The intraseasonal oscillations(ISOs) were first found in tropical regions.However,the ISOs are not limited in tropical areas,and in the subtropical regions and middle and high latitude regions,the strength of the ISOs is also very strong.There are some obvious differences of ISOs between the tropical and extratropical areas,including the typical oscillation period,three-dimensional structure,and propagation characteristics.The existing study does not involve the winter ISOs in the middle and high latitude in the north of the Pacific.In this paper,the NCEP/DOE Reanalysis data(geopotential height,air temperature,zonal wind,meridional wind,and so on,at pressure levels) are chosen to study the intraseasonal variations and ISOs of the geopotential height in the north of the Pacific in the troposphere,especially in the upper troposphere. The study results show that the strength of the intraseasonal variation of the geopotential height in the north of the Pacific increases with the height in the troposphere,and the strongest center is located at 250 hPa.The dominant intraseasonal period of the geopotential height is about 10—40 days.The EOF decomposition method is used to analyze the distribution of the 10—40 day filtered geopotential height.The first mode of EOF shows that the oscillation center is located in the northern Pacific and Arctic Oceans,and the second mode of EOF presents the anti-phase oscillation between northeast Asia and northwest USA.101 positive oscillation events and 95 negative oscillation events occurred from 1979 to 2012.The composite analysis is used to study the propagation characteristics of the oscillation.Based on the time series of the first EOF mode,the 10—40 day filtered geopotential height moves westward:the negative center first appears in the northern Pacific,then moves to Asia;at the same time,a positive geopotential height moves from the Americas to the west;and the leading-lagging correlation coefficients show that the first EOF mode is related to the second EOF mode,and in fact,they are the results of the different phases of the geopotential height oscillation.Different from the intraseasonal time scale,the synoptic time scale(shorter than 10 days) geopotential height propagates eastward.The intraseasonal oscillation of the geopotential height occurs along with the evolution of the Pacific ridge and East Asian trough.It is worth noting that in the peak(or break) day of the intraseasonal geopotential height,the distribution of the synoptic geopotential height is similar to that of the 10—40 day filtered geopotential height,which strengthens the Pacific ridge and East Asia trough. The mechanism of the geopotential height ISOs is further analyzed.Due to the relationship between the geopotential height and vorticity,the vorticity equation is diagnosed.The results show that the 10—40 day filtered vorticity tendency is strongest at the 250 hPa level.The vorticity tendency is determined by the relative vorticity advection,geostrophic vorticity advection,vertical vorticity advection,tilt item and divergence term.In the intraseasonal time scale,the geostrophic vorticity advection contributes the most greatly,and from the low to upper troposphere the distributions are similar.The divergence term has the same magnitude as the geostrophic vorticity advection,but for the vertical structure the divergence term in the upper troposphere is opposite to that in the low troposphere.The divergence term strengthens the oscillation of the geopotential height in the upper troposphere,and weakens it in the low troposphere.In the peak day of the geopotential height,the vorticity tendency is negative in the west of the key regions,namely positive height tendency,which guides the wave to spread toward the west.Although the relative vorticity advection has a negative contribution to the vorticity tendency,the geostrophic vorticity advection and divergence term are negative in the west of the key regions,and positive in the east of the key regions.Therefore,the geostrophic vorticity advection and divergence term are the main reasons for the westward propagation of the geopotential height ISOs.