Figure 4

Top panel: (a) time series of a variety of circulation indices during period of 1 July–20 August, 2018. Indices include: GHGS and GHGN strengths for atmospheric blocking variability, Atlantic Low (AL), Azores High (AZH), and North Atlantic Oscillation (NAO). Dots denote both GHGS and GHGN strengths meet the criteria (BLs and BLn; see text in “Methods”), indicating blocked day. Gray shading region covers the period of 12 July–8 August, 2018. The GHGS strength is also termed as BL in this study. Central and bottom panels: linear regression of daily anomalies of 2-m temperature (shading, °C) and 500-hPa geopotential height (contour, m) onto the standardized circulation indices (b) BL, (c) AL, (d) AZH, and (e) NAO. Solid and dashed contours for positive and negative regression coefficients, respectively, and gray contours for zero. Contour interval is 150 m per standard deviation change in index for 500-hPa geopotential height anomalies. Contour interval for 2-m temperature anomalies is denoted by label bars, the units is °C per standard deviation change in index. The linear regression analyses were conducted for period of 1 July to 31 August, 2018. The maps were generated using software NCAR Command Language (https://www.ncl.ucar.edu/)³² with the built-in Ncarg4_0 database.