Characteristics of atmospheric fields (a),(d) for recent summers, (b),(e) for positive AO, and (c) for negative AO. From the summer AO index (Fig. 10b), the positive AO summers are 1989, 1996, 2002, 2010, 2012, 2016, 2018, 2021, and 2022, in which the index exceeds +1.0σ, and the negative AO summers are 1958, 1960, 1965, 1968, 1976, 1977, 1978, 1985, 1987, 1995, 2009, and 2015, in which the index exceeds −1.0σ. Rows show (a)–(c) composites of geopotential height (in m) at 250 hPa and (d)–(f) composite latitude–height cross sections of zonal-mean zonal winds (unit: m s−1) and geopotential height (unit: m). Contours indicate (a)–(c) the mean values for respective mean values and (d)–(f) zonal-mean zonal winds. In (a)–(f) color shading indicates that deviations from the mean for the years other than the selected years are statistically significant at the 10% level according to the applied t test.
Citation: Journal of Climate 36, 23; 10.1175/JCLI-D-23-0191.1
