Assessment of a Decreasing Wind Speed Trend at Mount Washington, New Hampshire

Several global wind studies mentioned in the Intergovernmental Panel on Climate Change Fifth Assessment Report (McVicar et al. 2012 and references therein) report decreasing wind speed throughout most of the northern mid-latitudes. However, the causes of the decreasing trends are still uncertain and likely vary by site. Possible causes outlined by McVicar et al. (2012) include Arctic amplification (e.g., Ren 2010), increased vegetation height around the site (e.g., Vautard et al. 2010), poleward movement of large storms (e.g., Frederiksen and Frederiksen 2007), and a lower Bowen ratio (e.g., Shuttleworth et al. 2009). Increased vegetation and other local environmental changes around a site can impose a challenge on assessing the impact of the other factors on wind speed changes. The summit of Mount Washington, New Hampshire (44.27°N 71.30°W, 1917 m ASL) is well above treeline and the surficial landscape has not changed significantly in recent decades. Therefore, minimal local environment changes and the large prominence of the summit suggest the Mount Washington wind record is representative of regional wind speed changes that are driven by external factors (e.g., Arctic amplification, poleward movement of large storms, and a lower Bowen ratio).

Here, we assess diel, seasonal, and multi-decadal wind speed and direction variability and drivers using hourly wind observations collected by the Mount Washington Observatory from 1935-2011. The 1981-2011 period was specifically examined for climatological trends because of the consistency of instrument location, anemometer used, and surficial landscape of the summit cone.

A Theil-Sen's slope trend analysis of the Mount Washington wind record from 1981-2011 reveals a decrease of 0.43 m s^-1 decade^-1 that is statistically different from zero at the 90% confidence interval. Wind speed and directional diel patterns exist during all seasons with the most consistent diel pattern seen in the summer. Summer wind speed peaks before sunrise and decreases to a minimum in the early afternoon indicating frequent boundary layer exposure. Regular exposure to the free troposphere in the winter explains the weak winter diel wind cycle; synoptic scale patterns dominate winter wind variability. The influence of large-scale atmospheric circulation patterns (e.g., Arctic Oscillation) on Mount Washington wind speed is strongest during the cold season, but is relatively weak. Based on these results, we propose a new hypothesis that increased boundary layer exposure at the summit is partially responsible for decreasing wind speed.