The convolution of dynamics and moisture with the presence of shortwave absorbing aerosols over the southeast Atlantic

Adeyemi A. Adebiyi, Paquita Zuidema, Steven J. Abel

Research output: Contribution to journalArticle

41 Scopus citations

Abstract

Biomass burning aerosols seasonally overlie the subtropical southeast Atlantic stratocumulus deck. Previous modeling and observational studies have postulated a semidirect effect whereby shortwave absorption by the aerosol warms and stabilizes the lower troposphere, thickening the low-level clouds. The focus herein is on the dynamical and moisture effects that may be convoluted with the semidirect effect. Almost-daily radiosonde data from remote St. Helena Island (15.9°S, 5.6°W), covering September-October 2000-11, are combined with daily spatial averages (encompassing the island) of the MODIS clear-sky fine-mode aerosol optical depth (τaf ). Increases in τaf are associated with increases in 750-500-hPa moisture content. The net maximum longwave cooling by moisture of almost 0.45Kday-1 reduces the aerosol layer warming from shortwave absorption. ERA-Interim spatial composites show that polluted conditions are associated with a strengthening of a deep land-based anticyclone over southern Africa, facilitating the westward offshore transport of both smoke and moisture at 600 hPa. The shallower surface-based South Atlantic anticyclone exhibits a less pronounced shift to the northeast, strengthening the low-level coastal jet exiting into the stratocumulus deck and cooling 1000-hPa potential temperatures. Warm continental outflow further increases the 800-hPa potential temperatures (θ800), reinforcing the lower tropospheric stability (θ800 - θ1000) over the stratocumulus deck. Enhanced southerly dry air advection also strengthens the cloud-top humidity inversion. The increased stability helps explain an observed decrease in cloud-top heights despite an anomalous reduction in subsidence. The changes to the horizontal dynamics enhance low-level cloudiness. These are separate but not necessarily distinct from an aerosol semidirect effect, encouraging care in attribution studies.

Original languageEnglish (US)
Pages (from-to)1997-2024
Number of pages28
JournalJournal of Climate
Volume28
Issue number5
DOIs
StatePublished - 2015

Keywords

  • Aerosols
  • Anticyclones
  • Clouds
  • Dynamics
  • Humidity
  • Radiative forcing

ASJC Scopus subject areas

  • Atmospheric Science

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