What can Hydrography Tell Us about the MOC on Centennial Timescales?

The meridional overturning circulation (MOC) in the North Atlantic consists of two overturning cells of comparable strength, a shallow one with convective overturning in the subpolar gyre (the Labrador and Irminger Seas), and a deep one fed by dense overflow water from the Nordic Seas. The shallow cell has a complex horizontal circulation that shifts and expands in response to the state of the North Atlantic Oscillation. The deep cell involves convective overturning and substantial heat loss, but also considerable buffering in the Nordic Seas. Recent studies of overflow from the Nordic Seas suggest little if any long-term trend. Here we use hydrography to extend the observational window of Nordic Seas transport back to the early 1900s with focus on the very longest timescales.

The data show no evidence for a change in the strength of MOC transport toward the Nordic Seas. To determine this, all available hydrographic data in two quiet areas south and north of the Scotland Greenland Ridge, the Rockall Trough and southern Norwegian Sea, respectively, are lumped into two sets. These are chosen to bracket both the Iceland-Faroe and Faroe-Scotland inflows into the Nordic Seas. From an examination of dynamic height in the two sets, it is evident that any trend in transport since the 1930s would at most be a few percent of the total, ~8 Sv. Additional hydrographic data from the first decade of the 1900s are consistent with this conclusion even though the limited number of casts do not provide as tight a constraint. The above geostrophic transport relative to 600 m, that of the deepest inflow into the Nordic Seas in the Faroe-Shetland Channel, gives a transport remarkably close to recent directly measured estimates for the combined Iceland-Faroe and Faroe-Scotland inflows. A stable MOC and very little long-term trend in temperature in this region also means that the net heat flux into the Nordic Seas should have been stable over the past century at the 270 TW level.