Diagnosing Excessive Subsidence Across the Southeast Atlantic in the Goddard Earth Observing System (GEOS) During ORACLES‐1

Abstract

AbstractComplicated by seasonal transport of biomass burning aerosol plumes primarily overlaying the marine stratocumulus‐to‐cumulus transition, the complexity of the atmosphere over the southeast Atlantic Ocean is difficult to represent in models. Biases with respect to the height of the aerosol plume and excessive subsidence have previously been documented in MERRA‐2. This paper further diagnoses these biases, and addresses the unreasonable subsidence over the southeast Atlantic region currently simulated by the Goddard Earth Observing System (GEOS) model during the first deployment of the ORACLES campaign in September 2016. Our study is based on model simulations where the meteorological state is constrained by the GEOS‐based MERRA‐2 reanalysis, as well as by ERA5, using the so‐called replay technique. Free running simulations with the GEOS model show excessive subsidence that begins over land such that aerosol transported by the easterly jet reaches a strip of complex terrain and is forced downward; a similar behavior can be found in the MERRA‐2 reanalysis. This excessive subsidence has been somewhat reduced by improved parameterizations found in recent versions of the GEOS model. However, any replay constrained by MERRA‐2 meteorology, even those based on recent versions of GEOS, suffer from the same excessive subsidence. An additional set of simulations in which the GEOS model was free‐running was used to quantify the role of aerosol itself in adjusting the profile of vertical motion. While self‐lofting of biomass burning aerosol reduces the subsidence over the ocean, the magnitude of its impact is only a fraction of the bias in MERRA‐2.