Tropospheric air trajectories that occurred during the Southern African Fire‐Atmosphere Research Initiative (SAFARI) in August–October 1992 are described in terms of a circulation classification scheme and the vertical stability of the atmosphere. Three major and frequently occurring stable discontinuities are found to control vertical transport of aerosols in the subtropical atmosphere at the end of the dry season. Of these, the main subsidence‐induced feature is a spatially ubiquitous and temporally persistent absolutely stable layer at an altitude of about 5 km (3.5 km above the interior plateau elevation). This effective obstacle to vertical mixing is observed to persist without break for up to 40 days. Below this feature an absolutely stable layer at 3 km (1.5 km above the surface) prevails on and off at the top of the surface mixing layer for up to 7 days at a time, being broken by the passage of regularly occurring westerly wave disturbances. Above the middle‐level discontinuity a further absolutely stable layer is frequently discerned at an altitude of about 8 km. It is shown that five basic modes can be used to describe horizontal aerosol transportation fields over southern Africa. Dominating these is the anticyclone mode which results in frequent recirculation at spatial scales varying from hundreds to thousands of kilometers. In exiting the anticyclonic circulation, transport on the northern periphery of the system is to the west over the Atlantic Ocean via a semistationary easterly wave over the western part of the subcontinent. On the southern periphery, wave perturbations in the westerly enhance transports which exit the subcontinent to the east into the Indian Ocean. Independently derived data suggest that during SAFARI only 4% of the total transport of air from three locations south of 18°S was into the Atlantic Ocean. Over 90% of the transport was into the Indian Ocean across 35°E. This result reflects circulation fields typical of the extremely dry conditions prevailing in 1992. The integrated effect of the control exerted by atmospheric stability on vertical mixing, on the one hand, and the nature of the horizontal circulation fields, on the other, is to produce a distinctive suite of transport patterns that go a long way to explain the observed high concentrations of tropospheric aerosols and trace gases observed over the subcontinent in winter and spring, as well as over the tropical South Atlantic and southwestern Indian Oceans.

This study focuses on the interplay between mean sea level pressure (MSLP), sea surface temperature (SST), and wind and cloudiness anomalies over the Indian Ocean in seasonal composite sequences prior to, during, and after strong, near-global El Niñ o and La Niñ a episodes. It then examines MSLP and SST anomalies in the 2-2.5-year quasi-biennial (QB) and 2.5-7-year low-frequency (LF) bands that carry the bulk of the raw ENSO signal. Finally, these fields were examined in conjunction with patterns of correlations between rainfall and joint spatiotemporal empirical orthogonal function (EOF) time series band pass filtered in the QB and LF bands. The seasonal composites indicate that the El Niñ o-1 (La Niñ a-1) pattern tends to display a more robust and coherent (weaker and less organized) structure during the evolution towards the mature stage of the event. The reverse tends to be apparent in the cessation period after the peak phase of an event, when El Niñ o events tend to collapse quite quickly. Climatic variables over the Indian Ocean Basin linked to El Niñ o and La Niñ a events show responses varying from simultaneous, to about one season's lag. In general, SSTs tend to evolve in response to changes in cloud cover and wind strength over both the north and south Indian Ocean. There are also strong indications that the ascending (descending) branch of the Walker circulation is found over the African continent (central Indian Ocean) during La Niñ a phases, and that the opposite configuration occurs in El Niñ o events. These alternations are linked to distinct warm-cool (cool-warm) patterns in the north-south SST dipole over the western Indian Ocean region during the El Niñ o (La Niñ a) events. An examination of MSLP and SST anomaly patterns in the QB and LF bands shows that signals are more consistent during El Niñ o-1 and El Niñ o sequences than they are during La Niñ a-1 and La Niñ a sequences. The QB band has a tendency to display the opposite anomaly patterns to that seen on the LF band during the early stages of event onset, and later stage of event cessation, during both El Niñ o-Southern Oscillation (ENSO) phases. El Niño events tend to be reinforced by signals on both bands up to their mature phase, but are then seen to erode rapidly, as a result of the presence of distinct La Niñ a anomalies on the QB band after their peak phase. During La Niña events, the opposite is observed during their cessation phase. Both QB and LF bands often display SST dipole anomalies that are not clearly evident in the raw composites alone. An eastern Indian Ocean SST dipole shows a tendency to occur during the onset phase of particular El Niño or La Niñ a episodes, especially during the austral autumn-winter (boreal spring-summer) and, when linked to tropical-temperate cloud bands, can influence Australian rainfall patterns. Analyses of seasonal correlations between rainfall and joint MSLP and SST EOF time series on QB and LF bands and their dynamical relationship with MSLP and SST anomalies during El Niñ o and La Niñ a events...

Significant differences in rainfall over central South Africa are known to occur between opposite extremes in the phase of the Southern Oscillation, but details of both temporal and spatial aspects of the modulation of South African rainfall with the phase changes of the Oscillation remain to be described. Zero‐lag correlations between the Tahiti—Darwin Southern Oscillation Index and monthly and 3‐month seasonal rainfall over South Africa suggest that the rainfall—Southern Oscillation Index association is best defined in the late summer season January—March and in a north‐west to south‐east aligned zone across the central summer rainfall region of South Africa. Rainfall in this zone is directly related to the Southern Oscillation Index, increasing during high phase summers. An apparent semi‐annual cycle in the rainfall—Southern Oscillation Index correlations over central South Africa is in phase with the November and February turning points of a semi‐annual cycle in atmospheric circulation parameters over southern Africa. A plausible circulation mechanism is suggested to account for some of the spatial and temporal characteristics of the association between the Southern Oscillation and South African rainfall.

Early warning of weather conditions conducive to outbreaks of Ross River virus disease is possible at the regional level with a high degree of accuracy. Our models may have application as a decision tool for health authorities to use in risk-management planning.

This study focuses on the interplay between mean sea level pressure (MSLP), sea surface temperature (SST), and wind and cloudiness anomalies over the Indian Ocean in seasonal composite sequences prior to, during, and after strong, near-global El Niñ o and La Niñ a episodes. It then examines MSLP and SST anomalies in the 2 -2.5-year quasi-biennial (QB) and 2.5-7-year low-frequency (LF) bands that carry the bulk of the raw ENSO signal. Finally, these fields were examined in conjunction with patterns of correlations between rainfall and joint spatiotemporal empirical orthogonal function (EOF) time series band pass filtered in the QB and LF bands.The seasonal composites indicate that the El Niñ o-1 (La Niñ a-1) pattern tends to display a more robust and coherent (weaker and less organized) structure during the evolution towards the mature stage of the event. The reverse tends to be apparent in the cessation period after the peak phase of an event, when El Niñ o events tend to collapse quite quickly.Climatic variables over the Indian Ocean Basin linked to El Niñ o and La Niñ a events show responses varying from simultaneous, to about one season's lag. In general, SSTs tend to evolve in response to changes in cloud cover and wind strength over both the north and south Indian Ocean. There are also strong indications that the ascending (descending) branch of the Walker circulation is found over the African continent (central Indian Ocean) during La Niñ a phases, and that the opposite configuration occurs in El Niñ o events. These alternations are linked to distinct warm -cool (cool-warm) patterns in the north-south SST dipole over the western Indian Ocean region during the El Niñ o (La Niñ a) events.An examination of MSLP and SST anomaly patterns in the QB and LF bands shows that signals are more consistent during El Niñ o-1 and El Niñ o sequences than they are during La Niñ a-1 and La Niñ a sequences. The QB band has a tendency to display the opposite anomaly patterns to that seen on the LF band during the early stages of event onset, and later stage of event cessation, during both El Niñ o -Southern Oscillation (ENSO) phases. El Niño events tend to be reinforced by signals on both bands up to their mature phase, but are then seen to erode rapidly, as a result of the presence of distinct La Niñ a anomalies on the QB band after their peak phase. During La Niña events, the opposite is observed during their cessation phase.Both QB and LF bands often display SST dipole anomalies that are not clearly evident in the raw composites alone. An eastern Indian Ocean SST dipole shows a tendency to occur during the onset phase of particular El Niño or La Niñ a episodes, especially during the austral autumn -winter (boreal spring -summer) and, when linked to tropical-temperate cloud bands, can influence Australian rainfall patterns.Analyses of seasonal correlations between rainfall and joint MSLP and SST EOF time series on QB and LF bands and their dynamical relationship with MSLP and SST anomalies during El Niñ o and La Niñ ...