Interannual variability of wind speed and wind power at five tall-tower sites in Minnesota (1996-2001)

We use wind speed measurements from 1996-2001 at 70 m above ground (a typical wind turbine height) to investigate interannual variability in wind speed and wind power at five sites in Minnesota. Our results show that, over the period of record, there was an 8-12% difference between the lowest and highest mean annual wind speeds (depending on the station); differences for wind power were 12-33%. Years with the highest mean annual wind speed did not always have the highest mean wind power, and some "fast" years had less hour-to-hour variability than "slow" years. We also estimated the available wind power at each site, which we defined as the energy that can be captured by a particular wind turbine. Accounting for turbine characteristics reduced the mean annual wind power by as little 2% at low wind-speed sites, to as much as 13% at sites with higher mean speeds. Mean annual 70-m wind speeds at these five Minnesota sites show more spatial coherence than speeds at lower measurement heights, and their time series are similar to the pattern of temporal variation in mean annual indices of the Arctic, North Atlantic, and Southern Oscillations. We suggest that interannual variability in the large-scale circulation may be an important contributor to the variability of turbine-level wind speeds.