Relation of Atmospheric ISOs at Mid–High Latitude Eurasia to the European Blocking Frequency and Their Co-effect on Extreme Hot Events during Boreal Summer

Using the NCEP reanalysis daily data during 1979–2018, the spatiotemporal evolution of the 10–30-day atmospheric intraseasonal oscillations (ISOs) at mid–high latitude Eurasia and their effects on the European blocking frequency are examined. Furthermore, the co-effect of the blocking and ISOs on extreme hot event frequency is studied. The ISO shows two modes, namely, the eastward and westward propagating modes. During the eastward (westward) propagating mode, the northwest—southeast tilted quadrupole (east—west dipole) quasi-barotropic geopotential height anomaly, together with the air temperature anomaly at the troposphere, propagates southeastward (westward). Phase composite reveals that, during both modes, the mid–high latitude low-frequency Rossby wave trains substantially influence the European blocking frequency during the propagating journey. The most frequent European blocking emerges in phases 6–7 (5–6) during the eastward (westward) mode, which is referred to as E-P67 (W-P56). During E-P67 (W-P56), western Europe, the eastern European plain, the Ural Mountains, and the northeast plain of China (Europe and the Ural Mountains) are controlled by quasi-barotropic + − + − (+ −) height anomalies, respectively, thereby resulting in considerable positive frequency anomalies of extreme hot events over western Europe and the Ural Mountains (Europe) and negative anomalies over the eastern European plain and the northeast plain of China (Ural Mountains). If blocking occurs during E-P67, the positive height anomaly intensity over western Europe increases substantially, and the positive or negative ones over the Ural Mountains, eastern European plain, and northeast plain of China weaken; meanwhile, a negative height anomaly emerges in the south of Europe, causing a negative extreme hot frequency anomaly. During W-P56, the positive and negative height anomalies are increased, and they materialize over the northeastern plains of China and south of Europe, decreasing and increasing the hot events in the two regions, respectively. Therefore, during E-P67 and W-P56, the European blocking increases (decreases) the frequency of extreme hot events in Europe and the northeast plains of China (south of Europe and the Ural Mountains). Thus, European blocking activities considerably control the influences of the two propagating ISO modes on the extreme hot events over the mid–high latitudes of Eurasia.