Statistical Characteristics of Blocking Highs and their Modulation on the Surface Air Temperature at 2 m during Stratospheric Northern Annular Mode Events

In this paper, the statistical characteristics of blocking highs and their modulation on surface air temperature anomalies (SATAs) at 2m during stratospheric northern annular mode (NAM) events are analyzed using the 1979–2020 ERA5 daily reanalysis data. The results show four blocking highs tend to occur in the northern hemisphere during different stages of NAM events. Greenland blocking (GB) and North Pacific blocking (NPB) occur frequently during the development stage of positive NAM events and the decay stage of negative NAM events. Meanwhile, Ural blocking (UB) tends to occur in the decay stage of positive NAM events and the development stage of negative NAM events. Finally, Atlantic–Europe blocking (AEB) is more likely to occur in the decay stage of positive and negative NAM events. Although the number of days with blocking highs is considerably lower than that without blocking highs during NAM events, blocking highs have a considerable modulation effect on the spatial SATA pattern, which is related to NAM events. In particular, when GB occurs during positive NAM events, the SATAs in Eurasia change from “overall warm” or “north warm–south cold” to “north cold–south warm” in the extratropics, whereas those in North America change from “overall cold” or “north cold–south warm” to “north warm–south cold.” UB in the decay stage of negative NAM events reverses the SATA pattern from “north cold–south warm” to “north warm–south cold” in Eurasia and from “north warm–south cold” to “south warm–north cold” in North America. Meanwhile, AEB and NPB have minimal modulation effect on the spatial SATA patterns in most stages of NAM events; that is, the spatial SATA patterns are mainly dominated by NAM events. Under the influence of the NAM and modulation of blocking high, Eurasia is colder and North America is warmer in the mid-latitude regions from the peak stage of positive NAM events to the developing stage of negative NAM events, while opposite trends are observed from the decay stage of negative NAM events to the initial stage of positive NAM events. The dominant SATA patterns in Eurasia and North America follow the South–North dipole pattern between mid- and high-latitude areas during NAM events. These conclusions are verified using the Monte Carlo random sampling test.