Recent rapid Arctic sea ice loss was documented as combined results from anthropogenic forcing and climate system internal variability. However, the role of internal variability is not well understood. Here, we propose that the Asian-Pacific Oscillation (APO), an intrinsic atmospheric mode featuring out-of-phase variations in upper-tropospheric temperatures between Asia and the North Pacific, is one driver for autumn sea ice variability in the eastern Arctic. The positive summer APO favors warming of the mid-latitude North Atlantic sea surface temperatures. This warming persists to autumn and in turn triggers strong anticyclonic anomalies over the Barents-Kara-Laptev Seas and weak lower-tropospheric cyclonic anomalies over the East Siberian Sea, enhancing moisture transport into the eastern Arctic. Such changes consequently increase lower-tropospheric humidity, downwelling longwave radiation, and surface air temperature in the eastern Arctic, thereby melting sea ice. Hence, a recent tendency of the summer APO towards the positive phase accelerates autumn sea ice loss in the eastern Arctic.

The Beijing-Tianjin-Hebei (BTH) region of above 110 million people, located in North China, is confronted with high risks of precipitation-related disasters during the summer. Efforts to better understand diurnal variation characteristics of summer precipitation and associated physical driving processes are of vital importance to accurate forecast of short-time precipitation. Based on hourly gridded precipitation data at a fine resolution of 0.1° newly developed by China Meteorological Administration (CMA), we investigate diurnal variations of summer precipitation and their linkages with the topographical conditions in the BTH region for the period of 2008–2018. Summer precipitation amounts are shown to nonlinearly change with the topographical height, the largest values occurring at the altitudes of around 350 m in the BTH region. As a whole, diurnal variation of summer mean precipitation in the BTH region exhibits an S-shape structure with the peak appearing around 20:00 LST. While the mountainous precipitation largely triggers the precipitation peak with contribution from coastal and plain areas, the large precipitation in early morning is dominated by the precipitation over coastal and plain areas. Heavy and very heavy precipitation frequencies generally decrease with topographical height while light precipitation frequency increases in a nonlinear manner. The physical processes explaining the tight precipitation-topography linkages are also discussed. Our findings are expected to provide useful information for the improvement of short-time precipitation forecast over the BTH region.