Discussion Of The Distribution Characteristics And Influence Facts Of Stratospheric Quasi-zero Wind Layer In The Northern Hemisphere

The data employed for this study come from the ERA-Interim reanalysis provided by the ECMWF for the period from March 1979 to February 2015 at a total of 36 years. They include the daily mean and the monthly mean zonal wind, meridional wind, temperature, and geopotential height data on 37 pressure surfaces from 1000 hPa to 1 hPa,with a horizontal resolution of 1.5°×1.5°. In this paper, I have used the composite analysis and harmonic analysis method, to discuss the distribution characteristics of stratospheric quasi zero wind layer (QZWL) and the influencing factors. Main conclusions are as follows:(1) The distribution characteristics of lower stratospheric zonal wind in the northern hemisphere is that easterly wind is prevailing in the southward of 20° N all the year round and the wind in middle-high latitude has obvious seasonal change. During the winter, westerly wind is prevailing in middle-high latitude, and the westerly wind is strongest in a year. During the summer, except for the wind at 100 hPa, easterly wind is prevailing in the stratosphere and is strongest in a year. Spring and autumn are the season transition period, the distribution characteristics of stratospheric zonal wind in spring and autumn is about the same as the wind in winter, except that it has weaker westerly wind in middle-high latitude and stronger easterly wind in low latitude.(2) Except the area from 100° E to 100° W in the middle and low latitude in the lower stratosphere, along a certain latitude, the distribution of QZWL does not change much with longitude. With the change of latitude, the distribution of QZWL varies with different seasons. In winter, QZWL is mainly located in the area from 10°N to 20°N in low latitude on the pressure levels between 50 hPa and 100 hPa, mostly between 50 hPa and 70 hPa. In Spring, QZWL is located in in the area from EQ to 30° N between 30 hPa and 100 hPa, mainly from 20° N to 30° N. In summer, QZWL is mainly located in the northward of 30° N in middle-high latitude between 50 hPa and 100 hPa, mostly in the nearby of 70 hPa. In autumn, QZWL is mainly located in area from 20° N to 40° N in middle and low latitude between 50 hPa and 100 hPa.(3) In the northern hemisphere, the distribution of zonal wind is in a winter type from November to March in the following year and in a summer type from June to August. Under the climate average state, the conversion time of zonal wind from a winter type to a summer type is from April 8 to May 18, and that of zonal wind from a summer type to a winter type is from August 21 to September 21. The latitude location of QZWL varies with the season conversion of zonal wind.(4) When we divide QBO into the westerly phase and the easterly phase using the sign of equatorial zonal wind on different pressure levels, no matter whether it’s in summer or in winter, the distribution of stratospheric zonal wind in low latitude will change. The divisions of QBO phases mainly influence the distribution of QZWL in low latitude. In winter the distribution of zonal wind changes with the level selected. When the division of QBO phases is changed, the location of QZWL will change,as well is the wind beneath or above the QZWL. In summer, no matter how we divide the QBO phases and no matter what phase the QBO is at, QZWL in middle-high latitude is always located in the northward of 30° N between 50 hPa and 100 hPa.(5) Changes in the winter stratospheric meridional temperature gradient make the closed zero wind line often appear in the subtropical area from 100° E to 100° W. In June and July, the changes of stratospheric meridional temperature gradient make QZWL in middle-high latitude move northward, and in August they make the easterly wind in high latitude decrease and turn into westerly wind. During the spring and autumn, the stratospheric temperature will undergo a adjustment, the temperature changes in spring make for the formation of QZWL in middle-high latitude, and that in autumn in favor of moving QZWL from middle latitude to low latitude.(6) During the winter, the transportation of planetary wave eddy flux is strongest. The divergence anomaly of planetary wave EP flux for wavenumber land 2 in high latitude in the lower stratosphere accelerates stratospheric westerly wind, making QZWL can not be formed in middle-high latitude. During the summer, the transportation of planetary wave eddy flux is weakest. The weak convergence anomaly of planetary wave EP flux for wavenumber 1,2 and 3 in middle-high latitude in the lower stratosphere prompts QZWL move downward. The transportation of planetary wave eddy flux in spring and autumn is stronger than in summer and weaker than in winter. And in autumn the transportation of planetary wave eddy flux is stronger than in spring. During the spring, the weak convergence anomaly of planetary wave EP flux for wavenumber 1,2 and 3 in middle-high latitude in the stratosphere is beneficial to the formation of QZWL in middle-high latitude. During the autumn, the divergence anomaly of planetary wave EP flux for wavenumber land 2 in high latitude in the stratosphere makes QZWL can not be maintained in high latitude.