NEW ZEALAND CLIMATE: THE IMPACT OF MAJOR VOLCANIC ERUPTIONS

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1 Weather and Climate (1998) 18 (1): NEW ZEALAND CLIMATE: THE IMPACT OF MAJOR VOLCANIC ERUPTIONS M. James Salinger National Institute of Water and Atmospheric Research ABSTRACT Major volcanic eruptions which inject significant amounts of dust and sulphate aerosols into the atmosphere produce discernible climate signals in the New Zealand region. A superimposed epoch (compositing) method was used to determine the effects on regional temperatures of the six major volcanic eruptions since the 1880s that have been likely to have affected New Zealand climate. Regional atmospheric circulation anomalies during the three significant late twentieth century volcanic eruption events were examined. As El Nilio - Southern Oscillation (ENSO) events significantly effect temperature and circulation anomalies, these climatic effects were removed. The results show that the effects on temperature and atmospheric circulation in the New Zealand region commenced rapidly, in the first few months after the volcanic eruption event, and lasted 24 months. These events were found to depress surface temperatures in the region by 0.3 to 0.4 C from 1 to 21 months after the eruption.atmospheric circulation anomaly patterns were very distinct and show more surface southerlies and troughs near the Chatham Islands in the first two seasons (1-6 months) after the eruption, followed by stronger west to south west flow anomalies in seasons three to five (7 to 15 months) over the region. Finally, a period of more troughs over the North Island of New Zealand occurs in seasons six to eight (16 to 24 months) after the eruption episode. The magnitude of the volcanic signal in the New Zealand region is consistent with previous larger-scale global studies. INTRODUCTION Major explosive volcanic eruptions inject massive amounts of dust and sulphur-rich gases (mainly SO9) into the atmosphere (Rampino and Self,1982). Once reaching the stratosphere, some gases rapidly oxidise to sulphuric acid and condense with water to form an aerosol haze. The volcanic aerosols increase the planetary albedo and the dominant radiative effect is an increase in scattering of solar radiation, which reduces the net radiation available to the surface/ troposphere, thereby leading to a cooling (Hartmann 1994, Lamb 1970, 1977). A direct calculation of the tropospheric impact of Mt Pinatubo was a radiative cooling of about -4 W m-2 around one year after the eruption, decaying to -1 W m-2 after 2 years (IPCC, 1994). For the two most recent eruptions where there are observations (El Chichon and Pinatubo), the distribution of aerosols was similar from the second season, spreading fairly uniformly from the tropics to the poles (Strong, 1984; Stowe et al. 1992). Both these eruptions occurred in tropical latitudes. Earlier studies (e.g Lamb 1970) for which there were no aerosol distribution observations, have assumed that eruptions in the tropical belt (30'N to 30'S) produce aerosols that spread throughout the global stratosphere, whereas eruptions poleward of these latitudes only affect the hemisphere where the eruption occurred. This same assumption will be made here. Various studies have examined possible cooling in global and hemispheric mean air temperature following major volcanic eruptions (Mass and Schneider, 1977; Kelly

2 12 N e and Sear, 1984). Mass and Portman (1989) detected a cooling of C in zonal mean surface air temperature after the very largest of these eruptions. This study also recognised that E l Nifio-Southern Oscillation (ENSO) related v a r i a b i l i t y can mask the volcanic climate signal. Robock a n d M a o (1995) examined variations associated with the 12 largest volcanic eruptions between the years 1883 and By removing ENSO signals, and regionalising the eruptions, w a r m i n g signals were found over Eurasia and North America i n t h e f i r s t boreal w i n t e r a f t e r tropical eruptions (<30' latitude), and in the first or second winter after higher latitude eruptions, w h i l e c o o l i n g occurred o v e r northern Africa, the Middle East, southern Asia and Australia. These studies show that the larger scale regional signal is a perturbation around general global trends in climate. There is still considerable uncertainty regarding smaller scale regional climatic effects associated with major volcanic eruptions Robock and Mao (1992, 1995). Detection of such effects is made harder because some major explosive volcanic eruptions have occurred around ENSO events, in particular those o f Agung i n 1963, E l Chichon in 1982 and Pinatubo in 1991, which correspond to the ENSO episodes o f 1963, 1982/83 and Gordon (1985, 1986) and Mullan (1995) have shown significant response of New Zealand climate patterns to ENSO events. Gordon (1986) showed t h a t strong pressure anomalies over New Zealand occur, w i t h i n c r e a s e d s o u t h - w e s t e r l y anomalies for El Nifio episodes in the autumn and spring seasons. I n summer, anomalous westerly flow is more prevalent, and in winter southerly flow occurs. This paper will describe the regional and seasonal patterns o f surface temperature effects and atmospheric circulation anomaly patterns produced by several major volcanic eruptions. To isolate the volcanic signal in the New Zealand region, the ENSO climatic signal was r e m o v e d b y a p p l y i n g r e g r e s s i o n relationships b e t w e e n t h e S o u t h e r n Oscillation I n d e x ( S O T ) a n d m e a n temperature and sea level pressure. These results a r e compared w i t h t h e r e g i o n a l climatic signal o f volcanoes as seen i n the general circulation model of Robock and Liu (1994). w Zealand Climate DATA The New Zealand monthly mean surface air temperature series (Salinger, 1981) and the newly homogenised monthly mean sea level pressure series data were used in this study. The temperature and pressure records were carefully refined b y a number of methods. They were screened for inhomogeneities by examining station histories and comparisons were made between neighbouring stations to identify unrecorded site changes o r other environmental changes near the climate station site. Procedures used to homogenise the d a t a (Rhoades a n d Salinger, 1993) included c u m u l a t i v e s u m p l o t s a n d neighbouring station comparisons. For a few early records where neighbouring stations did not exist, other techniques were used t o evaluate t h e significance o f, a n d m a k e adjustments for, suspected inhomogeneities (Rhoades and Salinger, 1993). Folland and Salinger (1995) found mostly very good agreement over the period between the variations i n mean land surface air temperatures in New Zealand and the rigorously q u a l i t y controlled m a r i n e temperature d a t a m e a s u r e d o v e r t h e surrounding ocean surface on time scales down to a season. The study is limited to volcanic eruptions between the years 1870 to 1995, a period for which reliable land surface air temperature records are available f r o m a t least f o u r locations. The examination o f atmospheric circulation anomalies is further limited to the period , when enough station data is available to reconstruct surface pressure fields. The most commonly used measure for El Nifio episodes is the Southern Oscillation index (SOI), the standardised difference in the sea level pressure between Ta h i t i (18'S, 150'W) and Darwin (12'S, 131"E). The SOT series o f Ropelewski a n d Jones (1987), extended back to 1866, is used here. METHODS The superposed epoch (compositing) method (Mitchell, 1961; Mass and Portman, 1989) was used to determine regional climatic effects of volcanic eruptions. Anomalies are calculated over various t i m e periods and averaged. These t i m e periods, w h i c h are usually equivalent i n duration, are defined

3 New Zealand Climate with respect to the specific key date of the month and year of an explosive volcanic eruption. Monthly New Zealand temperature series averages and station mean sea level pressure averages are composited and averaged for the ten years immediately prior to the key dates of the selected volcanic eruptions. Anomalies from the 10 year mean are then calculated for 8 three-month periods (2 years) before, and 12 three month-periods (3 years) after the month of the volcanic eruption, so as to detect any systematic signal that is associated with the volcanic eruption. The atmospheric pressure analysis was not performed on a "standard season" basis (i.e DJF, MAM, JJA and SON) as mean atmospheric circulation in the New Zealand region shows little seasonal variation. ENSO events were present in four of the six volcanic episodes analysed. These effects were removed by regressing the SOI with the New Zealand temperature series to establish relationships between ENSO events and three-monthly New Zealand temperature. For each three month period ENSO effects were removed by using the current value of the SOI to adjust the three-monthly New Zealand temperature anomaly value using the regression relationship. Regression relationships between station pressure and the SOI were also established, and departures of seasonal station mean sea level pressure were scaled by subtracting the SOI induced pressure anomaly value for each season. SELECTION OF VOLCANIC ERUPTION EVENTS Years of major volcanic eruptions were identified on a combination of the Dust Veil Index (DVI > 550) of Lamb (1970, 1977 and 1983), and the Volcanic Explosivity Index (VEI > 5) of Newhall and Self (1982). Care must be taken in the application of these criteria, because it has become evident in the last decade (Rampino and Self, 1982) that the effect of a volcanic eruption on climate is most directly related to the sulphur content of emissions injected into the stratosphere and not the direct explosivity of the eruption. Robock (1991) has discussed the relative merits of each index, the Lamb DVI assessing the fraction of the globe covered by the volcanic dust veil, and the VET, and index of volcanic explosivity. Robock and Free (1995) have shown that the DVI and VEI are all highly correlated and all indicate the same large volcanic eruptions. By consulting these sources six years of major explosive volcanic eruptions that would produce a volcanic dust veil over the Southern Hemisphere were identified, and listed in Table 1. The criteria chosen were DVI > 550 or VEI > 5, provided the eruption occurred at low latitudes south of 30'N. During each of the years identified, evidence of increased stratospheric aerosols was first reported in the vicinity of the erupting volcano and associated aerosol clouds spread over midlatitude regions of the Southern Hemisphere within the first six months. Of the episodes, five were major eruptions in tropical latitudes, (between 30'N and 30'S) and one at southern midlatitudes (Tarawera). Using higher values of the two criteria does indicate the largest eruptions which are most likely to be climatically important in the New Zealand region. Of the six eruptions selected, three had El Nino episodes during the eruption year (Agung, El Chichon and Mt Pinatubo), one a 13 Volcano Month/Year of Eruption Latitude DVI VEI Krakatau August 'S Tarawera June S Santa Maria October 'N Agung March 'S El Chichon April N Mt Pinatubo June 'N Table 1. List of the 6 largest stratospheric-aerosol-producing volcanoes since 1866 with a DIU > 550 (Lamb, 1970) or VEI > 5 (Newhall and Self 1982), that have occurred south of 30W.

4 14 La Nina event (Tarawera) and two neutral conditions (Krakatoa and Santa Maria). Tarawera had the complicating factor of being 35 months after a larger event (Krakatoa). Because the Tarawera eruption was a large mid-latitude Southern Hemisphere eruption, it was expected to influence New Zealand climate. VOLCANIC SIGNALS IN SURFACE TEMPERATURE VARIATIONS Temperature anomalies relative to the time of eruption were computed, and results are displayed for the 6-eruption composite (Figure 1) and for each eruption separately (Figure 2). The volcanic signal can be seen in the seasonal-average SOI adjusted data for seven seasons after the eruption event (Figure 1 and Table 2a and 2b). The most immediate impact is in the first two seasons (1-6 months) after the event, where temperatures are New Zealand Climate Table 2a. Seasonal New Zealand mean surface air temperature anomalies before and after major volcanic eruptions. The climatology period is the ten seasons immediately prior to the eruption month. Anomalies (A) and SOI-adjusted anomalies (B) are shown for each eruptive episode. Event Krakatau Tarawera Santa Maria Season A B A B A LO E Unadjusted SOI adjusted Season Relative to Eruption Figure I. New Zealand seasonal temperature before and after eruption events unadjusted and adjusted for the Southern Oscillation Index (SOI). The values are averaged from six explosive volcanic events.

5 New Zealand Climate significantly depressed by -0.7 C, compared with the 10 years prior to the eruption. Surface temperature anomalies of the six events then show anomalies of -0.2 C for seasons 3 to 6 (7-18 months), with a significant anomaly of C in season 7. In this sample of 42 seasons (Figure 2), counts of above average temperatures only occur in 9 of the threemonth periods, and only two of these were more than 1 C above average. Although seasons 9 and 10 gave below-average temperatures for the composite, the number 15 of above average cases was higher. Therefore, the volcanic signal is only coherent in New Zealand temperatures up to 7 seasons (21 months) after the eruptive event. Of the six volcanic episodes only Tarawera and the Santa Maria events gave significant positive temperature anomalies in season 3 (+2.0 C) and season 5 (+1.1 C) respectively. In the case of the Tarawera eruptive pattern, the averaging period ten years prior to the eruption date included the Krakatoa event. The first strongly positive anomaly significantly reduced the average temperature anomaly in season 3.Apart from the two large positive temperature departures noted, all six events showed the general pattern of stronger negative temperature anomalies in the first 3 seasons, followed by weaker negative temperature anomalies in the next 3 seasons. The increased negative temperature departures in season 7 is consistent throughout the six events Eruption Krakatoa Tarawera ID Santa Maria Agung El Chichon E Pinatubo Season Relative to Eruption Figure 2. New Zealand seasonal temperature before and after six eruptions. SOI effects have been removed.

6 16 N e w Zealand Climate Table 2b. Seasonal New Zealand mean surface air temperature anomalies before and after major volcanic eruptions. The period for comparison is the ten seasons immediately prior to the eruption month. Anomalies (A) and SOI-adjusted anomalies (B) are shown for each eruptive episode. For all six, the standard deviation is shown (B), as well as the number displaying negative anomalies (C). Anomalies of three-month periods of 0.5 C or more (denoted by an asterisk) are significant at the 0.05 confidence level, when compared with the normal distribution of three-monthly temperatures. Event Season Agung A B El Chichon A B Mt Pinatubo A B A Average ± ± ± ± ± ± ± ± OM * -0.7 ± 0.8* * -0.7 ± 0.6* ± ± ± ± * -0.7 ± 0.5* ± ± ± ± ± Counts of negative anomalies for the 8 three-month periods prior to the eruption vary between 2-4, averaging 3. For seasons 1-7 after the eruption, negative anomaly counts average 4.7 per event, ranging between 3-6. From season 8 onwards, the negative anomaly count was 3.2, and ranged between 2-5. The significance of the number of counts of negative anomaly three-month periods was tested using the Chi-square statistic, which is the measure of the discrepancy between observed and expected frequencies of events. In this application, the observed frequency of negative anomalies was tested against the expected frequency of negative anomaly temperatures. This was greater than expected at the 1% level of significance for seasons 1 to 7 after the eruption event. ATMOSPHERIC CIRCULATION PATTERNS SOI-adjusted mean sea level pressure anomalies relative to the time of each individual eruption were computed for the Agrung, El Chichon and Pinatubo eruptions. As seasons 1 and 2, 3 to 5, and 6 to 8 showed similar circulation anomalies, these were combined to form multi-season composites. The results are displayed as 3-eruption composite for seasons 1 and 2 (Figure 3), 3 to 5 (Figure 4) and 6 to 8 (Figure 5). No clear discernible atmospheric pressure anomaly patterns were detectable prior to the eruption date. The striking feature of the first 6 months (seasons 1 and 2, Figure 3) following the date of eruption is the strong cyclonic mean sea level pressure anomalies near the Chatham Islands, with more frequent south easterly flow over the South Island, and south westerly flow over the North Island. In season 1, cyclonic southerly flow prevails, which is followed in season 2 by cyclonic south westerly flow over the North Island, and south easterlies over the far south with more depressions centred near the Chatham Islands, located to the east of New Zealand (latitude 44'S, 177'W). Seasons 3 to 5 (7 to 15 months) after the event were characterised by strong west to

7 New Zealand Climate 17 Figure 3. Mean sea level pressure anomaly averages (0.1 hpa) for Agung, El Chichon and Pinatubo eruptions for seasons 1 and 2 (1-6 months after eruption episodes). The anomalies are with respect to the reference period, with SOI effects removed. Figure 4. As for Figure 3, but for seasons 3 to 5 (7-15 months). south west flow anomalies over the New Zealand region (Figure 4). In season 3 strong west to south west flow occurs. In season 4 the flow is more westerly, but particularly cyclonic near the Chatham Islands with a return to south west flow in season 5. In the final group of seasons (months 16 to 24) mean sea level pressure anomalies showed frequent cyclonic development over New Zealand, with easterlies over southern New Zealand (Figure 5). Frequent troughs occur over New Zealand, particularly over the North 1 I Figure 5. As for Figure 3, but for seasons 6 to 8 (16-24 months). Island, in season 6, and in season 7 this pattern continues but weakens. In season 8 the troughing pattern strengthens again, but this time over central New Zealand. The first anticyclonic anomaly patterns develop in season 9, with more anticyclones over northern New Zealand and to the east, with westerly flow anomalies over the South Island. Atmospheric anomaly patterns are weak in the following three seasons (10 to 12). DISCUSSION These results show a discernible temperature signal in the New Zealand region following volcanic eruptions that inject volcanic aerosols into the stratosphere. The spatial spread of such aerosols in the atmosphere after the Mt Pinatubo eruption was very rapid, and along with the eruption of Cerro Hudson in South America in August 1991, had spread over New Zealand latitudes by September 1991 (Rosen et al. 1994, McCormick et al. 1995). In comparison, El Chichon aerosols stayed in the latitude band from the equator to 30'N during the first season (Strong, 1984). This was the only eruption that did not produce a depression in New Zealand temperatures (Figure 2 and Table 2b) in the first season following the eruption. No data are available for the aerosol spread of the four earlier events. However, they all occurred at low latitudes, except Tarawera at 38'S. From the immediate cooling response

8 18 New Zealand Climate in the first season one possibility is that the spread of stratospheric aerosols was sufficient to increase the optical depth over the New Zealand region thereby reducing insolation. Very few studies have been made of local effects of volcanic signals in temperature and atmospheric circulation, that cover the South Pacific. Robock and Mao's (1995) global study does include all latitudes, and uses the 5' x 5' grided monthly surface temperature dataset of Jones e t a l (1986). T h i s s h o w e d a temperature decrease of greater than 0.25 C in the New Zealand area in season 2 following six volcanic eruptions. Also examined were seasons 6 and 10, which showed no discernible signal over New Zealand. The sample of six e r u p t i o n episodes d i f f e r e d f r o m t h o s e examined here in that Tarawera was excluded, and Katmai, at 58'N, was included. The s p a t i a l a n d temporal response o f climate t o aerosol forcing has also been modelled using the Goddard I n s t i t u t e f o r Space Studies (GISS) general circulation model (GCM). The volcanic signals from seven individual simulations, with spread of aerosols were composited (Robock and L i u 1994). Details of the stratospheric aerosol injections in the model are given in Hansen et al. (1988). The zonal mean results showed negative mean temperature anomalies i n N e w Zealand latitudes o f C commencing 4 months after the eruption, and increasing to -0.5 C for the period 6 to 18 months. The global pattern from the model showed cooling of 0.25 C in season 2, increasing to C i n season 3, then C in season 4 and C in season 5. Temperature anomalies then recovered to above C 21 months after the eruption event. These results compare favourably with regional observations such as presented in this paper, which demonstrate negative temperature anomalies from 1 to 21 months after the eruptions. However, t h e largest cooling occurs in seasons 1, 2 and 7 after the eruptions. The small sample size of six events from the observed record could account for the seasonal differences. CONCLUSIONS Major explosive volcanic eruptions which inject s i g n i f i c a n t a m o u n t s o f d u s t a n d sulphate aerosols into the atmosphere produce discernible climate signals in the New Zealand region. All the six major explosive eruptions, preselected by size and location t h a t have occurred since the 1870s, have a demonstrated local effect in the New Zealand region. These are Krakatoa (1883), Tarawera (1886). Santa Maria (1902),Agung (1963), El Chichon (1982) and M t Pinatubo (1991). Five o f these six occurred at low latitudes (between 30'N and S), whilst only Tarawera occurred i n New Zealand. The common pattern of behaviour of New Zealand temperatures, once SOT adjustments have been made, is one of a cooling of between -0.2 and -0.7 C (average -0.4 C) for the first seven three-month periods (1 to 21 months) after the eruption.all temperature departures were normalised using the ten years prior to each eruption event as the reference period. The largest cooling occurs i n the first t w o seasons, f o l l o w e d b y a m o d e r a t i o n subsequently. A l t h o u g h some cooling i s detectable in seasons 9 and 10, the variability between the six cases does not allow such a conclusion to be drawn. Atmospheric circulation patterns for the three events examined also show a discernible signal in the New Zealand region. In the first two seasons more southerly flow prevails over the country, whilst the next three seasons are characterised by anomalously strong west to south west flow over the country. The final three seasons (seasons 6 to 8) showed a very distinctive p a t t e r n o f troughs over N e w Zealand and more frequent easterly flow over the far south of the country. The results indicate that the response of the climate signal in the New Zealand region to volcanic forcing is very rapid. Both cooling in temperature and southerly flow anomalies occurred in the first season after the eruption date, from seasons which did not display any consistent climatic patterns before the event. This suggests t h a t t h e sulphate aerosol dispersal around the stratosphere occurred rapidly enough to cause negative radiative forcing in the mid-latitudes of the Southern Hemisphere within months of the eruption, or that tropical radiative forcing induced a Southern Hemisphere dynamical response within approximately 3 months. The magnitude o f the volcanic signal is consistent with previous larger-scale studies which demonstrated some cooling i n t h e second season after the eruption. Because the sample was o n a coarser scale and used different events, previous studies did not note

9 New Zealand Climate the scale o f local response. T h e results presented in this study compared well with the GISS GCM simulations, which gave a temperature depression of at least 0.25 C in New Zealand latitudes from season 2 to season 7. These findings indicate the importance of examining regional climate records to detect volcanic signals. For the New Zealand region, this initial study can be extended f u r t h e r once local climate records of mean sea level pressure, temperature and precipitation have been homogenised. ACKNOWLEDGMENTS Thanks are due to Drs Brett Mullan and Jim Renwick for providing useful comments in the preparation of this manuscript. The research w a s f u n d e d b y N e w Z e a l a n d Foundation f o r Research, Science a n d Technology c o n t r a c t C , a n d i s a contribution to the International CLIVAR programme REFERENCES FoHand, C. K. a n d S a l i n g e r, M. J., Surface temperature t r e n d s i n N e w Z e a l a n d a n d t h e surrounding ocean, Int. J. Climatol, 15, pp Gordon, N.D., The Southern Oscillation: a New Zealand perspective. J. R. Soc. New Zealand, 15, Gordon, N.D., The Southern Oscillation and New Zealand weather. Mon. Wea. Rev., 114, Hansen, J.I., Fling, A., Rind, D., Lebedeff, S., Ruedy, R., Russell, G. and Stone, R, Global climate changes as forecast by the GISS 3-D model. J. Geohpys. Res., 93, Hartmann, D.L., Global Physical Climatology Academic Press, San Diego, 411pp. IPCC, Climate Change 1994: Radiative forcing of Climate Change and an evaluation of the IPCC IS92 Emission Scenarios. J. T. Houghton, L.G. Meira Filho, J. Bruce, Hoesung Lee, B. A. CaBandar, E. F. Haites, N. Harris and K. Maskell (eds). Cambridge University Press, Cambridge, U.K. Jones, P.D., Raper, S.C.B. and Wigley, T.M.L., Southern Hemisphere surface a i r temperature variations: J. Climate Appl. Meteor., 25, Kelly, P M. and Sear, C.B., Climatic impact o f explosive eruptions. Nature, 311, Lamb, H. H., Volcanic dust in the atmosphere with a chronology and assessment o f its meteorological significance. Philos. Trans. Roy. Soc. London, 266, Lamb, H H, Supplementary volcanic dust veil index assessments. Climate Monitor, 6, Lamb, , U p d a t e o f t h e c h r o n o l o g y o f assessments of the volcanic dust veil index. Climate Monitor, 12, McCormick, M.P., Thompson, L.W. and Trepte, CIL, Atmospheric effects o f the M t Pinatubo eruption, Nature, 373, Mass, C.F. and Portman, D.A., Major volcanic eruptions and climate:a critical evaluation. J. Climate, 2, Mass, C.F. and Schneider, SR., Statistical evidence on the influence of sunspots and volcanic dust on longterm temperature records. J. Atmos. Sci., 34, Mitchell, M., Recent secular changes of the global temperature. A n n a l s o f the New York Academy o f Science, 95, Mullan, A.B., O n the linearity and stability o f Southern Oscillation-climate relationships for New Zealand. Int. J. Climatol, 15, Newhall, C.G. and Self, S., The volcanic explosivity index (VEI): A n estimate of explosive magnitude for historical volcanism. J. Geophys. Res., 87(C2), Rampino, M.R. and Self, S., Historic eruptions of Tambora (1815), Krakatoa (1883) and Agung (1963): Their stratospheric aerosols and climatic impact. Quat. Res., 18, Rhoades, D.A. and Salinger, M.J., Adjustment of temperature and rainfall records for site changes. Int. J. Climatol, 13, Robock, A., The volcanic contribution to climate change of the past 100 years. Greenhouse-Gas-Induced climatic change: A critical appraisal of simulations and observations, U K Schlesinger, Ed., Elsevbier, Robock, A, and Free, M.P., Ice cores as an index of global volcanism from 1840 to the present. J. Geophys. Res, 100, Robock, A. and Liu, Y., The volcanic signal i n Goddard Institute for Space Studies three-dimensional model simulations. J. Climate, 7, Robock, A and Mao, J., Winter warming from large volcanic eruptions. Geophys. Res. Lett., 19, Robock, A. and Mao, J., The volcanic signal i n surface temperature observations. J. Climate, 8, Ropelewski, C.F. and Jones, P.D., A n extension of the Tahiti-Darwin Southern Oscillation Index. Mon. Weath., Rev., 115, Rosen, J.M., Kjome, MT., McKenzie, R.L. and Liley, B.J., Decay of Mount Pinatubo aerosol at midlatitudes in the northern and southern hemispheres, J. Geophys. Res., 99, (D12), Salinger, M.J The New Zealand temperature series, Climate Monitor, 9, Stowe. L I., Carey, R.M. a n d Pellegrino, P.P., Monitoring the Mt. Pinatubo aerosol layer with NOAAJ 11 AVHRR data. Geophys. Res. Lett., 19, Strong, A. E., M o n i t o r i n g el Chichon aerosol distribution using NOAA-7 satellite AV H R R sea surface temperature observations. Geofis. Int., 23,

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