Nonlinear Processes in Geophysics www.nonlin-processes-geophys.net 1023-5809 1607-7946 15 1 2008 10.5194/npg-15-13-2008 http://www.nonlin-processes-geophys.net/15/13/2008/ http://www.nonlin-processes-geophys.net/15/13/2008/npg-15-13-2008.html http://www.nonlin-processes-geophys.net/15/13/2008/npg-15-13-2008.pdf 13 24 2008-01-18 North Atlantic climate variability in coupled models and data S. K. Kravtsov kravtsov@uwm.edu W. K. Dewar M. Ghil P. S. Berloff J. C. McWilliams Department of Mathematical Sciences, University of Wisconsin-Milwaukee, P. O. Box 413, Milwaukee, WI 53201, USA Dept. of Atmospheric and Oceanic Sciences,& Institute of Geophysics and Planetary Physics, University of California at Los Angeles, Los Angeles, CA 90095–1565, USA Dept. of Oceanography, Florida State University, Tallahassee, FL, 32306, USA Département Terre-Atmosphère-Océan, & Laboratoire de Météorologie Dynamique (CNRS and IPSL), Ecole Normale Supérieure, F-75231 Paris Cedex 05, France Dept. of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA DAMTP, University of Cambridge, Cambridge, UK We show that the observed zonally averaged jet in the Northern Hemisphere atmosphere exhibits two spatial patterns with broadband variability in the decadal and inter-decadal range; these patterns are consistent with an important role of local, mid-latitude ocean–atmosphere coupling. A key aspect of this behaviour is the fundamentally nonlinear bi-stability of the atmospheric jet's latitudinal position, which enables relatively small sea-surface temperature anomalies associated with ocean processes to affect the large-scale atmospheric winds. The wind anomalies induce, in turn, complex three-dimensional anomalies in the ocean's main thermocline; in particular, they may be responsible for recently reported cooling of the upper ocean. Both observed modes of variability, decadal and inter-decadal, have been found in our intermediate climate models. One mode resembles North Atlantic tri-polar sea-surface temperature (SST) patterns described elsewhere. The other mode, with mono-polar SST pattern, is novel; its key aspects include interaction of oceanic turbulence with the large-scale oceanic flow. To the extent these anomalies exist, the interpretation of observed climate variability in terms of natural and human-induced changes will be affected. 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