Dec. 16, 1999 BOULDER -- New results from the National Center for Atmospheric Research with implications for climate and space weather are being presented this week during the fall meeting of the American Geophysical Union in San Francisco. Three presentations are highlighted below. NCAR's primary sponsor is the National Science Foundation.
--Agriculture Keeps the United States Cooler--
Converting forest to cropland in the central United States has cooled daily maximum temperatures for deforested areas by 2 to 3 degrees Celsius (4 to 5 degrees Fahrenheit), according to an NCAR land-surface model and a new analysis of observations.
Gordon Bonan analyzed daily maximum and minimum temperatures over the past decade in the Midwest, which is primarily cropland, and compared them with the same information for the Northeast, which is primarily forested. The analysis provides support for Bonan's model predictions: croplands reduce daily maximum temperatures, especially in summer and autumn, and the maximum temperatures are reduced more than daily minimums, resulting in a decrease in the diurnal (24-hour) temperature range in the Midwest.
"You would expect diurnal temperature ranges to decrease as you go from the Midwest to the East," says Bonan, because clouds, which keep nighttime temperatures warmer, are heavier in the East. "But the observations show the opposite, and that's consistent with what the model suggests."
Bonan's findings suggest that human-induced changes other than greenhouse emissions are affecting the climate. Some climate-change critics have found reassurance in cooler temperatures over the United States. However, Bonan says, "When people say that temperatures in the United States have not warmed, we have to look at all the reasons why they haven't warmed."
--Observations Tie El Nino to Major Volcanic Eruptions over the Past 400 Years--
Large volcanic eruptions in the tropics over the past 400 years have often been followed by an El Nino pattern around the globe, according to a new climate data analysis at NCAR.
Paleoclimate student Caspar Ammann used local eruption observations and ice-core data to track the years of major volcanic eruptions in the tropics. He then used a data set based on tree rings and coral growth layers to arrive at a global picture of the earth's climate the following year. He found a tendency for major tropical eruptions to be followed by an El Nino-like pattern of temperature changes in the tropical waters of the eastern Pacific, Alaska, the southeastern United States, the tropical Atlantic, the Indian Ocean, and the western Pacific. He defined major eruptions as those powerful enough to eject sulfates into the stratosphere, where they would remain for several years.
"It appears that a major volcanic eruption in the tropics can somehow tweak the normal interannual cycle to favor an early onset of El Nino," says Ammann, "but we don't understand the mechanism." El Ninos tend to recur naturally every three to seven years. The next step is to model the climate over the past centuries using the volcanic data to see if the relationship is reproduced, then investigate why it occurs.
--Model Breakthrough Explains Length-Strength Relationship in Solar Cycle--
New computer models of the Sun's dynamics are helping to explain the length and strength of the solar cycle. The NCAR modeling effort could help provide a stronger physical basis for predicting the intensity of upcoming solar maxima, such as the one expected this winter.
The Sun's activity tends to peak about every 11 years, but some cycles are separated by as little as 9 years or as much as 14. Stronger cycles tend to be shorter than average ones, and weak cycles longer than average. Most nonlinear dynamo models of the solar cycle have been unable to explain this behavior until now. NCAR researchers Mausumi Dikpati and Paul Charbonneau believe they may have found the cause: variations in the meridional (north-south or south-north) flow of plasma across the Sun.
The meridional flow is typically quite weak, but over time it carries large amounts of magnetic field poleward and helps recharge the solar dynamo for its next activity peak. In a model that captures solar magnetic field transport, Dikpati and Charbonneau have argued that the meridional flow can be easily disrupted by the intense vertical motions of convection (parcels of heat rising from the Sun's surface). This convection is naturally variable, which helps explain why its disruption of the flow varies from cycle to cycle. Yet the amount of plasma transported by the flow over the course of a cycle remains relatively constant; thus, at times when the flow is disrupted more than usual, the resulting solar cycle is weaker but lasts longer. "It's been said that the Sun has a clock and the clock is always adjusted," says Dikpati.
NCAR is managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.
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