Aerosol Forcing Masks and Delays the Formation of the North Atlantic Warming Hole by Three Decades

Abstract

The North Atlantic warming hole (NAWH) is referred to as a reduced warming, or even cooling, of the North Atlantic during an anthropogenic-driven global warming. A NAWH is predicted by climate models during the 21st century, and its pattern is already emerging in observations. Despite the known key role of the North Atlantic surface temperatures in setting the Northern Hemisphere climate, the mechanisms behind the NAWH are still not fully understood. Using state-of-the-art climate models, we show that anthropogenic aerosol forcing opposes the formation of the NAWH (by leading to a local warming) and delays its emergence by about 30 years. In agreement with previous studies, we also demonstrate that the relative warming of the North Atlantic under aerosol forcing is due to changes in ocean heat fluxes, rather than air-sea fluxes. These results suggest that the predicted reduction in aerosol forcing during the 21st century may accelerate the formation of the NAWH. Plain Language Summary Anthropogenic aerosols are particles suspended in the atmosphere, which were released due to anthropogenic activity. These particles have a general cooling effect on the Earth due to their interactions with radiation and with clouds. Here we show that the surface temperature in the North Atlantic Ocean is predicted to increase due to aerosol forcing (despite the global cooling). This trend is the opposite of the surface temperature trend predicted due to increase in greenhouse gases (global warming with a warming "hole" in the North Atlantic, trend known as the North Atlantic warming hole-NAWH). Using state-of-the-art climate models, we show that aerosol forcing delays the formation of the NAWH by about 30 years. This trend could have important climatic impacts due to the key role of the North Atlantic surface temperatures in setting the Northern Hemisphere's climate and due to the predicted reduction in aerosol forcing in the next few decades.

Detecting Climate Signals Using Explainable AI With Single‐Forcing Large Ensembles

“…As industrial aerosols are reduced over the 21st century, there is a net warming trend globally in AER+ through 2080 (Figures 2c and 2d). Notably, the temperature trend in the North Atlantic reverses and resembles the “North Atlantic Warming Hole.” In agreement with earlier studies (e.g., Dagan et al., 2020), this suggests an important role for aerosols in North Atlantic climate variability. Figures 2e–2h reveals the global warming signature due to the dominant greenhouse gas forcing in GHG+ (time‐evolving greenhouse gases; constant aerosols), along with a cooling patch in the North Atlantic.…”

“…However, there is a notable statistically significant region of warming over parts of the North Atlantic and Southern Ocean (Figure 2b). These areas of warming may be connected to a strengthened Atlantic Meridional Overturning Circulation (Dagan et al., 2020; Keil et al., 2020; Menary et al., 2020). The global signature of cooling prior to 2000 is associated with an increase in industrial aerosol emissions.…”

“…While greenhouse gas forcing dominates the overall climate change signal (net warming), an abundance of anthropogenic aerosols can also influence Earth's surface temperature (net cooling) by scattering or absorbing incoming solar radiation (Bellouin et al., 2020). Further, recent studies have found an influence of anthropogenic aerosols on tropospheric temperatures (e.g., Mitchell et al., 2020; Santer et al., 2019), oceanic internal variability (e.g., Dagan et al., 2020; Haustein et al., 2019; Meehl, Hu et al., 2020; Qin et al., 2020), the hydrologic cycle (e.g., Bonfils et al., 2020; Marvel et al., 2019), and the large‐scale atmospheric circulation (e.g., Allen & Sherwood, 2011; Wang et al., 2020). Meanwhile, less attention has been given to comparing regional climate trends to individual anthropogenic external forcings relative to the influence of internal variability (see examples by Bonfils et al., 2020; Chemke et al., 2020; Deser, Phillips et al., 2020; Polvani et al., 2011; Santer et al., 2019).…”

How Does the North Atlantic SST Pattern Respond to Anthropogenic Aerosols in the 1970s and 2000s?

“…The negative anthropogenic aerosol radiative effects (Figure ) causes the expected northern hemisphere SST reduction in SP‐75 compared to PI, but the eastern part of the sub‐polar gyre in the North Atlantic responds with a SST increase (Figure 1c), consistent with other data (Dagan et al., 2020). Regional SST reductions associated with anthropogenic aerosols are at the order of −0.5°C.…”

“…Relative to the 1970s, the NAWH changed little while the northern hemisphere in the 2000s generally warmed more than the southern hemisphere (e.g., Figure ). Despite the importance of understanding near‐surface warming patterns, the underlying physical mechanisms of the NASST patterns are still poorly understood, so much so that different plausible explanations exist (e.g., Booth et al., 2012; Clement et al., 2015; Dagan et al., 2020; Delworth & Mann, 2000; Keil et al., 2020; Kim et al., 2018; Otter et al., 2010; Wang et al., 2012).…”