01.10.2012: FB1-Seminar

11:00 h, Hörsaal (West)

Abstract:

 Multi-decadal variability of Atlantic Meridional Overturning Circulation (AMOC) is examined from a 700-year 1990 control integration of the NCAR Community Climate System Model version 3 with T85 atmospheric resolution (CCSM3). AMOC variability in CCSM3 exhibits two distinct regimes, i.e. periods with very regular and strong decadal (~20-years) variability versus irregular red-noise-like variability with persistence longer than 10 years, with an abrupt transition between them. The differences between the two regimes are examined based on the comparison between the AMOC calculated with the depth and the density as the vertical coordinate, respectively. Depth-based AMOC exhibits maximum near 40degN at the crossover between the northward Gulf Stream/North Atlantic Current and southward Deep Western Boundary Current. On the other hand, the density-based AMOC shows maximum near 55degN associated with the deep convection in the Labrador Sea. However, the leading EOF modes of AMOC variability in both representations have maxima near the gyre boundary at ~40degN, suggesting the AMOC multi-decadal variability is dominated by the fluctuations of the boundary between the subtropical and subpolar gyres. The connectivity of the AMOC multi-decadal variability is examined from the time series of the various components of AMOC based on both vertical coordinates. The comparison between the depth and density spaces reveals distinct contributions from the upper and deep ocean circulations to the AMOC variability. For example, primarily due to the contribution from the upper ocean subpolar gyre circulation, the multi-decadal variability in the time series of the maximum AMOC in density coordinate lags that in the depth coordinate by 1-2 yrs.