Impact of El Niño on the delineation of tropospheric cooling due to volcanic eruptions

It is well known that there has been a close relation between sea surface temperature (SST) variations in the eastern equatorial Pacific (El Nino region) and tropospheric temperature variations in the tropics, with the amplitude of the SST variations averaging about twice that of the tropospheric temperature variations. In particular, it is shown in this paper that during 1960–1986 there was a correlation of 0.66 (significant at the 1% level) between SST in the region 0°–10°S, 180°–80°W and zonally averaged 850- to 300-mbar temperature in the equatorial zone one season later. In the north and south subtropics the maximum correlations between this SST and 850- to 300-mbar temperature were 0.48 and 0.52 (significant at the 5% level) at a lag of two seasons. In temperate and polar latitudes the correlations were a maximum (though nonsignificant except in the south polar zone) at lags of two to three seasons. There is the implication of a tropospheric thermal pulse originating at the equator and spreading poleward, decaying as it does so. On the basis of the magnitude of these lagged correlations in the tropics it appears that the reason tropospheric temperatures cooled following the Agung eruption in 1963 but warmed following the El Chichon eruption in 1982 is the occurrence of a very strong El Nino after El Chichon but only a relatively weak El Nino after Agung. When tropical tropospheric temperatures are adjusted by “subtracting out” the usual relation with equatorial SST (by displacing the tropospheric temperatures two seasons earlier with respect to SST and dividing the SST deviations from the mean by 2 before subtraction from the tropospheric temperature deviations), the magnitude of the tropical tropospheric cooling following El Chichon becomes similar to that following Agung (about 0.5°C) but is of shorter duration. When a similar adjustment procedure is applied to the five major volcanic episodes since 1880, the eruptions of the tropical volcanoes of El Chichon, Agung, Santa Maria-Pelee-Soufriere, and Krakatau are all indicated to have brought about a decrease in northern hemisphere continental surface temperatures of about 0.3°C, whereas the eruption of Katmai in Alaska is indicated to have decreased this temperature by only about 0.1°C. It is proposed that the reason the evidence for volcanically induced cooling of the Earth's surface has been so uncertain and controversial in the past is that such cooling may or may not be observed depending on the extent of sea surface warming in the eastern equatorial Pacific after the volcanic eruption.

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