Transport Variability of Very Short Lived Substances From the West Indian Ocean to the Stratosphere

Abstract

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.

Importance of seasonally resolved oceanic emissions for bromoform delivery from the tropical Indian Ocean and west Pacific to the stratosphere

“…For our transport calculations, we use the Lagrangian particle dispersion model FLEXPART of the Norwegian Institute for Air Research in the Atmosphere and Climate Department (Stohl et al, 2005), which has been evaluated in previous studies (Stohl et al, 1998;Stohl and Trickl, 1999). The model includes moist convection and turbulence parameterizations in the atmospheric boundary layer and free tro-posphere (Stohl and Thomson, 1999;Forster et al, 2007). In this study, we employ version 9.2 of FLEXPART, which has been modified to incorporate atmospheric lifetime profiles for the decay of transported VSLSs.…”

“…In most atmospheric models, convection, which occurs on scales smaller than the grid scale, is parameterized. The FLEXPART convection scheme was described and evaluated by Forster et al (2007). FLEX-PART ERA-Interim simulations have previously been used to diagnose the VSLS transport and good agreement with aircraft measurements of bromoform, dibromomethane, and methyl iodide up to 13 km above the tropical west Pacific and methyl iodide in the UTLS was achieved.…”

Importance of seasonally resolved oceanic emissions for bromoform delivery from the tropical Indian Ocean and west Pacific to the stratosphere

“…Based on the first measurements of enhanced surface seawater concentrations of bromoform and dibromomethane from the subtropical and tropical west Indian Ocean in 2014, Fiehn et al (2017) calculated strong emissions and diagnosed stratospheric injection of these two VSLS in the southeastern part of the Asian monsoon anticyclone in July and August 2014 with Lagrangian model calculations. VSLS tracers with different lifetimes revealed a strong seasonality in the transport strength from the tropical west Indian Ocean to the stratosphere, with maximum transport in boreal spring, when the main uplift occurs over this ocean basin (Fiehn et al, 2018).…”

Answer to the Referee comments

“…In most atmospheric models, convection, which occurs on scales smaller than the grid scale, is parameterized. The FLEXPART convection scheme was described and evaluated by Forster et al (2007). FLEX-PART ERA-Interim simulations have previously been used to diagnose the VSLS transport and good agreement with aircraft measurements of bromoform, dibromomethane, and methyl iodide up to 13 km above the tropical west Pacific (Fuhlbrügge et al, 2016) and methyl iodide in the UTLS (Tegtmeier et al, 2013) was achieved.…”