NASA researchers using 22 years of satellite-derived data have confirmed a theory that the strength of "long waves," bands of atmospheric energy that circle the earth, regulate the temperatures in the upper atmosphere of the Arctic, and play a role in controlling ozone losses in the stratosphere. These findings will also help scientists predict stratospheric ozone loss in the future.

These grids depict the strength and shape of planetary-sized waves or long waves in both 1984 and 1997 in the northern mid-latitudes. In 1984, the long waves were strong, as depicted by the solid black lines. The stronger waves provided the fuel for the atmospheric circulation to warm the stratosphere in the north polar region. Because the stratosphere was warm, reactive chlorine levels were low, and less ozone was lost. The orange and red colors represent more ozone present in the upper atmosphere. In 1997, the waves were weaker, as depicted by the broken black line. The weaker long waves provide little fuel for the atmospheric circulation that normally warms the polar stratosphere, making it colder than usual. Colder temperatures cause polar clouds to form, which lead to chemical reactions that affect the chemical form of chlorine in the stratosphere. In certain chemical forms, chlorine can deplete the ozone layer. The reduced ozone, or "ozone hole" is depicted in the blue and purple. Photo: NASA

These long waves affect the atmospheric circulation in the Arctic by strengthening it and warming temperatures, or weakening it and cooling temperatures. Colder temperatures cause polar clouds to form, which lead to chemical reactions that affect the chemical form of chlorine in the stratosphere. In certain chemical forms, chlorine can deplete the ozone layer. One theory is that greenhouse gases may be responsible for decreasing the number of long waves that enter the stratosphere, which then thins the ozone layer.

Just as the weather at the Earth's surface varies a lot from one year to the next, so can the weather in the stratosphere. For instance, there were some years like 1984, in which it didn't get cold enough in the Arctic stratosphere for significant ozone loss to occur. "During that year, we saw stronger and more frequent waves around the world, that acted as the fuel to a heat engine in the Arctic, and kept the polar stratosphere from becoming cold enough for great ozone losses," said Paul Newman, lead author of the study and an atmospheric scientist at NASA's Goddard Space Flight Center, in Greenbelt, Md.

"Other years, like 1997, weaker, and less frequent waves reduced the effectiveness of the Arctic heat engine and cooled the stratosphere, making conditions just right for ozone destruction," Newman said. The paper appears in the September 16 issue of Journal of Geophysical Research-Atmospheres.

The temperature of the lower level of the stratosphere over the poles is also controlled by the change in seasons from winter to spring, and by gases such as ozone, water vapor and carbon dioxide.

A long wave or planetary wave is like a band of energy, thousands of miles in length that flows eastward in the middle latitudes of the upper atmosphere, and circles the world. It resembles a series of ocean waves with ridges (the high points) and troughs (the low points). Typically, at any given time, there are between one and three of these waves looping around the Earth.

These long waves move up from the lower atmosphere (troposphere) into the stratosphere, where they dissipate. When these waves break up in the upper atmosphere they produce a warming of the polar region. So, when more waves are present to break apart, the stratosphere becomes warmer. When fewer waves rise up and dissipate, the stratosphere cools, and the more ozone loss occurs.

These are actual satellite images of ozone loss from 1984 and 1997, showing the difference between years where long waves are strong and weak. The colors represent total ozone amounts with purple and blue representing the low end and orange and red representing the high end. The black lines represent the average February long-wave wave structure. 1984 was an active year with strong planetary waves and the ozone levels are relatively high at the pole. 1997 was an extremely quiet year for long waves and the ozone levels are relatively low at the pole (as seen in blue and purple). Photo: NASA

Weaker "long waves" over the course of the Northern Hemisphere's winter generate colder Arctic upper air temperatures during spring. By knowing the cause of colder temperatures, scientists can better predict what will happen to the ozone layer.

The temperature of the polar lower stratosphere during March is the key in understanding polar ozone losses - and the temperature at that time is usually driven by the strength and duration of "planetary waves" spreading into the stratosphere.

This discovery provides a key test of climate models that are used to predict polar ozone levels. "This then lends itself to adjusting climate models, and increasing their accuracy, which means scientists will have a better way to predict climate change in the future," Newman said.

The stratosphere is an atmospheric layer about 6 to 30 miles above the Earth's surface where the ozone layer is found. The ozone layer prevents the sun's harmful ultra-violet radiation from reaching the Earth's surface. Ultra-violet radiation is a primary cause of skin cancer. Without upper-level ozone, life on Earth would be non-existent.

The research used temperature measurements of the stratosphere from the Upper Atmospheric Research Satellite (UARS).