Downstream development during ridging South Atlantic Ocean anticyclones

Ndarana, Thando; Lekoloane, Lesetja E.; Rammopo, Tsholanang S.; Reason, C. J. C.; Bopape, Mary-Jane M.; Chikoore, Hector; Engelbrecht, François

doi:10.1007/s00382-023-06717-7

Cited by 5 publications

(7 citation statements)

References 74 publications

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“…Why some waves in the South African domain enter non-linear regimes and others remain in more linear regimes is still an open question. However, Ndarana et al (2023) showed that baorclinic conversion observed during the letter is less intense that in the case of former, meaning that when COLs (in general wave breaking) do not occur it is suppressed.…”

Section: Introductionmentioning

confidence: 93%

“…Linear Waves the weaker increase in eddy kinetic energy generation during linear-regime waves in the South African domain (Ndarana et al 2023).…”

Section: Introductionmentioning

confidence: 99%

“…This extensive analysis of non-linear wave breaking regimes associated with weather extremes has diagnosed the key role of downstream development Piva 2013, 2016;Ndarana et al 2021a, b;Pinheiro et al 2022;Ndarana et al 2023), which is largely facilitated by the strength of the jet streak located immediately upstream of ridge to the west of the main trough axis. Ndarana et al (2023) diagnosed that during ridging SAOH events which are associated with a COL aloft, the upstream jet streak is stronger than the one located downstream of the trough axis. This configuration then increases strain rates so that the waves may break (Nakamura and Plumb 1994;Akahori and Yoden 1997).…”

Section: Introductionmentioning

confidence: 99%

“…A working hypothesis is that the barotropic governor mechanism (James and Gray 1986;James 1987) might be responsible for this. Moon and Feldstein (2009) showed that it leads to weaker upward fluxes of wave activity during the initial states of their weak barotropic wave life cycle case, which might explain the weaker increase in eddy kinetic energy generation during linear-regime waves in the South African domain (Ndarana et al 2023).…”

Section: Introductionmentioning

confidence: 99%

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A quasi-geostrophic analysis of summertime southern African linear-regime westerly waves

Ndarana,

Rammopo,

Bopape

et al. 2023

Preprint

Self Cite

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Linear-regime westerly waves that propagate across the South African domain are often linked to well-known rainfall producing systems such as tropical temperate troughs and synoptic scale tropical low-pressure systems, and ridging South Atlantic Ocean anticyclones at the surface. It is accepted that the baroclinic waves that propagate across the domain provide the lifting mechanism that causes the required vertical motion for rainfall to occur. This study shows that there exists a jet streak embedded in these waves that is located downstream of the trough axis, to the east of which vertically upward motion is expected to occur. The entrance of the jet streak passes just south of the country, as the waves propagate past the domain. The study further shows that for this class of waves, the vertical motion that causes rainfall to occur is induced by the thermally direct transverse ageostrophic circulation that is located at this jet entrance. This is instead of the conventional upper air divergence that is located at the inflection point east of the trough axis. Using a method of decomposing the Q-vector into its transverse (Qs) and shear (Qs) components, the divergence fields of which are used to decompose the vertical motion into the corresponding components, i.e ωn and ωs, respectively; it was shown that the vertical motion over South Africa is explained more by the former than the latter. Therefore, the uplift over the country and that located at the infection point east of the trough are dynamically distinct processes. Taking the limitations of the quasi-geostrophic framework into consideration, the study concludes that during the passage of linear-regime waves vertical motion that might lead to rainfall is caused by the circulation at the jet entrance and not the divergence in the baroclinic wave.

show abstract

Section: Introductionmentioning

confidence: 93%

“…Linear Waves the weaker increase in eddy kinetic energy generation during linear-regime waves in the South African domain (Ndarana et al 2023).…”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

A quasi-geostrophic analysis of summertime southern African linear-regime westerly waves

Ndarana,

Rammopo,

Bopape

et al. 2023

Preprint

Self Cite

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show abstract

“…4c). These anomalies indicate the presence of extratropical waves that can favour precipitation (Ivanciu et al, 2022;Ndarana et al, 2022Ndarana et al, , 2023, through their effects on the AL (Pascale et al, 2019).…”

Section: The Effect Of Internal Climate Variabilitymentioning

confidence: 99%

Internally-driven variability of the Angola Low is the main source of uncertainty for the future changes in southern African precipitation

Monerie,

Dieppois,

Pohl

et al. 2024

Preprint

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Variations in southern African precipitation have a major impact on local communities, increasing climate-related risks and affecting water and food security, as well as natural ecosystems. However, future changes in southern African precipitation are uncertain, with climate models showing a wide range of responses from near-term projections (2020-2040) to the end of the 21st century (2080-2100). Here, we assess the uncertainty in southern African precipitation change using five Ocean-Atmosphere General Circulation single model initial-condition large ensembles (30 to 50 ensemble members) and four emissions scenarios. We show that the main source of uncertainty in 21st Century projections of southern African precipitation is the internal climate variability. In addition, we find that differences between ensemble members in simulating future changes in the location of the Angola Low explain a large proportion (~60%) of the uncertainty in precipitation change. Together, the internal variations in the large-scale circulation over the Pacific Ocean and the Angola Low explain ~64% of the uncertainty in southern African precipitation change. We suggest that a better understanding of the future evolutions of the southern African precipitation may be achieved by understanding better the model’s ability to simulate the Angola Low and its effects on precipitation.

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A quasi-geostrophic analysis of summertime southern African linear-regime westerly waves

Ndarana,

Rammopo,

Bopape

et al. 2024

Clim Dyn

View full text Add to dashboard Cite

Linear-regime westerly waves that propagate across the South African domain are often linked to well-known rainfall producing systems such as tropical temperate troughs and synoptic scale tropical low-pressure systems, and ridging South Atlantic Ocean anticyclones at the surface. It is accepted that the baroclinic waves that propagate across the domain provide the lifting mechanism that causes the required vertical motion for rainfall to occur. This study shows that there exists a jet streak embedded in these waves that is located downstream of the trough axis, to the east of which vertically upward motion is expected to occur. The entrance of the jet streak passes just south of the country, as the waves propagate past the domain. The study further shows that for this class of waves, the vertical motion that causes rainfall to occur is induced by the thermally direct transverse ageostrophic circulation that is located at this jet entrance. This is instead of the conventional upper air divergence that is located at the inflection point east of the trough axis. Using a method of decomposing the Q-vector into its transverse ($${Q}_n$$ Q n ) and shear ($${Q}_s$$ Q s ) components, the divergence fields of which are used to decompose the vertical motion into the corresponding components, i.e $$\omega _n$$ ω n and $$\omega _s$$ ω s , respectively; it was shown that the vertical motion over South Africa is explained more by the former than the latter. Therefore, the uplift over the country and that located at the infection point east of the trough are dynamically distinct processes. Taking the limitations of the quasi-geostrophic framework into consideration, the study concludes that during the passage of linear-regime waves vertical motion that might lead to rainfall is caused by the circulation at the jet entrance and not the divergence in the baroclinic wave.

show abstract

Downstream development during ridging South Atlantic Ocean anticyclones

Cited by 5 publications

References 74 publications

A quasi-geostrophic analysis of summertime southern African linear-regime westerly waves

A quasi-geostrophic analysis of summertime southern African linear-regime westerly waves

Internally-driven variability of the Angola Low is the main source of uncertainty for the future changes in southern African precipitation

A quasi-geostrophic analysis of summertime southern African linear-regime westerly waves

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