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This study aims to understand the source of surface air temperature bias over the contiguous United States (CONUS) during boreal summer (June–September) in the Unified Forecast System (UFS) coupled model prototype 8 (P8), developed by the National Centers for Environmental Prediction (NCEP) and the National Oceanic and Atmospheric Administration (NOAA). The focus is on the subseasonal variability defined as a weekly average in weeks 2–5 of forecast leads (total 224 cases; 4 weeks × 2 initialization dates × 4 months × 7 years). The large-scale surface air temperature bias pattern is extracted using the empirical orthogonal function (EOF) analysis. The associated principal component describes the variability of bias for each reforecasting week throughout the 2011–17 reforecasting period. The leading EOF of surface air temperature bias exhibits an east–west dipole pattern over the CONUS, explaining 31.6% of the total variability of weekly temperature bias. This bias pattern is strongly related to the upper-level Rossby wave induced by a bias in convection over the central tropical Pacific. Furthermore, the mean bias of background flow in the extratropics degrades the representation of teleconnections from the tropics to the midlatitudes. UFS P8 has weaker zonal wind over the North Pacific with stronger vertical wind shear than the ERA5 reanalysis. The weak zonal wind hampers the Rossby wave's propagation, while strong vertical shear reduces its amplitude. [ABSTRACT FROM AUTHOR]

In South Korea, an unprecedented heatwave occurred in August 2016. That year, sea surface temperature (SST) in the Korean marginal seas (KMS) was remarkably higher than that in climatology, and upper‐level anticyclonic circulations favoured this heatwave. More recently, similar conditions that can expect the frequent occurrence of heatwaves prevailed in August 2022. However, the opposite extreme phenomenon, heatwave days below climatology, occurred with the emergence of a stationary rainband in South Korea. This study aims to understand the two different behaviours of heatwaves in Korea under the same hot SST conditions in the KMS. The most pronounced difference between these two extreme cases is the existence of moisture flux related to the western North Pacific summer monsoon (WNPSM). Although the WNPSM is not generally highly correlated with temperature and precipitation in Korea, their relationship is particularly strong when hot SST conditions develop in the KMS. This SST condition, combined with the WNPSM, plays a role in determining this anomalous moisture flux in Korea. Enough tropical moisture flux occurred to build a stationary front with declining heatwave days during August 2022. In contrast, extremely dry and hot weather was more likely in 2016 due to inactive moisture fluxes from the western North Pacific. [ABSTRACT FROM AUTHOR]