The quasi-biennial oscillation and the El Niño Southern Oscillation

E. Linacre and B. Geerts

9/'98

The QBO is a regular variation of zonal (i.e. east-west) stratospheric winds above the equator (Section 12.3). The ENSO, on the other hand, is highly irregular and involves near-surface winds as well as the ocean. Both oscillations have a clear effect on the frequency of hurricanes in the Atlantic, amongst others.

Notwithstanding their obvious differences, there appears to be a weak correlation between the QBO and the ENSO. An examination of the QBO signal over the Atlantic and Africa shows a correlation with the Southern Oscillation Index in the early part of the year (1). Strong La Niña events tend to occur when the QBO is in its westerly phase over the equatorial Pacific.

A record of SOI values for 1876 - 1995 reveals that the ENSO can be decomposed in two cycles: a quasi-biennial rhythm and a low frequency (5-10 year) variation (2). Both signals were notably weak during 1921-41 (when ENSO activity was suppressed), and both were strong prior to 1920 and after the mid 1960s. The quasi-biennial ENSO rhythm appears to be a harmonic oscillation in equatorial Pacific atmosphere-ocean system, and it was in sync with the QBO in 1879-99 and 1963-83. For the quasi-biennial ENSO rhythm, sea-level pressures (the SOI) and SST values (El Niño) are notably well correlated, in particular in 1879-99 and 1963-83.

 

References

(1) Jury, M.R. , C. McQueen and K. Levy 1994. SOI and QBO signals in the African region. Theor. Appl. Climatol., 50, 103-15.

(2) Allan, R., J. Lindesay and D. Parker 1997. The global ENSO picture: interannual and decadal variability. Paper to 7th Conf. Climate variations, Long Beach, Calif., February (Amer. Meteor. Soc.), J24-J27.