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The effects of external forcing variation on the intrinsic variability in the upper-layer circulation occurring within the East Sea (Sea of Japan) and its physical mechanism are analyzed using numerical experiments. In this study, the experiments were conducted with climatological annual/monthly mean forcings (constant/seasonal forcings). The intrinsic variability is mainly distributed in the meandering regions around the main current path with the comparatively large variability limited to the southern region. The reason of greater intrinsic variability mainly in the southern part of the East Sea than in the northern part is that more energy is required from external forcings to change the thicker upper layer formed in the northern part due to seasonal forcings (strong wind stress and surface heat flux). Although the experiments show slight differences, westward propagation of the Rossby wave appears in areas where the variability is large. The transport of the eddy momentum flux associated with the Rossby wave modulates the strength of the eastward jet and the north–south shift of its axis. Among the external forcings, the volume transport through the Korea/Tsushima Strait is the most important driver of intrinsic variability, and wind stress plays an important role in expanding and strengthening intrinsic variability. Significance Statement: Intrinsic variability is an important factor in understanding the total variability in the upper-layer circulation in the East Sea (Sea of Japan). However, the physical mechanisms of intrinsic variability remain unclear. This study investigates the physical mechanisms of intrinsic variability occurring within the East Sea. The intrinsic variability occurs mainly around the main current path, especially the meandering current. Based on numerical experiments, the mechanism of the intrinsic variability is represented by the fluctuation in the strength of the eastward current or the north–south movement of its axis caused by the Rossby wave. [ABSTRACT FROM AUTHOR]

Spatial and temporal distributions of Clear‐Air Turbulence (CAT) in the Northern Hemisphere were investigated using 41 years (1979–2019) of the European Centre for Medium‐range Weather Forecast Reanalysis version 5 (ERA5) data. We used two groups of CAT diagnostics to determine occurrence frequencies: (a) commonly used empirical turbulence indices (TI1, TI2, and TI3) and their components [vertical wind shear (VWS), deformation, ‐divergence, and divergence tendency], and (b) theoretical instability indicators [Richardson number (Ri), potential vorticity (PV), and Brunt‐Vӓisӓlӓ frequency]. The empirical indices showed high frequencies of moderate‐or‐greater (MOG)‐level CAT potential over the East Asian, Eastern Pacific, and Northwestern Atlantic regions in winter. Over East Asia, the entrance region of strong upper‐level jets showed the highest frequencies in TI1, TI2, and TI3 due mainly to strong VWS. The Eastern Pacific and Northwestern Atlantic areas near the exit region of jets had relatively high frequencies of these indices and also Ri. PV frequency was high on the southern side of jet primarily due to negative relative vorticity. Long‐term increasing trends of MOG‐level CAT potential also appeared in those three regions mainly due to warming in lower latitudes. The most significant increasing trend was found over East Asia, due to the strengthening of the East Asian jet and increased VWS due to the strong meridional temperature gradients in the mid‐troposphere induced by warming in the tropics and cooling in eastern Eurasia. These trends over East Asia, if continued, are expected to be of importance to efficient aviation operations across the northwestern Pacific Ocean. Plain Language Summary: Strong bumpiness of cruising aircraft can lead to physical injuries for crews and passengers, which is mostly occurred due to atmospheric turbulence by enhanced wind shear near upper‐level jet stream. Spatial characteristics of turbulence and their long‐term trends at cruising altitude in Northern Hemisphere were examined using the up‐to‐date global reanalysis data. The result showed that East Asia, Eastern Pacific, and Northwestern Atlantic regions have a higher frequency of turbulence than other areas. In particular, East Asia was found to be the highest frequency and the largest increasing trend of strong turbulence, which is collocated with a heavy air‐traffic region between East Asia and North America across the North Pacific Ocean. This implies that the potential of encountering turbulence over East Asia could be increased further, if the jet stream will be strengthened continually due to climate change in this region. Key Points: Spatial and temporal distributions of Clear‐Air Turbulence (CAT) indices for 41 years in the Northern Hemisphere are derived using the highest resolution of reanalysis dataThe CAT potentials are high in three regions: the East Asian, Eastern Pacific, and Northwestern Atlantic regions due to upper‐level jetsIncreasing trend of CAT potentials is the largest in East Asia due to warming in tropics and cooling over the Eurasian continent, which have a high impact on transpacific flights [ABSTRACT FROM AUTHOR]