Jim Hurrell (Climate and Global Dynamics Division) analyzes decadal trends of the North Atlantic Oscillation (NAO) in the 4 August issue of Science. The NAO is one of the primary modes of year-to-year atmospheric variability. It is a north-south oscillation in atmospheric mass with centers of action near the semipermanent Icelandic low and Azores high. When the NAO is in a positive phase, pressures are higher than normal across the North Atlantic south of 55 degrees N and lower than normal across the Arctic. The resulting westerly flow brings maritime warmth to Europe during the winter and allows the northwest Atlantic to cool below normal.
Jim Hurrell. (Photo by Bob Bumpas.)
Jim took a data set for the years 1899-1993 and compared surface pressures with sea-surface and continental temperatures. The comparison revealed a strong correlation between the NAO pressure signature and the presence of European warmth and northwest Atlantic cold, with unprecedented strongly positive values of the NAO index since 1980. Jim then linked the decadal variations in the mean circulation patterns to changes in the atmospheric moisture budget that agree with observed variations in regional precipitation. The message from these and other analyses performed by Jim is that the NAO is largely responsible for the unusually warm and wet winters observed across much of Europe over the past 15 years or so.
What might be causing these unusually intense positive phases of the NAO? Jim notes that the large warm wintertime anomalies over Europe resemble some results obtained by coupled atmosphere-ocean models forced with steadily increasing greenhouse gases. However, these models are not as likely to project cold anomalies over the North Atlantic. One reason may be the absence of sulfates in the models.
Along these lines, David Erickson (Atmospheric Chemistry Division) has put the effects of sulfate aerosols into version 1 of NCAR's community climate model. The results appear in the 1 August issue of Geophysical Research Letters. With coauthors Robert Oglesby (Purdue University) and Susan Marshall (University of North Carolina at Charlotte), David introduced a cloud-albedo function that varies based on industrial sulfate forcing. The emissions are mainly confined to Europe, China, and eastern North America.
David Erickson. (Photo by Curt Zukosky.)
Though one might expect cooling over these regions due to the increased reflection of sunlight by the aerosols, David and colleagues found otherwise. The model's results are remarkably similar to the NAO pattern analyzed by Jim: wintertime warming over Europe and cooling across the northwest Atlantic and eastern United States. Summertime signals are much weaker in the model runs used to evaluate the climate response to sulfate aerosols.
"The model results suggest that it is possible that the way the North America continent responds to the [sulfate] forcing is fundamentally different than how Eurasia climate evolves under the same forcing," writes David and his coauthors. They note that the competing influences of greenhouse gases and sulfates may be producing a persistent change in the "hemispheric wave train" of storm systems. Jim and Dave now plan to collaborate and examine how well Dave's model simulates the NAO itself and its recent variations.
Another researcher, John Mitchell (Hadley Centre for Climate Prediction and Research), recently published the first results with both greenhouse gases and sulfates incorporated in a coupled ocean-atmosphere model. The paper appears in the 10 August issue of Nature, along with a commentary by CGD senior scientist Tom Wigley. The model confirms a finding by Tom and colleagues at Lawrence Livermore National Laboratory that models with aerosols do a much better job than models without aerosols in simulating the patterns of near-surface temperature change from 1861 to date. Tom heralds the results of Mitchell et al. as "a turning point in our ability to both understand past changes and predict the future."
With the support of the NSF's Atmospheric Science Division, scientists at the University NAVSTAR Consortium (UNAVCO) have designed a climate and meteorological (CLAM) pack to be used with current Global Positioning System (GPS) receivers. Approximately the size of a shoe box, the device--with a parts cost of about $2,500--is outfitted with sensitive temperature, pressure, and humidity sensors to measure surface conditions at regular intervals. The measurements are fed into the GPS receiver to become part of the GPS data stream. The CLAM design includes a microcontroller, data and program storage, and serial, digital, and analog inputs. It can accommodate other data types such as surface winds, rainfall, snowfall, ceiling, and runway visual range. A prototype of the CLAM sensor, built by UNAVCO, has been sent to NASA's Jet Propulsion Laboratory for a trial run.
The surface meteorology measurements, in conjunction with the GPS measurements, allow scientists to calculate precipitable water vapor (PWV), the amount of water that would result if all atmospheric water vapor in a vertical column of air were condensed to liquid. Knowledge of PWV levels could greatly increase the accuracy of climate and meteorological studies, as well as that of weather forecasting.
See the May issue of Staff Notes Monthly for more details on meteorological applications of GPS signals. Additional information on the CLAM project can be found on the World Wide Web at this address: http://w ww.unavco.ucar.edu/articles/pots_pap.html