How do tropical-temperate troughs form and develop over southern Africa?

par Clémence Macron, Benjamin Pohl, Yves Richard (CRC) et Miloud Bessafi (LE2P, univ. de la Réunion)

accepté dans Journal of Climate

This paper aims at separating the respective influences of tropical and mid-latitude variability on the development and life cycle of tropical-temperate troughs (TTT) over Southern Africa in austral summer (November through February). Cluster analysis is applied to 1971-2000 ERA40 daily outgoing longwave radiation (OLR) anomalies to identify TTTs and monitor tropical convection. The same analysis applied to the zonal wind stretching deformation at 200hPa (ZDEF) characterizes mid-latitude transient perturbations. Results based on the comparison between these two classifications first confirm that mid-latitude baroclinic waves are a necessary condition for TTT development, but they are not sufficient. Roughly 40% of those occurring in austral summer are associated with a TTT. They tend to be stronger than the baroclinic waves not associated with TTT development. In the tropics, additional conditions needed to form a TTT consist of an excess of latent energy over the Mozambique Channel, mostly due to moisture advections and convergence from the Atlantic and Indian Oceans. Taken together, these conditions are highly favorable for deep atmospheric convection over and near Southern Africa and seem to explain a large fraction of TTT variability.

KEY WORDS

Southern Africa / Southwest Indian Ocean; Tropical-temperate-troughs; mid-latitude transient perturbations; tropical convection; Cluster Analysis

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How do tropical-temperate troughs form and develop over southern Africa?

par Clémence Macron, Benjamin Pohl, Yves Richard (CRC) et Miloud Bessafi (LE2P, univ. de la Réunion)

accepté dans Journal of Climate

This paper aims at separating the respective influences of tropical and mid-latitude variability on the development and life cycle of tropical-temperate troughs (TTT) over Southern Africa in austral summer (November through February). Cluster analysis is applied to 1971-2000 ERA40 daily outgoing longwave radiation (OLR) anomalies to identify TTTs and monitor tropical convection. The same analysis applied to the zonal wind stretching deformation at 200hPa (ZDEF) characterizes mid-latitude transient perturbations. Results based on the comparison between these two classifications first confirm that mid-latitude baroclinic waves are a necessary condition for TTT development, but they are not sufficient. Roughly 40% of those occurring in austral summer are associated with a TTT. They tend to be stronger than the baroclinic waves not associated with TTT development. In the tropics, additional conditions needed to form a TTT consist of an excess of latent energy over the Mozambique Channel, mostly due to moisture advections and convergence from the Atlantic and Indian Oceans. Taken together, these conditions are highly favorable for deep atmospheric convection over and near Southern Africa and seem to explain a large fraction of TTT variability.

KEY WORDS

Southern Africa / Southwest Indian Ocean; Tropical-temperate-troughs; mid-latitude transient perturbations; tropical convection; Cluster Analysis