NEP should be also viewed in the context of long-term variability of main parameters. (2016) showed distinct synoptic-scale eddy regions at 850 hPa in winter in the eastern and western Pacific. By using the EOF computation of NCEP/NCAR reanalysis data ( Kalnay et al., 1996) for 1948–2011, Xia et al. Some of the differences are because of specifics of atmospheric forcing that generates a separation of the North Pacific (NP) in winter into the eastern and western parts. It is unclear why marine fog often occurs in winter in the NEP. However, the MFF over the northeast Pacific (NEP) is higher than the MFF over the NWP, with a maximum of 11% ( Figure 1). In a cold season (November–February), fog rarely occurs over the NWP, and the MFF is close to zero, under the influence of the cold winter monsoon from the Asian continent ( Wang, 1985). (2015) suggest that the position and orientation of the NPSH is the most important factor influencing the MFF in the NWP. (2013) indicate that the strengthened Okhotsk high and suppression of the northward extension of the northern Pacific surface high (NPSH) were responsible for the declining trend of MFF during 1931–2010 at Kushiro, Hokkaido, in July. The large-scale circulation associated with marine fog in the NWP has been analyzed in previous literature. The annual marine fog frequency (MFF) is up to 23% ( Fu and Song, 2014) and reaches 59.8% in summer (June–August) ( Dorman et al., 2017). The midlatitude region of the northwest Pacific (NWP) is the foggiest area worldwide. The low visibility in marine fog may cause ship damages, casualties, and economic losses ( Gultepe et al., 2007). Marine fog occurs over oceans and coastal regions when tiny water droplets sustain in the atmospheric boundary layer and cause the degradation of atmospheric horizontal visibility to less than 1 km ( Wang, 1985). Dry air enhances longwave radiative cooling from the fog top, favoring cooling of the fog layer, gradually causing SAT to fall below SST. Composite analysis of the latter shows lower specific humidity above the inversion bottom compared to the former. Approximately 68% of all fog cases (42242) show positive differences between surface air temperature (SAT) and sea surface temperature (SST), while 32% are negative, during southerly winds. The lower (at 925 to 875 hPa) and stronger (up to 0.08 K hPa −1) inversion layer, compared with cloudy cases and the turbulence in the lower atmosphere (below 975 hPa), also promotes fog formation and evolution. The smaller upward latent heat flux ( ∼10 W m −2) compared to the surrounding area ( >60 W m −2) demonstrates that the moisture originates from the advection instead of local evaporation. The air near the sea surface in foggy areas is cooled by the downward sensible heat flux. Under such conditions, warm and moist air flows through a cooler sea surface and facilitates the formation of advection-cooling fog. Composite analysis shows that the eastern flank of the Aleutian low and the northwestern flank of the Pacific subtropical high jointly contribute to a northward air flow over the NEP. By synthesizing observations and reanalysis results from 1979 to 2019, this study investigates the atmospheric circulation and marine atmospheric boundary layer structure associated with marine fog over the NEP in winter. Observations show that the northeast Pacific (NEP) is a fog-prone area in winter compared with the northwest and central Pacific where fog rarely occurs in winter. 3Division of Atmospheric Sciences, Desert Research Institute, Reno, NV, United States.2Physics Department, Faculty of Science, University of Split, Split, Croatia.1Physical Oceanography Laboratory, Qingdao Collaborative Innovation Center of Marine Science and Technology, Ocean-Atmosphere Interaction and Climate Laboratory, Ocean University of China, Qingdao, China. Xinbei Li 1, Suping Zhang 1*, Darko Koračin 2,3, Li Yi 1 and Xin Zhang 1
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