Tracing the Potential Existence of the Asian Tropopause Aerosol Layer
(ATAL) Prior to the Late 1990s through Observations
Abstract
An enhanced aerosol layer, known as the Asian Tropopause Aerosol Layer
(ATAL), has been observed within the seasonal Asian monsoon anticyclone
(AMA) since the late 1990s. Given the apparently abrupt appearance of
this layer based on observations, it has been speculated that it
originates from increasing human made emissions in Asia. However, the
ATAL confinement is a result of a dynamical feature and does not purely
consist of human made components. We herein investigate the possible
existence of an ATAL earlier than the late 1990s. We exploit earliest
possible, high quality space-based aerosol observations from
Stratospheric Aerosol and Gas Experiment, or SAGE (1979-1981), SAGE
III/ISS (2017, ongoing) and revisit SAGE II (1984-2005) data analysis.
We find that seasonal averaged solar occultation aerosol measurements
(past and present) can neither be used to exclude the existence of the
ATAL, nor to infer a significant trend. However, first CAM5-MAM7
simulations indicate the presence of an ATAL signal for the tested years
1979 and 1980, with a human made component. We hypothesize that the
human made component of the ATAL likely occurred since at least the
1970s, while the natural ATAL component (e.g. from dust) has always
existed. Extended simulation based ATAL evolution studies are therefore
the most reliable source for early ATAL investigations.