Abstract 
The subinertial, climate relevant variability of the large-scale ocean circulation in the northern North 
Atlantic and its integral key parameters such as the advective transports of mass (volume), heat and 
freshwater are determined from observations alone using the hydrographic data from seven realisations 
of the so-called “48°N”-section between the English Channel and the Grand Banks of Newfoundland. 
The data consist of five available sets of the W0CE/A2-section during the Nineties for the years 
1993,1994, 1996, 1997, 1998 and of two previous transatlantic cruises in April of 1957 and 1982. The 
realisations of the WOCE/A2-section were carried out in the same season (May to July), except for 
the cruise in October 1994. The “48°N”-section follows the divergence zone of the mainly wind-driven 
subtropical gyre and the more complex, with respect to the forcing, subpolar gyre. In the central 
Westeuropean and Newfoundland Basins the section runs a few degrees south of the line of zero 
wind stress curl (curl-r). In the West, the W T OCE/A2-section turns northwest to cross the boundary 
current regime perpendicularly. Therefore this quasi-zonal hydrographic section covers all large-scale 
circulation elements on the regional scale that contribute essentially to the ocean circulation on the 
global scale - the Meridional Overturning Circulation (MOC). 
The transport estimates are given as the sum of the three transport components of a quasi-steady, 
large-scale ocean circulation: the ageostrophic Ekman-, and the two geostrophic components, the 
depth-independent, barotropic or Sverdrup- and the baroclinic component. To maintain the mass ba 
lance over the plane of the section the compensation of each component is assumed. In the case of the 
baroclinic component the balance is achieved through a suitable choice for a surface of “no-motion”. 
The absolute meridional velocity as a function of the zonal distance along the section and depth is 
therefore the sum of the three components and their compensatory components, respectively, at every 
point of the area integral of the mass transport. 
The recent almost-annual data sets exhibit a short-term variability that appears to be a close reflection 
of the extreme interannual behaviour of the NAO-index since 1993. The meridional heat transport 
drops from 0.70 PW in 1996 to 0.30 PW just one year later, following the decline of the NAO-index 
between winter 1995 and 1996, and recovers equally rapidly to 0.60 PW in summer 1998 when the 
NAO-index returns to positive values. The two earlier estimates of the heat transport in 1957 and 
1982 are not inconsistent with this behaviour of a fast oceanic response to atmospheric fluctuations. 
The observed temporal changes in the heat transport seem to reflect changes in the NAO-index with 
a time-lag of about one year. For this phase lag the changes in the NAO-index explain 70% of the 
changes in heat and 65% in freshwater transports as well as 60% in the meridional overturning rate - 
the transport of the upper layer of the MOC. Though based on only few occupations, this dramatic 
behaviour may be a first indication that a short-term change in atmospheric forcing, however extreme, 
may be matched by an equally rapid response in the large-scale ocean circulation. The “dynamical 
response” across the “48°N”-section is most pronounced in the central Newfoundland Basin. 
Using different forcing values for the two wind-driven parts of the total heat transport explains only 
30% of the observed temporal changes. The baroclinic part is mainly responsible for the observed 
changes. It contributes more than 80% to the total heat transport across the “48°N”-section including 
20% mesoscale variability due to eddies. This mesoscale variability is independent of the choice of the 
reference level and seems dynamically relevant for the large-scale ocean circulation at “48°N” in the 
North Atlantic. 
The line of zero curl 2 r shifts its position in concert with changes in the NAO-index. This shift produces 
primarily a deformation and/or an acceleration of the subpolar gyre, with a generation of meanders of 
the North Atlantic Current, baroclinic instabilities or Rossby wave interaction with the mean current, 
ultimately mesoscale hydrographic variability. Still up for discussion is which of these mechanisms 
contribute to the observed changes along the “48°N” -section in the North Atlantic.