Fig. 4: Geographical deconstruction and inter-decadal variation of the overturning circulation.

a The overturning transports at sections FULL and EAST (for experiments CTRL and SENS), SILL and 48N (CTRL only for clarity: SENS does not differ from CTRL at SILL, and the difference between SENS and CTRL at 48N is similar to that difference at FULL); the blue shading highlights the difference between CTRL and SENS at FULL; the grey shading indicates the first phase of the OSNAP observations. b FULL minus EAST transport for CTRL and SENS (dashed), indicating the net contribution to the basin-wide overturning cell (AMOC) by downwelling in the Labrador Sea. c Map summarising the evolution of the lower limb of the AMOC for three different 5-year periods of CTRL: section transports (values in rectangles; in Sv) and diapycnal downwelling from the upper to the lower limb (values in ovals; in Sv) implied by the transport divergence between EAST and SILL (oval ‘a’), and FULL and EAST (oval ‘b’). The horizontal current speed averaged between the potential density surface 27.55 kg m⁻³ and the bottom (colour bar; in cm s⁻¹) illustrates the concentration of the lower limb transport along the continental slopes of Greenland and North America. The transport evolves from two main sources, the outflow from the Nordic Seas and its enhancement by downwelling in the northeastern basins between SILL and EAST. After minor augmentation during its passage through the Labrador Sea, it reaches its maximum strength near 52°N (cf. Fig. 3). The increase of ~4 Sv at FULL and 48N between the 1970s and 1990s originated mainly from enhanced downwelling between SILL and EAST, with a contribution of ~1 Sv from enhanced downwelling in the Labrador Sea.