Fig. 1: Global residual meridional overturning streamfunction.
From: Controls of the global overturning circulation of the ocean
Observationally-constrained estimate based on the Estimating the Circulation and Climate of the Ocean version 4 release 4 (ECCOv4r4) reanalysis product40, computed in a depth coordinate and b density coordinate (based on potential density σ2). The contour interval is 10 Sv (1 Sv = 106 m3 s−1). Zonal mean density contours are indicated (thin dashed black curves) on the depth-coordinate streamfunction. Similarly, zonal mean depth contours are shown on the density-coordinate streamfunction. Residual streamfunctions include the Eulerian and bolus (eddy-induced) components of the flow which advect tracers. Two major cells structure the present overturning circulation, an upper cell exchanging properties between the North Atlantic and the Southern Ocean, referred to as the AMOC, and a lower cell associated with the circulation and transformation of Antarctic Bottom Water (AABW)185. These cells interact in a number of ways and are thought to be connected in a global figure-of-eight structure186. Both the upper and lower overturning cells reach the surface in the Southern Ocean, making it an important place for the global climate. In addition, shallow tropical cells contribute to a large heat transport at low latitudes187,188. The two streamfunctions provide broadly similar pictures, except in the Southern Ocean where a strong Deacon cell present in the depth space is absent from the density space42. The ECCOv4r4 product is a physically-consistent reconstruction of the ocean state on a 1° global grid for the period 1992–2017. It was created using an ocean model that assimilates a large number of observations from across this time period. It is presented as an illustration of the GOC structure and pathways, yet it is associated with many uncertainties and biases189 and should therefore be considered critically.
