Executive Summary

The project on moisture transport, sources and sinks, seeks to investigate the underlying dimension of atmospheric moisture on which the South African rainfall is inherently dependent. This aspect of the South African climate system has been under-investigated, with past work providing some insight, yet at times contradictory interpretations. This project thus focuses on clarifying the issues involved in order to better understand the implications for rainfall in the context of climate variability and possible implications of climate change. At the same time, the results provide a foundation for further research avenues. The original project objectives, as proposed, were as follows:

These objectives have been achieved, and in many respects, exceeded.

The research follows two complementary pathways, adapting the research agenda in light of the evolving results and findings. Firstly, a trajectory model is developed and then applied to investigate the time-evolving transport paths of the atmospheric moisture, and the related sources of moisture. The trajectory model takes advantage of low cost computing infrastructure, and is developed to specifically address large-volume trajectory calculations. From this is developed a 20-year climatology of moisture transport for southern Africa that is then examined in terms of seasonal and sub-seasonal attributes. Secondly, the large-scale seasonal mean moisture dynamics are examined to identify key source regions of moisture contribution to the atmosphere. Both approaches develop different insights into the climate system, yet complement and support one another.

Overall, the results lead to a number of key conclusions:

  1. For summer rainfall over the sub-continent, and particularly for the key economic agricultural region of the South African interior, at the seasonal time frame there is a significant moisture transport pathway from the east (elevated over the ocean) with a dominant source of moisture from east of Madagascar.
  2. The ocean regions to the southeast of the continent, in particular over the Agulhas gyre, are a strong source of moisture for the atmosphere. However, there is some question over how much of this moisture makes direct or indirect contributions to continental rainfall.
  3. There are clear indications that local continental feedbacks may be of significant importance in modulating intra-seasonal variability of rainfall.
  4. Intra-seasonal atmospheric moisture content over the continent does not show large variability, suggesting the dynamics of circulation processes may have a greater role in modulating rainfall than the moisture transport itself.
  5. Spatially extensive heavy rainfall events appear to have moisture transport pathways and mean atmospheric moisture contents that are not dissimilar to the seasonal means, again suggesting that other dynamic processes may be more important.
  6. Climate change implications for moisture transport remain uncertain due to issues of skill in the GCMs used to project future climate regimes. However, current consensus is that significant changes are indeed expected, with a tentative inference for wetter conditions on the eastern margins of South Africa, and drier conditions to the west.

These results suggest, in particular, three main research avenues in need of further investigation. Firstly, to distinguish the relative roles of the identified large scale sources of moisture to the atmosphere (particularly the Agulhas gyre) on a seasonal basis. This work would require the application of regional climate models to better address the regional scale process controls.

Second is the question of identifying the role of local land-surface feedbacks and moisture recycling in the intra-seasonal time frame. There are suggestions that these may be of significant importance but it remains unclear whether their role is dominant in modulating individual events, or how these impact the seasonal mean, such as by playing a role in determining onset of summer rains, or influencing the intensity of extensive heavy rainfall.

Third, it remains unclear as to how credible global climate models are in representing the foundational moisture transport processes, and consequently their ability to credibly simulate projections of future climate. This requires new methodological approaches to determining model skill at time and space scales of relevance to the continental hydrology.

In terms of building capacity, the project has developed a new trajectory model that may be readily applied in a wide variety of contexts, and has already been adopted as part of other post-graduate thesis work. Linkages with the SA Weather Service have improved with increased collaboration as a direct consequence of this project. In addition, a core group of scientists and students has been developed who are increasingly addressing the key question of regional hydrology in the context of a coupled ocean, land, and atmospheric system, reflecting complex feedbacks and coupling mechanisms, and which necessitates a coupled systems approach. A number of post-graduate projects have evolved from this that will, in part, address the key questions outlined above.

The primary recommendation to come from this project is for continued support of complementary climate research activities, focused on coupled climate system dynamics, and in particular through the use of climate models. Key to this is developing a sustainable research thrust that builds on accomplished work to support an understanding of the collective aspects of the climate system, the consequent impact on surface hydrology, and ultimately, the relevance to society.

Overall the project has successfully met and exceeded the original objectives, and represents a significant advance in our understanding of this key component of the climate system.