Reassessing the Role of the Indo‐Pacific in the Ocean's Global Overturning Circulation

Abstract

Surface buoyancy fluxes in the Southern and North Atlantic Oceans are presumed to disproportionately influence the ocean's residual global overturning circulation (GOC) with respect to those in the Indo‐Pacific. Here, this assumption is challenged through an assessment of global buoyancy transport in the Community Earth System Model 1.0, which reveals that the steady state GOC is equally constrained by surface buoyancy flux everywhere. Further, an unacknowledged aspect of the GOC is demonstrated: it transports buoyancy from where it is gained at the surface, predominately in the Indo‐Pacific, to where it is lost, predominately in the Atlantic and Southern Oceans. This global buoyancy transport requires zonal structure in the GOC, linking the Atlantic and Indo‐Pacific within the Southern Ocean, asymmetry and interbasin coupling absent from many conceptual descriptions of overturning dynamics. These results compel a more nuanced appreciation for an Indo‐Pacific influence in GOC evolution.

Atlantic Ocean Heat Transport Enabled by Indo‐Pacific Heat Uptake and Mixing

“…[PsiAI]e Talley, 2003;Ferrari & Ferreira, 2011). Note that the net heat transports in the individual basins (Figure 3), as well as the temperature structure of these transports (Figure 2), compare well with the buoyancy transports discussed by Newsom & Thompson, 2018 (their Figures 2 and 4) outside the high-latitude Southern Ocean, showing that heat transport generally dominates the buoyancy transport in the upper ocean in these regions. The southward heat transport out of the subtropical Indo-Pacific across 34 • S exceeds the global southward heat transport there (compare black and blue lines in Figure 3), meaning that a significant fraction of this heat (∼0.5 PW) must ultimately be a source for the northward heat transport in the Atlantic.…”

“… Θ , the vertical vector component of the heat function contours in the temperature-latitude plane (equation (13)), contains contributions from surface forcing (Figures 2g-2i) and mixing (Figures 2j-2l). This heat uptake is dominated by the eastern equatorial Pacific cold tongue (1.09 PW of heat enters the region 10 • S to 10 • N, 165-70 • W, not shown, see Figure 1a of HZE19), with a much smaller fraction entering the low-latitude Atlantic (0.36PW between 10 • S-10 • N across the Atlantic, compare Figs. [PsiAI]g-i, also see Newsom & Thompson, 2018). This heat uptake is dominated by the eastern equatorial Pacific cold tongue (1.09 PW of heat enters the region 10 • S to 10 • N, 165-70 • W, not shown, see Figure 1a of HZE19), with a much smaller fraction entering the low-latitude Atlantic (0.36PW between 10 • S-10 • N across the Atlantic, compare Figs. [PsiAI]g-i, also see Newsom & Thompson, 2018).…”

“…Forget & Ferreira, 2019;Newsom & Thompson, 2018;Toggweiler et al, 2019). There are a growing number of studies with this emphasis (e.g.…”

“…The drivers of this circulation still remain under discussion, with debate centering around the proportion of water mass transformation between light and dense waters occurring in the high-latitude North Atlantic and the Southern Ocean and through diapycnal upwelling at lower latitudes (Cessi, 2019;Ferrari & Wunsch, 2009;Ferrari et al, 2017;Gnanadesikan, 1999;Talley, 2013;Lee et al, 2018;Marshall & Speer, 2012;Thompson et al, 2016;Toggweiler et al, 2019;Newsom & Thompson, 2018). The drivers of this circulation still remain under discussion, with debate centering around the proportion of water mass transformation between light and dense waters occurring in the high-latitude North Atlantic and the Southern Ocean and through diapycnal upwelling at lower latitudes (Cessi, 2019;Ferrari & Wunsch, 2009;Ferrari et al, 2017;Gnanadesikan, 1999;Talley, 2013;Lee et al, 2018;Marshall & Speer, 2012;Thompson et al, 2016;Toggweiler et al, 2019;Newsom & Thompson, 2018).…”

“…The drivers of this circulation still remain under discussion, with debate centering around the proportion of water mass transformation between light and dense waters occurring in the high-latitude North Atlantic and the Southern Ocean and through diapycnal upwelling at lower latitudes (Cessi, 2019;Ferrari & Wunsch, 2009;Ferrari et al, 2017;Gnanadesikan, 1999;Talley, 2013;Lee et al, 2018;Marshall & Speer, 2012;Thompson et al, 2016;Toggweiler et al, 2019;Newsom & Thompson, 2018). In particular, Newsom and Thompson (2018) highlighted the importance of asymmetries in the net buoyancy forcing between the Atlantic and Indo-Pacific, emphasizing the often overlooked role of water mass transformation in the tropical Indo-Pacific for the global residual circulation. In particular, Newsom and Thompson (2018) highlighted the importance of asymmetries in the net buoyancy forcing between the Atlantic and Indo-Pacific, emphasizing the often overlooked role of water mass transformation in the tropical Indo-Pacific for the global residual circulation.…”