Varying planetary heat sink led to global-warming slowdown and acceleration

Abstract

A vacillating global heat sink at intermediate ocean depths is associated with different climate regimes of surface warming under anthropogenic forcing: The latter part of the 20th century saw rapid global warming as more heat stayed near the surface. In the 21st century, surface warming slowed as more heat moved into deeper oceans. In situ and reanalyzed data are used to trace the pathways of ocean heat uptake. In addition to the shallow La Niña-like patterns in the Pacific that were the previous focus, we found that the slowdown is mainly caused by heat transported to deeper layers in the Atlantic and the Southern oceans, initiated by a recurrent salinity anomaly in the subpolar North Atlantic. Cooling periods associated with the latter deeper heat-sequestration mechanism historically lasted 20 to 35 years.

Mid‐2000s North Atlantic shift: Heat budget and circulation changes

“…Regarding the first mechanism, modeling simulations show that La Niña surface cooling in the eastern and central tropical Pacific can decrease the rate of increase in atmospheric temperature (although the similarity of observed and simulated deceleration is still debated [ Trenberth , 2015; Karl et al , 2015]) and induce an anomalous heat flux into the ocean [ Kosaka and Xie , 2013; England et al , 2014; Balmaseda et al , 2013]. Associated to this anomalous heat flux into the ocean, subsurface heat uptake in the western tropical Pacific and Indian Oceans occurs [ Goddard , 2014; England et al , 2014; Meehl et al , 2013; Lee et al , 2015] mostly above 300 m (see Figure S1 in the supporting information) [ Chen and Tung , 2014, Figures 1 and 3; Nieves et al , 2015], thus not largely involving transfer of heat into the deep ocean.…”

“…Regarding the second mechanism, vertical heat inventories of the North Atlantic show notable heat transfer from the upper (0–300 m) to intermediate (300–700 m) and deep (>700 m) layers since the mid‐2000s (see Figure S1 and corresponding comments) [ Alexander et al , 2013, Figure 3.1; Chen and Tung , 2014, Figure 6; Williams et al , 2014, Figure 1c]. Such recent downward heat transfer follows decades of sustained surface warming and increasing ocean heat content in the basin [ Drijfhout et al , 2014; Häkkinen et al , 2015; Williams et al , 2014; Durack et al , 2014].…”

“…Such recent downward heat transfer follows decades of sustained surface warming and increasing ocean heat content in the basin [ Drijfhout et al , 2014; Häkkinen et al , 2015; Williams et al , 2014; Durack et al , 2014]. However, concomitant with the downward heat export after the mid‐2000s, the increase in ocean heat content in the upper 2000 m of the North Atlantic paused [ Chen and Tung , 2014, Figures 1 and 3; Häkkinen et al , 2015, Figure 1], making the bulk rate of change of ocean heat content per unit area in the basin almost zero. Bulk rates of change in ocean heat content per unit area are considered an estimation of the ocean heat uptake ( Q , W m −2 ) at ocean basin scales [e.g., Mauritzen et al , 2012; Guemas et al , 2013; Chen and Tung , 2014].…”

“…However, concomitant with the downward heat export after the mid‐2000s, the increase in ocean heat content in the upper 2000 m of the North Atlantic paused [ Chen and Tung , 2014, Figures 1 and 3; Häkkinen et al , 2015, Figure 1], making the bulk rate of change of ocean heat content per unit area in the basin almost zero. Bulk rates of change in ocean heat content per unit area are considered an estimation of the ocean heat uptake ( Q , W m −2 ) at ocean basin scales [e.g., Mauritzen et al , 2012; Guemas et al , 2013; Chen and Tung , 2014]. Changes in ocean circulation can generate ocean heat content anomalies of opposite signs between different ocean layers and regions (e.g., subtropical versus subpolar gyre), but the anomalies compensate when integrated at ocean basin scales and decadal timescales [ Lozier et al , 2008; Mauritzen et al , 2012].…”

“…Deep convection in the subpolar North Atlantic under high‐salinity conditions might account for the quick transfer of heat to the deep ocean [ Chen and Tung , 2014]. According to these authors, recent high‐salinity conditions may be caused by a strengthening of the Atlantic Meridional Overturning Circulation (AMOC).…”