Sammendrag
Halogen- and sulfur-containing compounds are supersaturated in the surface ocean, which
results in their emission to the atmosphere. These compounds can be transported to the stratosphere,
where they impact ozone, the background aerosol layer, and climate. In this study we calculate the seasonal
and interannual variability of transport from the West Indian Ocean (WIO) surface to the stratosphere for
2000–2016 with the Lagrangian transport model FLEXPART using ERA-Interim meteorological fields. We
investigate the transport relevant for very short lived substances (VSLS) with tropospheric lifetimes
corresponding to dimethylsulfide (1 day), methyl iodide (CH3I, 3.5 days), bromoform (CHBr3, 17 days), and
dibromomethane (CH2Br2, 150 days). The stratospheric source gas injection of VSLS tracers from the WIO
shows a distinct annual cycle associated with the Asian monsoon. Over the 16-year time series, a slight
increase in source gas injection from the WIO to the stratosphere is found for all VSLS tracers and during all
seasons. The interannual variability shows a relationship with sea surface temperatures in the WIO as well as
the El Niño–Southern Oscillation. During boreal spring of El Niño, enhanced stratospheric injection of VSLS
from the tropical WIO is caused by positive sea surface temperature anomalies and enhanced vertical uplift
above the WIO. During boreal fall of La Niña, strong injection is related to enhanced atmospheric upward
motion over the East Indian Ocean and a prolonged Indian summer monsoon season. Related physical
mechanisms and uncertainties are discussed in this study.
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