an air transport climatology for subtropical southern africa - Royal

INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 16, 265-291 (1996)

AN AIR TRANSPORT CLIMATOLOGY FOR SUBTROPICAL SOUTHERN AFRICA PD.TYSON Climatology Research Group, University of the Witwatersmnd,Johannesburg 2050, South Afnca M. GARSTANG AND R. SWAP

Depariment of Environmental Sciences, Universiry of Virginia, Clark Hall, Charlottesville, VA 22903, USA p. KALLBERG

Swedish Meteorological and Hydrological Institute, Norrkiping, Sweden

AND M. EDWARDS

South Afiican Weather Bureau, Pretoria, South Afiica Received 26 January 1995 Accepted 2 7 June I995

ABSTRACT An air transport climatology is derived for subtropical southern Africa (Africa south of 15OS) by classifying daily synoptic situations into predominant circulation types. The annual variation of these provides the basis for determining month-by-month transport. Percentage zonal transport in easterly and westerly directions, levels of transport, and times of transit are derived from forward trajectory analyses using European Centre for Medium-range Weather Forecasts (ECMWF) data for a 7-year period. It is shown that semi-permanent subtropical continental anticyclones, transient mid-latitude ridging anticyclones and midlatitude westerly disturbances produce major transport into the south-westem Indian Ocean in the Natal plume. Only quasistationary tropical easterly waves result in appreciable transport into the tropical South Atlantic Ocean in the Angolan plume. Total transport is a function of circulation type and frequency, as well as plume dimensions. Transport in continental highs follows an annual cycle reaching peak values in excess of 70 per cent in winter. That in easterly waves also exhibits an annual cycle, but one peaking in summer, when up to 55 per cent transport may occur in north-western regions. Transport in ridging highs and westerly perturbations is much less and occurs throughout the year, with a slight tendency to peak in spring. Recirculation of air is shown to be considerable when anticyclonic conditions prevail. Monthly, seasonal, and annual mass fluxes over and out of southern Africa are determined from transport fields, frequency of occurrence of circulation types and from measurements of aerosol concentrations. An annual mass flux of aerosols some 134 Mtons is generated over the subcontinent. About 60 Mtons year-' are deposited, and approximately 29 Mtons year-' are exported westward over the Atlantic Ocean and 45 Mtons year-' eastward over the Indian Ocean. Twenty-six million tons of the 74 Mtons of aerosols exported annually to the adjacent oceans on each coast are a product of recirculation. Deposition within 10" latitude of the coast is nearly 10 times greater on the east than on the west coast. KEY WORDS:

air transport; trajectories; southern Africa.

INTRODUCTION Long-range transport and horizontal and vertical mixing of aerosols and trace gases in the atmosphere have important implications for global change. Overall global concentrations of atmospheric constituents are a function not only of their regional production but also of their transport and mixing. Most investigations of long-range transport have been based on case studies. It has been shown that Saharan dust may be carried to the Caribbean (Prosper0 et al., 1981; Muhs et al., 1990), the Amazon basin (Swap et al., 1993) and Northern Europe and Fennoscandia (Pitty, 1968; Franzen et al., 1994). Transport of radioactive material from Chernobyl in the Ukraine CCC 0899-841 81961030265-21 0 1996 by the Royal Meteorological Society

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to Sweden and elsewhere has been well documented (Persson et al., 1987). Transport over long distances of continentallyderived particulate carbon in the marine atmosphere was demonstrated quantitatively by Cachier et al. (1986). Long-range transport of aerosols and trace gases associated with biomass burning from Africa south of the Equator into the tropical south Atlantic Ocean and south-western Indian Ocean has been reported (Crutzen and Andreae, 1990; Watson et al., 1990; Levine, 1991; Fishman, 1991; Fishman et al., 1991; Moody et al., 1991; Pickering et al., 1992, 1994; Krishnamurti et al., 1993). More recently, the horizontal and vertical transport of air from subtropical southern Africa during the Southern Africa Fire-Atmosphere Research Initiative investigation (SAFARI)has been examined in detail (Garstang et al., 1995; Swap et al., 1995). Most of the studies quoted above used trajectory analyses of one kind or another to determine transport pathways. The majority of analyses refer only to specific transport episodes or to field observation periods associated with particular programmes or experiments, such as SAFARI. A few comprehensive attempts have been made to determine an air transport climatology showing seasonal variations of transport patterns (e.g. Moody et al., 1991; Dorling et al., 1992). In this paper, a regional air transport climatology is developed for subtropical southern Africa. This is done by classifying daily synoptic conditions into dominant circulation types and by determining their annual variation. Thereafter, air transport fields associated with specific circulation types are linked to the month-by-month variation of the circulation. Mean regional transport pathways are determined and their variation as a function of timedependent circulation fields is shown. Monthly, seasonal and annual mass fluxes over and out of southern Africa are determined from the transport fields and from measurements of aerosol concentrations. Two major pathways, the Natal plume to the east and the Angolan plume to the west, are identified. Substantial recirculation confines a significant amount of the air transport to the confines of the subcontinent. Estimates of annual deposition of aerosols over the continent and into the adjacent oceans are calculated. DATA AND METHODOLOGY South African Weather Bureau 0000 UT and 1200 UT synoptic charts at 850 and 500 hPa over the 5-year period 1988-1992 are used to classify circulation types and their seasonal variation. Air transport fields have been derived from forward trajectories of air parcels originating from six localities in subtropical southern Afnca, defined for purposes of this study as Africa south of 15"s. The localities are 20"E, 15"s; 30"E, 15"s; 25"E, 20"s; 20"E, 25"s; 30"E, 25"s; and 25"E, 30"s. Trajectories have been determined using the method of the Swedish Meteorological and Hydrological Institute (KAllberg, 1984) and the European Centre for Medium-range Weather Forecasts (ECMWF) operational analyses of the six-hourly, three-dimensional windfield available globally at 3 1 levels, 13 of which are below 500hPa. Bengtsson (1985) describes the ECMWF data assimilation system. Pickering et al. (1994) found the fields generated by ECMWF to be superior to others available for such analyses. Likewise, Garstang et al. (1995) and Swap et al. (1995) showed that transport percentages and subsidence rates determined from ECMWF operational analysis fields compared well with estimates derived independently. It is believed that ECMWF data can be used with confidence for large-scale trajectory climatology studies. Forward trajectories are calculated over 10-day periods using the principle of Lagrangian advection. The u, v, and w wind components at the point of origin for 1200 UT on a desired date are used to compute new downstream locations 15 minutes later. New positions are then redetermined using the components at the new location to calculate the next position, and so on. The procedure is repeated for 11 days to obtain a 10-day trajectory. Vertical velocities are determined from adiabatic non-linear normal mode initialization, which permits the diabatic as well as adiabatic processes which influence vertical motion to be considered. The boundary condition at the surface of zero vertical motion, together with the consideration of atmospheric stability, permits trajectories to follow the terrain or go around obstacles when the trajectories approach the surface. Trajectories have been determined at the surface, 850, 800, 700, and 500 hPa levels and then vertically integrated from the surface to 800 hPa and from 700 to 500 hPa to derive low- and mid-level transports. The transport is partitioned into zonal and meridional components by counts of all individual trajectory passages through vertical planes on meridians and parallels at 5" and 10" intervals. Percentage transports through all meridional and latitudinal planes at specific heights and times of travel to reach these locations are determined. Zonal transport is separated into easterly and westerly components, meridional into northerly and southerly.

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Because meridional transports are small in relation to zonal, discussion of the latter will provide the focus of this paper. The detail of the preceding computational procedure is provided in Appendix A. Examination of synoptic charts over the 7-year period 1986-1993 allowed a large number of occasions when particular circulation types were stable for a week or so to be determined. Ten-day trajectories have been determined from the ECMWF data for these cases. A total of 1230 trajectories were calculated and analysed. The final transport climatology is based, not on the synoptic situation prevailing over the subcontinental region as a whole, but on the circulation type prevailing at the locality of origin of the transport. This has had to be done because the considerable north-to-south extent of the subcontinent ensures that more than one circulation type may occur with a particular synoptic situation. Thus it is possible that northern areas (latitude 15OS) may be under the influence of quasi-stationary easterly waves, while to the south in mid-latitudes (35"s)the passage of westerly disturbances may be occurring at the same time. CIRCULATION TYPES On occasions, transport of air out of subtropical southern Africa may be directly south and may reach Antarctica in 4-5 days. On others it may be to the north and reach India in 10 days. On yet others, the lower, middle, and upper atmospheres may be decoupled to the extent that transport out of the region may be in completely different directions at different heights. Notwithstanding, from inspection of the forward trajectory data, and from SAFARI findings (Garstang et al., 1995), it would appear that air transport over and out of subtropical southern Africa is highly organized. The nature of the circulation of the atmosphere over southern Africa has been reviewed in detail (Tyson, 1986; Preston-Whyte and Tyson, 1989). Four major circulation types dominate. They occur with different frequencies throughout the year and are, first, semi-permanent subtropical anticyclones characteristic of the large-scale subsidence region occumng between the Hadley and Ferrel cells of the Southern Hemisphere general circulation (Newell et al., 1972). These continental highs are deepest and strongest in winter (June-July), when they occur with a frequency exceeding 70 per cent of days (Figure 1). In summer (December-January) their frequency falls to below 20 per cent. Secondly, transient mid-latitude ridging anticyclones originate in mid-latitudes on the southern margin of the South Atlantic Anticyclone and ridge to the east over the southern parts of southern Africa in the wake of travelling westerly disturbances. They show little annual variation and occur with a frequency of 10-1 5 per cent, with a slight maximum in the austral spring (September-October). If both types of anticyclonic circulation are taken together, then high pressure cells are found to prevail on more than 80 per cent of all days in July. Thirdly, westerly baroclinic disturbances take the form of travelling Rossby waves or, more occasionally, cut-off lows. They exhibit little seasonal variation and have a combined frequency of 20-30 per cent, showing a spring (October) maximum of occurrence, when the frequency reaches 40 per cent. In the region of southern Africa the passage of westerly wave disturbances occurs with a quasi-regular period of about 6 days (Preston-Whyte and Tyson, 1973). Finally, barotropic quasi-stationary tropical easterly waves are primarily a summer phenomenon. They occur on 5 M O per cent of days in January and show a clear annual cycle with a mid-winter minimum in July, when daily frequency falls to below 5 per cent. Easterly waves occur over the north-western desert and semi-desert areas of southern Africa in response to strong surface heating in summer. Consequently, they are shallow and seldom extend to the 500 hPa level. Unlike their counterparts over A h c a north of the Equator, they are not travelling waves in the easterlies. SYNOPTIC SITUATIONS, CIRCULATION TYPES, AND AIR TRANSPORT When linking air transport to circulation types, it is not possible to do this for the subcontinent as a whole. Instead, the link has to be effected by specific locality of origin of the transport and thus region depending on the area in which the circulation types occur preferentially. An example illustrates the point. On 17 April 1991 two circulation types prevailed in one synoptic situation (Figure 2, upper left). To the north-west an easterly wave occurred; elsewhere over the subcontinent a large continental anticyclone was evident. In the region of the easterly wave, transport into the Atlantic Ocean along the pathway suggested by the circulation type (pathway A) corresponded to actual 875-700 hPa trajectories (Figure 2, upper right). Air originating near the centre of the anticyclone could

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I Ridging anticyclone

Y

Figure 1. Major circulation types affecting southern Africa (after Tyson 1986) and their monthly frequency of occurrence over the 5-year period 1988-1992. Heavy lines represent conditions at 500 hPa; light lines give surface conditions (as sea-level isobars over the oceans and contours of the 850 hPa surface over the subcontinent)

have been expected to recirculate (pathway B). Trajectories show that it did (Figure 2, lower left). Those originating on the south-westem side of the anticyclone could have been expected to exit directly into the south-westem Indian Ocean (pathway C); they did (Figure 2, lower right). Given the high frequency of occurrence of anticyclonic conditions over southern Africa and the tendency for recirculation to occur under such conditions, the issue of recirculation needs to be considered further. Recirculation occurs when air is transported away from a point of origin and then returns in the opposite direction from which it has travelled after having recurved or rotated cyclonically or anticyclonically. Both directions of curvature occur. However, the overwhelming majority of recirculation events occur with anticyclonic recurvature of transported air. Recirculation occurs on a variety of scales ranging from subcontinental, where it is not uncommon for the radius of circulation to be of the order of 40" of longitude (i.e. ca. 4400 km),to regional, with recirculation radii the order of 15" (ca. 1650 km), and local (less than Y , ca. 550 km) (Figure 3, upper, middle, and lower respectively). Annegarn et al. (1993) and Held et al. (1994) report similar recirculation on local or smaller scales.


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Figure 2. An illustration of how different transport modes may occur with the same synoptic situation. Pressure levels are given in italics at I-day intervals along foward trajectories, which are labelled according to their levels of origin at different localities over southern Africa

TRANSPORT BY CIRCULATION TYPE Continental highs

Vertically integrated, surface to 800 hPa transport fields determined from daily transport over 10-day periods from individual localities show distinctively organized spatial patterns of both direct and recirculated transport. In the case of transport from 25"E, 20"s (in northern Botswana on the northern, equatorward limb of the continental anticyclone), two modes of transport can be expected with the continental high circulation type. The first can be expected to exit into the Atlantic Ocean via the Angolan plume, the second into the Indian Ocean via the Natal plume (Figure 4,inset). Easterly flow predominates with this circulation pattern. In the first hour of transport away from the point of origin, 90 per cent of the 1200 UT trajectories are to the west (i.e. easterly); none start out toward the east. Only 7 per cent go north or south and 3 per cent stagnate under calm conditions. (These initial transports on time-scales of an hour are shown in the inset in Figure 5 , upper.) The climatological pattern of the easterly component of the percentage transport is a spatially continuous and coherent contour field (Figure 4, upper). The locus of the core of maximum transport, which defines the maximum frequency pathway, crosses the west coast at about 18"s. At 20"E, 57 per cent of forward trajectories cross the meridian at the 850 hPa level, having left their point of origin 2.6 days before. After crossing the coast with a travel time of 6.7 days the stream attenuates to 7 per cent and subsides to 950 hPa. Easterly zonal recirculation is evident (broken heavy lines in Figure 4,upper) in two streams returning to the area of origin from the east. At 30"E, 17 per cent of the transport is air recirculating at the 750 hPa level, having taken 6.3 days to recurve to this point.

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Figure 3. Forward trajectories to illustrate scales of recirculation occurring at different levels from different localities of origin

Zonal transport to the east (Figure 4, lower) consists only of recirculated transport (because no initial westerly movement occurred in the first hour of transport). At 20"E a maximum of 43 per cent recirculation occurs at 700 hPa, with a travel time of 3.8 days (i.e. air having moved away from the point of origin to the west is now returningfrom the west 3.8 days later). The maximum frequency pathway shows 91 per cent of trajectories cross the east coast at the 750 hPa level after 4.0 days. Thereafter, the stream rises to 450 Wa as it is transported progressively eastward in the westerlies. After 5.7 days 48 per cent of air transported from central southern Africa reaches 70"E at latitude 45"s


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Figure 4. Isopleths of percentage vertically integrated surface-800 hPa zonal transport in easterly and westerly directions with the confinenfa1 anticyclonecirculation type. Locality of origin is 25"E, 20"s. Solid heavy lines indicate the core of direct maximum frequency transport; broken lines indicate the core of maximum frequency recirculated air transport. Bold figures give the total horizontally and vertically integrated percentage transport at a particular meridian; light figures give the height of this transport @Pa) at the location of maximum frequency transport and italic figures the mean transit time for all trajectories to reach the meridian

Analyses from three other localities that identify transport in semi-permanent subtropical continental highs (namely 20"E, 25"s; 3WE, 25"s; and 25"E, 30"s)reveal similar transport fields (Figure 5). Maximum frequency pathways from all four points of origin show the extent and position of the Angolan and Natal plumes (Figure 5, upper). Total zonal percentage transport and times of travel are given at each lo" meridian. It is clearly evident that little air is transported from subtropical southern Africa into the Atlantic Ocean when continental highs prevail. However, as will be seen later, the frequency of occurrence and the horizontal and vertical extent of plumes embedded within anticyclonic circulation do result in a large total transport in months of high frequency occurrence and on the annual and seasonal time-scales. By contrast, transport by continental highs into the Indian Ocean is substantial. Because recirculation keeps occurring en route, more than 100 per cent transport is evident at 35"E from origins at 20"E, 25"s and 25"E, 30"s.

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Figure 5 . Vertically integrated surface-800 hPa transport associated with continental anticyclones. (Upper) Cores of maximum frequency pathways. Heavy solid lines indicate direct transport; broken lines indicate recirculated transport. Within the inset, initial forward-trajectory percentage directions within the first hour of transport are given. (Lower) Percentage recirculated transport for different localities. The figure to the right of the station gives percentage initial stagnation (calm conditions within the first hour); those above and below the arrows give easterly and westerly percentage recirculated transport and the bold figures to left indicate the total percentage recirculation for the station. The heavy bold figure gives the areal mean. Within the insets the variation of transport with pressure level and latitude is given. The remaining symbol convention is as defined in Figure 4

Substantialrecirculation of air occurs before transport takes place out of the subcontinent (Figure 5 , lower). At 30"E, 25"s (over the plateau to the east of Pretoria on the Eastern Plateau Highveld) the recirculation index (defined as the sum of the percentage recirculation to the east and that to the west, together with the percentage calms) reaches 71 per cent. The average figure for subtropical southern Africa as a whole is 64 per cent. With transport to the west over the Atlantic Ocean, air generally sinks as it comes under the influence of the subsidence that occurs in the South Atlantic Anticyclone; with transport to the east air rises as it is entrained into the unstable baroclinic westerlies (Figure 5 , lower inset). Ridging highs

Maximum transport out of transient ridging highs originating in mid-latitudes can be expected to be towards the southern Atlantic Ocean, whereas recirculated transport is likely to occur to the south into the Indian Ocean (Figure 6, inset). Vertically integrated surface to 800 hPa transport from 30"E, 25"s in the first hour of travel is to the

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Figure 6. lsopleths of percentage vertically integrated surface-800 hPa zonal transport in easterly and westerly directions with the ridging anticyclone circulation type. Locality of origin is 30"E, 25"s. Definitions as in Figure 4

extent of 44 per cent to the west, 39 per cent to the east, 0 per cent to the north, 6 per cent to the south and 11 per cent stagnant. The transport to the west (Figure 6, upper) follows the hypothesized pathway, with 19 per cent maximum transport into the Atlantic Ocean at 10"E, at a height of 850 hPa after a transit time of 5.1 days. In this case, the Natal plume comprises recirculated air streaming to the south-east. At 35"E, 52 per cent of the original air remains in the stream, is being transported at 700 hPa and takes 4.2 days to reach the meridian at about 38"s. It reaches 70"E at a height of 450 hPa in 6.2 days, i.e. taking slightly longer than air originating in continental highs. Three stations are positioned to record transport from ridging highs. They are: 20"E, 25"s; 30"E, 25"s; and 25"E, 30"s.All three exhibit similar maximum frequency pathways (Figure 7, upper) and indices of recirculation (Figure 7, lower). Transport to the west is at lower levels than in air moving to the east owing to the different stability regimes prevailing on the two sides of the subcontinent (Figure 7, lower insets).

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Figure 7. Vertically integrated surface-800 hPa transport associated with ridging anticyclones. (Upper) Maximum frequency pathways. (Lower) Recirculation and variation of transport with pressure level and latitude. Definitions as in Figures 4 and 5

Westerly waves Perturbations in mid-latitude westerly flow to the south of the subcontinent constitute the strongest regularly occurring disturbances affecting circulation over southern Africa. Westerly Rossby waves are always deep enough to disturb the circulation over the interior plateau, which has an average altitude of about 1500 m. Often they disturb the entire troposphere over the region. Cut-off lows are stronger and deeper and always perturb the whole troposphere. All westerly disturbances are associated with steep pressure gradients, strong winds, and diminished atmospheric stability to the rear. Only one of the six stations used in the study, that at 25"E, 30"s (on the interior plateau of South Africa just south of Kimberley) is situated to characterize air transport associated with westerly wave disturbances. Zonal transport fields associated with westerly disturbances are given in Figures 8 and 9. They show that little air is transferred into the Atlantic Ocean by these systems. That which is transported to the Atlantic is recirculated air, because in the first hour no air is transported directly to the west. Most air is transported quickly into the mid-latitudes of the Indian Ocean, reaching 35"E in about half the time taken to transport air in anticyclonic circulations. The index of recirculation over the subcontinent is only 17 per cent, ie., about a quarter that associated with continental or ridging highs. What is surprising is that recirculation occurs at all. When recirculated air enters the Atlantic Ocean, it does so at about 650 hPa owing to temporary destruction by the frontal system of the subsidence regime prevailing most of the time over the cold Benguela Current off western southern


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Figure 8. lsopleths of percentage vertically integrated surface-800 hPa zonal transport in easterly and westerly directions with the westerly wave circulation type. Locality of origin is 25"E,30"s. Definitions as in Figure 4

Africa. By comparison, anticyclonic systems produce transport at much lower levels over the Atlantic Ocean and this is always capped by a subsidence inversion or absolutely stable discontinuity (Figure 9, lower insets). Easterly waves

Low-level transport patterns associated with quasi-stationary barotropic tropical easterly wave disturbances are distinctly different from those discussed previously. Air transported from 30"E, 15"s (in southern Zambia ) in the maximum frequency pathway with easterly wave conditions is carried to the west such that 92 per cent reaches the 15"E meridian after 5.4 days (Figure 10, upper). Tropical convection raises the stream to 550 hPa before it subsides quickly to 850 hPa once it reaches the Atlantic Ocean. Thereafter attenuation of transport is rapid. Only 8 per cent of the air reaches 1O"W at 850 hPa after 8.4 days. Some air recirculates back into the tropics in the tropical westerlies at about 5"s (Figure 10, lower). Most of the recirculating air recurves anticyclonically to the south such

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Figure 9. Vertically integrated surface-800 hPa transport associated with westerly waves. (Upper) Maximum frequency pathways. (Lower) Recirculation and variation of transport with pressure level and latitude. Definitions as in Figures 4 and 5

that about 20 per cent is transported in a westerly stream into the mid-latitudes across south-westem South Africa. By the time the stream reaches 30"E only 14 per cent of the original air remains. It takes 6.7 days to reach this locality at a height of 700 hPa. Total zonal transport and maximum frequency pathways for the three points of origin that can be used to assess the effects of easterly wave transport are given in Figure 1 1. Much more air is transported into the Atlantic Ocean in the Angolan plume than reaches the Indian Ocean via the Natal plume. Transport to the west over the Atlantic Ocean is below 800 ma; to the east over the Indian Ocean it reaches about 500 hPa. Recirculation over the northwestern continental areas of Namibia, Angola, and Zambia is significantly less (with an index of 39 per cent) than that occurring to the south-east over South Africa, Zimbabwe, and Mozambique when high pressure conditions prevail (Figure 11, lower inset).

EFFECTS OF THERMODYNAMIC STRUCTURE OF THE ATMOSPHERE No data are available yet by synoptic circulation type on the thermodynamic structure of the atmosphere for Africa south of 15"s. However, mean atmospheric stability has been determined for a 77-day period, including the field observation period of SAFARI during August-October 1992 (Garstang et al., 1995). Three stations were used for trajectory analysis in SAFARI, Kruger National Park (KNP) in South Africa, Victoria Falls (VF) in Zimbabwe, and


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Figure 10. lsopleths of percentage vertically integrated surface-800 hPa zonal transport in easterly and westerly directions with the easterly wave circulation type. Locality of origin is 30"E, 15's. Definitions as in Figure 4

Etosha National Park (ENP) in Namibia. Continental anticyclones prevailed on most days during SAFARI. The vertically integrated, low-level zonal transport from surface to 800 hPa from central southern Africa during the austral spring of 1992 may be illustrated by the easterly and westerly transport fields for Victoria Falls (Figure 12). Maximum frequency pathways for zonal transport for all three stations are given in Figure 13. It is clear that little air is transported into the Atlantic Ocean; the transport reaching 1O"W does so at low levels and takes just over 7 days to reach the meridian. Most air is conveyed into the Indian Ocean and at higher levels, taking about 6 days to reach 70"E at heights between 650 and 400 hPa (Figure 13, lower insets). During SAFARI absolutely stable discontinuitiesprevailed at four levels in the troposphere for much of the time over the whole of southern Africa. Two stable layers will be considered here. The first occurred at the top of the mixing layer (over land) at about 750-700 hPa and prevailed for up to 7 days at a time, being broken only by the passage of westerly disturbances. The second was associated with the large-scale subsidence that occurs in the semi-permanent continental high pressure cells and was observed within the layer 60CL500 hPa. Persistent

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Figure 1 1. Vertically integrated surface-800 hPa transport associated with easterly wuves. (Upper) Maximum frequency pathways. (Lower) Recirculation and variation of transport with pressure level and latitude. Definitions as in Figures 4 and 5

absolutely stable layers were found to prevail for up to 40 consecutive days within these limits on one occasion. The extent to which these layers, particularly the one at about 600-500 hPa, control transport over and out of southern Africa is clear (Figure 13, lower). Recirculation under such conditions reaches an index of 74 per cent. Easterly transport is confined to the layer below the stable discontinuity associated with the mixing layer. Recirculated, anticyclonically recurving air, moving in from the east at a higher level, is constrained by the same stable layer. Westerly transport is capped by the subsidence-induced 600-500 hPa stable discontinuity. Only when the transporting airstream reaches offshore mid-latitudes over the Indian Ocean it is lifted above the level of the stable layer. Recirculated air from the easterly airstream, having subsided over the Atlantic Ocean, approaches the subcontinent from the west at lower levels than the air being transported directly to the east. At middle levels, 700-500 hPa, zonal transport from Victoria Falls shows the same basic pattern as its low-level counterpart, but with some differences (Figure 14). Recirculation of air transported initially in an easterly direction, but thereafter approaching central southern Afiica from east of its point of origin, reaches 36 per cent at 25"E with a transit time of 5.5 days. Dispersion of transport in the Natal plume is slightly greater than at lower levels. The recirculation index is slightly less than in the surface-800 hPa layer, but is still considerable at 69 per cent (Figure 15). Recirculated air, as with low-level transport, moves into the subcontinent from the west at lower levels than direct transport to the east, whereas that moving in from the east does so at higher levels than direct transport to the


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Figure 12. Isopleths of percentage vertically integrated surface-800 hPa zonal transport in easterly and westerly directions from Victoria Falls during the SAFARI field observation period, August-October 1992. Definitions as in Figures 4 and 5

west. The absolutely stable layers effectively cap horizontal transport and inhibit any vertically upward transfer of air over the subcontinent.

MEAN TRANSPORTS AND THEIR ANNUAL VARIATION Transport by circulation type may be summarized by determining the spatially averaged characteristicsof vertically integrated low- and mid-level air transport from the regions bounded by the stations in the areas predominantly affected by specific circulation types. These are the quadrilateral 20"E, 25"s; 25"E, 20"s; 25"E, 30"s and 30"E, 25"s for continental highs; the triangle 20"E, 25"s; 30"E, 25"s and 25"E, 30"s for ridging highs, and the triangle 20"E, 15"s; 30"E, 15"s and 20"E, 25"s for easterly lows. The point 25"E, 30"s has to suffice for westerly waves. In each case recirculation within the areas and transport out of the continent to 10"E, just off the west coast of

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Figure 13. Vertically-integrated surface-800 hPa transport from Etosha National Park, Victoria Falls, and Kruger National Park during SAFARI. (Upper) Maximum frequency pathways. (Lower) Recirculated transport and variations of transport with pressure level, latitude and absolute stability of the atmosphere (occuning when the observed lapse rate is less than the saturated adiabatic lapse rate). Ninety-eight per cent confidence limits are given for the horizontal and vertical dispersion of transport

southern Africa, and 35"E,just off the east coast, will be considered. The meridians 10"E and 35"E will be taken as the limits to define transport exiting the subcontinent. Material exiting to the west does so in the Angolan plume; that to the east in the Natal plume. Total percentage transport across the meridians will be given, together with pressure levels and the mean transit times for transport on maximum frequency pathways. Low-level transport

In the case of the semi-permanent subtropical continental highs, the mean recirculation index is 64 per cent. Ninety per cent of air is exported to 35"E in the Natal plume; 4 per cent is conveyed to 10"E in the Angolan plume (Figure 16). At 35"E the air exits the continent to the east at the 775 hPa level with a transit time of 3.7 days; at 10"E air is exported out of the region towards the west at 750 hPa after a transit of 7.2 days. Pansient ridging highs that originate in mid-latitudes show a mean recirculation index of 63 per cent. An average of 5 1 per cent of air is transported in the Natal plume to 35"E where it exits at 725 hPa with a travel time of 3.1 days. Twenty-four per cent of air is transported westward to 10"E in the Angolan plume reaching the meridian at the 875 hPa level after 4.5 days.


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Figure 14. Isopleths of percentage vertically integrated 70G500 hPa zonal transport in easterly and westerly directions from Victoria Falls during the SAFARI field observation period, August-October, 1992. Definitions as in Figures 4 and 5

Westerly wave disturbances in mid-latitudes produce an 87 per cent transport into the Indian Ocean at 35"E and only 7 per cent into the Atlantic Ocean. The transport over the east coast occurs at about the 650 hPa level and the strong winds of these disturbances ensure a rapid travel time of 2-1 days in air of minimal or zero stability to the 35"E meridian. The air recirculating into the Atlantic Ocean does so at 600 hPa after taking 7.7 days to reach 1O"E. In contrast to the above, quasi-stationary tropical easterly waves produce their maximum transport from subtropical southern Africa into the tropical Atlantic Ocean between latitudes 10"-20"s at 10"E at the 850 hPa level after a transit time of 5.1 days. Only 18 per cent transport takes place into the Indian Ocean via the Natal plume (at 35"E at 600 hPa after 5.7 days).

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Figure 15. Vertically integrated 70C500 hPa transport from Etosha National Park, Victoria Falls, and Kruger National Park during SAFARI. (Upper) Maximum frequency pathways. (Lower) Recirculated transport and variations of transport with pressure level, latitude, and absolute stability of the atmosphere (occurring when the observed lapse rate is less than the saturated adiabatic lapse rate). Ninety-eight per cent confidence limits are given for the horizontal and vertical dispersion of transport

Mid-level transport

With both continental and ridging highs, recirculation is less in the vertically integrated 70&500 hPa layer than in the surface-850 hPa layer. Sixty per cent of the transport takes place into the atmosphere over the Indian Ocean via the Natal plume when continental anticyclones prevail over southern Africa (Figure 17). The amount of westerly transport resulting from perturbations in the mid-latitude westerlies is 57 per cent. The main contrast between low- and middle-level transport fields occurs with tropical easterly waves. Between the surface and 800 hPa, mean transport to the Atlantic Ocean at 1O"E is 60 per cent in 5.1 days. In the 700500 hPa layer it is 16 per cent in 4.7 days. Thus, significantly different percentage transports occur, with transit times not significantly different. With easterly waves, low-level transport into the Indian Ocean in the Natal plume is 18 per cent in 5.7 days; middle-level transport is 37 per cent in 4.7 days. The differences in transport proportions reflect the fact that easterly wave disturbances are features largely induced by surface heating in summer and are shallow, a fact well known from synoptic experience. At 10"E, over the west coast, transport into the Atlantic Ocean with continental highs, ridging highs, and easterly waves occurs at or below 775 hPa. With the less stable conditions associated with westerly waves air exits the

283


Figure 16. Mean percentage surface800 hPa transport and recirculation by circulation type over and out of southern Ahca. Mean heights of transport and transit times to reach beyond the west coast at 10"E and beyond the coast at 35"E are given

subcontinent at the 600 hPa level. Over the east coast, at 35"E,with all systems, transport is confined to 525 hPa or below, i.e. below the level of the persistent subsidence-induced absolutely stable discontinuity that was shown to inhibit vertical transport during the 1992 SAFARI field experiment. Annual cycle of air transport

Given the annual variation of circulation types over southern Afnca that was established at the beginning of the paper (Figure l), it is possible to infer, by circulation type, with some confidence the month-by-month frequency of air transport in the Angolan plume to the northern South Atlantic Ocean and in the Natal plume to the southwestern Indian Ocean (Table I).

Table I. Monthly percentage frequencies of vertically integrated surface-800 hPa and 700-500 hPa air transport by circulation type over subtropical southern Africa

Continental highs Ridging highs Westerly waves Easterly waves

J

F

M

A

M

J

J

A

S

O

N

D

15 7 21 55

21 10 18 45

42 14 25 15

55 12 22 9

62 10 24 4

56 14 27 3

70 10 22 1

48 15 33

41

21 20 41 16

25 17 29 26

18 14 27 36

1

18

38 4

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P. D. TYSON ET AL.

Figure 17. Mean percentage 70C500 hPa transport and recirculation by circulation type over and out of southern Africa. Mean heights of transport and transit times to reach beyond the west coast at 10"E and beyond the east coast at 35"E are given

Transport associated with continental highs occurs all year round, but with considerably greater frequency in winter. Such transport exhibits a clear annual cycle peaking in July. Likewise, transport from easterly waves exhibits a clear annual cycle, but 180" out of phase with its continental-high counterpart. Easterly wave transport peaks in mid-summer (January) and diminishes to almost nothing in winter. By contrast, disturbances associated with the mid-latitude westerlies, both ridging highs and westerly waves, show little annual variation except a tendency to peak in the austral spring. MASS FLUX AND DEPOSITION RATES The preceding analysis provides a way to estimate aerosol mass fluxes and deposition rates over and out of the subcontinent of southern Africa south of latitude 15"s. The envelope through which air is being transported is determined within the 98 per cent confidence limits from the horizontal and vertical distributions of the pathways of maximum transport given in Figures 5 , 7, 9, and 11. This envelope of air transport is determined for each circulation class for transport to the east and west across the meridional planes at 10"E and 35"E in m'. The horizontal advection velocity of the air through each meridional plane is determined by the mean travel time of the trajectories to reach that plane and the distance from the centroid of origin of the air to the plane. The volume flux of air through the meridional plane is given by the plume area on the plane multiplied by the advectionvelocity and is determined in m3 d-' as shown in Table 11. The volume flux is important because it defines the carrying capacity of the transporting medium. The total volume flux into the Atlantic Ocean in the Angolan plume is 43 x loL4m3 d-l, whereas that into the Indian Ocean in the Natal plume is considerably greater at 73 x l O I 4 m3 d-l.

285


Table 11. Volume flux of air through the meridional planes at 10" and 35"E together with the horizontal advection velocity at each meridional plane, the mean aerosol concentration at that plane and the mass flux of aerosols through the plane Circulation type

Continental highs 1O"E 35"E Ridging highs

10"E 35"E Westerly waves 10"E 35"E Easterly waves 10"E 35"E

Area on meridional

Advection velocity

12.0 11.0

2.1 3.4

1.8 9.0

Volume flux (m3 d-' x

Mean concentration 042 m-3)

Mass flux (tons d-' x lo4)

22.0 32.0

69 69

15.2 22.1

4.0 2.6

5.4 17.0

69 69

3.7 11.7

2.1 1.6

3.3 5.5

6.0 7.6

45 45

2.7 3.4

3.8 6.3

3 .O 2.8

9-5 15.5

45 45

4.3 7.0

Mean aerosol concentrations are based on simultaneous filter samples taken from the three sites: Etosha National Park in Namibia, Victoria Falls in Zimbabwe, and Kruger National Park in South Africa during SAFARI. Metals, organic and inorganic carbon, and total nitrogen were determined using PIXE and TRMS analysis (Maenhaut et al., 1993, 1994; Swap, 1995). Mean near-surface, free-air concentrationsobserved at Etosha National Park for each of the circulation types prevailing during the SAFARI field observation period were used to adjust concentrations for the other stations accordingly. The mean aerosol concentrations used in each class are given in Table 11. These values are derived from daily measurements (24 h), which are subject to large variability (plus or minus 66 per cent). Daily zonal mass fluxes have been converted to monthly and annual equivalents (Table 111) by using the percentage occurrence of each circulation type given in Table I. Only in this way are seasonal variations in aerosol loadings taken into account. During transportation no settling of aerosols is taken into account. Monthly and annual mass fluxes into both the Atlantic and Indian Oceans are dominated by frequency of occurrence and spatial extent of continental high pressure systems. Maximum transport into both oceans occurs during the end of the Southern Hemisphere dry season in July, with the strongest seasonal signal present on the west coast. Zonal mass flux into the Indian Ocean is nearly 60 per cent greater than that into the Atlantic Ocean.

Table 111. Monthly zonal mass flux into the Atlantic and Indian Oceans by circulation type (Mtons) J

Continental highs Ridging highs Westerly waves Easterly waves Total monthly mass flux Total monthly flux

F

M

A

M

J

J

A

S

0

N

D

Annual total

1.9 2.8

1.0

1.5 1.7 0.1 0.6 0.3 0.3 0.4 0.5

0.8 1.2 0.1 0.6 0.2 0.3 0.5 0.8

22.2 32.0 1.7 5.5 2.4 3.2 2.5 4.6

1.6 2.9

28.8 45.2

5.3 4.5

74.0

Atlantic at 10"E Indian at 35"E Atlantic at 10"E Indian at 35"E Atlantic at 10"E Indian at 35"E Atlantic at 10"E Indian at 35"E

0.7 0.8 1.0 1.2 0.0 0.1 0.0 0.3 0.2 0.2 0.2 0.2 0.7 0.5 1.2 0.9

0.1

0.1

0.6 0.2 0.3 0.2 0.4

0.4 0.2 0.3 0.2

2.2 3.3 0.1 0.1 0.1 0.1 0.4 0.6 0.4 0.6 0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.4 0.0 0.0 0.0 0.0 0.1 0.1 0.0 0.0

Atlantic Indian

1.6 1.7 2.4 2.3 2.6 4.2

2.9 4.5

3.3 5.1

2.8 4.6

3.6 2.6 5.5 4.2

2.3 3.8

1.6 2.3 2.6 3.0

3.9 4.3

7.3

8.4

7.4

9.1

6.1

4.2

1.9 2.5 3.0 3.0 3.6 4.3

6.6

0.1

2.5 3.3 3.7 4.8

6.8

1.4 0.1 0.3 0.6 0.7 0.3 0.2 0-4 0.2 0.0 0.2 0.1 0.4

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P. D. TYSON ET AL.

Table IV Mean annual deposition as a percentage (italic) and absolute amount (roman) (Mtons year-') as a function of the total zonal transport through the meridional planes of 1O"E(west coast of southern Africa) and 35"E (east coast of southern Africa) ~~

Atlantic Ocean 1O"E-O" 0" -1O"W

Continental highs Ridging highs Westerly waves Easterly waves Total deposition

86.7 18.8 67.6 1.1 100 2.4 52.9 1.4

0 0 100 0.6 0 0 43.2 0.6

23.7

1 .o

Indian Ocean Onward transport (to west)

35"4O"E

40"-50"E

50"40°E

60"-70"E

10.0 3.0 3.6 0.1 0 0 16.7 0.7

6.9

15.7 3.7 3.9

20.0

0.1

5.9

0.6

6.8 2.1 5.3 0.3 16.1 0.5 0 0

4.0

2.9

3.9

3.4 0 0

1.8

7.5 0.4 0 0 29.6 1.2

8.2 0.6 4.8 0.1

3.4

4.6

Onward transport (to east)

2.0 2.6 30.5

The total zonal mass flux into both Southern Hemisphere oceans from Africa south of 15"s is 74 Mtons year-', which is about half the reported mass flux into the North Atlantic from North Africa (D'Almeida, 1978; Duce et al., 1991). Some measure of seasonality in aerosol production is retained because production of aerosols is a strong function of the seasonality of circulation type, e.g. when production under anticyclonic flow conditions reaches a maximum in winter. Deposition of aerosol mass is implied by the transports shown through the west and east coast meridional planes at 10" and 35"E respectively and subsequent planes downstream. Table IV is based upon the annual zonal mass fluxes of Table 111 and on the percentage change in the frequencies along the major pathways shown in Figures 5,7, 9, and 11. Large deposition occurs between longitudes 10"E and the Greenwich meridian. Some 23.7 Mtons is deposited on the Atlantic Ocean surface in this longitudinal band per year. The deposition rate drops off markedly westwards and only 4.0 Mtons year-' continues into the Atlantic beyond longitude 10"". In contrast, over the Indian Ocean some 30.5 Mtons year-' is transported eastward beyond longitude 70"E, with low deposition occurring between the east coast and 70"E. Percentage recirculation indices over the subcontinent are given in Figure 16, together with the percentage occurrence of each circulation type. The area-averaged, annual recirculation index for the subcontinent is 55 per cent. Forty-five per cent of the air leaves the subcontinent on a direct path. Estimates of mass fluxes of aerosols in air exiting on direct pathways and fluxes in air exiting on recirculation pathways can be made from Figures 5 , 7,9, and 11 and Table 111. The total zonal mass flux into both oceans from southern Africa is approximately 74 Mtons year-'. Of this amount about 26 Mtons yew-' is recirculated mass. Over the continent approximately 60.5 Mtons yew-' of mass is recirculated without export. The total mass flux over the continent is thus about 134 Mtons year-'.

CONCLUSIONS Until recently it was a well-established fact that mid-latitudes in the Southern Hemisphere were characterized by a distinctive semi-annual cycle in pressure fields (van Loon, 1972; van Loon and Rogers, 1984a,b). This cycle was known to affect South Africa and the annual variation of the frequency of passing perturbations in the westerlies (Vowinckel, 1956; Taljaard, 1985; Harrison, 1984). Until the late 1970s, the occurrence of both westerly waves and cut-off lows exhibited a clear semi-annual cycle, with peaks in the austral autumn (March-April) and spring


287

(September-October) (Taljaard, 1985). Recently, van Loon et al. (1993), Hurrel and van Loon (1994) and van Loon and Tourpali (1994) have shown that the semi-annual cycle in many parts of the mid-latitudes of the Southern Hemisphere began weakening in the early 1980s. The 1988-1 992 classification of circulation types over southern Africa definitely supports this contention. The autumnal peak that would give a semi-annual cycle in the occurrence of westerly waves and cut-off lows is missing from the recent record. Transport over and out of subtropical southern Afnca south of 15"s varies both quantitatively and qualitatively with the circulation type prevailing over the subcontinent. All circulation systems, except easterly waves, result in major transport of air in the Natal plume into the atmosphere over the south-westem Indian Ocean through a relatively deep column. The transport of air by circulation types, however, must be scaled by the frequency of occurrence of each class. With stable continental high pressure systems, transit times for vertically integrated surface to 800 hPa air to reach the east coast at around 35"E are of the order of 4 days. The systems are large and affect most of the subcontinent. By contrast, with unstable westerly waves, the transit time over land is half that produced by continental highs and the area affected by the systems is only the southern part of the subcontinent. The only system to produce major transport of air from latitudes poleward of 15"s into the tropical South Atlantic Ocean is the easterly wave type. Transit time for air in these systems to reach 10"E in the vicinity of the west coast is of the order of 5 days in the vertically integrated surface-800 hPa layer. The systems affect only the north and north-western parts of the subcontinent. Air being transported to the tropical south Atlantic Ocean does so in the Angolan plume at low levels owing to subsidence over the western subcontinent and south Atlantic Ocean. Air being carried in the Natal plume to the south-western Indian Ocean is transported at higher levels at a mean height of 525 hPa, but at no time higher than the 400 hPa level. Air recirculation back towards southern Afnca from the west occurs at lower levels than that recirculating back from the east. Throughout the year, given the high incidence of anticyclonic circulations, atmospheric stability may be pronounced. This is particularly so in autumn, winter, and spring (Taljaard, 1955; Preston-Whyte et al., 1977; Tyson et al., 1988; Hamson, 1993). During SAFARI the stability of atmosphere was shown to exert an absolute control on both vertical and horizontal transport of air and aerosols and trace gases carried therein (Garstang et al., 1995; Swap et al., 1995). The outcome of the 7-year climatological study reflects on an inter-annual scale the strong role that stable layers played on a day-to-day basis in capping horizontal air transport during SAFARI. The effect of the 600-500 hPa absolutely stable layer is particularly evident. It is only in the baroclinic mid-latitudes of the south-western and southern Indian Ocean that stability fails to dominate. By the time the Natal plume has reached 70"E, after a transit time of up to 6 days depending on the circulation type dnving the transport, it has been lifted to 500-400 hPa levels. Moody et al. (1991) and Cachier et al. (1986) have shown, from chemical analysis of rainfall over the period 198&1987 at Amsterdam Island (ca. 38"S, 78"E), that radon, together with non-seasalt sulphates and nitrates, may be transported from South Africa and Madagascar to the island. It is now clear that these must precipitate in the form of wet deposition in rain from the middle-toupper troposphere and not the lower atmosphere as was thought previously. The transport pathways and times of transport, scaled by the volume of air transported, frequency of occurrence of the controlling circulation patterns, and the associated concentrations of aerosols, allows estimates to be made of the zonal mass flux over and out of southern Afnca. The continental anticyclonic circulation fields, which are dominant in both time and space, result in the transport of the greater proportion of aerosol mass. Thus, maximum zonal mass flux occurs in the Southern Hemisphere dry season in July into both the Atlantic and Indian Oceans. Annual zonal mass flux into the atmosphere over the Atlantic Ocean is just over 29 Mtons and just over 45 Mtons into the Indian Ocean for a total zonal mass flux of 74 Mtons out of the subcontinent south of 15"s. Estimates of recirculation suggest that recirculation over the subcontinent occurs more than half of the time, resulting in a large input into the atmosphere which is not exported from the continent. Some 60 Mtons of material may be injected into the continental atmosphere per year and deposited locally. About 26 Mtons year-' of the 74 Mtons year-' exported is due to the recirculated products. The subcontinent thus generates on an annual basis approximately 134 Mtons of aerosols. Details of the sources of these aerosols and their role in radiant, nutrient, and other budgets is crucially important to regional and global climate and ecosystems. Increasing accuracy in the determination of fluxes and deposition rates under present climatic conditions will allow comparison with past conditions and estimates of future trends. Knowledge of the physical processes is essential if such progress is to be made.

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ACKNOWLEDGEMENTS

This work has been supported in part under grant ATM92-07924 from the National Science Foundation to the University of Virginia and by funds from the South African Foundation for Research Development, Core Programme, granted to the first author. The trajectory calculations were carried out with the support of the Swedish Meteorological and Hydrological Institute and the European Centre for Medium-range Weather Forecasts. The climatological analyses of circulation types were supported by the South African Weather Bureau and the field measurements were part of the IGBP-STARE Southern African Fire-Atmosphere Research Initiative with support from the Etosha Ecological Research Institute at Okaukuejo in Namibia. Work carried out by the first author of the University of Virginia during the spring of 1994 was supported by the Department of Environmental Sciences of the University of Virginia. We wish to express our thanks and appreciation of this support to all of these organizations. Matt Cobbett's assistance with the development of programmes and execution of the computational work and Philip Stickler's completion of final diagrams were likewise greatly appreciated.

APPENDIX A: COMPUTATIONAL PROCEDURES USED TO DETERMINE VERTICALLY AND HORIZONTALLY INTEGRATED ZONAL AND MERIDIONAL TRANSPORTS Input data Circulation type. One of the four circulation types (Figure 1) is selected. Points of origin. A specific locality or localities idare selected which is/are representative of transport under the chosen circulation type. This location becomes the point of origin for the trajectories that will characterize transport under that circulation type. Trajectory calculation. Trajectories are calculated starting at the point of origin at the surface, 850, 800,700 and 500 hPa every 15 minutes for 11 days in order to obtain a 10-day trajectory for each height. Horizontal and vertical distribution of the trajectories. Meridional planes are erected along 12 lines of longitude (lOOW, 0, 10"E, 15"E, 20"E, 25"E, 30"E, 35"E, 40"E, 50"E, 60"E, and 70"E) extending from 45"s to 10"N latitude and from the surface to 500 hPa (Figure Al). Latitudinal planes are erected every 12 lines of latitude from 45"s to 10"N at 5" latitude intervals extending from 1o"W to 70"E longitude and from the surface to 500 hPa. This paper focuses upon the zonal flow (E-W, W-E) that intersects the meridional planes. All M h e r discussion will be directed at the zonal flow through the meridional plane. The meridional plane is divided into 5" and 10" latitude by 100-hPa grid cells as shown in Figure Al. All trajectories from all levels from the point of origin(s) for each circulation type passing through each grid cell of all the meridional planes are recorded. The total number of trajectory hits in each cell is then assigned to the centre of that cell (Figure Al). Horizontal integration

The field of numbers for each circulation type from each point of origin on each meridional plane is then converted to percentages of the total number of trajectories starting from the point of origin. The field of trajectory frequencies is then contoured on the meridional plane and the core of the transport determined (y, z coordinate of maximum frequency). The core of maximum frequency transport is located vertically (z) and horizontally (y) on each meridional plane and is then converted to percentages of the total number of trajectories starting from the point of origin. The field of trajectory frequencies is then contoured on the meridional plane and the core of the transport determined (y, z coordinate of maximum frequency). The core of maximum frequency transport is located


289

Figure A.l Framework for determining vertically and horizontally integrated zonal flow across a meridional plane. The example shown is a plane on the 20"E meridian from 45"s to 1O"N and from the surface to 500 hPa

vertically (z) and horizontally (y)on each meridional plane and is the sum of the percentage frequency at that level on that meridional plane located at the contoured centre. The core is then connected between planes as the pathway of maximum frequency transport (heavy lines in Figures 4-14). The core maximum frequency at each meridional plane refers to the horizontal level of maximum frequency transport and thus differs from the vertically integrated transport of frequencies described below and displayed in the same figures (Figures 4-14). Distinction is made on the meridional plane between direct transport and recirculation where direct transport is defined as flow from the point of origin flowing east or west directly through the grid element of the meridional plane. Recirculation is defined as trajectories that have recurved cyclonically or anticyclonically and returned through a grid element of the meridional wall from the opposite direction. The recirculated trajectories are then separated from the direct transport (broken lines in Figures 4-14). At each meridional plane the core of the direct maximum frequency transport can be identified in terms of its percentage frequency (the sum of the percentage frequencies across the meridional plane at the level of maximum transport), height (in Wa) and transit time (in days). Similarly, information can be obtained for the core of the maximum frequency of recirculated air transport. For clarity the zonal flow through the meridional planes is broken down into easterly transport (westerly zonal flow) and westerly transport (easterly zontal flow). Vertical integration

Vertical integration of the transport through each meridional plane is carried out in order to depict the horizontal distribution of the transport. Vertical integration is carried out for (i) low levels-surface-850 Wa (ii) mid-levels-700-500 Wa (iii) the entire column-surface-500

hPa

and is accomplished by vertically summing all of the percentages registered at the centre of each grid cell on each meridional plane for each circulation type and point of origin. The vertical sum of the percentage values is carried out in each 10" latitude column (Figure Al).

290

F! D. TYSON ET A L

A horizontal field of percentage frequencies results when the vertical integration is carried out on all meridional planes. This horizontal frequency is then contoured and a frequency field is depicted for each circulation type and each point of origin (Figures 4, 6, 8, 10, 12 and 14). REFERENCES Annegam, H. J., Kneen, M. A., Piketh, S. J., Home, A. J., Hlapolosa, H. S.F! and Kirkman, G. A. 1993. ‘Evidence for large-scale circulation of anthropogenic sulphur over South Africa’, Paper presented at the National Associationfor Clean Air Conference, Brits, 11-13 November, 1993. Bengtsson, L. 1985. ‘Medium-range forecasting at ECMWF’, Adv. Geophys., 238, 3-56. Cachier, H., Buat-Menard, F!, Fontugne, M. and Chesselet, R. 1986. ‘Long-range transport of continentally-derived particulate carbon in the marine atmosphere: evidence from stable carbon isotope studies’, Tellus, 38B, 161-177. Crutzen, F! J. and Andreae, M. 0. 1990. ‘Biomass burning in the tropics: impact on atmospheric chemistry and biogeochemical cycles’, Science, 250, 1669-1678. D’Almeida, G. A. 1987. ‘Desert aerosol characteristics and effects on climate’, in Leinen, M. and Samthein, M. (eds), Palaeoclimatology and Palaeometeorology: Modern and Past Patterns of Global Atmospheric Transport, Kluwer Academic Publishers, Dordrecht, 909 pp. Dorling, S. R., Davies, T. D. and Pierce, C. E. 1992. ‘Cluster analysis: a technique for estimating the synoptic meteorological controls on air and precipitation chemistry-results from Eskdalemuir, South Scotland’, Atmos. Environ., 26A,2583-2602. Duce, R. A,, Liss, P. L., Memll, J. T., Atlas, E. L., Buat-Menard, F!, Hicks, B. B., Miller, J. M., Prospero, J. M., Arimoto, R., Church, T. M., Ellis, W., Galloway, J. N., Hansen, L., Jickels, T. D., Knap, A. H., Reinhardt, K. H., Schneider, B., Soudine, A,, Tokos, J. 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Global Biomass Burning: Atmospheric, Climate and Biospheric Implications, MIT Press, Cambridge, MA, 551 pp. Maenhaut, W. I., Salma, M., Garstang, M. and Meixner, F. 1993. ‘Size-fiactioned atmospheric aerosol composition and aerosol sources at Etosha, Namibia and Victoria1 Falls, Zimbabwe’, Pvoceedings I993 American Geophysical Union Fall Meeting, San Francisco, 6-10 December 1993, p. 104. Maenhaut, W. I., Salma, M., Cafmeyer, J., Annegam, H. J. and Andreae, M. 0. 1994. ‘Regional atmospheric aerosol composition and sources in the Eastem Transvaal, South Afnca and the impact of biomass burning’, J: Geophys. Res., in press. Moody, J. L., Pszenny, A. A. P., Grandry, A., Keene, W. C., Galloway, J. N. and Polian, G. 1991. ‘Precipitation composition and its variability in the southern Indian Ocean: Amsterdam Island, 1980-1987’, 1 Geophys. Res., 96,20769-20786. Muhs, D. R., Bush, C. A. and Stewart, K. C. 1990. ‘Geochemical evidence of Saharan dust parent material for soils developed in quaternary limestones of Caribbean and western Atlantic Islands’, Quat. Res., 33, 157-177. Newell, R. E., Kidson, J. W., Vincent, D. G. and Boer, G. J. 1972. The General Circulation of the Tropical Atmosphere and Interactions with Exhvlhopical Latitudes, Vol. 1, MIT Press, Cambridge, MA, 258 pp. Persson, C., Rodhe, H. and DeGeer, L.-E. 1987. ‘The Chernobyl accident-a meteorological analysis of how radionuclides reached and were deposited in Sweden’, Ambio, 16,2&3 1. Pickering, K. E., Thompson, A. M., McNamara, D. P., Schoeberl, M. R., Lait, L. R., Newman, F! A,, Justice, C. 0. and Kendal, J. D. 1992. ‘A trajectory modelling investigation of the biomass burning tropical ozone relationship’, in Proceedings of the Quad. Ozone Symposium, NASA Goddard Space Flight Center, Greenbelt, MD, in press. Pickering, K. E., Thompson, A. M., McNamara, D. P. and Schoeberl, M. 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