The dynamics and energetics of tropical temperate troughs over Southern Africa
Date
1992
Authors
D'Abreton, Peter Charles
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Abstract
Water vapour content and transport over southern Africa and adjacent oceans
are examined. Early summer rainfall over the northern and central interior of
South Africa tends to be associated with baroclinic controls whereas late-summer
rainfall is barotropic in origin. This is reflected in the northwesterly water vapour
transport from an Atlantic Ocean source by middle and upper tropospheric
westerly waves in early summer. A thermally indirect Ferrel cell, indicated-from
energetics, COpIU1nStheĀ· temperate nature of the early-summer atmosphere over
southern Africa. Late summer water vapour transport, in contrast, is strongly
from the tropics, with' a reduced eddy component, indicating an important
tropical control on late SUmmerrainfall especially in terms of fluctuations in the
position of the ascending limb of .the Walker cell Over southern Africa. The
Hadley cell is of importance to the late summer rainfall in that dry (wet) years
are associated with an anomalous cell OVereastern (central) South Africa such
that low level vapour transport is southerly (northerly). The anticyclone over the
eastern parts of southern Africa, coupled with. a trough over the interior
(especially at the 700 hPa pressure level), is important for the introduction of
water vapour over the subcontinent in wet and dry years and for
tropical-temperate trough case studies. Water vapour source regions differ from
early summer (Atlantic Ocean) to late summer (Indian Ocean), which reflects the
temperate. control on early and the tropical control on late summer circulation.
The convergence of water vapour over southern Africa in wet years and during
tropical-temperate troughs is not only important for cloud formation and
precipitation, but also for latent heat release associated with convergent water
vapour. Diabatic heating decreases the stability of the tropical atmosphere
thereby resulting in increased vertical motion. It also forces an anomalous Badley
circulation during wet late summers and tropical-temperate trough .cases as a
result of complex energy transformations. Heating increases eddy available
potential energy which is converted to zonal available potential energy by a
thermally indirect circulation found in the tropics. The zonal potential energy is
then converted to kinetic energy by the thermally direct Badley cell. Water
vapour and its variations are thus important for the precipitation, heating and
SUbsequent energy of the subtropical southern African atmosphere,