In this article, the climate background of wind speed was analyzed in Beijing-Tianjin-Hebei (BTH) region from 1979 to 2017 by using the daily near-surface wind speed from meteorological stations. The standard strong wind speed and regional strong wind day are defined on the frequency distribution characteristics of near-surface wind speed in the winter half-year. The annual variation (including linear trend and periodic characteristics) of strong wind frequency and speed in the study region were analyzed. The atmospheric circulation and sea-surface temperature corresponding to strong wind frequency and speed also are researched. The mechanism of influence on the strong wind is further discussed. Based on the ERA-Interim reanalysis data, the K-means method and hierarchical clustering method are used to classify the circulation patterns of strong wind days in Eurasia, and discuss the spatial distribution and temporal distribution of the main circulation patterns. The main results obtained are as follows:

The statistical results of meteorological station wind speed data show that the daily average wind speed in BTH region from 1979 to 2017 reached a maximum of 18.3 m/s and a minimum of 0 m/s. From the regional daily average wind speed, the maximum is 8.9 m/s, the minimum is 0.5 m/s, the standard deviation is 0.77 m/s, the result show that the fluctuation is small. The annual average wind speed shows a significant decrease trend, with a linear trend of -0.05 m/s/10yr (p<0.0001) in BTH region. The proportion of northerly and southerly winds in the wind direction throughout the year are similar.

The statistical results of the seasonal average wind speed in the BTH region show that the maximum wind speed in spring is 8.9m/s, the maximum wind speed in winter is 7.4 m/s, and the maximum wind speeds in autumn and summer are 6.5 m/s and 4.7 m/s, respectively. Among them, the average daily wind speed in winter shows strong fluctuations with a standard deviation of 0.81 m/s. The standard deviation in spring is 0.76 m/s. The liner trends of wind speeds in spring, summer, autumn, and winter are -0.09 m/s/10yr, -0.03 m/s/10yr, -0.06 m/s/10yr and -0.03 m/s/10yr. The wind speeds in spring and winter show the characteristics of large maximum wind speed and strong fluctuation.

The statistical results of the winter half-year wind speed show that the annual average wind speed decreases at a linear rate of -0.01 m/s/10yr in BTH region. The wind direction corresponding to the daily maximum wind speed is mainly northerly and northerly wind, accounting for 59% on average. According to the frequency distribution of wind speed, 3.5 m/s (90% percentile) is defined as the standard strong wind speed, and at least 60% of the stations' wind speed achieve the wind speed of 3.5 m/s as a regional strong wind day in BTH region. The annual frequency of strong wind days decrease significantly at a rate of -1.77 d/10yr (p<0.1); The linear rate of the annual average strong wind speed is -0.07 m/s/10yr (p<0.05). The average non-strong wind speed increases at a rate of 0.02 m/s/10yr. Increasing frequency of regional strong wind events is the main reason for the reduction of average wind speed in the study region. In addition to significant long-term trends, there are significant inter-annual and inter-decadal period of strong wind frequency and speed, the strong wind frequency has an inter-annual period of 3-4 years and an inter-decadal period of 14-15 years. The inter-annual period of strong wind speed is the most significant which has 2-3 years period.

The atmospheric circulation corresponding to the strong wind in BTH region mainly shows that the intensity of the westerly jet stream in East Asia has a significant positive correlation with the frequency of the regional strong wind events (r