BIENNIAL, QUASI-BIENNIAL, AND DECADAL OSCILLATIONS OF POTENTIAL VORTICITY IN THE NORTHERN STRATOSPHERE

Seasonal and interannual variations of the northern hemisphere stratos phere on the 600 K isentropic surface (20-35 hPa) are investigated by using observations for 1964-1996. An area diagnostic is defined in ter ms of Ertel's potential vorticity (PV), which measures the area enclos ed by PV contours as a function of equivalent latitude and is not tied to the spherical coordinate system. Data from all seasons are examine d for oscillations with periods of 6 months to more than a decade. Spe ctral analysis of PV at equivalent latitudes from 20 degrees N to the pole reveals several possible signals: (1) In addition to the well-kno wn annual cycle of the polar vortex there is a striking semiannual osc illation of PV in middle equivalent latitudes. At 34 degrees-53 degree s N the semiannual oscillation is larger than the annual cycle. This o scillation arises from the formation of a surf zone in winter, due to planetary wave breaking, superposed on the annual cycle. (2) A signal associated with the quasi-biennial oscillation (QBO) of the equatorial stratosphere, with average period slightly less than 30 months, is st rongest at low equivalent latitudes and is apparent in middle equivale nt latitudes up to 67 degrees N. Through interaction with the annual c ycle, oscillations with periods of 20.2 and 8.5 months are generated. These three spectral peaks at approximately 30, 20, and 8.5 months hav e been observed in ozone and wind data and are seen here, for the firs t time, in PV. (3) At middle to high equivalent latitudes, on the othe r hand, we observe biennial and associated 8-month oscillations relate d to the occurrence of stratospheric warmings and strong, undisturbed vortices in midwinter to late winter. The data record is remarkable in that, at high equivalent latitudes, strong and weak polar vortices al ternated from year to year, producing the biennial signal and a signif icant negative lag-1-year autocorrelation. The addition of an 8-month harmonic generates a much better fit to the sharp late winter anomalie s. (4) South of 60 degrees N a spectral peak at 10.6 years is observed , with the dearest signal near 20 degrees N. This spectral peak may be caused by the solar cycle, or it may arise through interaction of bie nnial and quasi-biennial oscillations.