Publication Abstract

Stirring times in the Atlantic

R.R. Dickson

• Past article stressed the importance of open ocean convection in the Labrador Sea one of the few spots south of the Greenland Scotland ridge where the signal of ocean climate change could be transferred to depth to affect the deep limb of the MOC.
• Below that lateral injection by overflows might do the same job.
• Convective activity, then at its height was regarded as a rather simple function of the NAO. High positive NAO =strong wintertime Nwerlies out of the Canadian arctic = strong convection
• NAO was recently at or near its extreme positive state in 600 years
• Steady incr in density cooling and deep excavation of the salty NADW sublayer
• Since then a number of things have happened to change the plot. First we became aware from the sort of hosing experiments carried out by Vellinga on HadCM 3 which confirmed/did nothing to disprove that the MOC overturning was in control of our climate
• Freshwater poured out of the Arctic, in the near surface water from Pacific, ice melt, runoff (up 20%; Dickson et al 2003) where it was mixed down by strong NAO convection and in the overflows (Dickson et al 2002), which injected the freshening signal at depth.
• Then following a strong NAO-negative episode in 1996, the established and expected pattern of events failed to materialize.
• Though NAO Index recovered the NAO pressure pattern itself shifted subtly east.
• Like the extreme NAO positive state itself in the 1990s, this eastward shift was predicted in some model runs to be the result of GHG forcing. Hurrell points out though that it could just be what you get intrinsically from NAO-positive since that mode, established over 100 years of data, always tends to be eastward-shifted cf the NAO-negative pattern
• And Jung show its been going on for a while
• Whatever the cause, its brought change to the Lab Sea. Southerlies there rather than Nwerlies in winter and Nwerlies fllow out from SE Greenland.
• LSW production shuts down and warmer & saltier) jumps to a lighter density
• Nwerlies over S Greenland associated with storms tracking thro Dk Str cause tip jet events. So unsurprising that these short term extreme forcings would have been more noticeable (Sheldon and Pickart this vol).
• So maybe the deep stirring has moved in character. LSW production weakens but Irminger Sea conv starts up.
• Problem is that the basis for our knowledge of hydro changes in the nW atlantic is the annual transect and these will still contain plenty of evidence of the spreading of LSW from the old days. 2 years to the Irminger andf 6? Year to the iceland basin.
• Its against that background of pan-oceanic change that we have to try and see the switch to tip jet events as a source of deep mixing. Not easy until short term profiling sensors are available to help detect such short term changes.
• Its still important, though the theory hasn’t caught up, Curry and McCartney have already demonstrated that the gradient of potential energy anomaly across the gyre ; gyre boundary is what drives the eastgoing baroclinic mass transport of the N Atlantic and on a larger scaleThorpe et al describe the s-N gradient of steric height through the deep basins of the N and S Atlantic as the feature of the ocean that correlates most closely to MOC overturning rate in Had CM 3---one of the best of the CCMs.
• So whatever changes the density of the NW Atlantic watercolumn matters.

Reference:

R.R. Dickson, 2003. Stirring times in the Atlantic. Nature, 424: 141-142.