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University of Johannesburg (UJ) (2009)

El Niño Southern Oscillation (ENSO) and atmospheric transport over Southern Africa

Kanyanga, Joseph Katongo

Titre : El Niño Southern Oscillation (ENSO) and atmospheric transport over Southern Africa

Auteur : Kanyanga, Joseph Katongo

Université de soutenance : University of Johannesburg (UJ)

Grade : PhD. (Environmental Management) 2009

Résumé
This research investigates associations between El Niño Southern Oscillation (ENSO) phases and the occurrence of an unusual atmospheric transport patterns over southern Africa, observed during the SAFARI 2000 field campaign. This transport pattern manifested itself as a smoke and hazeladen flow, originating from the sub-equatorial tropical regions over northern Angola and western Zambia, and traversing the subcontinent with exit off the south-east coast over southern Mozambique and northern KwaZulu-Natal into the Indian Ocean. This pattern has been dubbed The River of Smoke. This pattern is distinctly different from the continental re-circulation pattern associated with continental high-pressure systems over the subtropical sub-region, which is the prevalent system according to current literature. The period during which the SAFARI 2000 campaign was conducted happened to be at the height of the strong 1999/2000 La Niña episode, associated with excessively wet conditions over the sub-continent. In contrast, during an earlier field campaign, SAFARI’92, the continental re-circulation transport mode, termed the gyre, was observed to prevail over the subtropical sub-region. SAFARI’92 coincided with the 1991/1992 El Niño episode that resulted in drought conditions over much of southern Africa. These observations triggered the current investigation of the causal associations and frequencies of the River of Smoke flow with phases of the ENSO. This study has employed conventional and modeled methods of meteorological analysis to investigate the influence of ENSO on atmospheric transport and synoptic coupling mechanisms, Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) forward trajectory computations supported by an enhanced method of cluster analysis were used to determine and characterize the mean atmospheric transport modes over the subcontinent. Trajectories were calculated originating at two source points, representative of the tropical and subtropical subregions respectively (Mongu, Zambia ; and Tshane, Botswana). Three study years were selected, corresponding to three distinct phases of ENSO : an El Niño episode during 1991/1992, a neutral ENSO episode during 1996/1997, and a La Niña episode during 1999/2000. The National Center for Environmental Prediction (NCEP)/ National Center for Atmospheric Research (NCAR) Reanalysis model is employed to reproduce mean composite synoptic circulation fields over southern Africa during the three periods. Findings show that for airflows originating over tropical southern Africa, the Indian Ocean fast exit pathway is unique to the neutral ENSO phase, while the continental re-circulation transport mode exhibits zero occurrences during the neutral phase. A continental re-circulation transport mode in the lower troposphere was observed during the El Niño and La Niña extremes, but with low annual frequencies (less than 30%). The direct Atlantic Ocean, the north-west and the Indian Ocean slow exit pathways are common to all three ENSO phases. For airflows originating over the subtropical sub-region, the continental re-circulation transport mode has the highest vi frequencies during all three ENSO phases, more especially during the El Niño episode, with more than 50% annual frequency, as observed during SAFARI’92. The north-west, Indian Ocean slow and Indian Ocean fast pathways are observed during all the three ENSO phases, with lower overall frequencies. The anticyclonic circulation flows are driven by the strengthened high-pressure systems over southern Africa : the semi-permanent subtropical high-pressure systems over the adjacent oceans (Atlantic Ocean High and Indian Ocean High) and the transient cut-off continental high-pressure systems. Airflows exiting toward the Indian Ocean have been described previously. In this work, a novel distinction is made between the Indian Ocean fast and the Indian Ocean slow exit pathways. The Indian Ocean slow corresponds to the conventional off-coast flow, such as observed during SAFARI’92. The Indian Ocean fast exit pathway is a newly described transport mode, which results from strong coupling between the subtropical circulation systems and the mid-latitude westerly flows. This strong-coupling induced transport mechanism provides favourable conditions for transport of mass fluxes, such as fire emissions originating from the tropical sub-region, towards the Indian Ocean. The Indian Ocean fast exit transport mode is characterized by higher speeds while exiting off the south-east coast, and extends further into the temperate latitudes towards Australia. When coupled with tropical sub-region, this flow mode is responsible for the occurrence of the River of Smoke phenomenon identified during SAFARI 2000. This Indian Ocean fast transport mode is intermittent in nature, and couples to the tropical sub-region only during the neutral ENSO episode, during which it exhibits low monthly frequencies, mainly during the dry season July through October. The intermittent nature, dependent on ENSO phases, and the associated low frequencies, may explain why it was observed in SAFARI 2000, but escaped observation and comment during previous studies, such as SAFARI’92.

Mots clés : El Niño current ; Southern oscillation ; Atmospheric thermodynamics (Southern Africa)

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