STRATOSPHERIC WARMING DUE TO AGUNG, EL CHICHON, AND PINATUBO TAKING INTO ACCOUNT THE QUASI-BIENNIAL OSCILLATION

Radiosonde data are used to estimate the impact on stratospheric tempe rature of Agung, El Chichon, and Pinatubo volcanic eruptions, taking i nto account the quasibiennial oscillation (QBO). This is done by compa ring the temperature variations following the three eruptions with the average quasi-biennial variation in temperature obtained by applying the superposed epoch method to the 13 QBO cycles (centered on Singapor e 50 mbar east wind maxima) between 1958 and 1996 which do not include these three eruptions. Confidence estimates are based on the 2-standa rd deviation value of the 13 comparisons. Shown are estimates of the v olcanic impact and approximate 95% confidence intervals in 300-100 and 100-50 mbar layers of polar, temperate, subtropical, and equatorial z ones (and the globe as a whole), as well as in north subtropical and e quatorial zones at pressure surfaces of 50, 30, 20, and 10 mbar. Based on this procedure, and ignoring other possible influences on stratosp heric temperature, all three eruptions warmed the tropical low stratos phere by nearly 2 degrees C, with the largest warming 3 degrees C at 5 0 mbar in the equatorial zone following Agung and Pinatubo. Above 50 m bar, the volcanic warming usually decreases with height in the tropics , an exception being the 2 degrees C warming of the 10 mbar surface in the north subtropics following El Chichon. While the volcanic warming is most frequently a maximum 2 seasons after the eruption, the warmin g duration can be 2 years. In the polar zones there is evidence of coo ling of stratospheric 300-100 and 100-50 mbar layers following the eru ptions. Comparisons are made with other volcanic warming estimates.