Variations in Summer Extreme High-temperature Events over Northern Asia and the Possible Mechanisms

In this study, the variations in the extreme high-temperature event (EHE) over Northern Asia (NA) and the associated possible mechanisms are explored. On an interdecadal timescale, NA EHE frequency experienced a significant interdecadal increase around the mid-1990s, which could be associated with the phase shift of the Atlantic Multidecadal Oscillation. On an interannual timescale, the first two empirical orthogonal function modes of the NA EHE frequency exhibit meridional dipole pattern (EOF1) and diagonal tripolar pattern (EOF2), respectively. Further analysis reveals that the EOF1 mode is related to the Polar-Eurasian teleconnection pattern (POL), while the EOF2 mode is associated with North Atlantic Oscillation (NAO) and Pacific-Japan/East Asia-Pacific pattern (PJ/EAP). The fitted EHE frequency based on the atmospheric factors (POL, NAO and PJ/EAP) can explain the interannual variation in the regionally averaged EHE frequency by 33.8%. Furthermore, three anomalous sea surface temperature (SST) patterns over the North Atlantic-Mediterranean Sea region and around the Maritime Continent are associated with the two EHE modes by intensifying the pronounced atmospheric teleconnections. Analysis on the simulation of five models in the Atmospheric Model Intercomparison Project experiment further confirms the impact of the pronounced SST patterns on the POL, NAO and PJ/EAP. From a synoptic perspective, the atmospheric patterns responsible for the NA EHE are investigated. By applying a hybrid regionalization approach to the daily maximum temperature, three subregions of NA can be identified: western NA, central NA, and southeastern NA. To better understand the mechanism for the formation of EHE in each subregion of NA, the EHE-related synoptic circulation patterns over each subregion are further categorized into two types. These six synoptic circulation patterns influence the NA EHE occurrence through different radiation and advection processes. From the forecasting perspective, six wave train patterns are explored as the precursors of the six synoptic circulation patterns, separately. These wave train patterns appear over the upstream regions of NA subregions with at least three-day lead, and provide potential forecasting information for the NA EHEs. The results may deepen our understanding of the NA EHE formation and provide information for the prediction and forecast of NA EHE.