African Easterly Jets Dynamics over Central Africa.

Abstract

This thesis characterized the representation of two regional atmospheric climate features, AEJ-N and AEJ-S, in reanalysis data sets and climate model outputs. It has shown that the dynamics of these systems are fundamental to the climate distribution and simulation of rainfall over central Africa. The AEJ-S in particular is an important feature of the central African mid-level circulation and has been identified as a key contributor to convection over the region, but yet less is known about its dynamic and structure. By examining AEJ-S mechanisms, this thesis will improves the foundation of new mechanisms that help to identify suitable metrics for the evaluation of Global models over the Central Africa region. As climate models are vital to the assessment of the impacts of climate change, establishing how well models reproduce key processes is important to our confidence in these tools. In the first part of the results in this thesis, we used reanalysis data to establish mechanisms related to AEJ-S dynamics. Results demonstrate that AEJ-S is dominated by rotational circulation and is maintained by a mid-level high that forms over the Kalahari region from September to November. At the core of this high, anticyclonic circulation is induced and maintains AEJ-S located at the northern flank of the high. A link between AEJ-S dynamics and southern subtropical westerly waves is also revealed. It is shown that, when waves amplify over southern subtropics, they modify lower tropospheric heating. Depending on the phase of the wave, this modifies the latitude temperature gradient throughout equatorial regions, therefore modulating the intensity of the jet. At the intraseasonal scale, we describe the meteorological characteristics directly associated with the upper wave flow such as wave structure and periodicity. Results show that the eastward propagation of the wave takes about 10 days to reach the west coast of South Africa and its passage over the Continent takes about 06 days. Its impact on the warming of the atmosphere and on the mid-level circulation is associated with phases, structure and wave periodicity when it crosses the sub-continent. By clarifying mechanisms that govern the AEJ-S, this research work contributes to insight into central Africa climate dynamics and suggests a link between central Africa and southern Africa climate systems. We examine in the second part the model representation of the SON characteristics, such as location and intensity, of the AEJ north and south. The analysis evolves to assess key drivers of the AEJ from energetic interactions, the characteristics of mid-level highs and thermal heat lows and the nature of surface thermal heating. In general, the spread of simulated AEJ locations around reanalyses is larger for the CMIP5 sample compared to CMIP6 equivalent models, indicating improvement from CMIP5 to CMIP6. However, this improvement in some CMIP6 models (e.g. GISS-E2-1-G and MIROC6) is not related to a maximum surface meridional gradient in temperature. Most CMIP5 and CMIP6 models underestimate the surface temperature gradient over the AEJ-N region. As a first-order diagnostic of the jet’s acceleration, most coupled models better simulate the atmospheric energetic interactions over the AEJ-N region which leads to its strong contribution to AEJ-N maintenance compared to AEJ-S. This study strengthens our understanding of the mid-level circulation over Central Africa by detecting gaps in the mechanisms maintaining the AEJ in coupled models and highlights processes that should be improved in future ensembles.