期刊
QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY
卷 142, 期 697, 页码 1574-1589出版社
WILEY
DOI: 10.1002/qj.2765
关键词
Southern African easterly jet; aerosols; stratocumulus cloud; large-scale dynamics; subsidence; ageostrophic circulation
资金
- NSF Large-Scale Dynamics Program [AGS-1233874]
- Directorate For Geosciences
- Div Atmospheric & Geospace Sciences [1233874] Funding Source: National Science Foundation
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 degrees S and 15 degrees 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 degrees S and 15 degrees 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.
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