Northern Hemisphere stratospheric winds in higher midlatitudes: 1 longitudinal distribution and long-term trends

9 The Brewer-Dobson circulation (BDC, mainly meridional circulation) is very 10 important for the stratospheric ozone and, thus, for the overall state of the stratosphere. There 11 are some indications that the meridional circulation in the stratosphere could be longitudinally 12 dependent, which would have an impact on ozone distribution. Therefore, we analyze here the 13 meridional component of the stratospheric wind at northern middle latitudes to search for its 14 longitudinal dependence. The analysis is based on the NCEP/NCAR-1 (National Centers for 15 Environmental Prediction and the National Center for Atmospheric Research), MERRA 16 (Modern Era-Retrospective Re-Analysis ) and ERA-Interim (European Centre for Medium- 17 Range Weather Forecasts (ECMWF) Re-Analysis Interim) reanalysis data. The well- 18 developed, two-core structure of strong but opposite meridional winds, one at each 19 hemisphere at 10 hPa at higher northern middle latitudes, and a less-pronounced five-core 20 structure at 100 hPa, are identified. In the peak areas of the two-core structure the meridional 21 and zonal wind magnitudes are quite comparable. The two-core structure at 10 hPa is 22 practically identical for all three different reanalyzes in spite of the different time periods 23 covered. The two-core structure is not associated with tides. However, the two-core structure 24 at the 10 hPa level is related to the well-pronounced Aleutian pressure high at 10 hPa. Zonal 25 wind, temperature and the ozone mixing ratio at 10 hPa also exhibit the effect of the Aleutian 26 high, which thus affects all parameters of the northern middle stratosphere. Long-term trends in the meridional wind in the “core” areas are significant on the 99% level. Trends are 28 negative during the period of ozone depletion development (1970-1995), while they are 29 positive after the ozone trend turnaround (1996-2012). They are independent of the Sudden 30 Stratospheric Warming (SSW) occurrence and the Quasi-Biennial Oscillation (QBO) phase. 31 The influence of the 11-year solar cycle on stratospheric winds has been identified only 32 during the west phase of QBO. The well-developed two-core structure in the meridional wind 33 illustrates the limitations of application of the zonal mean concept in studying stratospheric 34 circulation.


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Stratospheric winds play an important role in stratospheric chemistry through the 39 transportation of long-lived species, but they could also create transport barriers -which could 40 isolate the polar vortex in winter (Shepherd, 2007(Shepherd, , 2008. Simultaneously with the chemical 41 processes, the trace gas distribution modulates the radiative forcing in the stratosphere. The 42 changes of stratospheric wind, namely the strengthening of the westerly polar vortex and its 43 poleward shift, are coupled with ozone depletion and temperature changes (Scaife et al., 44 other parameters in the stratosphere based on the long-term re-analysis data serieswhich is 80 the aim of this paper. 81 Our study of longitudinal distribution of meridional and zonal wind, which we found 82 to be substantial, should reveal where the meridional wind is a substantial component of the  To test the temporal stability of longitudinal distribution, long-term trends at latitudes 91 of the most pronounced longitudinal structures are calculated. Ozone concentration in the 92 northern middle latitudes changed its trend in the mid-1990s (e.g., Harris et al., 2008). Since references herein). Impact of these phenomena on stratospheric circulation, particularly on the 104 observed longitudinal structures in meridional wind, deserves attention and analysis. 105 This paper focuses on two topics: 106 (1) Longitudinal distribution of the meridional wind component at different pressure 107 levels and the possible reasons for its behavior. Therefore the longitudinal distributions of 108 geopotential height and zonal wind component will also be calculated. This will be 109 accompanied by trend analysis of observed longitudinal structures. The results are described 110 in Section 3.1. Long-term trends in the longitudinal distribution of meridional wind are also 111 examined and the results are presented in Section 3.2.

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(2) Trend analysis of stratospheric total horizontal wind and meridional component 113 with connection to QBO, SSW (mainly wave driven) and solar activity. The results are 114 described in Section 3.3.

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The structure of the paper is as follows. In Section 2, the data and methods are 116 described. Then, in Section 3, the results of analysis are shown and, in Section 4, they are 117 briefly discussed. Section 5 summarizes conclusions.

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The NCEP/NCAR reanalysis was described in detail by Kistler et al. (2001). This reanalysis 134 provides data from 1948 onwards (but the data is more reliable from 1957 onwards, when the 135 first upper-air observations were established) and from 1979 onwards, due to the start of 136 satellite data assimilation. Data is available on the 2.5° to 2.5°grid at 00, 06, 12 and 18 UTC.

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Vertical resolution is 28 levels with the top of the model at 2.7 hPa. The NCEP/NCAR  The trend analysis is focused on middle latitudes (50°-60°N), again at the pressure 166 level of 10 hPa, in order to investigate the behavior of wind in the two-core structure area. We 167 also analyze the connection between QBO, SSW and solar activity versus dynamics 168 (stratospheric wind) 10 hPa. In trend analyzes we have used either total horizontal wind or v 169 (meridional) components separately. The total horizontal wind speed is calculated from 170 gridded u (zonal) and v (meridional) components.

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The selected latitudes are separated into four sectors (100°E-160°Epoleward wind 172 core, 160°E-140°Wthe sector of the Aleutian height, 140°W-80°Wequatorward wind core 173 and 80°W-100°Ethe sector not affected by the two-core structure, see Fig. 1). 174 We look for trends or differences between different groups in each sector at 10 hPa.

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The statistical significance threshold of trends has been set on the 95% level, which is the 176 standard significance level for analyzes in meteorology (wind, temperature, etc.), and in trend according to QBO (east or west QBO phase) or solar cycle influence (solar maximum years 179 and solar minimum years) and for the trend analysis we divided data into two periods  1995, with decreasing ozone, and 1995-2012, with increasing ozone). We compute trends 181 separately for all these groups with a significance threshold of 95% or 99%. five-core structure, which is much less pronounced than the two-core structure at 10 hPa. The 199 same analysis as in Fig. 1 is shown in Fig. 3 for July at 10 hPa. not display any well-pronounced structure and, therefore, no pronounced structure is 225 developed in meridional wind (Fig. 3). At 100 hPa on the western hemisphere (not the eastern 226 hemisphere) the distribution of geopotential height resembles the five-core structure in winds in Fig. 2 but, again, this structure is much less pronounced than that at 10 hPa (not shown 228 here).   The trends are significant on the 99% level (in a few cases only on the 95% level) in 264 the two sectors where the core structure occurs (100°E-160°E and140°W-80°W). There are 265 only a few significant trends (95% level) in the other two sectors. There are generally stronger 266 negative significant trends (99% level) in Table 1 than in Table 2

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The results on longitudinal distribution of the meridional and zonal components of 289 stratospheric wind show that the meridional wind forms a well pronounced two-core structure 290 at 10 hPa in winter. This two-core structure is revealed by NCEP/NCAR, ERA-Interim and 291 MERRA reanalyzes in a very similar form, despite the different time periods used (Fig. 1).

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The wintertime longitudinal distribution at 10 hPa can be explained neither by diurnal, nor by 293 semidiurnal tides, because there are no differences between the longitudinal distribution of 294 meridional winds at 00, 06 and 12 UTC (Fig. 4). However, the geopotential height analysis 295 reveals the reason for this longitudinal distribution. The well-developed large Aleutian high at 296 10 hPa in Fig. 5 can block the zonal flow (see Fig. 6) and pushes the winter eastward winds to  (Table 2). We can connect this with 344 changes of ozone trends. The trends in core structure areas are significant (mainly 99% level) 345 for all four SSW/QBO combinations (Table 2) as well as for all years trend (Table 1). In areas 346 not containing the core structure, more significant trends (95% level) occur for years with  winds, one at each hemisphere (eastern and western) at 10 hPa, and a much less pronounced 370 five-core structure at 100 hPa. All three reanalyzes provide the same pattern. In summer, such 371 a well-pronounced core structure is absent. The two-core structure at 10 hPa is not caused by 372 tides, as no differences exist between 00, 06 and 12 UTC results. We have identified the explaining qualitatively the two-core structure in the longitudinal distribution of meridional    (1970-1995 and 1996-2012). Major SSW-only years when the major SSWs