Trouet et 2009 posit a positive NAO as the “explanation” of the Medieval Climate Anomaly, pausing only briefly to ask what might have caused a centuries long (“temporally pervasive”) positive NAO, falling back on an arm-waving attribution to a stronger Atlantic meridional overturning circulation:
The persistently strong winter MCA NAO and its weakening during the LIA raise question about the mechanism responsible for producing such a temporally pervasive atmospheric state over the North Atlantic, as well as MCA-LIA climate anomalies elsewhere … Stronger westerlies associated with a prolonged positive NAO phase may have enhanced the Atlantic meridional overturning circulation (AMOC) (27), which, in turn, generated crossequatorial salinity and SST anomalies in the tropical Atlantic and a related northward migration of the intertropical convergence zone (28). An enhanced AMOC may have accommodated a constructive feedback mechanism, proxy evidence for which is provided by North Atlantic records (Fig. 4B and table S2), that reinforced the La Niña–like conditions in the tropical Pacific (22).
Trouet et al 2007 here recycle similar language from Esper the Non-Archiver 2007:
Association of a strengthened THC with a poleward shift of westerly storm tracks was recently suggested [Seager et al., 2007] to be teleconnected with persistent La Nina-like conditions in the tropical Pacific [Cobb et al., 2003], the major driver for medieval drought in N America [Cook et al., 2007; Graham et al., 2007]. Similar considerations might also account for the persistently drier conditions now reconstructed for NW Africa.
Anyway, let me compare excerpts from two recent diagrams. On the right is Trouet’s diagram of a positive NAO phase, showing strong westerlies, Azores high and dry Morocco. On the left is a similar diagram that I located in a non-peer reviewed Internet publication (though by a scientist with a longer resume than Trouet). Both diagrams clearly associate dry Morocco (and a wrm MWP) with positive NAO.
However, the author of the left diagram did not attribute a persistently positive NAO to a strong Atlantic circulation, but to a weaker Atlantic circulation.
A weaker Atlantic Ocean Thermohaline Circulation (THC) and weaker Surrounding Antarctic Subsidence (SAS) or Meridional Overturning Circulation (MOC = THC + SAS) for the period of a decade or a decade-and-a-half can bring about a gradual global upper ocean warming. When a weaker MOC occurs, the Southern Hemisphere tropical oceans upwell less cold water into the thermocline, and there is generally less global rainfall. With a lag of 5-10 years a modest globe warming ensues.
I think that it’s worth noting that the argument in favor of a strong Atlantic MOC as presented by Trouet et al 2009 in
Reader’s Digest for Scientist Science contains no levels of proof greater than the internet article attributing the same phenomenon to a weak Atlantic MOC. Even the arm-waving in Trouet is pretty perfunctory, no elaborate choreography.
At this time, I have no views on which theory is more strongly supported – whether persistently positive NAO due to strong Atlantic MOC; or whether it is due to weak Atlantic MOC. HOwever, it’s interesting that both camps seem content with the idea that changes in ocean circulation underlie the Medieval Climatic Anomaly and both camps associate it with a positive NAO and dry Morocco.
To the extent that Cobb’s Pacific corals – (22) in the first quote – are relied on as evidence for a “cool” medieval Pacific, it seems to me that they are a very thin reed, given that it appears that they may also be explainable by a slight northward migration of the ITCZ (see my post on this) – and such slight northward migration of the ITCZ is already contemplated in the Trouet “explanation”.
The diagram on the left occurs in William Gray 2009, online here. Given that Gray and Trouet agree on key elements – positive NAO, dry Morocco,…- I’m a little puzzled as to how Trouet et al purport to refute any of Gray’s views. Just another climate mystery, I guess.