Grape-harvest dates hold climate clues. (Earth Science).

The vintner's age-old habit of picking no grapes before their time may give scientists a tool for determining European climate patterns for the past 500 years.

Grapevines are particularly sensitive to the temperature and amounts of precipitation they're exposed to at various phases of their 2-year fruit-production cycle. A pair of French researchers contend that an analysis of the yearly grape-harvest dates in Europe could help climatologists fill in the blanks of the continent's postmedieval climate records.

Europe's weather, especially in the winter, is driven by the so-called North Atlantic Oscillation (NAO). The NAO index measures the difference in pressure between a region of nearly permanent low atmospheric pressure hovering near Iceland and a region of high pressure that typically resides off the northwestern coast of Africa. Changes in the index affect the paths of storms and low-pressure systems crossing the Atlantic and moving into Europe (SN: 10/25/97, p. 268).

When the NAO index is high, weather systems generally are stronger than normal and invade the continent on a more northerly path. This typically heralds a winter with milder temperatures and more-humid conditions than normal in northern Europe, says Annie Souriau, a climatologist at the National Center for Space Studies in Toulouse, France.

Souriau and atmospheric scientist Pascal Yiou of the French Atomic Energy Commission in Gif-sur-Yvette report in the Oct. 15 GEOPHYSICAL RESEARCH LETTERS that the monthly average of the NAO index for the past 175 years correlates with the prime grape-picking time. A low NAO index for December, which typically signifies a colder-than-normal winter, was usually associated with an early grape harvest the next year. A high NAO index in May, which indicates a warm, humid spring, also seemed to be linked to an early ripening.

Although European scientists measured atmospheric pressure even in the mid-17th century, the weather data needed to calculate the NAO index didn't become regularly available until 1825. Souriau and Yiou suggest that climatologists may be able to use the archival data of grape-harvest dates to reconstruct a much longer NAO history. In some regions of northern Europe, village archives detailing grape harvests go back to the late 1400s. --S.P.