Characterizing the Synoptic Expression of the Angola Low

Howard, Emma; Washington, Richard

doi:10.1175/jcli-d-18-0017.1

Cited by 55 publications

(66 citation statements)

References 31 publications

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“…In the top row of Figure , the increased latitudinal spread in ERA‐Interim compared to the other reanalyses is again evident. Across the reanalysis products, 33.5% of tropical lows form in the Angola Low region (11–19°S, 14–25°E, consistent with Howard & Washington, ) and 23.5% lysis there. Meanwhile, the track density in the Angola Low region composes of 37% of the total track density.…”

Section: Characteristics Of Tropical Lowssupporting

confidence: 92%

“…Ratna et al () characterize this response as a shift in the Walker Circulation over equatorial regions, which leads to enhanced upper level convergence (divergence) over southern Africa during the El Niño (La Niña) phase. This response would lead to a less (more) favorable environment for tropical low formation, since tropical lows feature surface convergence and upper level divergence (Howard & Washington, ). Meanwhile, Lindesay et al () and Vigaud et al () find that the El Niño phase is associated with a northward shift in the subtropical jet and high‐pressure belt, leading to increased subsidence over southern Africa.…”

Section: Discussionmentioning

confidence: 99%

“…Hurley and Boos () conducted a global study of land‐based depressions in the tropics and found similar results over northern Australia and India. Howard and Washington () found that the radii and vertical structures of Australian and Indian monsoon lows were qualitatively comparable to those of tropical lows in southern Africa.…”

Section: Introductionmentioning

confidence: 95%

“…A large, elongated cyclonic vorticity anomaly in Namibia is also evident in the vorticity map. This is the Kalahari heat low (Howard & Washington, 2018) and is a shallow feature, disappearing at 600 hPa. Based on the above analysis, we conclude that the track algorithm works acceptably well over southern Africa, and we proceed to examine its results in the following sections.…”

Section: Validationmentioning

confidence: 99%

“…The contrast between the very strong El Niño of 1998, when the Angola Low was strong and the forecasted drought did not occur, and 2015, when the Angola Low was unusually weak and the summer rainfall was the lowest in recorded history, demonstrates this relationship (Blamey et al, ). Howard and Washington () found that it is the contribution of tropical lows to the Angola Low and not that of heat lows, which influences the relationship between rainfall and ENSO.…”

Section: Introductionmentioning

confidence: 99%

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Tropical Lows in Southern Africa: Tracks, Rainfall Contributions, and the Role of ENSO

2019

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Southern African tropical lows are synoptic‐scale cyclonic vortices that propagate westward across southern Africa in the Austral summer. They strongly influence local rainfall and aggregate in the climatological December, January, and February mean to form the Angola Low. In this study, tropical lows are identified and tracked using an objective feature tracking method. The statistics of tropical low tracks over southern Africa are presented and compared across three reanalysis products. Findings are compared to the literature of tropical low‐pressure areas elsewhere in the world, where it is found that most tracking statistics compare well but that the tendency of tropical lows to become semistationary over Angola is unique to southern Africa. The hypothesis that tropical lows in Angola have a causal relationship with Tropical Temperate Troughs is tested, and a correlation between occurrence frequencies is found at interannual but not daily time scales. Precipitation is attributed to the tropical lows, and it is found that tropical lows are associated with 31% of rainfall across tropical southern Africa, based on gridded precipitation products. The interannual variability of the number of tropical lows that form per year (σ=6 events/year) is linked to El Niño–Southern Oscillation (ENSO) and the tropical easterly jet. The mean latitude of tropical lows is shifted northward during El Niño and southward during La Niña. Much of the interannual precipitation variability maximum in Angola is attributed to rainfall associated with tropical lows. These results provide insights into the southern African response to ENSO and into the mechanisms of rainfall in the southern African tropical edge.

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Section: Characteristics Of Tropical Lowssupporting

confidence: 92%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 95%

Section: Validationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Tropical Lows in Southern Africa: Tracks, Rainfall Contributions, and the Role of ENSO

2019

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Tracking mesoscale convective systems in central equatorial Africa

Hartman¹

2020

Intl Journal of Climatology

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The Congo basin in central equatorial Africa is home to some of the most intense convection in the global tropics. Mesoscale convective systems (MCSs) provide much of the annual rainfall over this region during the March-April-May (MAM) and September-October-November (SON) rainy seasons. Features of these systems are essential to rainfall variability in this region and greatly impact human health, agriculture, livestock, and drought monitoring. Knowledge of variability is hindered by the lack of in-situ observations and meteorological stations. The present study identifies and tracks MCSs for the 33-year period 1983-2015 for MAM and SON. MCS and environmental parameters are calculated for the rainy seasons using satellite and reanalysis data. Spatial distributions of MCS parameters and diurnal cycles for select MCS parameters are compared to prior research. Statistical significance testing is performed to determine if there are meaningful differences between the seasons. Seasonal differences are briefly discussed. 650 hPa relative vorticity patterns suggest localized terrain effects may play a role near a local maximum in MCS initiation frequency in the lee of the Great Rift Valley. Spatial distributions of 33-year MCS counts, trajectories, speeds, sizes, maximum intensities, and durations, based on initiation locations, agree well with prior research. Differences between seasons are statistically significant and variable and latitude dependent. There is high interannual variability among all MCS and environmental parameters.

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The flow and moisture fluxes associated with ridging South Atlantic Ocean anticyclones during the subtropical southern African summer

Ndarana

Mpati

Bopape

et al. 2020

Intl Journal of Climatology

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Using 38 years of ERA-Interim reanalyses, this study examines the flow and associated moisture fluxes induced by ridging South Atlantic Ocean anticyclones over South Africa. The flow and moisture fluxes are divided into their geostrophic and ageostrophic components. Composite analysis reveals that ridging high pressure systems are modulated by Rossby wave trains that develop upstream in the middle latitudes near South America and the South Atlantic Ocean near the Greenwich Meridian. The wave trains are similar to those that are associated with tropical temperate troughs (TTTs) and nearly all TTTs are followed by ridging, whilst 20% of the ridging events are linked to TTTs. Composite analysis also shows that ridging-induced moisture fluxes affecting South Africa originate from different areas of the surrounding oceans at different times during the evolution of ridging highs. In the early stages, moisture enters the country along the southeastern coast through ageostrophic processes from a moisture divergence region located adjacent to the coast and remains largely south of 30 S latitude line. The moisture fluxes have a northeastern orientation on land, following the geometry of the eastern coast. These ageostrophic fluxes contribute to the occurrence of rainfall during the ridging process in the region located between Lesotho and Swaziland. Ridging events are associated with about 60% of rainfall days in summer over southern Africa whilst TTTs contribute about 21% summer rainfall days. During the later stages of the evolution of ridging, geostrophic fluxes enter the southern parts of Mozambique and contribute the accumulation of moisture in that region. The associated moisture divergence region is always located ahead of its ageostrophic counterpart, with a local maximum eventually forming in the Mozambique Channel.

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Characterizing the Synoptic Expression of the Angola Low

Cited by 55 publications

References 31 publications

Tropical Lows in Southern Africa: Tracks, Rainfall Contributions, and the Role of ENSO

Tropical Lows in Southern Africa: Tracks, Rainfall Contributions, and the Role of ENSO

Tracking mesoscale convective systems in central equatorial Africa

The flow and moisture fluxes associated with ridging South Atlantic Ocean anticyclones during the subtropical southern African summer

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