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Formation of the North Atlantic Warming Hole by reducing … – Nature.com

In this study, sensitivity experiments were conducted with no anthropogenic SO2 emissions (Sulf × 00) and double anthropogenic SO2 emissions (Sulf × 2) compared to those in the base experiment; and for comparison, additional experiments were also carried out wherein CO2 concentrations of 0.9 (CO2 × 09) and 1.2 (CO2 × 1p2) times, respectively, were changed to produce nearly equivalent global mean surface temperature changes (see “Methods” in detail). When global warming occurs, the NAWH appears in the North Atlantic region (Fig. 1a,b), while the reversed NAWH occurs in the same region under global cooling (Fig. 1c,d). The distribution of the sea surface temperature (SST) anomalies is similar to that of surface air temperature anomalies, and the NAWH appears in the same region, as shown later. Compared to the CO2 sensitivity experiments, the SO2 emission change results in larger temperature changes in the North Pacific, Asia, Europe, and the United States, which are regions with high industrial activity.

Figure 1
figure 1

Anomalies of the annual mean surface air temperature for CO2 × 1p2 (a), Sulf × 00 (b), CO2 × 09 (c), and Sulf × 2 (d) experiments from the base experiment. Areas of dots indicate that the change is statistically significant. The area enclosed by the square represents the North Atlantic region defined in this study. The global mean temperature changes are + 0.32 K (a), + 0.41 K (b), − 0.28 K (c), and − 0.34 K (d), respectively. The maps were generated with GrADS 2.2.1 (URL: http://cola.gmu.edu/grads/).

Figures 2 and 3 show the anomalies in SST and sea surface salinity (SSS), respectively, in the North Atlantic region (0°–80°W, 20°–80°N). In the NAWH region, the temperature anomaly is reversed from the trend of global change in all the experiments. Under global warming (Sulf × 00), SST cooling in the Labrador Sea also occurs in addition to the formation of the NAWH inside the NASG (Fig. 2b). The distributions of the regions where temperature changes are reversed are almost the same for CO2 × 1p2 (Fig. 2a) as Sulf × 00. The reversed NAWH with global cooling (Sulf × 2) also has a geographic distribution similar to that of the NAWH (Fig. 2d). The anomaly in the SSS from the base experiment decreases north of 40°N with global warming and increases with global cooling (Fig. 3b,d). This is due to changes in the amount of high-salinity surface seawater transported from the south by the North Atlantic Current. However, as discussed later, since the current velocity from the Labrador Sea to the interior of the NASG is enhanced with global warming, the decrease in SSS during the formation of the NAWH can also be due to an increase in the inflow of low-salinity seawater from the polar region.

Figure 2
figure 2

Anomalies of the annual mean sea surface temperature for CO2 × 1p2 (a), Sulf × 00 (b), CO2 × 09 (c), and Sulf × 2 (d) experiments from the base experiment in the North Atlantic. Areas of dots indicate that the change is statistically significant. LS and WH show the Labrador Sea and NAWH regions, respectively. The maps were generated with GrADS 2.2.1 (URL: http://cola.gmu.edu/grads/).

Figure 3
figure 3

Same as Fig. 2 but for the annual mean sea surface salinity. The maps were generated with GrADS 2.2.1 (URL: http://cola.gmu.edu/grads/).

The SST anomalies of the global and regional mean in the NAWH region are shown in Table 1. The global mean SST anomalies show the similar level of warming and cooling between CO2 × 1p2 and Sulf × 00 and between CO2 × 09 and Sulf × 2, respectively. However, the temperature increase inside the NAWH region with global cooling is approximately twice as large for Sulf × 2 than for CO2 × 09. The reason for this difference can be explained by the fact that the sulphate aerosols produced from SO2 are more unevenly concentrated in the mid-latitudes of the Northern Hemisphere than CO2. The NAWH region is consistent with areas of air pollutant outflow from North America. This will be discussed in more detail later, along with a discussion of the main factors in the formation mechanism of NAWH.

Table 1 Anomalies of the annual mean sea surface temperature averaged over global and the NAWH region for each sensitivity experiments from the base experiment.

The vertical profiles of ocean temperature, salinity and density averaged in the NAWH region are shown in Fig. 4a–c. The NAWH and reversed NAWH are clearly shown to occur mainly in the surface layer shallower than 100 m depth (Fig. 4a). The increase in ocean temperature causes a decrease in density, and on the other hand, the increase in salinity causes an increase in density. Therefore, the effect of the salinity change is dominant in the NAWH region (Fig. 4b,c). The effect of this density change can alter the vertical mixing in the NAWH region and weaken the heat exchange with the warm seawater in the subsurface layer, leading to further cooling of the SST.

Figure 4
figure 4

Anomalies of annual mean vertical profiles of (a,d) potential ocean temperature (K), (b,e) salinity (psu), and (c,f) potential density (g cm–3) averaged in the (a–c) NAWH region (20°–40°E, 40°–60°N) and (d–f) Labrador Sea (52°–60°E, 55°–63°N) for each experiment from the base experiment.

It should be noted here that the changes in temperature, salinity, and density in the NAWH region are larger under adjustments to sulphate aerosol concentration changes than under CO2 concentration changes. While there is little difference between them (Sulf × 00 and CO2 × 1p2) in regards to temperature change under global warming, the difference in temperature change under global cooling between Sulf × 2 and CO2 × 09 is clear. The change in salinity and the associated change in density are larger in the case of sulphate aerosol concentration changes in both the NAWH and reversed NAWH cases. This can be attributed to the fact that anthropogenic sulphate aerosols are concentrated in the mid-latitudes of the Northern Hemisphere. Temperature changes in the mid-latitudes of the Northern Hemisphere due to anthropogenic sulphate aerosols are then greater than in other latitudinal zones11.

Changes in the North Atlantic heat flux

Figure 5 show the distribution of horizontal heat flux and ocean temperature anomalies relative to the base experiment averaged below 100 m depth. The formation of the NAWH with global warming can be attributed to the change in heat transport from the south to the interior of the NASG due to the change in the flow path of the North Atlantic Current and the increase in the inflow of cold water from the surface layer of the Labrador Sea into the interior of the NASG4. On the other hand, during the formation of the reversed NAWH under global cooling condition, caused by the decrease in CO2 concentration and increase in SO2 emissions, the flow path of the North Atlantic Current is strengthened towards the interior of the NASG, which increases the inflow of warm seawater from the south. Then, the surface water inside the reversed NAWH flows into the inner part of the Labrador Sea by the enhanced NASG, causing a positive temperature change in the Labrador Sea. Although heat fluctuates in the ocean occur vertically, their contribution to the formation of NAWH is negligible because the amount of change is more than three orders of magnitude smaller than that of horizontal heat fluxes. Additionally, the change in horizontal heat flux between the Labrador Sea and the NAWH region is less than 0.5 MW m–2, which is less than the change in heat flux due to changes in the Gulf Stream and North Atlantic Current, and therefore it has a limited role in NAWH formation (Fig. 5a,b). With global cooling, the enhanced NASG increases the heat flux from the reversed NAWH region to the Labrador Sea. The change in the horizontal heat flux is approximately twice as large in the Sulf × 2 experiment (Fig. 5d) than in the CO2 × 09 experiment (Fig. 5c), which is consistent with the larger temperature anomaly in Sulf × 2 than in CO2 × 09 (Table 1).

Figure 5
figure 5

Anomalies of annual mean horizontal heat flux (vectors) with ocean temperature (colours) for CO2 × 1p2 (a), Sulf × 00 (b), CO2 × 09 (c), and Sulf × 2 (d) experiments from the base experiment in the North Atlantic. They are averaged below 100 m depth. Anomalies of the horizontal heat flux below 0.25 MW m–2 are not shown. The maps were generated with GrADS 2.2.1 (URL: http://cola.gmu.edu/grads/).

The AMOC weakens with global warming and strengthens with global cooling, resulting in changes in oceanic heat transport (Fig. S1). In particular, the AMOC stream function strengthens approximately twice in Sulf × 2 than in CO2 × 09 (Fig. S1c,d), which is consistent with the difference in the regionally averaged temperature change in the NAWH region under global cooling conditions (Table 1). This confirms that heat transport from the south in the surface layer is dominated by the AMOC and is consistent with changes in the Gulf Stream and the North Atlantic Current.

In the case of global cooling, reversed NAWH appears stronger in the increased SO2 emissions (Sulf × 2) than in the decreased CO2 concentration (CO2 × 09), while in the case of NAWH associated with global warming, the difference between increased CO2 concentration (CO2 × 1p2) and decreased SO2 emissions (Sulf × 00) is not clear (Table 1). The reason can be that although a difference in the strength of the AMOC stream function changes appears between CO2 × 1p2 and Sulfx00 (Fig. S1a,b), there is little difference between them for the change in horizontal heat flux in the surface layer, the main mechanism for NAWH formation (Fig. 5a,b). That is, the degree of weakening of the North Atlantic Current flux into the interior of the NASG due to the northward shift of the Gulf Stream caused by global warming is similar in both cases. Reduced SO2 emission means that spatial heterogeneity in the radiative forcing due to sulfate aerosols is eliminated, thus nonlinear changes are mitigated not only in the atmosphere but also in the ocean surface layer directly affected by the atmosphere.

The changes in horizontal heat fluxes are mainly due to changes in the velocity and the path of the Gulf Stream and the North Atlantic Current, and the changes in the currents are also due to changes in the Deep Western Boundary Current (DWBC), which is the Labrador Deep Water Current that flows southward around the Island of Newfoundland6. The DWBC weakens with global warming and strengthens with global cooling (Fig. S2). The DWBC has a significant influence on the path of the Gulf Stream through changes in the NRG vorticity.

Figure 4d–f show the vertical profiles of anomalies of temperature, salinity, and density from the base experiment averaged in the Labrador Sea. The effect of salinity change is the primary factor that changes the density of seawater, as seen in the NAWH region (Fig. 4a–c). The salinity changes at depths below 200 m are approximately twice as large with global warming and approximately three times as large with global cooling in the sulphate sensitivity experiments. This can be related to the fact that the changes in the mixed layer depth in the Labrador Sea are greater in the sensitivity experiments for sulphate concentration than for CO2.

Changes in freshwater fluxes in the North Atlantic

Freshwater fluxes were analysed to investigate the causes of low salinization in the surface waters of the Labrador Sea. The freshwater and horizontal freshwater fluxes were defined as FW = 1–(S/Sref) and FW = FWV, respectively, where S is the salinity of seawater, Sref = 34.7 psu, and V is the current horizontal velocity vector4. The anomaly of the freshwater flux from the base experiment in each sensitivity experiment was analysed by decomposing it into two components, one originating from the change in freshwater volume and the other from the change in current velocity, as follows.

$$Delta {mathbf{FW}} = , Delta ({text{FW}} cdot {mathbf{V}}) , = , Delta {text{FW}} cdot {mathbf{V}} + {text{ FW}} cdot Delta {mathbf{V}} .$$

(1)

Freshwater fluxes are averaged below 100 m depth in the analysis.

With global warming, the freshwater inflow from the Arctic increases mainly due to the flux through the Canadian Arctic Archipelago (CAA) into the Labrador Sea and the inner NASG (Fig. S3a). It is shown that the freshwater volume increases from the Arctic Ocean into Baffin Bay through the CAA (Fig. S3b) and that the increase in current velocity southward in Baffin Bay increases the freshwater flux into the Labrador Sea (Fig. S3c). Conversely, with global cooling, the freshwater flux from the Arctic Ocean and Labrador Sea into the NASG decreases (Fig. S3d–f).

Figure 6a and b show the change in sea ice thickness in the Arctic. With global warming, there are particularly large decreases north of Greenland and around the CAA. Conversely, with global cooling, large increases in sea ice thickness occur in the Greenland Sea and around the CAA. With global warming, the melting of arctic sea ice brings freshwater into the North Atlantic through two major entry points: the Fram Strait and the CAA. Figure 6c shows the meridional components of the freshwater fluxes through the surface layer of the four straits (Fram Strait, CAA, Denmark Strait, and Davis Strait shown in Fig. 6a). The southward freshwater flow through the Fram Strait and the CAA is enhanced by the melting of sea ice in the Arctic Ocean. The flux through the Fram Strait is divided into the flow to the North Atlantic through the Denmark Strait and northward again to the Barents Sea, of which the component flowing into the North Atlantic is reduced. The southward freshwater flux through the CAA is found to pass through the Davis Strait and into the Labrador Sea without changing its magnitude. With global cooling by increasing sulphate aerosols (Sulfx2), the change in freshwater flow through the Denmark Strait is also very limited.

Figure 6
figure 6

Anomalies of the annal mean Arctic sea ice thickness for Sulf × 00 (a) and Sulf × 2 (b) experiments from the base experiment. The dashed and solid blue lines are the locations of the annual mean sea ice thickness of 0 m in the base and each sensitivity experiments, respectively. The letters A, B, C, and D in (a) are the locations of the Fram Strait, Canadian Arctic Archipelago (CAA), Denmark Strait, and Davis Strait, respectively. The maps were generated with GrADS 2.2.1 (URL: http://cola.gmu.edu/grads/). The annual mean southward freshwater flux through each strait below 100 m depth is in (c).

Therefore, the freshwater supply from the Arctic region to the North Atlantic Ocean is dominated by the change in the freshwater flux through the CAA. The freshwater fluxes in the North Atlantic region from the melting of sea ice are more than one order of magnitude larger than other freshwater flux changes, including precipitation, evaporation from the sea surface, and river inflow.

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