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Records and Patterns.

By: Dr. Ricky Rood, 6:53 PM GMT on August 30, 2007

Records and Patterns

It is hot in Phoenix, Arizona, where the record was just broken for the number of consecutive days above 110 degrees F.

Here's a story or two.

Christian Science Monitor

Washington Post

Both of these stories talk about the urban heat island effect, and mention CO2 global warming as well. Phoenix is one of the places where the heat island effect is most obvious. There have been huge increases in the nighttime temperatures.

Here is the link to the National Climatic Data Center's Summary Report for the U.S. in July 2007.

Here is the dot diagram of the July 2007 temperature anomalies.

July 2007 Temperature Anomaly (NCDC)

The eastern half of the U.S. was cooler than normal, which provided the core for some of the Rush Limbaugh discussion in early August. The western half of the U.S. was very hot, especially in the states of Idaho-Montana. Taken as a average, the country was above the long-term average, the 15th warmest July since 1895. (This is based on preliminary data.)

This sort of warm-cold pattern is familiar to those who look at atmospheric observations. The patterns are related to high and low pressure centers, and are often best described as a wave. The highs and lows are the peaks and troughs of the wave. Much of the weather in the U.S. is due to waves that are traveling from, approximately, west to east.

There are many types of waves in the atmosphere. One type of wave has a very long wavelength, with only 1 - 3 ups and downs making a belt around the Earth. These are called planetary waves. The shorter waves directly responsible for the weather travel along these very long waves; weather is, approximately of course, steered by these very long waves. On TV and in weather forecasting the "jet stream" is often used to describe weather patterns. The jet stream is related to the planetary wave patterns, and when it gets down to modeling the weather patterns it is wave theory that works best.

When you see a pattern like in the July figure above, it says that the wave is stationary, rather than moving from west to east. When you get a persistent pattern, and that pattern lasts for a very long time, then you are likely to get heat waves and cold snaps, and droughts and floods.

The question becomes, then, what causes these persistent patterns? Not an easy question to answer, but some insight can be gained by thinking about planetary waves and what causes them. Planetary waves arise because in middle latitudes the wind flows primarily from west to east, and when this flow encounters mountains or because of temperature differences between land and ocean, a wave is set up. These waves are said to be forced. They usually wiggle a little, keeping the hot and cold, the wet and the dry, from becoming extreme. But if they don't wiggle, for whatever reason, then the extremes can develop.

Every year sees extremes. What about climate and climate change? One way to think of climate change is - are the extremes more intense? Is the heat wave exceptional? Is the drought worse because the temperature is warming? Another way is to think about the waves. With climate change, the mountains stay the same, but the land-ocean temperature difference, which also forces the waves in the atmosphere, changes. It is intuitive, that if this forcing becomes more prominent and dominant in a warmer climate, then stationary wave patterns will be more prevalent. The waves lose their ability to wiggle.

Long waves. The Arctic Oscillation. The Bermuda High. Yes these are sources of variability, but they are not independent of the climate. The variability associated with these waves does not stand in contradiction to climate change; it confounds attribution.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.