The role of the southern African easterly jet in modifying the southeast Atlantic aerosol and cloud environments

Adeyemi A. Adebiyi, Paquita Zuidema

Research output: Contribution to journalArticle

22 Citations (Scopus)

Abstract

The westward transport of biomass-burning (BB) aerosols by mid-tropospheric winds over the southeast Atlantic stratocumulus deck has long been recognized, but the coupling to the large-scale circulation has yet to be investigated fully. This goal is furthered here using satellite observations and reanalysis datasets spanning 2001-2012, as well as 10 day forward trajectory calculations of satellite-detected smoke emissions. The results highlight the important role of a mid-tropospheric zonal wind maximum, the Southern African Easterly Jet (AEJ-S), in transporting BB aerosol west off the African continent. The AEJ-S, defined through daily-mean 600 hPa easterly wind speeds exceeding 6 m s-1 between 5°S and 15°S and centred zonally on the coastline, is most pronounced during September-October. The AEJ-S is part of a meridional circulation that is diabatically forced by the temperature-moisture gradient between the southern hot-dry and northern cool-moist convective structures over land. 45% of 24 264 total identified smoke trajectories exit the continent to its west between 5°S and 15°S. These thereafter follow three major pathways: northwestward (8%), directly westward (55%) and anticyclonically recirculated (37%). The AEJ-S induces an upward motion directly below the jet that enhances prevailing updraughts over land, lifting emissions and transporting aerosols more efficiently over the southeast Atlantic. Offshore, the prevailing large-scale mean subsidence is reduced, with an associated increase in the nearby cloud-top heights and reduction in the nearby marine low-level cloud fraction. Further from the jet, increased warm continental temperature advection at 800 hPa associated with the strengthened land-based anticyclone decreases mean low-level cloud heights.

Original languageEnglish (US)
JournalQuarterly Journal of the Royal Meteorological Society
DOIs
StateAccepted/In press - 2016

Fingerprint

aerosol
biomass burning
smoke
trajectory
stratocumulus
anticyclone
updraft
meridional circulation
zonal wind
advection
subsidence
wind velocity
temperature
moisture
coast
land
continent
observation satellite
calculation

Keywords

  • Aerosols
  • Ageostrophic circulation
  • Large-scale dynamics
  • Southern African easterly jet
  • Stratocumulus cloud
  • Subsidence

ASJC Scopus subject areas

  • Atmospheric Science

Cite this

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title = "The role of the southern African easterly jet in modifying the southeast Atlantic aerosol and cloud environments",
abstract = "The westward transport of biomass-burning (BB) aerosols by mid-tropospheric winds over the southeast Atlantic stratocumulus deck has long been recognized, but the coupling to the large-scale circulation has yet to be investigated fully. This goal is furthered here using satellite observations and reanalysis datasets spanning 2001-2012, as well as 10 day forward trajectory calculations of satellite-detected smoke emissions. The results highlight the important role of a mid-tropospheric zonal wind maximum, the Southern African Easterly Jet (AEJ-S), in transporting BB aerosol west off the African continent. The AEJ-S, defined through daily-mean 600 hPa easterly wind speeds exceeding 6 m s-1 between 5°S and 15°S and centred zonally on the coastline, is most pronounced during September-October. The AEJ-S is part of a meridional circulation that is diabatically forced by the temperature-moisture gradient between the southern hot-dry and northern cool-moist convective structures over land. 45{\%} of 24 264 total identified smoke trajectories exit the continent to its west between 5°S and 15°S. These thereafter follow three major pathways: northwestward (8{\%}), directly westward (55{\%}) and anticyclonically recirculated (37{\%}). The AEJ-S induces an upward motion directly below the jet that enhances prevailing updraughts over land, lifting emissions and transporting aerosols more efficiently over the southeast Atlantic. Offshore, the prevailing large-scale mean subsidence is reduced, with an associated increase in the nearby cloud-top heights and reduction in the nearby marine low-level cloud fraction. Further from the jet, increased warm continental temperature advection at 800 hPa associated with the strengthened land-based anticyclone decreases mean low-level cloud heights.",
keywords = "Aerosols, Ageostrophic circulation, Large-scale dynamics, Southern African easterly jet, Stratocumulus cloud, Subsidence",
author = "Adebiyi, {Adeyemi A.} and Paquita Zuidema",
year = "2016",
doi = "10.1002/qj.2765",
language = "English (US)",
journal = "Quarterly Journal of the Royal Meteorological Society",
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T1 - The role of the southern African easterly jet in modifying the southeast Atlantic aerosol and cloud environments

AU - Adebiyi, Adeyemi A.

AU - Zuidema, Paquita

PY - 2016

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N2 - The westward transport of biomass-burning (BB) aerosols by mid-tropospheric winds over the southeast Atlantic stratocumulus deck has long been recognized, but the coupling to the large-scale circulation has yet to be investigated fully. This goal is furthered here using satellite observations and reanalysis datasets spanning 2001-2012, as well as 10 day forward trajectory calculations of satellite-detected smoke emissions. The results highlight the important role of a mid-tropospheric zonal wind maximum, the Southern African Easterly Jet (AEJ-S), in transporting BB aerosol west off the African continent. The AEJ-S, defined through daily-mean 600 hPa easterly wind speeds exceeding 6 m s-1 between 5°S and 15°S and centred zonally on the coastline, is most pronounced during September-October. The AEJ-S is part of a meridional circulation that is diabatically forced by the temperature-moisture gradient between the southern hot-dry and northern cool-moist convective structures over land. 45% of 24 264 total identified smoke trajectories exit the continent to its west between 5°S and 15°S. These thereafter follow three major pathways: northwestward (8%), directly westward (55%) and anticyclonically recirculated (37%). The AEJ-S induces an upward motion directly below the jet that enhances prevailing updraughts over land, lifting emissions and transporting aerosols more efficiently over the southeast Atlantic. Offshore, the prevailing large-scale mean subsidence is reduced, with an associated increase in the nearby cloud-top heights and reduction in the nearby marine low-level cloud fraction. Further from the jet, increased warm continental temperature advection at 800 hPa associated with the strengthened land-based anticyclone decreases mean low-level cloud heights.

AB - The westward transport of biomass-burning (BB) aerosols by mid-tropospheric winds over the southeast Atlantic stratocumulus deck has long been recognized, but the coupling to the large-scale circulation has yet to be investigated fully. This goal is furthered here using satellite observations and reanalysis datasets spanning 2001-2012, as well as 10 day forward trajectory calculations of satellite-detected smoke emissions. The results highlight the important role of a mid-tropospheric zonal wind maximum, the Southern African Easterly Jet (AEJ-S), in transporting BB aerosol west off the African continent. The AEJ-S, defined through daily-mean 600 hPa easterly wind speeds exceeding 6 m s-1 between 5°S and 15°S and centred zonally on the coastline, is most pronounced during September-October. The AEJ-S is part of a meridional circulation that is diabatically forced by the temperature-moisture gradient between the southern hot-dry and northern cool-moist convective structures over land. 45% of 24 264 total identified smoke trajectories exit the continent to its west between 5°S and 15°S. These thereafter follow three major pathways: northwestward (8%), directly westward (55%) and anticyclonically recirculated (37%). The AEJ-S induces an upward motion directly below the jet that enhances prevailing updraughts over land, lifting emissions and transporting aerosols more efficiently over the southeast Atlantic. Offshore, the prevailing large-scale mean subsidence is reduced, with an associated increase in the nearby cloud-top heights and reduction in the nearby marine low-level cloud fraction. Further from the jet, increased warm continental temperature advection at 800 hPa associated with the strengthened land-based anticyclone decreases mean low-level cloud heights.

KW - Aerosols

KW - Ageostrophic circulation

KW - Large-scale dynamics

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KW - Stratocumulus cloud

KW - Subsidence

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