INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 14, 691 704 (1994) 551 S10.41 1.355 1.557.33

INTERANNUAL VARIABILITY OF SOME TRACE ELEMENTS A N D SURFACE AEROSOL

R . P. KANE Insriruro Nocioncil de Pesquisas Espcicitris ( I N P E I . 12201-971) SLio Josk dv.7 Cntnpos. SP, Bmsil

Received 23 Noveinher 1992 Accepted 7 August 1993

ABSTRACT After correcting for seasonal variations and long-term (linear or quadratic) trends, the residual interannual anomalies (variations) of the trace elements and surface aerosol show periodicities in the Quasi-biennial Oscillation (2-3 years) and 3-5 years range. Quasi-biennial Oscillations (QBO) of some elements match with the QBO of equatorial 50-hPa wind, but for some others, matching is not good. For some elements, matching with Southern Oscillation index is good. The relationship is probably through the sea-surface temperature changes, which affect the role of the ocean as a source/sink for these trace elements. Interaction between equatorial wind QBO and extratropical phenomena should also be involved.

K ~ Y WORDS Quasi-biennial Oscillation Trace elements

INTRODUCTION

The Geophysical Monitoring for Climatic Change (GMCC) programme (name recently changed to Climate Monitoring and Diagnostics Laboratory, CMDL) of the National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory, Boulder, Colorado, USA, formed in 1972, involves the measurement of several trace elements, such as carbon dioxide, ozone, nitrous oxide, and halocarbons and aerosols. Monthly means have been reported in their various summary reports (GMCC, 1972-1988; CMDL, 1989-1990) and also in some publications (e.g. Komhyr et al., 1985, 1989a; Thoning et al., 1989). Several characteristics of the seasonal, interannual, and long-term variations have already been reported. In this communication we update the earlier analyses and present some new results.

CARBON DIOXIDE MEASUREMENTS

The basic data used are monthly means. These indicate a strong seasonal variation which has been studied in great detail. For example, for Mauna Loa, Hawaii (MLO, 20"N, 156"W), Bacastow et al. (1985) demonstrated a maximum in May-June and a minimum in September-October. These are attributed to the seasonal changes in the metabolic activity of terrestrial plants, although seasonal changes related to fossil fuel CO, increase and seasonably dependent transport of CO, should also be taken into consideration. The seasonal amplitude, although showing considerable scatter from year to year, showed a clear signal for an increase with time. In this note, we do not propose to study this annual wave but concentrate on longer period variations. For this, the annual wave needs to be eliminated, i.e. data are to be deseasonalized. The usual procedure is to estimate an auerage seasonal variation from a few years data and subtract the same from the monthly values of each year. However, the seasonal variation varies considerably from year to year and hence the above procedure leaves a lot of scatter in the residual data. We prefer the alternative

CCC 0899-841 8/94/060691-14 $3 1994 by the Royal Meteorological Society

692 R. P. KANE

procedure of calculating 12-month running means, which eliminates or reduces the annual wave con- siderably and leaves the data smooth. It affects the amplitudes of nearby waves. Thus, amplitudes of the 2-year (biennial) and 3-year (triennial) waves are reduced to 65 per cent and 85 per cent, respectively, whereas larger periodicity (4 years or more) amplitudes are almost unaffected. Also, phases are unaffected and, because of the smoothness, visual estimations are better.

Figure 1 illustrates the procedure we have followed. Figure l(a) shows the seasonal (3-monthly) means of CO, at BRW (Barrow, Alaska, 71"N, 157"W). Figure l(b) shows the running means over four consecutive seasons (12 months), one season (3 months) apart, as a thin line. The thick line is a 2 degree polynomial fit (exponential fit gave a similar result) and represents the long-term secular increases, presumably due to the combustion of fossil fuel (petrol, diesel, etc.) and possibly clearing and harvesting (also burning) of forest land. The difference between the thin and thick curves represents the interannual 'anomaly' or 'variability' shown in Figure l(c).

Figure 2(a, b, c, d) solid lines show the anomalies for Barrow (BRW), Mauna Loa (MLO), Samoa (SMO, 14"S, 171"W) and the South Pole (SPO, 90''s). The crosses represent the time derivatives of the anomaly curves for MLO and SPO as given in Bacastow (1979). Komhyr et al. (1985) have calculated a similar parameter and called it growth rate, which is shown in Figure 2(a, b, c, d) as crosses for about 1976 onwards,

The anomaly as well as the growth rates show considerable variations from year to year. Also, the variations are almost similar at the four locations, which are geographically far apart, indicating a global effect. Apart from the above four locations, many more measurement programmes were started in 1982. A global growth rate for CO, was obtained by averaging the growth rates, weighted by latitudes, from the various stations and is given in the various GMCC reports and is reproduced in Figure 2(f) as a solid line for 1981 onwards, whereas the crosses in Figure 2(f) represent the average time derivative for Mauna Loa and South Pole for 1958-1980.

The time derivatives (or the growth rates) had been reported to be correlated with the Southern Oscillation index (SOI) long ago (Bacastow, 1976). Because SO phenomena are associated with El Niiio events, attempts have been made to relate CO, anomalies to sea-surface temperature and wind perturbations in the tropics and at higher latitudes (e.g. Bacastow, 1976; Bacastow et al., 1980). As Komhyr et al. (1985) mention, the relationship seems to be that minima in SO1 almost always coincide with inflection points of the growth-rate curves, which exhibit maxima about 6 months after the SO1 minima. Figure 2(e) shows a plot of Tahiti minus Darwin sea-level atmospheric pressure (as a measure of SOI), and El Niiio events are indicated by rectangles (filled, strong; shaded, weak). As can be seen, every SO1 minimum is followed by a global growth-rate maximum within a few months. Thus, this relationship seems to be well established even on a global scale. The cause of this relationship is not quite clear. It is probably indicative of significant temporal

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INTERANNUAL ANOMALY

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Figure 1. Carbon dioxide mole fraction (ppm) at Point Barrow, Alaska. (a) Seasonal values, (b) 12-month running averages (full line) and the long-term trend (thick line), (c) difference = interannual anomaly (variability)

QBOs AND TRACE ELEMENTS 693

TAHITI MINUS DARWIN PRESSURE L

x x

50 60 62 64 66 60 70 72 74 76 70 00 02 04 06 00 90 YEAR

Figure 2. (a-d) (full lines) Anomalies a t Barrow (BRW), Mauna Loa (MLO), Samoa (SMO), and South Pole (SPO) and (crosses) time derivatives or growth rates. (e) Southern Oscillation (SO) index represented by Tahiti minus Darwin atmospheric pressure.

(f) Global growth rate. Rectangles mark El Niiio events, full = strong and moderate, shaded =weak

and spatial variability in the exchanges of carbon between the atmosphere and oceans and between the atmosphere and the marine and terrestrial biospheres. Bacastow (1979) noted that, after correcting for the SO1 relationship, both Mauna Loa and South Pole CO, showed a dip in the 1960s (see Figure 2b and d) and concluded that the eruption of volcano Agung in March 1963 put dust into the stratosphere, which reduced solar transmittance, which reduced sea-surface temperatures, which lowered CO, levels. Tans et al. (1989) used a two-dimensional (latitude and height) atmospheric transport model to calculate the CO, sources and sinks needed to explain the observations. The model cannot identify the processes but gives an idea of temporal variability of major global-scale sources and sinks, as illustrated by Tans et al. (1991, figure 4.5) for four geographical regions, namely 0-30°N, 3&90"N, 0-3OoS, and 3&90°S. The model indicates that the tropical source (outgassing from tropical oceans) is about 1 Gt C year-' and hence the estimates of biomass burning in the tropics as 2-5 Gt C year-' given by Crutzen and Andreae (1990) are probably overestimates. Also, the relative magnitudes of the Northern and Southern Hemisphere CO, sources and sinks seem to be changing with time (Novelli et al., 1990). To check whether there were any periodicities in the CO, data, the anomalies were subjected to a

maximum entropy spectral analysis (MESA; Ulrych and Bishop, 1975). Because MESA does not give amplitude estimates correctly (Kane, 1977; Kane and Trivedi, 1982), it was used only for locating possible peaks, & ( k = 1 to n), and these peaks were used in the expression.

f ( t ) = A , + [ ak sin( 271 i ) + b, cos( 2n i)] + E k = 1

where f ( t ) is the observed time series and E is the error factor. A multiple regression analysis (Bevington, 1969) gives the best statistical estimates of A,, (ak, bk) and their standard errors. From these, amplitudes rk and their standard errors can be calculated. Amplitudes exceeding thrice the standard errors would be significant at a 99 per cent a priori level.

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Figure 3 shows the amplitudes of about a dozen periodicities chosen from the MESA of each parameter. The abscissa scale is not the conventional frequency or periodicity but log,,T’ where T’ is the number of seasons, and T = (T1/4) is the number of years. As can be seen, the periodicities are not similar at all locations. However, T = ca. 7.5 years seems to be common for C 0 2 at all locations but not for the SOI. T = 4.5 years seen in the SO index (T - D) is seen only in BRW C 0 2 , whereas T = 3-7 years is seen in CO, at BRW and MLO. A Quasi-biennial Oscillation (QBO, T = 2-3 years) is seen strongly in the SO1 and in CO, at BRW, but very weakly in CO, at MLO, SMO, and SPO. Thus, in agreement with the findings of Tans et al. (1991), the sources and sinks at the four locations have different time evolutions, not completely parallel to the evolution of the SOI. Even for the global growth rate, Tans et ul. (1991) show that, for the E N S 0 events of 1982-1983 and 19861988, the source and sink patterns were different. The main task now is to identify the sources and sinks and distinguish between marine and terrestrial effects. Measurment of isotropic ratios and obtaining vertical profiles in different regions would help in quantifying the global carbon cycle and balancing the carbon budget.

HALOCARBONS AND NITROUS OXIDE

These are trace gases involved in global warming and stratospheric ozone depletion (see, for example, Ramanathan et al., 1985). Nitrous oxide (N,O) is produced both by natural processes and anthropogenic activities, and halocarbons are produced by anthropogenic activities and natural phenomena such as volcanic eruptions. The main removal mechanisms for N,O are photolytic conversion in the stratosphere and reactions with odd oxygen. Halocarbons are difficult to destroy and probably go on accumulating.

QBOs AND TRACE ELEMENTS 695

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Figure 4. Left half: 12-month moving averages of CFC-11, CFC-12 and N,O at BRW, MLO, SMO. Right half: deviations from the trends. The bottom plot is for the 50-hPa equatorial wind

Figure 4 shows the plots for the last 15 years. The left half shows the four-season (12-month) running means, one season (3 months) apart. A continuous rise is evident in all parameters at all the locations. The rise is not linear but slightly exponential. Data for the South Pole were intermittent and not suitable for the present type of analysis.

When the major linear trends were removed, the residual anomalies were as shown in the right half of Figure 4. The fluctuations do not seem to be random and are undulatory. The intervals (months) in successive peaks seem to be in the range 20-36 months, typical of QBO (Quasi-biennial Oscillation, 2-3 years). The bottom curve shows the tropical 50-hPa wind, indicating a strong QBO of 28-32 months. The trace gas peaks do not always match with the wind QBO and hence could be of a different origin. In a recent paper, Kane (1992) compared the wind QBO with QBOs for several other parameters and reported dissimilarities.

METHANE

Methane plays an important role in the radioactive and chemical balance of the Earth. The main sources are fossil fuels (natural gas, coal), domestic animals, wetland, landfills, tropical swamps, rice fields, biomass burning, termites, etc. Destruction is mainly by OH oxidation and a little by soil absorption. Using data from 19 sites in the NOAA/CMDL network, Fung et al. (1991) made a three-dimensional model synthesis

696 R. P. KANE

84 86 88 90 84 86 88 90 I I I I I I I I I I I I I I

1700 -

YEAR Figure 5. Left half 12-month moving averages of CH, (methane) at BRW, MLO, SMO, CG (Cape Grim, Tasmania), and SPO and their linear slopes. Right half: deviations from the linear trends. The top plot is for 50-hPa winds and the bottom plot shows the

global growth rate of methane

of the global methane cycle, and allotted budget figures for the annual emission and destruction rates for the various sources and sinks.

Figure 5, left half, shows the four-season (12 month) running averages, one season (3 months) apart. Linear trends of 14.1, 12.3, 12.0, 11.8 and 12.5 ppb year-' were obtained for BRW, MLO, SMO, CG (Cape Grim, Tasmania, Australia), and SPO respectively. When these were eliminated, the residuals were as shown in the right half of Figure 5. Here again, peaks are noticed separated by about 24-48 months, indicating Quasi-biennial and Quasi-triennial Oscillations. The top curve (crosses) shows tropical 50-hPa winds with a QBO of 36 months. Tans et al. (1991) have given a global growth rate for atmospheric methane, which is reproduced at the bottom (crosses). It indicates a gradual decrease in growth rate and also indicates a wave of 38 months similar to that of wind QBO. Thus, for some reason, QBO seems to be excited in the interannual variation of trace gases.

OTHER TRACE GASES AT MAUNA LOA

Khalil and Rasrnussen (1991) have given the annual average concentrations of trace gases at Cape Kumukahi and Mauna Loa, Hawaii. The data for Mauna Loa are more complete for CO, CH,, N,O, CFC-12, CFC-11, CH,CCl,, CCl,, CFC-22 and CH,Cl. Figure 6 (left half) shows the annual values. Most of these have a linear upwards trend as indicated. When removed, the residuals were as shown in the right half of

QBOs A N D TRACE ELEMENTS

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YEAR Figure 6. Left half: annual values of some trace gases, with slopes of linear trends. Right half deviations from the linear trends

Figure 6. Some peaks indicative of QBO are seen. However, annual means (one value per year) are not adequate for a proper study of QBO. Twelve-month means separated only a few months apart are needed. The above results are, therefore, only indicative and monthly (or at least seasonal) means would be needed for a better study.

CONDENSATION NUCLEI, SURFACE AEROSOLS

Figure 7 shows a plot of the seasonal values (crosses and dashes) of the condensation nuclei (CN) at BRW, MLO, SMO and SPO for the last 15 years. The full lines show the four-season (12 months) running averages, one season (3 months) apart. The seasonal variations are discussed in detail by Bodhaine et al. (1990). The anomalies (full lines) show some interesting features. There are no uniform long-term trends. At BRW there was a wavy structure but no appreciable long-term trend from 1976 to 1982. There was then a sudden large increase up to 1984, followed by a decrease in 1985, but the level did not drop to the 1976-1982 level. For 1985-1990 there was a wavy structure and a slightly rising trend. Could it be that aerosols from the volcanic eruption of El Chichon in 1982 reached Alaska by 1984 and have not yet disappeared? At MLO there was a wavy structure (QBO?) superposed on a rising trend from 1976 to 1983, a sharp fall from 1983 to 1984, followed by a wavy structure. Values in 1991 are roughly the same as in 1984 and in 1976. Thus, no long-term linear trend is seen. At SMO there was a steep rise from 1976 to 1982 and a steeper fall from 1982 to 1983. The 1983 level continued up to 1984, but in 1985 there was a steep rise, followed by a steep fall. Levels in 1987-1990 remained steady and similar to those in 1983-1984 and 1976-1977. Thus, no long-term linear trend is seen. At SPO the level was aimost constant from 1975 to 1983, fell slightly in 1984-1986, and rose sharply thereafter. The seasonal cycle has a maximum in austral summer and the average rise from 1987 to 1990 is due mainly to increasing values of the summer maxima.

The aerosol at SPO is greatly affected by transport of sea salt in the upper troposphere from stormy regions near the Antarctic coast to the interior of the continent (Hofmann et al., 1991). At MLO (and probably at SMO too), long-range transport of dust from the Asian desert in the upper troposphere seems

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QBOs AND TRACE ELEMENTS 699

to be dominating. At BRW, the well-known Arctic haze seems to control the aerosols during winter and spring. Thus, the sources of aerosol are different at the different locations. The long-term variations are not similar, indicating that the variations are not global but of local origin. Some indications of QBO (period 2-3 years) and longer period waves (4-5 years) are seen. The dashed curve between CN(BRW) and CN(ML0) is the Southern Oscillation index obtained as Tahiti minus Darwin (T - D) mean sea-level atmospheric pressure. The minima of CN(BRW) and SO index seem to match well, indicating that the aerosol variations at BRW may be related to the Southern Oscillation. The dashed curve between CN(ML0) and CN(SM0) represent the tropical 50-hPa wind. Its strong QBO seems to have some resemblance with peaks in CN(ML0) and CN(SM0). Thus, aerosol changes at low latitudes may be related to stratospheric winds. This needs further investigation.

ATMOSPHERIC SOLAR IRRADIANCE TRANSMISSION

Figure 8 shows a plot of the atmospheric transmission of the solar irradiance observed at MLO. These data have an annual cycle and their amplitude was reported to have a modulation in coherence with the well-known QBO of the equatorial stratospheric wind (Dutton, 1992). Here we check whether any QBO is seen in the deseasonalized data. Figure 8(a) (full lines) shows the four-season moving averages, one season apart. These data show enormous changes (decreases) after volcanic eruptions, e.g. that of Mount Agung in 1963. Data for 1981 to date are available but are not shown in Figure 8 because the El Chichon volcanic eruption of 1982 caused a still larger decrease, which recovered only recently. During 1958-1962, a Quasi- biennial Oscillation is seen. The crosses show 3-year running averages, adjusted manually for some intervals. Figure 8(b) shows the equatorial 50-hPa wind. Figure 8(c) shows the interannual variation of solar irradiance, obtained as a difference between the full lines and the crosses of Figure 8(a), the maxima following the wind minima by a few months, as shown by the slant dashed lines. However, the QBO effect is rather small.

For the South Pole (SPO), Dutton et al. (1991) showed that the irradiance during the months February and March decreased from 1976 to 1988. Owing to lack of sunlight in austral winter, data for April-August are not available. Using a value of zero for these months we calculated 12-monthly running means, 3 months apart, which are shown in Figure 9(a). The thick line is a long-term trend, which shows a decrease from

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Figure 9. (a) Solar irradiance at the South Pole, 12-month running averages (thin line), and 3-year running averages (thick line). The difference is shown in (c) and running averages of the difference in (d). (b) Sunspot cycle and (e) Southern Oscillation index (T - D)

700 R. P. KANE

1976 to 1981, a steady level from 1981 to 1985 and a rise thereafter. It is tempting to suspect a sunspot cycle effect, but as seen from Figure 9(b), which shows the sunspot cycle, there is no phase matching, even with a lag. Dutton et af. (1991) report that during this period the cloudiness during austral summer at SPO increased from 1976 to 1987. Thus, the decrease in solar irradiance seems to be due to increased cloudiness. Why the cloudiness increased, is a moot question.

If the thick line in Figure 9(a) is subtracted from the thin line, the difference is as shown in Figure 9(c). Figure 9(d) shows the same but with further smoothing by obtaining running averages over four consecutive seasonal values. Figure 9(e) shows the Southern Oscillation index obtained as Tahiti minus Darwin (T - D) pressure. There is an indication of a possible negative relationship, i.e. larger solar irradiances for lower (T - D), which is also known to be coincident with El Niiio events. The reason for such a relationship needs further investigation.

STRATOSPHERIC WATER VAPOUR

As shown earlier, the CH, content of the atmosphere has risen at a rate of ca. 18 ppb year-' from 1980 to 1990. If this increase is extended to the stratosphere the photolysis of CH, could cause an increase in the stratospheric water vapour content. Hofmann et af. (1991, Figure 5.12) have given the water vapour volume mixing ratios in the stratosphere at Boulder (40"N, 105"W) at three levels (25, 50, 80-hPa), which are reproduced in Figure 10. There are wavy structures with periodicities of 2-3 years (QBO) as well as 3-5 years. As pointed out by Komhyr et al. (1989, Figure 5.8), the two ENS0 events of 1982-1983 and 1987 (low T - D values) seem to be reflected as above-average water vapour amounts in the lower stratosphere (80-hPa), probably due to enhanced convection associated with warmer ocean temperatures.

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Figure 10. Annual values of the water vapour mixing ratio above Boulder, Colorado, at 25, 50, and PO-hPa levels. Southern oscillation index (T - D) and 50-hPa equatorial wind are also shown

QBOs AND TRACE ELEMENTS 70 1

STRATOSPHERIC OZONE

Solar UV radiation of wavelenghs shorter than ca. 240 mm produces ozone (0,) in the stratosphere, with a broad maximum at ca. 25-30km. About 85 per cent of the ozone is in the stratosphere and about 15 per cent in the troposhere, although some trickles to the surface of the Earth. Because ozone strongly absorbs UV in the range 240-310 nm, which is harmful biologically, there is great concern about the depletion of the stratospheric ozone layer in the last decade. In the Antarctic region, depletions as large as 50 per cent are observed during October and are attributed to the destruction of 0, by halocarbons, mostly produced by human activities. Estimates of the total ozone content of the atmosphere (mostly stratospheric) are obtained indirectly by Dobson spectrophotometers, which compare the relative intensities of selected pairs of wavelengths in the Huggins 0, absorption band (300-325 nm). More than 250 Dobson instruments are functioning all over the world and GMCC and CMDL report data for 16 instru- ments.

Monthly mean values of total ozone show large seasonal variations, with maxima generally in local spring and minima in local autumn, with a range of ca. 20 per cent. The deseasonalized data show interannual variations, as shown in Figure 11 as averages for different latitudinal zones (Kane, 1988, updated). Figure l l(a) shows the plot for North Pole (Resolute Bay) and Figure l l(b) for Northern Hemisphere temperate latitudes. Figure 1 l(c) shows the 50-hPa equatorial winds. The vertical lines mark the maxima of the westerly winds. As can be seen, the ozone values show strong QBO, with the maxima coinciding with the minima of the wind QBO and vice versa. However, the ozone QBO are somewhat irregular and very weak during 1972-1976. Figure l l(d) shows tropical ozone and Figure ll(e) shows the Southern Oscillation index represented by Tahiti minus Darwin (T - D) pressure. No clear relationship between ozone and SO1 (or the accompanying El Niiio events) is indicated. However, a spectral analysis showed that both SO1 and ozone had periodicities in the ranges QBO (2-3 years), 3.5-4, 6-7, and 10-11 years, but relative magnitudes were different in different longitude zones.

Figure 1 l(f) shows ozone variations for the Southern Hemisphere temperate latitudes and Figure 1 l(g) for the South Pole region. Irregular QBO are seen. In all the ozone plots the thick lines represent 3-year running averages. Figure l l (h) represents global average ozone, obtained by applying 1, 3, 4, 3, and 1 weightings to the above five latitudinal zone values. The variations (QBO) are now rather small, indicating that the QBO at all latitudes do not have exactly similar phases. The thick lines indicate large long-term changes in the Northern Hemisphere, namely an increase of ca. 3-5 per cent from 1958-1960 to 1969-1970, a fall from 1970 to 1976, a rise from 1976 to 1980, and a fall of ca. 3 per cent from 1980 to 1986-1987. There is considerable concern about the recent 3 per cent decrease, and it is attributed to destruction of ozone by halocarbons escaping from refrigerators and sprays. However, this process of destruction has been identified convincingly only in the Antarctic region (Anderson et al., 1989). Also, much larger changes (decreases) occurred in earlier years (1960-1962), when the use of halocarbons was much less. Thus, a possibility arises that the causes of these changes may lie elsewhere. Some of the changes are due to solar (sunspot) cycle effects, where larger ozone values would be expected near 1958, 1969-1970, 1979-1981 and 1989-1990 (sunspot maxima, see Figure 1 l(i)). Other possible cyclic causes (e.g. a 22-25-year periodicity) cannot be ruled out.

In the last two decades, satellite ozone measurements have been made by BUV and TOMS and SBUV spectrometers aboard the Nimbus 7 satellite. Some of these show some drift, but, after esti- mating and correcting for this, zonally averaged patterns can be obtained (Bowman, 1989; Gray and Dunkerton, 1990), which are reproduced in Figure 110). As can be seen, the curves at all latitudes (30"N to 30"s) show QBO, and Gray and Dunkerton (1990) point out that for the equatorial region the QBO maxima occur soon after the westerly onset of the 50-hPa wind. These authors, as well as Bowman (1989) and, earlier, Hamilton (1989), show that even the deseasonalized plots show an annual synchronization, i.e. subtropical ozone maximizes in the local winter-spring in both hemispheres. This is taken as evidence that the wind QBO acts to modulate a strong seasonal ozone transport from mid-latitudes to the tropics. Recently, Shiotani (1992) reported a longitudinal structure in the QBO and ENS0 ozone variations.

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YEAR Figure 11. (a, b, d, f, and g) (full lines). Twelve-month running averages of north polar, north temperate, tropical, south temperate and south polar total ozone. (h) Global ozone, obtained by applying 1, 3, 4, 3, and 1 weightings to the above five latitude zones. Thick lines are 3-year running averages. (c) The 50-hPa equatorial wind. (e) Southern Oscillation index (T - D). (i) Sunspots.

(j) Nimbus satellite BUV, SBUV, TOMS ozone. Vertical lines mark the maxima of the westerly 50-hPa winds

TROPOSPHERIC (SURFACE) OZONE

The source of almost all near-surface ozone is the stratospheric ozone layer and, hence, measurements of this are valuable for studying stratospheric-tropospheric exchange processes. Monthly values are not available to us, but the plots of Figure 5.3 in Komhyr et al. (1989b) indicate that the summer and spring

QBOs AND TRACE ELEMENTS 703

(June-November) values at BRW show a rising trend of 1.39 & 057 per cent year-’, the January values at MLO show a rising trend of 1.37 f 1.00 per cent year-’, whereas SMO and SPO show decreasing trends of -1.84 k 1.25 per cent year-’ and - 1.68 f 0.85 per cent year-’, respectively, in the austral summer months. In addition, these plots show wavy structures at all of these locations, which appear to be QBO. This needs further study by using 12-monthly running averages.

CONCLUSIONS

When seasonal variations are eliminated and long-term trends (linear or quadratic) are corrected for, the residual interannual anomalies for trace elements and aerosols show periodicities in the QBO (Quasi- biennial Oscillation, 2-3 years) and/or the 3-5-year region. For some elements, the QBO matches with the well-known QBO of equatorial 50-hPa winds; but for some other elements the matching is not good. In some cases, matching with the Southern Oscillation index (represented by Tahiti minus Darwin atmospheric pressure) is quite good.

For most of the trace gases the ocean is an important source or sink, depending on the temperature. The convective activity associated with sea-surface temperature (SST) should modulate the interaction between equatorial wind QBO and extratropical planetary waves, as seems to be happening for ozone (Komhyr et al., 1991). In addition, SST changes could modulate the Hadley cell circulation. In principle, similar mechanisms may be operative for all the trace gases. Recently, Gray and Chipperfield (1990) and Chipperfield and Gray (1 992) have presented an interactive radiative-dynamical-chemical model of the atmosphere that includes a parametrization of the mechanisms believed to be responsible for the QBO of the zonal winds. It predicts a QBO not only in temperature at 20-30 km, but in a range of chemical species, including ozone (see also Gray and Dunkerton, 1990) and a number of other trace gases. Our results agree with these predictions in general, but the varying phases need further investigation.

ACKNOWLEDGEMENTS

This work was partially supported by FNDCT, Brazil under contract FINEP 5373 CT.

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