Northern Hemisphere atmospheric stilling partly attributed to an increase in surface roughness

Journal name:
Nature Geoscience
Volume:
3,
Pages:
756–761
Year published:
DOI:
doi:10.1038/ngeo979
Received
Accepted
Published online

Surface winds have declined in China, the Netherlands, the Czech Republic, the United States and Australia over the past few decades1, 2, 3, 4. The precise cause of the stilling is uncertain. Here, we analyse the extent and potential cause of changes in surface wind speeds over the northern mid-latitudes between 1979 and 2008, using data from 822 surface weather stations. We show that surface wind speeds have declined by 5–15% over almost all continental areas in the northern mid-latitudes, and that strong winds have slowed faster than weak winds. In contrast, upper-air winds calculated from sea-level pressure gradients, and winds from weather reanalyses, exhibited no such trend. Changes in atmospheric circulation that are captured by reanalysis data explain 10–50% of the surface wind slowdown. In addition, mesoscale model simulations suggest that an increase in surface roughness—the magnitude of which is estimated from increases in biomass and land-use change in Eurasia—could explain between 25 and 60% of the stilling. Moreover, regions of pronounced stilling generally coincided with regions where biomass has increased over the past 30years, supporting the role of vegetation increases in wind slowdown.

At a glance

Figures

  1. Observed and reanalysis surface wind speed trends.
    Figure 1: Observed and reanalysis surface wind speed trends.

    a, 30-year surface (1979–2008) wind speed linear trend calculated over all of the available observations and each of the selected stations, in ms−1perdecade. The specific regions studied in this Letter are shown by rectangles delimiting the areas over which statistics are calculated. b, NCEP/NCAR reanalyses trends calculated using mean daily 10m wind speed values available over a grid of 193×47 grid points over the Northern Hemisphere (data available at http://www.cdc.noaa.gov/data/gridded/data.ncep.reanalysis.html). c, The same as in b for ERA-interim reanalyses trends calculated over the past 20years alone. The area boundaries for the four regions of focus are: Europe 20°W–40°E, 30°–75N, 276 stations; Central Asia 40°–100°E, 30°–75°N, 96 stations; Eastern Asia 100°–160°E, 30°–75°N, 190 stations; North America, 170°–50°W, 30°–75°N, 170 stations. The South Asia area quoted in the main text covers 40°–160°E, 0°–30°N, 40 stations.

  2. Wind speed distribution evolution.
    Figure 2: Wind speed distribution evolution.

    ad, Evolution, as a function of year, of the annual frequency of wind exceeding a given threshold (various curves, see legend), for each of the four regions identified in Fig. 1: Europe (a), Central Asia (b), Eastern Asia (c), North America (d). The numbers take into account all stations in the domains together, as well as all hours and seasons. The linear regression trend coefficients, respectively for the frequency (in %perdecade) of winds stronger than 1, 3, 5, 7, 9, 11, 13 and 15ms−1 are: Europe: 0,−1,−5,−7,−11,−12,−11,−12; Central Asia: 0,−6,−13,−18,−23,−24,−22,−23; Eastern Asia: 2,−4,−10,−15,−19,−23,−30,−37; Northern America: −1,−2,−4,−5,−3,−3,−3,−11.

  3. Upper-air wind speed trends.
    Figure 3: Upper-air wind speed trends.

    a, Trends of the 850hPa wind measured by rawinsonde data, over stations having at least half of the yearly data over more than 15years during the two-decade period between 1979 and 2008, in ms−1perdecade. b, Mean vertical profile of wind speed trend obtained from monthly averaged rawinsonde measurements (850hPa and above) and closest surface site, averaged over all sites in each region of Fig. 1a. c, The same as in b for wind speed trends normalized by mean wind speed at each site. Surface values are arbitrarily set to 1,015hPa pressure in the profile.

  4. Surface wind trends and their relationship with NDVI trends.
    Figure 4: Surface wind trends and their relationship with NDVI trends.

    a, Time evolution of annual average surface wind speed for each of the stations with the trend in the lower tercile of the wind trend distribution over the Central Asia area. Each curve corresponds to a station. The mean trend is −0.31ms−1decade−1. b, Spring–Summer (April–September) trends (in decade−1), of the average over a 24×24km area around the surface wind stations of the NDVI (as obtained from the GIMMS AVHRR product). c, Median surface wind trends over the stations of each decile of the NDVI trend distribution, together with the corresponding wind trend distribution (20–40% and 60–80% in yellow, 40–60% in red), versus the median of each decile of NDVI trends. The regression line for the median (y=−0.104–2.65x) is shown.

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Author information

Affiliations

  1. LSCE/IPSL, Laboratoire CEA/CNRS/UVSQ, 91191 Gif/Yvette Cedex, France

    • Robert Vautard,
    • Julien Cattiaux,
    • Pascal Yiou &
    • Philippe Ciais
  2. ECMWF, Shinfield Park, Reading RG2 9AX, UK

    • Jean-Noël Thépaut

Contributions

R.V. designed the experiments and carried out the observation analysis, J.C. did all model experiments and analysis of outputs, as well as NCEP/NCAR reanalysis calculations. J-N.T. extracted and analysed all data from ERA-interim reanalyses. P.Y. initially suggested systematic wind trends in observations, and P.C. suggested that land-cover changes due to vegetation could be a major driver of wind stilling and helped design the experiments with NDVI. All co-authors substantially contributed to the paper writing.

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The authors declare no competing financial interests.

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