QUASI-BIENNIAL AND SEMIANNUAL OSCILLATIONS IN EQUATORIAL WIND FIELDS CONSTRUCTED BY DATA ASSIMILATION

Since October 1991, the U.K. Meteorological Office has used the techni que of data assimilation to produce global meteorological analyses of the troposphere and stratosphere. These analyses are constructed from conventional meteorological measurements in the troposphere and lower stratosphere, and temperature soundings from NOAA polar-orbiting satel lites. The analyses show very clear quasi-biennial and semi-annual osc illations in equatorial winds. The quasi-biennial oscillation has its maximum amplitude of about 20 ms-1 near 30 hPa, and exhibits descendin g easterly and westerly shear zones. Analysed winds compare well with radiosonde observations in the lower stratosphere. The semi-annual osc illation has its peak amplitude of about 30 ms-1 near the stratopause. Winds near the equatorial stratopause show strong longitudinal variat ions on a planetary scale during both the westerly and easterly phases of the semi-annual oscillation.