the United States to describe the overall health of the New York Stock Exchange. The S&P and Dow Jones are called stock indices or market indices and they are used not only to explain the state of the market but also to make predictions about it. Climate indices work pretty much the same way.
The idea behind an index is that a particular set of values, measured from a particular set of locations, can be combined to create a general picture of overall climate for a region or even the entire planet. Like stock indices, analysis of long term trends in climate indices can help to make predictions about the future state of Earth’s weather.
All of the climate indices will be explored in depth, but an example of how the El Nino – Southern Oscillation (ENSO for short) works will be useful in understanding the function of a climate index.
ENSO makes use of El Nino and La Nina events, as well as atmospheric measurements, in the southern Pacific Ocean to understand overall climate trends for the planet. Here is how the three parameters work together to create an index:
- El Nino – An ocean phenomenon usually seen around Christmas, off of the west coast of South America, in which ocean water is warmer than average by 3-5 degrees.
- La Nina – The opposite of El Nino in which ocean water is cooler than average by 3-5 degrees
- Southern Oscillation – Atmospheric pressure oscillations over Australia and Indonesia in the Western Pacific.
By monitoring these parameters, it is possible to create a general idea of the weather across the globe. For instance, a strong El Nino event along with a weak Southern Oscillation leads to the following conditions:
- North America: Winter – Warm/Dry in Northern Regions, but Cool/Wet in Southern Regions (Southwest U.S., Mexico)
- South America: Summer – Warm/Wet in Northern regions with major flooding
- Africa – March to May conditions are wetter than normal, but December to February conditions are drier than normal
- Asia – Mostly drier conditions
- Australia – Drier conditions with increased brush fires
A strong La Nina will generally lead to:
- Africa – Wetter than normal December to February
- Asia – Increased formation of typhoons that make landfall
- North America – Cooler/Wetter Northern Regions, but Cool/Dry conditions in Southern Regions
- South America – Drought is a major issue.
So, clearly understanding these widespread indices can help to make predictions about the weather on an individual basis, but combining them can lead to even more powerful predictions.
Of particular importance to climatologists is the fact that changes across of range if indices can be used to determine the overall state of the Earth’s atmosphere and help predict how specific factors, like greenhouse gas emissions, will affect weather over the long term.