The Angola Low: relationship with southern African rainfall and ENSO

Julien Cretat, Benjamin Pohl, Bastien Dieppois, Segolene Berthou, Julien Pergaud

Research output: Contribution to journalArticle

5 Citations (Scopus)

Abstract

The main states of the Angola Low (AL) are identified using clustering analysis applied to daily anomalous patterns of 700-hPa wind vorticity over Angola and adjacent countries from November to March for the 1980/81–2014/15 period. At the daily timescale, we examine the extent to which the main states of the AL modulate daily rainfall over southern Africa. At the interannual timescale, we assess both the relationship between the occurrence of these AL states and El Niño southern oscillation (ENSO) and the role of the AL in explaining ENSO’s failure in driving southern African rainfall at times. Three reanalyses are considered to account for uncertainties induced by the scarcity of data available for assimilation over southern Africa. Three preferential states of the Angola Low are identified: AL state close to its seasonal climatology with slight zonal displacements, anomalously weak AL state and anomalously strong AL state with meridional displacements. These different states all significantly modulate daily southern African rainfall. Near-climatological AL state promotes wet rainfall anomalies over eastern subtropical southern Africa and dry rainfall anomalies over its western part. A slight westward shift in the near-climatological position of the AL leads to reversed zonal gradient in rainfall. The remaining regimes significantly modulate the meridional gradient in southern African rainfall. Anomalously weak and anomalously northward AL states promote wet rainfall anomalies over tropical southern Africa and dry rainfall anomalies over subtropical southern Africa. The reverse prevails for anomalously southward AL. At the interannual timescale, ENSO significantly modulates the seasonal occurrence of most AL states in the three reanalyses. Anomalously weak and southward AL states are more strongly correlated with regional rainfall than ENSO in all reanalyses, suggesting that accounting for AL variability may improve seasonal forecasts. Case study analysis of the major 1982/83 and 1997/98 El Niño events suggests that the weak rainfall anomalies and strong seasonal AL in 1997/98 may result from counteracting effects between ENSO and Indian Ocean coupled modes of variability.
LanguageEnglish
Pages1783–1803
Number of pages21
JournalClimate Dynamics
Volume52
Issue number3-4
Early online date8 May 2018
DOIs
Publication statusPublished - Feb 2019

Fingerprint

El Nino-Southern Oscillation
rainfall
anomaly
timescale
vorticity
El Nino
climatology
Southern Africa

Keywords

  • Angola Low
  • Clustering analysis
  • Daily rainfall
  • ENSO
  • Interannual variability
  • Southern Africa

ASJC Scopus subject areas

  • Atmospheric Science

Cite this

The Angola Low: relationship with southern African rainfall and ENSO. / Cretat, Julien; Pohl, Benjamin; Dieppois, Bastien; Berthou, Segolene; Pergaud, Julien.

In: Climate Dynamics, Vol. 52, No. 3-4, 02.2019, p. 1783–1803.

Research output: Contribution to journalArticle

Cretat, Julien ; Pohl, Benjamin ; Dieppois, Bastien ; Berthou, Segolene ; Pergaud, Julien. / The Angola Low: relationship with southern African rainfall and ENSO. In: Climate Dynamics. 2019 ; Vol. 52, No. 3-4. pp. 1783–1803.
@article{b047c8e29b564fccb3cc145c2a2c9b1a,
title = "The Angola Low: relationship with southern African rainfall and ENSO",
abstract = "The main states of the Angola Low (AL) are identified using clustering analysis applied to daily anomalous patterns of 700-hPa wind vorticity over Angola and adjacent countries from November to March for the 1980/81–2014/15 period. At the daily timescale, we examine the extent to which the main states of the AL modulate daily rainfall over southern Africa. At the interannual timescale, we assess both the relationship between the occurrence of these AL states and El Ni{\~n}o southern oscillation (ENSO) and the role of the AL in explaining ENSO’s failure in driving southern African rainfall at times. Three reanalyses are considered to account for uncertainties induced by the scarcity of data available for assimilation over southern Africa. Three preferential states of the Angola Low are identified: AL state close to its seasonal climatology with slight zonal displacements, anomalously weak AL state and anomalously strong AL state with meridional displacements. These different states all significantly modulate daily southern African rainfall. Near-climatological AL state promotes wet rainfall anomalies over eastern subtropical southern Africa and dry rainfall anomalies over its western part. A slight westward shift in the near-climatological position of the AL leads to reversed zonal gradient in rainfall. The remaining regimes significantly modulate the meridional gradient in southern African rainfall. Anomalously weak and anomalously northward AL states promote wet rainfall anomalies over tropical southern Africa and dry rainfall anomalies over subtropical southern Africa. The reverse prevails for anomalously southward AL. At the interannual timescale, ENSO significantly modulates the seasonal occurrence of most AL states in the three reanalyses. Anomalously weak and southward AL states are more strongly correlated with regional rainfall than ENSO in all reanalyses, suggesting that accounting for AL variability may improve seasonal forecasts. Case study analysis of the major 1982/83 and 1997/98 El Ni{\~n}o events suggests that the weak rainfall anomalies and strong seasonal AL in 1997/98 may result from counteracting effects between ENSO and Indian Ocean coupled modes of variability.",
keywords = "Angola Low, Clustering analysis, Daily rainfall, ENSO, Interannual variability, Southern Africa",
author = "Julien Cretat and Benjamin Pohl and Bastien Dieppois and Segolene Berthou and Julien Pergaud",
year = "2019",
month = "2",
doi = "10.1007/s00382-018-4222-3",
language = "English",
volume = "52",
pages = "1783–1803",
journal = "Climate Dynamics",
issn = "0930-7575",
publisher = "Springer Verlag",
number = "3-4",

}

TY - JOUR

T1 - The Angola Low: relationship with southern African rainfall and ENSO

AU - Cretat, Julien

AU - Pohl, Benjamin

AU - Dieppois, Bastien

AU - Berthou, Segolene

AU - Pergaud, Julien

PY - 2019/2

Y1 - 2019/2

N2 - The main states of the Angola Low (AL) are identified using clustering analysis applied to daily anomalous patterns of 700-hPa wind vorticity over Angola and adjacent countries from November to March for the 1980/81–2014/15 period. At the daily timescale, we examine the extent to which the main states of the AL modulate daily rainfall over southern Africa. At the interannual timescale, we assess both the relationship between the occurrence of these AL states and El Niño southern oscillation (ENSO) and the role of the AL in explaining ENSO’s failure in driving southern African rainfall at times. Three reanalyses are considered to account for uncertainties induced by the scarcity of data available for assimilation over southern Africa. Three preferential states of the Angola Low are identified: AL state close to its seasonal climatology with slight zonal displacements, anomalously weak AL state and anomalously strong AL state with meridional displacements. These different states all significantly modulate daily southern African rainfall. Near-climatological AL state promotes wet rainfall anomalies over eastern subtropical southern Africa and dry rainfall anomalies over its western part. A slight westward shift in the near-climatological position of the AL leads to reversed zonal gradient in rainfall. The remaining regimes significantly modulate the meridional gradient in southern African rainfall. Anomalously weak and anomalously northward AL states promote wet rainfall anomalies over tropical southern Africa and dry rainfall anomalies over subtropical southern Africa. The reverse prevails for anomalously southward AL. At the interannual timescale, ENSO significantly modulates the seasonal occurrence of most AL states in the three reanalyses. Anomalously weak and southward AL states are more strongly correlated with regional rainfall than ENSO in all reanalyses, suggesting that accounting for AL variability may improve seasonal forecasts. Case study analysis of the major 1982/83 and 1997/98 El Niño events suggests that the weak rainfall anomalies and strong seasonal AL in 1997/98 may result from counteracting effects between ENSO and Indian Ocean coupled modes of variability.

AB - The main states of the Angola Low (AL) are identified using clustering analysis applied to daily anomalous patterns of 700-hPa wind vorticity over Angola and adjacent countries from November to March for the 1980/81–2014/15 period. At the daily timescale, we examine the extent to which the main states of the AL modulate daily rainfall over southern Africa. At the interannual timescale, we assess both the relationship between the occurrence of these AL states and El Niño southern oscillation (ENSO) and the role of the AL in explaining ENSO’s failure in driving southern African rainfall at times. Three reanalyses are considered to account for uncertainties induced by the scarcity of data available for assimilation over southern Africa. Three preferential states of the Angola Low are identified: AL state close to its seasonal climatology with slight zonal displacements, anomalously weak AL state and anomalously strong AL state with meridional displacements. These different states all significantly modulate daily southern African rainfall. Near-climatological AL state promotes wet rainfall anomalies over eastern subtropical southern Africa and dry rainfall anomalies over its western part. A slight westward shift in the near-climatological position of the AL leads to reversed zonal gradient in rainfall. The remaining regimes significantly modulate the meridional gradient in southern African rainfall. Anomalously weak and anomalously northward AL states promote wet rainfall anomalies over tropical southern Africa and dry rainfall anomalies over subtropical southern Africa. The reverse prevails for anomalously southward AL. At the interannual timescale, ENSO significantly modulates the seasonal occurrence of most AL states in the three reanalyses. Anomalously weak and southward AL states are more strongly correlated with regional rainfall than ENSO in all reanalyses, suggesting that accounting for AL variability may improve seasonal forecasts. Case study analysis of the major 1982/83 and 1997/98 El Niño events suggests that the weak rainfall anomalies and strong seasonal AL in 1997/98 may result from counteracting effects between ENSO and Indian Ocean coupled modes of variability.

KW - Angola Low

KW - Clustering analysis

KW - Daily rainfall

KW - ENSO

KW - Interannual variability

KW - Southern Africa

UR - http://www.scopus.com/inward/record.url?scp=85046640734&partnerID=8YFLogxK

U2 - 10.1007/s00382-018-4222-3

DO - 10.1007/s00382-018-4222-3

M3 - Article

VL - 52

SP - 1783

EP - 1803

JO - Climate Dynamics

T2 - Climate Dynamics

JF - Climate Dynamics

SN - 0930-7575

IS - 3-4

ER -