Troposphere–stratosphere response to large-scale North Atlantic Ocean variability in an atmosphere/ocean coupled model
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The instrumental records indicate that the basin-wide wintertime North Atlantic warm conditions are accompanied by a pattern resembling negative North Atlantic oscillation (NAO), and cold conditions with pattern resembling the positive NAO. This relation is well reproduced in a control simulation by the stratosphere resolving atmosphere–ocean coupled Max-Planck-Institute Earth System Model (MPI-ESM). Further analyses of the MPI-ESM model simulation shows that the large-scale warm North Atlantic conditions are associated with a stratospheric precursory signal that propagates down into the troposphere, preceding the wintertime negative NAO. Additional experiments using only the atmospheric component of MPI-ESM (ECHAM6) indicate that these stratospheric and tropospheric changes are forced by the warm North Atlantic conditions. The basin-wide warming excites a wave-induced stratospheric vortex weakening, stratosphere/troposphere coupling and a high-latitude tropospheric warming. The induced high-latitude tropospheric warming is associated with reduction of the growth rate of low-level baroclinic waves over the North Atlantic region, contributing to the negative NAO pattern. For the cold North Atlantic conditions, the strengthening of the westerlies in the coupled model is confined to the troposphere and lower stratosphere. Comparing the coupled and uncoupled model shows that in the cold phase the tropospheric changes seen in the coupled model are not well reproduced by the standalone atmospheric configuration. Our experiments provide further evidence that North Atlantic Ocean variability (NAV) impacts the coupled stratosphere/troposphere system. As NAV has been shown to be predictable on seasonal-to-decadal timescales, these results have important implications for the predictability of the extra-tropical atmospheric circulation on these time-scales.
KeywordsNorth Atlantic oscillation (NAO) Northern annular mode (NAM) Stratosphere/troposphere coupling North Atlantic variability (NAV) Atlantic multidecadal variability (AMV) Atlantic multidecadal oscillation (AMO) Ocean–atmosphere interaction
We are grateful to Sandro Wellyanto Lubis, Hisashi Nakamura, Marco Giorgetta and Mojib Latif for many fruitful discussion. Computing resources at the Deutsche Klimarechenzentrum, and the Norddeutscher Verbund für Hoch—und Höchstleistungsrechnen are also acknowledged. We are also grateful to our reviewers for the very constructive comments. The work was supported by the Deutsche Forschungsgemeinschaft under the Emmy Noether—Programm (Grant KE 1471/2-1); also by the European Union SUMO (ERC Grant # 266722) and STEPS (PCIG10-GA-2011-304243) projects; the DecCen project funded by the research council of Norway; by the Centre for Climate Dynamics at the Bjerknes centre, Norway; by the Max-Planck-Society, and by the Federal Ministry of Education and Research in Germany (BMBF) through the research programme ‘‘MiKlip’’ (FKZ: 01LP1158A).
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