I am sure it is little more complex than you imagine. 'The Atlantic Ocean exhibits considerable multi-decadal variability with time scales of about 50 to 100 years (see Chapter 3). This multi- decadal variability appears to be a robust feature of the surface climate in the Atlantic region, as shown by tree ring reconstructions for the last few centuries (e.g., Mann et al., 1998). Atlantic multi- decadal variability has a unique spatial pattern in the SST anomaly field, with opposite changes in the North and South Atlantic (e.g., Mestas-Nunez and Enfield, 1999; Latif et al., 2004), and this dipole pattern has been shown to be significantly correlated with decadal changes in Sahelian rainfall (Folland et al., 1986). Decadal variations in hurricane activity have also been linked to the multi- decadal SST variability in the Atlantic (Goldenberg et al., 2001). Atmosphere-Ocean General Circulation Models simulate Atlantic multi- decadal variability (e.g., Delworth et al., 1993; Latif, 1998 and references therein; Knight et al., 2005), and the simulated space-time structure is consistent with that observed (Delworth and Mann, 2000). The multi-decadal variability simulated by the AOGCMs originates from variations in the MOC (see Section 8.3). The mechanisms, however, that control the variations in the MOC are fairly different across the ensemble of AOGCMs. In most AOGCMs, the variability can be understood as a damped oceanic eigenmode that is stochastically excited by the atmosphere. In a few other AOGCMs, however, coupled interactions between the ocean and the atmosphere appear to be more important. The relative roles of high- and low-latitude processes differ also from model to model. The variations in the Atlantic SST associated with the multi-decadal variability appear to be predictable a few decades ahead, which has been shown by potential (diagnostic) and classical (prognostic) predictability studies. Atmospheric quantities do not exhibit predictability at decadal time scales in these studies, which supports the picture of stochastically forced variability.'
http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch8s8-4-6.html 'Generally, the simulated late-20th century Atlantic MOC shows a spread ranging from a weak MOC of about 12 Sv to over 20 Sv (Figure 10.15; Schmittner et al., 2005). When forced with the SRES A1B scenario, the models show a reduction in the MOC of up to 50% or more, but in one model, the changes are not distinguishable from the simulated natural variability. The reduction in the MOC proceeds on the time scale of the simulated warming because it is a direct response to the increase in buoyancy at the ocean surface. A positive North Atlantic Oscillation (NAO) trend might delay this response by a few decades but not prevent it (Delworth and Dixon, 2000). Such a weakening of the MOC in future climate causes reduced sea surface temperature (SST) and salinity in the region of the Gulf Stream and North Atlantic Current (Dai et al., 2005). This can produce a decrease in northward heat transport south of 60°N, but increased northward heat transport north of 60°N (A. Hu et al., 2004). No model shows an increase in the MOC in response to the increase in greenhouse gases, and no model simulates an abrupt shut-down of the MOC within the 21st century. One study suggests that inherent low-frequency variability in the Atlantic region, the Atlantic Multidecadal Oscillation, may produce a natural weakening of the MOC over the next few decades that could further accentuate the decrease due to anthropogenic climate change (Knight et al., 2005; see Section 8.4.6). In some of the older models (e.g., Dixon et al., 1999), increased high- latitude precipitation dominates over increased high-latitude warming in causing the weakening, while in others (e.g., Mikolajewicz and Voss, 2000), the opposite is found. In a recent model intercomparison, Gregory et al. (2005) find that for all 11 models analysed, the MOC reduction is caused more by changes in surface heat flux than changes in surface freshwater flux. In addition, simulations using models of varying complexity (Stocker et al., 1992b; Saenko et al., 2003; Weaver et al., 2003) show that freshening or warming in the Southern Ocean acts to increase or stabilise the Atlantic MOC. This is likely a consequence of the complex coupling of Southern Ocean processes with North Atlantic Deep Water production.' http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch10s10-3-4.html Please note also the discussion on abrupt changes. On Apr 21, 10:56 am, "David B. Benson" <[email protected]> wrote: > On Apr 19, 7:04 pm, Robert I Ellison <[email protected]> > wrote:> The formula you use is ... > >Internal variability consists of ENSO, etc., but >by taking decadal averages all that is needed is >the AMO; if you bothered to read about it you would >have discovered it is strongly affected by MOC rate. > AE(d) = k(lnCO2(d-1) - lnCO2(1870s)) - GTA(1880s) > but in which an obvious right parenthsis was > missing. There are no undefined terms and the > constant k is estimated for best fit to the > data as is subsequently mentioned. > > The OGTR is a transient response and is, as > pointed out, in agreement with an equilibrium > sensitivity of about 3 K. > > > Arrhenius needs to be understood in the light of 21st century physics? > > Yes and all has been settled since the 1970s; the > Arrenhius approximation is still considered good > enough to appear in IPCC AR4. > > Internal variability consists of ENSO, etc., but > by taking decadal averages all that is needed is > the AMO; if you bothered to read about it you would > have discovered it is strongly affected by MOC rate. > > Othr forcings need not be considered as the steady > ones all cancel out; see IPCC AR4. The random ones > are reflected in the AMO. > > The abrupt shifts seen in Greenland ice cores are > clearly the results of the dynamics of those portions > of the cryosphere that we now longer have with us. > > -- > You received this message because you are subscribed to the Google Groups > Global Change ("globalchange") newsgroup. Global Change is a public, > moderated venue for discussion of science, technology, economics and policy > dimensions of global environmental change. > > Posts will be admitted to the list if and only if any moderator finds the > submission to be constructive and/or interesting, on topic, and not > gratuitously rude. > > To post to this group, send email to [email protected] > > To unsubscribe from this group, send email to > [email protected] > > For more options, visit this group > athttp://groups.google.com/group/globalchange -- You received this message because you are subscribed to the Google Groups Global Change ("globalchange") newsgroup. Global Change is a public, moderated venue for discussion of science, technology, economics and policy dimensions of global environmental change. Posts will be admitted to the list if and only if any moderator finds the submission to be constructive and/or interesting, on topic, and not gratuitously rude. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/globalchange
