Predictability of the mid-latitude Atlantic meridional overturning circulation in a multi-model system
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Predictability of the mid-latitude Atlantic meridional overturning circulation in a multi-model system Holger Pohlmann • Doug M. Smith • Magdalena A. Balmaseda Noel S. Keenlyside • Simona Masina • Daniela Matei • Wolfgang A. Mu¨ller • Philippe Rogel
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Received: 20 April 2012 / Accepted: 2 January 2013 / Published online: 1 February 2013 Crown Copyright 2013
Abstract Assessing the skill of the Atlantic meridional overturning circulation (AMOC) in decadal hindcasts (i.e. retrospective predictions) is hampered by a lack of observations for verification. Models are therefore needed to reconstruct the historical AMOC variability. Here we show that ten recent oceanic syntheses provide a common signal of AMOC variability at 45N, with an increase from the 1960s to the mid-1990s and a decrease thereafter although they disagree on the exact magnitude. This signal correlates with observed key processes such as the North Atlantic Oscillation, sub-polar gyre strength, Atlantic sea surface temperature dipole, and Labrador Sea convection that are thought to be related to the AMOC. Furthermore, we find potential predictability of the mid-latitude AMOC for the first 3–6 year means when we validate decadal H. Pohlmann D. M. Smith Met Office Hadley Centre, FitzRoy Road, Exeter EX1 3PB, UK H. Pohlmann (&) D. Matei W. A. Mu¨ller Max-Planck-Institut fu¨r Meteorologie, Bundesstraße 53, 20146 Hamburg, Germany e-mail: [email protected] M. A. Balmaseda ECMWF, Shinfield Park, Reading RG2 9AX, UK N. S. Keenlyside Geophysical Institute and Bjerknes Centre, University of Bergen, Postboks 7800, NO-5020 Bergen, Norway S. Masina Centro Euro-Mediterraneo sui Cambiamenti Climatici, Istituto Nazionale di Geofisica e Vulcanologia, Viale Aldo Moro 44, 40127 Bologna, Italy P. Rogel CERFACS/URA1875/CNRS, 42 Avenue Gustave Coriolis, 31057 Toulouse, CEDEX 1, France
hindcasts for the past 50 years against the multi-model signal. However, this predictability is not found in models driven only by external radiative changes, demonstrating the need for initialization of decadal climate predictions. Keywords Decadal prediction Atlantic MOC Predictability Multi-model comparison
1 Introduction The AMOC, i.e. the zonally integrated flow in the Atlantic Ocean, is important for local and global climate (Knight et al. 2005; Srokosz et al. 2012) due to its transport of heat to the mid and high latitudes. The AMOC consists of wind driven ocean currents, including the Gulf Stream, and a thermohaline circulation driven by the combined effects of temperature and salinity on the density of sea-water. Climate model simulations suggest that AMOC fluctuations are linked to decadal climate variability, with important socio-economic impacts including rainfall over Africa, North and South America and India, and Atlantic hurricane activity (Knight et al. 2006; Zhang and Delworth 2006; Smith et al. 2010). The strength of the AMOC can fluctuate through natural internal variability of the climate system, but is also expected to decline as a consequence of the
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