Long-term trends in spatial and temporal distribution of southern African aerosols over the southern Indian Ocean during the austral winter: Patterns and implications

Long-term analysis of trends and patterns in the spatial and temporal variability of southern African aerosol transports indicate that aerosol transport is episodic in nature and exhibits a discrete spatial form, resembling a filament. Filamentary aerosol transport events (FATEs) occur twenty-percent of the July through October dry-season. The start date of the transport season varies according to the phase of the El Nino Southern Oscillation (ENSO) with an average difference of three weeks between El Nino and La Nina years for transport season start date. Upon conducting analysis regarding the spatial nature of FATEs based on stratification by ENSO phase, it is clear that the outflow region does not shift over time, but rather shifts in magnitude. In order to contextualize long-term aerosol transport trends a thorough comparison of the synoptic, large-scale and climate conditions during the 1992 and 2000 dry-seasons is presented. The implications of the comparison results are discussed in terms of their influence on southern African dry-season circulations, particularly on the mechanisms responsible for westerly transport of aerosols over the southern Indian Ocean. Upon examining a wide range of meteorological fields, the most important fields for understanding ENSO-driven variability of the southern African dry-season were identified as the surface pressure field and upper level wind field (where jet streams are located). These fields vary according to ENSO phase, where the conditions for these fields during El Nino year dry-seasons most closely resemble conditions during years identified as normal ENSO phase. An important non-ENSO driven trend discovered in the course of the long-term analysis of the functioning of the southern African and southern hemispheric atmosphere, is the concomitant increase in wave number 4 and decrease in wave number 2 during the successive dry-seasons spanning the period 1979-2006. This largely synthetic work explains how previous ideas about the functioning of the southern African dry season atmosphere need to be addressed in the context of climate-driven changes in synoptic and large-scale circulation and how doing so leads to a more robust understanding of the nature and variability of aerosol transport over the southern Indian Ocean.