Beitr. Phys. Atmosph., February 1994, p. 3-13 Article 0005-8173/94/01 0003-11 $ 3.00/0 Vol.67,No.1 3
Northern Hemisphere Tropospheric Mid-Latitude Circulation after Violent Volcanic Eruptions
by H.-F. GRAF, J. PERLWITZ and I. KIRCHNER
Max-Planck-Institut für Meteorologie, Bundesstraße 55, 20146 Hamburg, Germany
(Manuscript received September 3, 1993; accepted January 26, 1994)
Abstract The strengths of the polar stratospheric vortex and geopotential height anomalies of the 500 hPa layer are studied that are observed after recent violent volcanic eruptions. After all tropical eruptions the polar stratospheric vortex was intensified. The tropospheric anomaly patterns after tropical eruptions are very similar to those of winter months with a very strong stratospheric vortex, irrespective whether volcanically forced or not. Hence, if they have any effect on the wintertime tropospheric circulation, tropical eruptions seem to force a natural mode of the stratospheric winter circulation which is associated with a specific response of the tropospheric circulation with maximum amplitude over the North Atlantic and adjacent continental regions. Therefore, it remains difficult to give statistical evidence of volcanic impact on climate on the basis of the few observations after these rare events. A combination of observational studies and model experiments may help to overcome these difficulties in the future.
Zusammenfassung Die troposphärische Zirkulation in mittleren Breiten der Nordhemisphäre nach starken Vulkanausbrüchen Die nach den jüngsten starken Vulkaneruptionen beobachteten Stärken des polaren Strat- osphärenwirbels und der Anomalien der geopotentiellen Höhe der $00 Pa Fläche werden untersucht. Nach allen tropischen Eruptionen war der polare stratosphärische Wirbel verstärkt. Die troposphärischen Anomalieverteilungen nach tropischen Eruptionen sind sehr ähnlich zu jenen in Wintermonaten mit einem sehr starken stratosphärischen Wirbel, unabhängig ob dieser vulkanisch angetrieben ist oder nicht. Das bedeutet, daß wenn sie überhaupt einen Effekt auf die winterliche troposphärische Zirkulation haben, tropische Eruptionen einen natürlichen Mode der stratosphärischen Zirkulation anregen, der mit einer spezifischen Reaktion der troposphärischen Zirkulation mit besonders starker Amplitude über dem Nordatlantik und den angrenzenden kontinentalen Gebieten verbunden ist. Deshalb bleibt es schwierig, auf der Grundlage der wenigen Beobachtungen nach diesen seltenen Ereignissen den statistischen Nachweis des Einflusses von Vulkanen auf das Klima zu erbringen. Eine Kombination von Beobachtungsstudi- en und Modell-Experimenten kann bei der Überwindung dieser Schwierigkeiten in der Zukunft helfen. I
Introduction aerosol-containing stratosphere in lower latitudes results in stronger westerlies near the polar circle, The recent eruption of the tropical volcano Groisman suggested that the westerlies were caused Mt. Pinatubo in June 1991 revived the public inter- by polar cooling. Following RM, the westwind est in the effects of violent volcanic eruptions on anomalies "penetrate down" into the troposphere I climate. Concerning wintertime surface tempera- and bring warm and moist air from the Atlantic over I ture effects, Groisman (1992) and Robock and Mao continental Eurasia. Here, and over the western (1992, RM hereafter) described a stable anomaly part of North America positive temperature anoma- pattern after volcanic eruptions. While RM suggest- lies are recorded during winters after violent volcan- ed a mechanism according to which heating of the ic eruptions. Temperatures decline over the east of
J 4 H.-F. Graf et al. Beitr. Phys. Atmosph.
North America and Greenland due lo the advection larger than 4. This leaves only six cases for which of Arctic air masses, and in the Northern Hemi- data are available (see Table 1). Observational data sphere subtropics due to reduced solar radiation. cover the period from January 1957 (for the 500 hPa Such observation is not new (see e.g. Spirina, 1973, level) and from January 1963 (for the other levels) Loginov, 1984; Groisman, 1985; Graf, 1986), but it through December 1989. never found much resonance in the literature. We use monthly means of the geopotential heights Here we shall investigate the tropospheric geo- of lower tropospheric pressure layers (850, 700 and potential height anomaly fields, observed for the 500 hPa) north of 20o N. We calculated anomalies of few cases of recent violent volcanic eruptions for the geopotential heights relative to the 1982-1989 which data have been available. We want to mean, since for the case of the winter of 1991/92 we examine whether the mechanism of advection of only have anomalies for this reference period. The warm moist air to the continents due to "downward data are based on the analyses of the National penetrating westerly winds", as proposed by RM, Meteorological Center of the U.S. Weather Service worked in any of these cases. In addition, the and were transformed to a 5o X 5o grid at Max- observed patterns of geopotential anomalies follow- Planck-Institut fur Meteorologie. The stratospheric ing the volcanic eruptions shall qualitatively be data are monthly means of the height of the 50 hPa compared with model simulations. layer on a 10°x 10o grid. We obtained these data The process of volcanic aerosol forcing was mod- from the Stratosphere Group of the Meteorological elled in a perpetual January General Circulation Institute of the Free University Berlin. Model experiment (Graf et al., 1992) including long- Four of the eruptions that we considered in our and short-wave radiative effects of the aerosol. In study are tropical ones, and only two (Bezymianny these simulations, differential stratospheric heating and St. Helens) exploded in higher northern lati- I due to the absorption of thermal and solar radiation tudes. RM studied Bezymianny's climate impact not by the stratospheric aerosol led to significant tem- before the winter of 1957/58, arguing that aerosol perature anomaly patterns in the lower troposphere from a high latitude source is transported but slowly of the Nor thorn Hemisphere. These pattern were into lower latitudes, where it can absorb solar very close to those found by RM and comparable to radiation and, thus, increase the meridional temper- those observed during the winters after the ature gradient during winter. St. Helens was not Mi. Pinatubo eruption. The mechanism can be considered by RM because of the low $02 concen- I explained with linear theory (e.g. Geller and Alpert, tration in the eruption plume. However, according 1980, Schmitz and Grieger, 1980). In case of a strong to McCormick and Trepte (1987), an increased polar vortex, vertically propagating tropospheric optical depth with a maximum of 'c = 0.06 was planetary waves are trapped more effectively in the observed over the North Pole during the year after troposphere than in periods with a weak vortex. this eruption. In addition, with its VEI of 5 and the Other numerical simulations with a similar forcing DVI of 500, it complies with the rule of RM andreas type (i.e. strong versus weak polar night jet) have therefore also considered in our study as an exam- the same effects (e.g. Boville, 1984) on the planetary ple of high latitude eruptions. Otherwise there wave patterns. would be no reason to include the 1956 Bezymianny Following RM, we used those volcanoes as key eruption in this study. For this eruption the relevant events which had either a Dust Veil Index (DVI) of data are simply unknown, and the DVI is very small at least 250 or a Volcanic Explosivity Index (VEI) (see Table 1).
Table 1 Volcanic eruptions considered in this study. DVI and VEI are taken from Tobock and Mao (1992), SO2 data arc from different sources ('Milan et al., 1985, 2Cadlc, 1980, 3Rosc at al., 1982. "Blunt cl al., 1991).
Volcano Latitude Longitude Date of Eruption DVI VEI SO2-Emission
Bezymianny 56o N 161o E March 30 1956 30 5 unknown Agung 8o N 116o E March 17 1963 800 4 12.0 M12 Fuego 14o N 91 o E October 10 1974 250 4 3.1 MI3 St. Helens 46o N 122o E May 18 1980 500 5 2,1 MY El ChichOn 17o N 93o E April 3 1982 800 5 7,0 M14 Mt. Pinatubo 15o N 120° E June 15 1991 1000 6 20.0 Mt'* Vol. 67, 1994 Mid-latitude circulation after violent volcanic eruption 5
Another aspect might be of interest concerning high Table 2 Pattern correlation coefficients (area weighted) latitude volcanos: The aerosol built up from such between the anomaly patterns shown in Figure 1a. To the eruptions is believed to remain in the stratosphere right of the matrix diagonal the area 90o W to 145o E, and to over the polar cap for a long time. This was the left the sector 145 o E to 90o W is used. observed, for instance, after the St. Helens eruption 63/64 75/76 82/83 91/92 in 1980 (McCormick and Trepte, 1987). If most of the aerosol remains over the pole, it may there 63/64 1.0 0.52 0.30 0.65 absorb thermal radiation. Thus, it will warm the 75/76 0.62 1.0 0.76 0.75 82/83 - 0.56 - 0.70 1.0 0.48 polar stratosphere, leading to a weaker polar vortex, 91/92 0.47 0.37 0.39 1.0 i.e. to an effect opposite to the one caused by tropical volcanos. Therefore, we also included in our analysis the first winters after these eruptions. geopotential height anomaly fields of the four winters displayed in Figure la. To the right of the Results matrix diagonal the PCC are given for the 90o W to 145o E sector, i.e. for the Atlantic and Eurasian We examined the height anomalies of the 850, 700 area, and to the left of the matrix diagonal the North and 500 hPa pressure levels observed during the Pacific and western North America (145o E to winters after strong volcanic eruptions. All anomaly 90 oW) area is used. In order to account for the patterns found are similar throughout the tropo- natural variability of planetary wave patterns we sphere, suggesting a barotropic response. The am- computed the PCCs allowing the patterns to shift up plitudes of the anomalies increase with height. to 10O in latitude and longitude. The maximum Here, we shall discuss the 500 hPa level results only, PCCs ("optimal PPC") are then given in Table 2. since the data set is longest for this level, it also The differences found in the PCCs for the different contains some of the possible effect of the 1956 regions can readily be explained by the impact of Bezymianny eruption. As a reference period we El Nico on the geopotential field in the mid- took the winters from 1982 to 1989 because this is latitudes of the Northern Hemisphere: During the also the reference period for the 1991/92 winter for very strong El Nico of 1982/83 the Aleutian low was which only anomalies were available. deep, while in the winter of 1975/76, when equatori- The observed height anomalies of the 500 hPa level al Pacific sea surface temperatures were low, an are shown in Figure 1 for the winters that immedi- anticyclonic anomaly developed in this region. The ately followed the tropical eruptions of Agung, two other winters had weak (1963164) or moderate El ChichOn and Pinatubo. For the extratropical (1991/92) positive sea surface temperature anoma- eruptions we studied the winters one year after the lies in the El Nico region and, therefore, negative eruptions. We decided to use the second winter and weak positive PCC, respectively. Thus the after the Fuego eruption, as that eruption was late in differences in the pattern correlations between the 1974, and normally it takes several months before single cases of tropical volcanic eruptions over the the initially gaseous SON is converted into sulphate North Pacific and western North America are aerosol (which interacts with radiation) and distrib- basically due to the impact of El Niflo on the uted quasihomogeneously in the tropics. Moreover, observed geopotential fields. For the Atlantic and the stratosphere responds but slowly to diabetic Eurasian region the El Nico influence is much heating anomalies produced by additional aerosol. smaller, and therefore the anomaly patterns are The pattern of the geopotential anomalies for the more similar for the single cases. Again, the PCC winter of 1974/75 (not shown here) has the same are weakest for any combination with the "Super basic features as the one for the winter of 1975/76, El Nico"-winter of 1982/83. but with smaller amplitude. The anomaly patterns over the North Atlantic and The 500 hPa height anomalies observed during the Eurasia agree well with perpetual January simula- winters immediately after the tropical eruptions are tions (Figure 2, right panel) of the climatic impact of very similar over the Atlantic and Eurasian regions, tropical volcanos (Graf et al., 1992). These patterns while those anomalies differ greatly between the are characterized by a low geopotential of the four winters over the North Pacific and the western 500 hPa layer in very high latitudes with a trough half of North America. Table 2 contains the pattern over Greenland during the winters after the Agung, correlation coefficients (PCC) between the 500 hPa Fuego, El ChichOn and Pinatubo eruptions. Over 6 Graf et al. Beitr. Phys. Atmosph.
(2) DJF 63/64 first winter of ter Agung DJF 75/16 second winter after Fuego
4 I. '.**. q .. .. . "\ 1. |:|.*.""- :;.: rip "0 .¢-e'u . ..." .'*.In . 4...... t' »~¢€».," ""*53?P,<"j"' ,iv - 40 H:.: ii . i .*p. :. . . - JL: E* ., 1 . #. I:2: a In .< _ 1:I: q I . Q l . w "I » I 41 J I' W go §§».u.q. . Q » 1 • . - . I . \ , .f . 1 VJ .l ::" s,-r : 1..xw3 `. . v . . r | I. \ . . l . • -: 11 .\ r l ';..~ r . . .r n . , f n ..». ~.'°- x. Q. l . . I ¢ ... * . _ I I | . " - *..;. \ I l 2% .. . .' » 'I \ .,. I | . l r"\ • . | I . . . . \ .. . 1 2 . • *\ ,.lf¢.'... .u A . . s • • 1' ;is». . ¢ ' I . 1 } n 1-._ r • 15 ° . *.4 . I t * | =: : ¢: .'. | o gc .I!-4°- . l I J \ q 1 l I > J o - I ..s I | .l . ..1. . I I 4 I . I f , I 1 . 4 \ . s . ¢. I 1 . " . I * 1 . A - r' .§.. r I . I ¢`l . U 4 Fu. v'*`"' _'\» r . /".»-"°`:" * \ _.¢.. "Di o • . . ¢ \ '\ .. . Q 1 \ fsr .. . I 1. r .. , . . . ¢ | • :R ' . s q . ' U .A | 4 -. s ¢4o{I • . I 1 . _ x' I . §,.:.: I a '\. : J . .; v -111- .. : \,, i, . . -..-.*' I 1 . rr . | 'lv . ; g ¢ I . f I ./' • .;,.-':- 4 ll .3 l . *1 n I 4 1... v | . . I 'I' \ " l . . .» I _ I | u 1 I h _ . • . . I . i. . I . g I .I . . I » . \ Au# .h.¢ .'¢ I , o. . I I ( I 1 ¢ . 1 I et*~é I \ . . .|.' . . , I '_ _Q . ' . . .. -2. .* .i- I . • . 1 t:u-1.l¢.''u,l l .;..: '. f • i 1 N • "\ . I u , . I v • .s . | "3... . ¥ I * Gang* .: . 'I ..;, . I \...;:: r w f \ I ,.,. :. 1 'G I :- .* : e . 1 . .I . 'QM i&*= . I ' a 1 n .:.\': *I .c i . I . I l (,' .of r I I . -. .\c"T :. HE . :.\*.. . .., : . s 5" 1 .. . I A ..I J. 4 . 1 ° l I p. . . : \ ...... * I Sal *52 :: ` 49 .-e . u . * r i I . . I 1 f E].itr f. 4 .. ¢.+ h . sI If .g l | I I I §2>N\.5-4 D .;. 3 I I :» | *h 1 S I . or .II . -q is ex i c .___ . i 3 . 'i . .I.I* y • \ I .r~ n `-. I .\ \ .Vu K < _ .f\° . .. s ~. . . I 35 e "E: 'l . v .J.l, \ .| . 1 I F *S *;.. :._.1g0._\ I - r ¢ .1 \ > °: . -4 / I .. . . 4 2 U."'-.__ . l 1 o G a *Q . . . . I .. <"ac 3. I . '\. II. '\_"! u .. r \ J . JM r. .. i - .0 l AS s \ . :=:~. \ a U S E. \ l 1:I*°l?*R \ I .S°. I • • L \ J . » (I _§\. . .I f l :: . "\ ..{ N . f . 3. x To, *2:R:... v * ; : j I . I . . .: L '°°~3 . I ..I \ *v-80 G ,|» . I, » L J . , . . .. 4 24§§.é \ 8.- • I _ : . I .-.).. I .-.,,_.b.. . I l .. I in L .:. U . ul || | 3 In;TIl"Fi;. I \ < \ | \ . r . |_ ae'I :l I I . l I . :- l • . .lr l * . of * I N.. "of . i ° or.. . . I 1. , 1-gpoc " .Fgig., Ag. %\r\ w I u . :. l I -:.:;:;"»:-:._ .r l I I* .".§.: . . I 3 of I ~:~ as. ' ...., I 4~ a l . , , . v \.. v . 2 A } 1 . . '" i 4 . . \ .. ' I f 's 4 ., . I 1 I s • '.' .`-. n _ . I * . .» Q . I x .:Ra i1' n, l I l. l \ . - I . . \ \"5'=; *IP "J, . u .- _.§» \ . .no " *" ;=w2e5e*'F7 1 a _\ . s .;!" .- "1 . s I . 1 1 x JI I °-. I s 3 | u *oh r .Ii '\ _. q . . )*.." '4 -. . l 9 . I ' .'. . lL\ 2 .l I .:.. : • ° * l. \ .. Q.. N :. . : : L .a. *I 1 . . ,. ,- I* l ==¥»;i'£°=;` ;,.. . j: E I .. . . .'.\ ic 5;'J~ I I »@ \ . . . :-::- . l*H»»h , ==~ .E s • Iv" P:j:1':;. . l . I 'a I 8\.-.M . ..£ -5' . .H. .,..,. . , . I r ».;:..j :-:." ... *. *-. A " . 1 » I . _ . . WI \ ` so *n . .. . y : - .| 4 . . . . . n . l. . j.j. . r . °°"1°l.mn-0 ."|\ . . "Q I a I a . l A . .a.
DJF 82/83 first winter after El Chi chon DJF 91/92 first winter after Pinatubo
.l : j:j;:., *bl aiH:§,' .: . 1 I Uh ll. | 7.;. g:::?::5§: F . " l c';l.,' '|°\u'|'| . . I2l2 1.'.. - :... r .g,;,.e *.. `:".. J ,*z' * 1. ,»I::: '- .2. i . * l,¢'@a¢. .,._.-: .4 . M ' |".' . ` __4,.,»¢)»¢~°-40-. ? . I I . A n _ 4 A 1. ..*"' ~.~"`\ . | go., . ,»-.L . (t ,. -?=:PE-:CI.9;-3'5"-. §:;.~i:* .*. 0 1-s;.-;:=; 1- ":: ' ..**:.'*..-°"l¢\¢l, I é ./' c ' ".°' °'.... -1 . I é 1 `*w ¢':3!" .:$"' |.: I *n I . o -1s,***='*§;¥> : 09 ofQ' a . ' *'».¢'-3 .:7.-:.: *\ \s 3 . *Q 5w».w-.:;' # ,...-5 ~.z» 1 \ iz ""~.-.1-q.< . '°*.* ¢.. l w .{."LE;:I,:I.,* . " ...... ~ I . . I E L _r a>§,;2='§i?"*><=e55..;=r === ). e.. I 1 I ? . r r'. . 7I * | .¢... ..,\.» "4 ..... - .y -. '-. . -..,;.'. Q-. . -"in »= .+ I . .:Q.§~ .: .. . | .¢L*. 1961 ' .0 ' •` '.Ra n. 1 . ;'- u f. ... °'!'r .s; .- . .5:~:' (~... - ;al':. • I **r 'U _.1'"r/.3 A a f/1 \ e. E' I I *1g4 "4 c . F"¢ I. .._ \ _,., . .»,=~ \g*}, I . *.I'Ij §.: h U 5 '~. .Q . m, 1 . "§i=é :.\ I, . I *.:. I . et. |. .q. .-.. I nx_. *\" 'Q 1 .l | I ;:, l., 9 • I In 1l I l »r .. f as \ • I A...¢=-#r.-Av» u 15 l »`: =I>' . |. : I.\ ,=+,a',§ Q 1 | . . . I 'ft . I \1 s go. 1 F '*i . *.,¢ 1 "~' c • §'.' .-» I Q | Q . | • II. E ¢ 1 :;'f'#3"" I l . I . 4 :-I? l 4o' e -.-L \ ` .>"". 0 1 5 I o Q . 4 .. o * . l. L _ . . f . j.]. I 101 *I l 53 1'.l' • I a . a F r 1' I . s "\ . I 14 . 1 ;,,, I ' In: .in: I . :." .. aL.u | n .n ' , - lg- . . • . go. . -: ..:. .» . - I Q "." 1 5 I .;. n I :-.' . .. .* .j.. | 1. .;» r • .. 'A .. a I .,f 1. I ,| '4'T» .4 • " I 3' s . NIH \ .1 N \ l • . < _.I I g.;.. 1 u . 4 1 • a ui . 1. in .. T.=.¢,..:: :». _ \ I .ii *Q \ It 40 :f ..-'*'*¥=1¢» v. :; - \ 4 o°°° "~¢ L z' 3 Y r .I a . 1 <1 ,l .1.1 ;. I , ..*' .I q l ¢..Z:. »9,5 I . > 3. -4 ¥. | .§ ».... , r*'°»~. 11 r ¢.'...jf' '~.*:» .. * . w 3, 4 . . l I .. 1\ 611 1 I .°. l 0 \ Q _ 4 ,. a . "z: I u .l r . . . rl" .. *.. r -. . 1 \ 1 I r .. » . . \,>* . _ I \ .ii ,§\ \ | a I .*-....- s . .. 1 u I :; ; or J , . a m.o I ¢ \ . . 1' is 1 \ . ¢»"* '_ . , • . . r j .;§ .A 4 . . I .. .n *'. I\. ;. " . . . I 1 a I g...... g.'• *S gr:.{, .. n I I . 1 ~=-<*= x . 6 ° .| -' I " . . \i. . s i.) al • -ea.-»...=;~ * I' -1-. . .3`. . a l I' . . ..= <+. . . I a '-ilu.-»~~"* §. 1 u . 1 I :1.Hir ' »;;-Q. | . ' _ I . • .r R ...... -c1-.. ' kg L"*) . | 4 .. \ 7.-. 1 . . . . b , .C (b) DJF 57/58 second winter of ter Bezymionny DJF 81/82 second winter after St. Helens * | -¥ * ' ..' . I 'Q a | .** ._._. Ur * ....» 0 . . , . i I ¢ | ... -£acl? *a ,w . . . l . I 1*\¢ . _..:' l': I II;°.l Q l*°, 2Ej Q | 'Y a I | • I i I H • » .- =:-'::..: . I l • I .. l . I • I .. , .1,,. .1. | s . I .rI>Q-5:i, . U ', ,| 4 A . 1 r *. iililat '|, Q* . I . I H . l l l l *.'*. • I 1 a .U 3'v.~»- I II. .*I'* 0 » *.: 7 I I . • 11*1 I I I.. 1* I 'FNEQ _ 1 \ 11 I ."* 'Q . | l . Fifi: 'I , Q I q 4 I • I i .3... . . u*' • - .u | l -I ,u . . I Q | t 4 ' *l • . '.. i z I . 5 l I s . ¢.¢.1 1 ::-':4-Q |: I n I I I .I 4 9* *. _ I .'* I F \ I • . . I 1 l l . I 11 I • .\ L=, IN: . a 1 l I 1 | . . | ' a I I l 0 1 -*:->1 1 . P . . l U .. 9 I"\ .. 'sl ¢ -:$ I I ."!. . /?"*. 'K . . I J . .. | *. . » H, ,5. a . . or' ". . . I l¢* . . ¢ *v 0 4 . _ '. I . Cr. 111.: H | .*= I 1» | * . q u * * . " u U . " 11 'I I . `\ I . I I *: "):.": \ ' 1 .J -:-* ,go . v ,.=::r u I 4 `-"'~=-.r . "£§:]> H ¥\\ c ,.. ,Ag | I `\;~»:~4»§¢.*'? *. I a ¢ | r" I c I . l I 1 : . "'°-. 1 | 1 1 | J d' * .. | - .s u I » , ., • | kw#-* q L . l r : l . . is 4 5 e 4 l\ A.-p I 9 • J I ; I I I I -QC ¢ » 1 . • 1 . . I .J , . f ¢ . l I l 1 I L . _ r ' - c '40 | I 1 1 \." I. 9 • » r u I . I . *. I /" \ ,1 f .I l. 0 5 . . | n a . I 1 1 ,=? r'. . . l ¢ . . I \ I I "*'\' 9 .° ".| 1 | • o n -: I . M 4 .05 * » U . I I i I 5 r \ • \ . . '.. • * u 1 . ' i 1* \ » q . 1 ,;;;:I°"~:=. 4 I | 1 I ¢ ..* . ...5 # ¥ | , » . ' *'I :r.-* ~@ `\ | i.45.!' ".:5 .| ; . ¥..'; , it' . no" r ,~° • | s lu 11 4 , » .;~ - . .I 3 \'b 1 » $4 .H-"*' f ., .\ .J ii . ; " J 41" Q 1 . g r :I I 1. \ . I ..... u?) - i., ,. .:. .j *we Q 1 -I5:f. .. J. , . . I ... .'. I n 12 :fig .,.\ 2: ..2 3122-; ;: :3<; . °" ; I . " > . "-1. T I -'C: . *) . E 5 .:":' . 6::-:.-:535'§¢.. x l .-.2 i» I 1 li-5. . . U -~r * ' w.. .I *_:.;;: r i I: : t> \ c-5g P+ I . 5, . It s¥nK§£= ' \ . u ' 5:.4' II":"2.2 r . - r ; '40-114 O*- ..:. _ , I:: ¢ ¢."'-..._-. i. .; .I +x a . *in 5? S. I-":.i:l* r 1 J. . :-, ¢ \ al '"\. 0 Q . go . L I-'-11. J 1- -NA r p '»~40 . \.2Z . 5 ,.8 1 I ... :§**; . . . ` '.=. . .,. ';!*. . I.' '-éEi» l .. 55. :,- .\ .I.:.\,'. .I .L . x,-" - f T?/I| ...w- P ...... !IL I. q:.'4l...'. . 4 ::: 1 . b v . \° :- . I pa .. 1 .:q.. »°l:»'*. a . . 'v `\. • 1. . . I a t"7 ¢ . \' ,.-,.5 l .W . , 4 I . • I"1 . .D *o G . l ¢ 9 . :°:*:¥": \ ¢ - . =s,' I I . . >-~"""*% • L I . j. , ` . u . >'* . "I-l¢ . I *. ;' I . I +-4g°"` 841 *Qr -r .....,.? * . . I * . . I .| .§..j.. . . . Sri '. | pa x" I or. :1 . Iv", ° l ' . . * * . : .1. ,. I 1 . n '~.4 =n-:-€' .. . I . .Ya l I *,| ' I \ 1 4 a 40 . I 1 . J a ,H I u \ l" I *R • . _ if' . . @ . . » ¢1\F\ . I Q I - - a L / . I I 1 . . l §._*. u u a s • . . I '*\ I I • 1 I I » l . 1 I l . I l I . . n s • I . a Q ?> I 1 N . 1 1 4 . . 0 . . » * A • . 'u 9 ** . . I . U .»" . l . . » I q 2 1 1 -. ° 1 I . . . . | 1 '~. . I l I u . . | . I | * ' 1 i Ur i In ' ._. . l §:." ,- av. .u I *Q I I I g .I Q | I J * - .. . _s ' . A I 'a o , n . I II.. .I *uO' . u ,I . - - .. :: I 2-' ..'.| lr 'o . . . . . \ Q /° 1 \j.IJ .. ' . "l.1:... | 1 I I.. 1 .-3513* l s. \ D . (c) DJF 56/57 first winter after Bezymionny DJF 80/81 first winter after SI. Helens _. an 1 . ~. . x *.1. so J?- "°~~'-'!: . ,...... *":¢:¢..'.T. 1'4\ 4 - . Figure 1 (continued) the eastern North Atlantic and West Europe, an aerosol with the observed SSTA of the EI Nico anticyclonic anomaly is prominent in all cases. This winter of 1982/83 (Figure 2, left panel). The Fuego anomaly extended all over the North Atlantic to the cloud eastern is by far the weakest of the tropical eruptions part of North America in the winter of 1982/ 83. under consideration. It was selected by RM because This is in good agreement with model calcula- it fits their rule of DVI >= 250 or VEI >= 4. An tions of Kirchner and Graf (1993) who conducted inspection of observed stratospheric temperatures GCM experiments combining an El ChichOn type (Figure 3) shows that the tropical temperature at 8 H.-F. Graf et al. Beitr. Phys. Atmosph. Ano EI Nina + V forcing 1500 [qpm].t: 99% Ano Volcano (V) forcing 2500 [pm] to 99% . a _ . Q 0 » 1 . l ' • I . . a I "U ¢ of • I I • . r .. * . »' 1 20 1 * . C l I I » 1 a .u I a ,, " . l • 9 • ' . 1 » I ,Ql' I . • . sm - * .1?~' . f is I l ~¢*'1.;'»?.-. 0 . | 0 I -'I 1 9 . l 4'° ii -\; :»,~.`,' 4 . . * C * I . ? |.' 9 *. • \ r . `.5. * \ E§§ ,so xi q r= .,J . I 'U' *Sq-9-aibnq 1 *I l • ' 'n 4 " y * * . . _. r'- ' 'A A" .' f ._I'. N • Q - \ 1 _ 1. • .-Curr ._l, - J.. v Gr \ re' 3 l 'Kiln h § . _- • l • * * I a 1 • u .-* . . - . .> " ' -G to# .:x . .. I . ' a 1 r 1 . , ,,,;~'-,- • * Q . . 20 r 20 1 'A '. ` ' ' - .'. . 1 " I .-1 0 ¢ 6 U * _ //> . of t _ 'I- I 1 Ja 20 |, • I • .. 114 .| . . I 2, , iv . »~ l . • J ,I \ • * • I .`} 4 1 . • | . 1 . . a l - \ ~,\ -.-f' I • I I l . 0 . * o I u . I I . I n . an 4 a 4 . . . u ¢ 1 s • n • I I 9 I I 1 Q 1 • '\ I • . Ii . U *- -\s l I | . 1 s *. g .: . I _ l. & *s l » f 0"\~.¢ • l . . 1 3. | a . . I . . 'I I 1 I n .Bur I" . ..` ,{7"'. - .\t HQ. \ u 1> -,> . P I 4 W | »* .V . a J 4; f \ . r u ~».?C u s r . J l I 1 . .of I .. 1*\ . • N 1 |. 1 "1 \. • r I .. 1 \` \ .r., ,- °\ I I , g .r .. » » L 1 \ ' pg ' c I 4 -n ¢ e o l! T » r! ' \ 1 '. » , I I Q . . \ .X . .\:. I * 0 . » l • I n ..| • Hni5 . I f 1. . . ] 1 x \_ . | • `¢ • ¢ . 11 . . | I `. 1 . • *I n u. l r" l » I 1 . ° I » 1 . -20 u o~ °» I P ,-1" , • I o 1 I C ' ,- - Q' . • _ I • . I I e . l .- . . ,r s .u o a s . ' . l » » u . - _' . . l • 9 _v I 48 v 1 1 u . ¢ I or . I . | a I f 'rP . | . .9 .. . . . - ¢ 60 . . U . \ . .1 - . . 4 ' 1 u* . NU . - r a ,6 . . _ 1 • . I . l ' » ' ' 20 . • . _ /" 20 s . . . . of' | 1 '. . ,J*»-*§» . a ' • ". A • K. » No ...|.., . '»I¢'0 • • . 1 » r s I u • 9 . | . • l .. l . .._ . pa l Figure 2 Mean anomalies of the height of the 500 Pa level over the Northern Hemisphere for 58 permanent January 1 simulations with El Nico and volcanic forcing (left panel, Kirchner and Graf, 1993), and with volcanic forcing only (right panel, Graf et al. 1992). The shaded areas indicate anomalies exceeding the 99 % confidence level for a local t~test. the 30 hPa level was slightly above normal, and the Discussion North Pole was very cold. Thus the meridional temperature gradient was enhanced producing a Winters following the few recent violent tropical stronger than normal polar night vortex (Figure 4). volcanic eruptions have had stronger than normal In this case the proposed dynamic mechanism can polar stratospheric vortices. The observations of the work independent of whether or not it is volcanical- mid-tropospheric geopotential field after those erup- ly forced. tions do not contradict the hypothesis of RM that The 500 hPa layer height anomalies of the second tropical volcanoes have a significant effect on winter following the northern hemispheric mid- wintertime tropospheric circulation over the Atlan- latitude eruptions (1957/58 and 1981/82) are com- tic and Eurasian regions. However, a statistical pletely different from those for the tropical erup» prove of this relationship based on observations can l sons (Figure lb). The same holds true for the not be given because of the small number of events. winters immediately after the high-latitude erupt After strong eruptions in high northern latitudes a sons (Figure lc) with only one exception: the winter different geopotential anomaly pattern develops, of 1980/81. This result either suggests an aerosol which is not consistent with RM's hypothesis. The behavior different from the one anticipated by RM positive temperature anomalies in middle and high (i.e. a stronger tendency of the aerosol to remain in northern latitudes found by RM also after high high latitudes) or it is due to a very weak aerosol. latitude eruptions therefore require another expla- Observations of an increase of the optical depth at nation than tropospheric advection of maritime air the North Pole from 0.0001 in 1979 to 0.06 after due to primary stratospheric dynamic forcing. l St. Helens (McCormick and Trepte 1987), besides It is suggested by GCM experiments (Boville, 1984; the obviously weak aerosol, support also the first Graf et al., 1992) that the basic mechanism underly- view. If this is true, it will be necessary to re- ing the response of tropospheric circulation to examine the climatic effects of violent historic tropical volcanic forcing is a strengthened strat- eruptions at least for the cold season. By doing so, ospheric polar winter vortex resulting from en- strong differences in the climate effect for the cold hanced differential heating by the aerosol-contain- season can be expected between tropical and high- ing layers. In conformity with linear theory (Geller latitude eruptions. This, on a regional scale, was and Alpert, 1980; Schmitz and Grieger, 1980), the reported also by Lough and Fritts (1987). change of the tropospheric planetary wave structure I Vol. 67, 1994 Mid-latitude circulation after violent volcanic eruption 9 Zonal Mean Temperature 30 hPa (Data from Labitzke, FU Berlin) (December + January)/2. -50.0 c ı ı ı I I I ı r r I' I' A ñllflııßld. -in» ..._ .. . .,...... I. . -«...... „...... „.. .. , ....„.„ ıı ıı ıııı lıı¬› ı ı. ı,. ı ı - 1' 1 r . .. . • ¬ -60.0 .~II ›I 5. I \ 2 5 oC) 1% ( O~ `. ä í' • / /` I f a ı A -70.0 \ in9 f'\ t* f ›' Ä \...... ı un..-. 1 μ x Q x f" z \ l / í l temperature • . • I * I / .`\l X .f x J \. .f li Ö U ..ı-• Ö 1 I ¬.ıı/ -80.0 xla › t T Ff EC Pı -90.0 I I I I l I I I I ll I I I 1 I ı I I I I I I I I 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 year 9 • 10oN (The label is the year of January.) . --1--..Q 90oN long time mean co O Q o C F I | l I I I I 1 I (K) o Q m difference _L O Q • I I lllllllllllllllllllllI I l l l temperature :) 0.0D 64 66 68 70 72 74 76 78 80 B2 84 86 88 90 92 year Figure 3 (a) Zonal mean temperature al the 30 hPa level al 10o N and over the Nor th Pole, December/January mean. The tropical eruptions covered by the data are indicated by arrows (Fu = Fuego, EC = El ChichOn, Pi = Pinatubo). The Fuego eruption is expected to have influenced the winter 1975/76 because of its late eruption date in 1974 (see text). The abscissa labels indicate the year of the January. (b) Temperature difference 10o N minus North Pole at the 30 hPa level, December/January mean. The abscissa labels indicate the year of the January. is due to the trapping of planetary wave energy in In order to study the relationship between strat- the troposphere as a result of the strengthened polar ospheric and tropospheric circulation, we can easily night jet. Since changes in the strength of the increase the number of cases in our investigation stratospheric polar winter vortex may occur also just by comparing the winters with strong polar without volcanic forcing, volcanos may just stimu- vortex to those with a weak one. late a natural mode of the stratospheric winter To select these winter months, we made an Empiri- circulation. cal Orthogonal Function (EOF) analysis of the . Graf et al. Beitr. Phys. Atmosph. I' r I' l` -I r r r | 5.0 Y | r r | I I' l' | 5.0 » . - _ 4.0 .- 4.0 3.0 3.0 2.0 2.0 . 1 "nuns n1u»u1~ nuns- nn-nun . -noun uuln11u Nunn-» » nun 1 1 --in 1.0 nun.-nv .nun l»»uu~ I ~-nun unn1111~ 11-nun 1.0 | 11 "in 0 1 .no nun n 1 .» » un n 1 n » -n »1 n. 1 | u no n nu n an a » - v 0.0 1 I . 0.0 nu..-nn nu-u 1~-unul nuoosll 1"1u1" a u..-¢»nl nnnuu l-»ul~»»~ u nun. 1 a lllnu uuuunu nununu -1.0 norm. norm. -1.0 t"\ i -2.0 -2.0 1 PC PC _ -3.0 st st -3.0 I I -4.0 -4.0 » \ I \ -5.0 l I l l l l | 1 I 1 I I I I I I -5.0 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 I . I' I' r I I' I 5.0 r | | I' r r I I | | - 5.0 4.0 4.0 » 3.0 _ 3.0 2.0 2.0 ¢ nun. I nun" 1 man-:nn u u 1nu- uuunn lnul11-.-¢u1n.u dunno uunnu 1.0 1 "nu n nuan- 1 on-nn nu-an 1.0 1 | I nun 1unn1u a in an \ Q 11 .| -u | » on un .1 u • 1 1 .-on Vu n I .. ga 11 n 0.0 0.0 un1- »»»¢».~»¢¢ nu"1-» .-u~»»-- .| lunllll .nnun nunuu . "nun unnnnu | -nun anno-nn . nn--- -1.0 norm. norm. -1.0 I -2.0 -2.0 ¢ PC PC w If -3.0 st st -3.0 l l -4.0 _ -4.0 I o -5.0 I l I » l I l l l l I I I I |. I I -5.0 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 Figure 4 Values of the first Principal Component of the 50 hPa layer geopotential for all months July 1957 through June 1993, normalized to the standard deviation. Negative values denote a stronger polar vortex and positive values indicate a weak vortex. Note that during summer months this mode practically does not exist. Vertical lines denote January height of the 50 hPa layer. The first EOF (explain- the difference of the two are shown in Figures 5 and ing 52.4 % of the total variance (area weighted) for 6 respectively. In these Figures the statistical signifi- winter months (DJF), and 44 % for all months after cance (99.5 % confidence level) of the amplitudes is extraction of the mean annual cycle) describes indicated by shading. The composite anomalies of variations in the polar vortex. Its Principal Compo- the 500 hPa layer height from the 1957-1989 mean nent (PC) is highly correlated (r = 0.97) with the of the 38 months with a strong polar stratospheric anomalies in the zonal mean geostrophic wind at the vortex represent a wave structure (Figure Sa), with polar circle. Starting out from the time series of the troughs over North-East America and the North- first PC, normalized by its standard deviation west Atlantic, over the Ural mountain range and, (Figure 4), we chose those months with values fairly weak, over the Bering Strait. Ridges are found larger than +_ 1 standard deviation. Most of these over the East Atlantic and West Europe, and over strong anomalies occur during the winter months of East Siberia. A pattern that is just the opposite in December through February, only few are found in many respects, but with smaller amplitudes, is found the transitional seasons and none during the period for the 41 months with a weak stratospheric vortex between May and September. We found 46 winter (Figure Sb), with the strongest signal over the months with a very strong, and 41 months with a western North Atlantic. The difference pattern weak stratospheric vortex between 1957 and 1993. (strong minus weak vortex, Figure Sc) indicates a The composite anomaly patterns for the 500 hPa deep polar low centered over the Davis Strait and geopotential and for the surface air temperature for an anticyclonic anomaly over the North-East Atlan- months with strong and weak polar vortex, and for tic resulting in enhanced westerlies over the North -I I Vol. 67, 1994 Mid-latitude circulation after violent volcanic eruption 11 (a) 2500 57/7-89/6 (strong vortex - all months) (c) 2500 57/7-89/6 (strong - weak vor tex) .. ¢ . I . . ' * . 1 . ¢ " . . I 1 O J ,.*had | .r|_.| s - l I . J 4 . '. u i I C 111 u .I I I I » . I . a I l I u I I .I Dr I i 11 11 I a ._ 'I K • * . . . • u u _1 » l I .C . a .'- . 1 I a - r • . | 1 I ¢ .I I I 'bQ..---i-"""'\ l '_ • .a . * . 4 • . lil l _ 1 . l • 0 1 PI '* . -9 I I I , ¢ '. | . U 41 l . lo l ¢ . • | I . • I • I ' I . I 'I .| l . I . • I I I I n l I . . _ u Q I . * 1 l * *Y \ * * • l I .I I | 1 I • 1 1 1 . _ 4 1 . . 4 . I - ¢ | . . 10 . C I * £11 . .g * . . . 1 1" * r . | a * l 11 1 l ' * O l u I . . l ¢ 1 1 . 1 | • * I a u I * . c I | . l I J n \ I | I ' I I I 1 l I . . l 5 .».-u- I n I. • l I Q I . * . . . » * I u . 0 . * u n ' . a 5.3. . n 4 J' . ,r calv 4 . I 1 .L .Is " I c a 0 Q . 1 n l I l n f . r 0 . / _ a | * ' . . 4 . 1 |» • P u D | . n ... a }-~ 1 an my I °\ . - • __..r--ur I'\1r »'*'| 1 lrz am_ . . • ,1.- 'Q l . a 10 a . q,,1," . 1 . . H l 9 ' e » H...»» .. _.f , ..'° , 1 n I E? 4 u , I __.-¢2u». I . I...-» *t D **-,Qr ¢:' u I 1 "*"\ ii ..l ..:.»»" l .~"' 0 '*.l***' ~ . . *~ 4"/r¢"' _.. . . • n i . ':`\x , . J" Q 'G _.f Ia 'r . |.- : 1 | s *_. . ,I - . • t are .s . t ,1 *.* .I . 'Iv ; . /if Uf' ` " . **¢ "3 l c 11 I f' -1 ' ,;':~: l l?'§.;E I a 40' I `-:fi-"-I-P1. J '=i~*wi: 1 I ;".. 5 I . 1 . Ililll-an .It - I a .1}:;,» 1\ .io ¢ I . ~. f' J' • -g; l 1--i K I in`j. . I I .'*... ~.__ -é a * . • l 3 I i 3 -J - l+'--. I ,: Y a' O ' rl'~.» N: .l ;~'!»'2.§*'.' J .-:to IJ 10 \~, '~,' .N,g . "-..°`_;'_'_°-10 'u . .of p* 4.5 "\ '~. 'H \. ; • 1 J .P 1 I J . I. \ \ lj . _ |¢|l' i . . 1 L. n I * | *w*-. hw- .- . '* 4 -30 I J' -:»'~ 5'I I- \ } "A-\..F¢§ be! ..--.;..~-ql I 1 <204 . _\ . s . I in I °.° ..' 4 I ..:.'~. . . . ° . '\1 u . I • _ s * . . . n l . ,//at'/" . . f l :°\; . | | . ;.9-4 *\ |. I .-, . 1 . . 0 § • l | I '~."-_ '!\' To *=- I '~\' o*'» v' e' * ».j • 5% ; ., I an | \. *'i1-34J~*;»-::.I . r J ' : • ,.:....r' |. . a . I ~§=Z'* n Y0le" .2 * » 0 -:-20-II-" l . \\ 'x l~ . |. e l _ in I ' a * _ f .10 l !i¢*'L * I I 9 h . I I n `., J'Lf *. *. t-§~ `l » , * . 311 #kg.:I .°l Q, 4 1. to. a _._ l "l . i -3' r' • \ a .-. r . ¢ I u I " l . I n . .'* I 1. 's QUO | . I "2 * .r Ut' ."\~\.-¢ . 1-" "p . . 4' 20 -._ --nu».1: . .. I 4 xi' . L * L .p ¢*~' 4 . Be*-** "-.__ 4 -wrfgq I » ,go 'G h .n .* /r '\.____ \ l \ \ ... 4 I _ . 1 __/' I . 2 . _/' .. 2"I*"° 'I .". Ln. • * * * I a °"-.,:_ "l . C 10 | ,?- . .l **ll-.*.` 1 .--alI . *is . - 1 .. .,|.----m, U _,a ' 7* ln- . in . I , . \ -*_,|O:~.-r"' $ I • P I l . I No \__..\.\¢..U Q . . . 1 l . Q.: . . 1.\ V*l u f ¢ I* 'Q » . . "`"; `l 4 i \ * 1 . . r Figure S Mean Northern Hemisphere anomaly (mean 1957- (b) 2500 51/7-89/6 (weak vortex - ull months) 1989) of the height of the 500 hPa layer for 38 month with strong (Sa), 41 months with weak stratospheric polar vortex .-a 'I : .. . 0* " » (Sb) and difference strong minus weak vortex (So). Shading *I* I ' " . . _»a I *. . 111 .l*"° ' -°'» _I 'a n * 1*1 • |*. » \ indicates statistically significant amplitudes (local t-Test, I 1 \ l I ..1. .u 11 1. I. .._* l-* *. |' *o \ -10.,...... - dl 'Q 1 99.5 % confidence level). I l . I 'o . - I I a . II. Q. I *. *a .* | I It U *i,*.___**.*** : ' 1 I I | I a * n t _ n .I o|*- . *n 11 _1-l1 I* '- I *u ' a 1 4 r . } I . . n l`. l*"*. | ' 11 * . . .a . * • " 'i.,....*¢¢1¢°'*¢,. I u . i o' a s . |', • \ \ .10 I | ¢ .. / I » . . ' n _ 4 . I I "41 . .`°'- ¢, 'I I . I s I I . . l l . | . .*u-* \ I* .*|'. 1 . 9 l ° a* . » I U 1 I I Q e . .»- n . * ` * . | I l in; l J | a ¢,.-» . s u s' - . I » :. _ 4 l I »$-T:'T;v.'§"{ i I g -u ' ' I L *,»-'-1»-. *J -1 . 0 I I -\. Elf:-. '* UP x \ "»'i . 'G .. *.. a L I ._* ....~§ 1 . .al I - L . ' al-, . . (44 months) polar vortex shows a very clear positive I e .'| g \_ Ig" 9 l -to. I i .\' * I . "~. _/ . J . temperature anomaly over middle and high lati- n ,g 1 * .|. I 4' I ' o I tudes of Eurasia (mainly Northern Europe and East 1 ;;~----""" »'.' \ .I* *' '\Q., °'1Q C.- »x "\'° l J' . Siberia), while negative temperature anomalies 1 0 l . of I *"""-. \1 / * U .. prevail over North-East America and over Arabia l l . I l I Q.II" and North Africa (Figure 6a). This pattern is similar n 1 \ . w»dv . O *J . 4 .. I a . * .0 1 u o l\ to RM's surface air temperature anomalies for the . O c . . . I c 12 strongest volcanic eruptions of the past century, and also similar to the GCM results of Graf et al. (1992). Again for the 42 months with weak polar Atlantic. Troughs develop east of the Rocky Moun- vortex (Figure 6b) a similar anomaly pattern with tains and over Central Russia. This observed pat- opposite sign develops, leading to quite substantial tern is consistent with the mean signal simulated and statistically significant temperature differences with a GCM for the Pinatubo aerosol in the winter (Figure 6c) between months with a large negative of 1991/92 (Figure 2, right panel). This is indicated First Principal Component (strong stratospheric by a pattern correlation coefficient, area weighted, polar vortex) and a strong positive one (weak of r = 0.65 for the Northern Hemisphere north of vortex). The patterns in Figures 5 and 6 are not 20o n. dominated by the winter months after strong vol- The anomaly pattern of the surface air temperature canic eruptions. This was tested by excluding these (Jones et al., 1988) for months with a strong months (not shown). 12 H.-F. Graf et al. Beitr. Phys. Atmosph. (a) Tsfc 57/8-92/7 (strong vor tex - all months) (c) Tsfc 57/8-92/7 (strong - weak or Tex) _J I 1 I l 1 . I I . .. l»l., °. I . I _.,r . • . | / n I n • I • 4 ° ' . | I' 1 \ I 9 • 0 .Q \ L . • 4 I °a ' \ n..:I' . I -*°'l . . \ i la r. • • \ r. A | I n I I • I G 'l u . I 1 I I 9 . 'I c l . 4 • • I l 0 . L _ a I q I I r u I I . I I . I l l I 1 O e l . ¢ I • 0 * I . .l I | I J I l » I I l I * " • 4 l a l »°* n I l * L . . I a 0 4 I a » 0 * 1 n l . * 1.f | I . , . 0 1 l l | . o 1 .I 1 * -l . f.. . | U a _-I C l - » I 4 O n .- o . I 1 . I _' ; C I . .' • 1 ¢ . a . .4 • , | J /i 1 *\ I g, J /~ l * O .I *~` *1 ,'--E au.. -Q e *_+- 0 n r' u# .*\ I 1 a ¢ 7°:. -. -_ » ».. `.. |*_. P -I' ! 1 _ ' ' 4 ." . of • r . '\H' .\ '* . 4 *, • I 1 61 _ . . F . -. E r 1 r °'i * . _./ ` 'l = .F » //, I . I *bl \ -i face air temperature anomalies in middle and higher /' l I .4 . Q _Vu 41 . 1 J 0 .Qn.Q§ I _IP'\ . no. latitudes as found by RM and others. The response ,I. * a*. 41_ '. ¢'.-""" `. ..»' I I I 1 1 .Q :h* *. . . \ I I iv iv. . ..> r .r, y ; a 'VR is less clear and may even have opposite sign for I . n I I m 1 x. 'a 'V¢ -.{.. r pi . eruptions in high latitudes. 0 f u.s"~-. . *J '- n . .) . ¢ > The possible mechanism leading to stronger wester- I * I . I 1, . | \. » . . 1 r 3 I I .4-- lies over the North Atlantic in winter and to positive ' 4 ( . :.-' . a »x f . - 1 . .4 . 1 h . al I I . \ temperature anomalies over large parts of Eurasia . ' + H. ~1 n I . %.-¥,-.. I * becomes evident when comparing GCM experi- I I . .1 I q I so • . I . ments with observations. It consists in exciting a ¢ ' T / I I . . u . u * . . . 1 I - » . natural mode of the stratospheric winter circulation. I .4 I QUO Differential heating between lower and higher latitudes by the aerosol-containing stratospheric layers increases the meridional temperature gradi- Conclusions ent and strengthens the polar stratospheric winter vortex. Stratospheric circulation affects the tropo- Though it is impossible to give statistical prove spheric circulation via vertical wave propagation. because of the limited availability of free atmos- These mechanisms have been imposed already phere data and the rareness of events, the mid- more than ten years ago with simple barotropic tropospheric geopotential anomaly patterns over models (Schmitz and Grieger, 1980, Geller and I Alpert, 1980). 4 the Northern Hemisphere observed after violent I tropical volcanic eruptions do not contradict the The anomalous surface air temperature patterns for I hypothetical mechanism suggested by RM to ex- winter months with strong polar stratospheric vor- plain the observed temperature pattern after violent tex for the years from 1957 to 1993 are nearly the _-I Vol. 67, 1994 Mid-latitude circulation after violent volcanic eruption 13 same as those analyzed by RM for the 12 strongest Cadle, R. D., 1980: A comparison of volcanic with other volcanic eruptions of the last century. This pattern is fluxes of atmospheric trace gas constituents. Rev. Geo- not dominated by the volcanically disturbed months. phys. Space Phys., 18, 3732-3740. Geller, M. A. and J. C. Alpert, 1980: Planetary wave cou- If they have any effect on the tropospheric winter pling between the troposphere and the middle atmos- circulation in middle and high northern latitudes, phere as a possible sun-weather mechanism. J. At nos. tropical volcanic eruptions enhance an inherent Sci., 371, 11972-1215. natural mode of the northern hemispheric winter Graf, H.-F., 1986: On El Nico/Southern Oscillation and circulation. Therefore, it is generally difficult to Northern Hemispheric temperature. Gerl. Beitr. Geo- prove the statistical significance of any observed phys. 95, 63-75. volcanic impact on global climate in winter. The Graf, H.-F., 1992: Arctic radiation deficit and climate variability. Climate Dynamics, 7, 19-28. type of tropospheric response is the same for all Graf, H.iF., I. Kirchner, A. Robock, and I. Sch uh, 1992: winter months with a strong polar vortex, irrespec- Pinatubo eruption winter climate effects: Model versus tive of whether or not an anomalous aerosol observations. MPI Report No. 94, 24 pp., also: Climate concentration was observed. Further, since volcanic Dynamics (1993) 9, 81-93. I eruptions are rare events the number of cases is too Groismann, P. Y., 1985: Regional climatic consequences of small to apply statistical tests. However, we can volcanic eruptions. Meteorology and Hydrology, No. 4, make use of GCM experiments (see e.g. Boville, 39-45. [in Russian]. Groismann, P. Y., 1992: Possible regional consequences of 1984, Rind et al., 1992, Graf et al., 1992) and study the Pinatubo eruptions: an empirical approach. Geophys. the stability of the tropospheric response found Res. Lott., 19, 1603-1606. there. Such simulations support the idea that the Jones, P. D., T. M. L. Wigley, C. K. Folland, D. E. Parker, correlations between the strength of the polar J. K. Angell, S. Lebedefji and J. E. Hansen, 1988: Evi- stratospheric vortex and the tropospheric geopoten- dence for global warming in the past decade. Nature, 332, tial field, or the surface air temperature, are due to 790. the stratosphere forcing the troposphere, and not Kirchner, I. and H.-F Graf, 1993: Volcanoes and El Nico - Signal Separation in Winter, MPI Report No. 121, 57pp. vice versa. Loginov, V. H, 1984: Volcanic eruptions and climate. (in Russian), Leningrad, Gidrometeoizdat, 64 pp. Lough, J. M. and H. C. Friars, 1987: An assessment of the possible effects of volcanic eruptions on North American Acknowledgements climate using tree-ring data, 1602-1900 A. D. Climatic Change, 10, 219-239. We thank Alan Robock for having supplied a McCormick, M. P. and C. R. Trepre, 1987: Polar stratospheric optical depth observed between 1978 and 1985. J. preprint of the RM paper and Mojib Latif for Geophys. Res. 92, 4297-4306. helpful discussions. Stratospheric data were kindly Million, M., A. J. Gallant, Y. Chung, and F. Fouad, 1985: provided by K. Labitzke of the Stratospheric Re- COSPEC observations of the Mt. St. Helens eruption search Group of the Meteorological Institute of the cloud of May 18, 1980 over Southern Ontario. At nos. Free University of Berlin. We received the 1991/92 Environ., 19, 265-283. data from John Janowiak, Climate Analysis Center Rind, D., N. K. Balachandran, and R. Suozzo, 1992: Climate and the air surface temperature data from Phil Jones, change and the middle atmosphere. Part II: The impact of volcanic aerosols. J. Climate, 5, 189-208. University of East Anglia. This work was partially Robock, A. and J. Mao, 1992: Winter warming from large supported by Bundesministerium fir Forschung volcanic eruptions. Geophys. Res. Lott., 12, 2405-2408. und Technologie (no. 07-KFT-86/2). Rose, W. I. Jr., R. E. Stoiber, and L. L. Ma lin conico, 1982: Eruptive gas compositions and fluxes of explosive volca- noes: Budget of S and Cl emitted from Fuego volcano Guatemala. In: R. Thorpe (Editor), Organic Andesites References and Related Rocks. Wiley, New York, NY, pp. 669-676. Schmitz, G. and N. Grieger, 1980: Model calculations of the Blulh, G. J. S., S. D. Doiron, C. C. Sch nelz ler, A. J. Krueger, structure of planetary waves in the upper troposphere and L. S. Walter, 1992: Global tracking of the SON clouds and lower stratosphere as a function of the wind field in from the June, 1991 Mount Pinatubo eruptions, Geophys. the upper stratosphere. Tellus, 32,, 207-214. Res. Left., 19, 151-154. Spiring, L. P., 1973: On the seasonal changes of the surface Boville, B. A., 1984: The influence of the polar night jet on air temperature field of the Northern Hemisphere after the tropospheric circulation in a GCM. Journ. Atmosph. volcanic eruptions (in Russian), Trans. of the Main Sci., 41, 1132-1142. Geophysical Observatory, 299, 3-7. `