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R. T. Sutton
,
G. D. McCarthy
,
J. Robson
,
B. Sinha
,
A. T. Archibald
, and
L. J. Gray

Abstract

Atlantic multidecadal variability (AMV) is the term used to describe the pattern of variability in North Atlantic sea surface temperatures (SSTs) that is characterized by decades of basinwide warm or cool anomalies, relative to the global mean. AMV has been associated with numerous climate impacts in many regions of the world including decadal variations in temperature and rainfall patterns, hurricane activity, and sea level changes. Given its importance, understanding the physical processes that drive AMV and the extent to which its evolution is predictable is a key challenge in climate science. A leading hypothesis is that natural variations in ocean circulation control changes in ocean heat content and consequently AMV phases. However, this view has been challenged recently by claims that changing natural and anthropogenic radiative forcings are critical drivers of AMV. Others have argued that changes in ocean circulation are not required. Here, we review the leading hypotheses and mechanisms for AMV and discuss the key debates. In particular, we highlight the need for a holistic understanding of AMV. This perspective is a key motivation for a major new U.K. research program: the North Atlantic Climate System Integrated Study (ACSIS), which brings together seven of the United Kingdom’s leading environmental research institutes to enable a broad spectrum approach to the challenges of AMV. ACSIS will deliver the first fully integrated assessment of recent decadal changes in the North Atlantic, will investigate the attribution of these changes to their proximal and ultimate causes, and will assess the potential to predict future changes.

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J.-P. Vernier
,
T. D. Fairlie
,
T. Deshler
,
M. Venkat Ratnam
,
H. Gadhavi
,
B. S. Kumar
,
M. Natarajan
,
A. K. Pandit
,
S. T. Akhil Raj
,
A. Hemanth Kumar
,
A. Jayaraman
,
A. K. Singh
,
N. Rastogi
,
P. R. Sinha
,
S. Kumar
,
S. Tiwari
,
T. Wegner
,
N. Baker
,
D. Vignelles
,
G. Stenchikov
,
I. Shevchenko
,
J. Smith
,
K. Bedka
,
A. Kesarkar
,
V. Singh
,
J. Bhate
,
V. Ravikiran
,
M. Durga Rao
,
S. Ravindrababu
,
A. Patel
,
H. Vernier
,
F. G. Wienhold
,
H. Liu
,
T. N. Knepp
,
L. Thomason
,
J. Crawford
,
L. Ziemba
,
J. Moore
,
S. Crumeyrolle
,
M. Williamson
,
G. Berthet
,
F. Jégou
, and
J.-B. Renard

Abstract

We describe and show results from a series of field campaigns that used balloonborne instruments launched from India and Saudi Arabia during the summers 2014–17 to study the nature, formation, and impacts of the Asian Tropopause Aerosol Layer (ATAL). The campaign goals were to i) characterize the optical, physical, and chemical properties of the ATAL; ii) assess its impacts on water vapor and ozone; and iii) understand the role of convection in its formation. To address these objectives, we launched 68 balloons from four locations, one in Saudi Arabia and three in India, with payload weights ranging from 1.5 to 50 kg. We measured meteorological parameters; ozone; water vapor; and aerosol backscatter, concentration, volatility, and composition in the upper troposphere and lower stratosphere (UTLS) region. We found peaks in aerosol concentrations of up to 25 cm–3 for radii > 94 nm, associated with a scattering ratio at 940 nm of ∼1.9 near the cold-point tropopause. During medium-duration balloon flights near the tropopause, we collected aerosols and found, after offline ion chromatography analysis, the dominant presence of nitrate ions with a concentration of about 100 ng m–3. Deep convection was found to influence aerosol loadings 1 km above the cold-point tropopause. The Balloon Measurements of the Asian Tropopause Aerosol Layer (BATAL) project will continue for the next 3–4 years, and the results gathered will be used to formulate a future National Aeronautics and Space Administration–Indian Space Research Organisation (NASA–ISRO) airborne campaign with NASA high-altitude aircraft.

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