Relying on the Ocean’s Long Term Memory
	The ultimate goal for atmospheric scientists studying the NAO is to be able to predict how positive on average or how negative on average the NAO will become on a yearly basis. If they knew that, then they could warn European farmers of when they should plant their crops, alert Mediterranean resorts as to the amount of rain they are likely to receive, and generally predict winter weather trends more accurately.
	Hurrell explains that the variability of the NAO occurs on several different time scales. From week to week, the NAO flip-flops between positive and negative phases seemingly at random, sending good and bad weather intermittently to both southern and northern Europe. Yet, each winter the NAO almost always shows an overwhelmingly negative or positive average for the year. When these yearly averages are put into an index and plotted next to one another, a clear pattern emerges. Since the 1960s, the differences in pressure between the Azores High and the Icelandic Low have followed a serpentine pattern, increasing for three to five years before decreasing for the same amount of time (World Climate Research Program, 1997). Despite its regular appearance, this long-term pattern is still too erratic to predict by simply looking at a chart of its history. "There are simply too many variables that go into these trends to make them easily predictable," says Hurrell. The only way scientists could forecast the dips and peaks in this trend would be if they understood exactly what was causing the two pressure systems to oscillate over long stretches of time, Mehta says. Currently, scientists only have a vague idea of what’s driving this rhythm in the NAO. Most researchers agree that the high and low would develop on their own over the Atlantic and that they would fluctuate in strength from week to week or even month to month. However, decadal patterns require some type of climatic "memory." For the NAO index to continue an upward climb over seven consecutive winters, there would have to be some mechanism in the atmosphere or the ocean that "reminds" the Azores High and the Icelandic Low of what part of the long-term cycle they were in the year before. "The hypothesis is that if you have long-lived climate anomalies that continue for more than a season or several years, then the land, ocean, or sea ice must be involved," says Mehta. Left to themselves, atmospheric currents change in temperature and density so rapidly over time that there is no way they could maintain a pattern into the spring and summer months after the low- and high-pressure systems break up. Ocean currents and ice formations, however, retain heat and can circulate for years without dissipating. Though the problem is still a ways from being solved, researchers believe that the ocean currents or ice sheets in the North Atlantic are somehow coupled with the atmosphere, influencing the positive and negative sign NAO will take each winter. Only by understanding the mechanisms that are driving the long-term decadal trend will scientists be able to forecast its behavior on a yearly basis. Mehta points out that knowledge of such systems is precisely what allowed scientists to predict the last El Niño nearly thirteen months in advance.    Mimicking Mother Nature  The Highs and Lows of the NAO		Although the relative pressure difference between Iceland and the Azores varies from month to month (top), the winter average of the months December, January, February, and March (above) cycles over a period of several years. For three to five years the NAO grows stonger, then for the next three to five years weakens. (Graphs by Robert Simmon, based on data from Hurrell)

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