A patch of ocean in the North Atlantic is experiencing cooling while the rest of the planet is warming, known as the “cold blob.” According to a study led by Penn State, changes in large-scale atmospheric patterns may be equally responsible for this anomaly, in addition to changes in ocean circulation.
The study’s corresponding author, Laifang Li, assistant professor of meteorology and atmospheric science at Penn State, stated that atmospheric circulation changes have a long-term impact on the climate system, contrary to popular belief that the atmosphere has a short memory.
Changes in sea surface temperatures in the subpolar North Atlantic, where temperatures have decreased by about 0.7 degrees Fahrenheit over the last century, may be attributed to a more frequent positive phase of the North Atlantic Oscillation (NAO). The NAO involves a low-pressure system near Iceland and a high-pressure system near the Azores Islands, influencing the direction of westerly winds over the ocean. In the positive phase, both pressure systems are stronger, resulting in a stronger jet stream and a northward shift of the westerly winds.
Li explained that the intensification of surface winds over the subpolar North Atlantic due to a positive NAO provides a direct cooling effect by promoting heat loss from the ocean surface, similar to stirring a cup of hot coffee to cool it down.
The researchers analyzed weather data and found a more dominant positive NAO in the past century, potentially caused by warming of the tropical Indo-Pacific and sea ice loss in the Labrador Sea. However, the exact causes of the shift remain unknown.
Using an idealized model, the researchers isolated the impact of increased winds on reducing sea surface temperatures via air-sea heat flux.
Yifei Fan, lead author on the study and a doctoral candidate at Penn State, explained that their model separated the processes of surface temperature change and the atmosphere’s response to it. The researchers found that the NAO alone explained 67% of the sea surface temperature cooling trend, with other atmospheric patterns partially offsetting this, resulting in a 44% overall impact of atmospheric circulation changes.
The study suggests that the NAO may play an equally important role as the Atlantic Meridional Overturning Circulation (AMOC) in the cold blob phenomenon. Previous studies have primarily focused on ocean circulation, but this research highlights the importance of considering atmospheric circulation changes.
The scientists emphasize that their findings do not discount the role of oceanic processes in the cooling, but suggest that atmospheric circulation changes should also be taken into account moving forward.
The unique cold blob region has potential climate impacts, as its cooling can increase atmospheric instability and facilitate the passage of storms, leading to extreme weather events in North America and Europe. This adds complexity and uncertainty to climate projections for densely populated areas.
Wei Liu from the University of California, Riverside; Ru Chen from Tianjin University, China; and Pengfei Zhang from Penn State also contributed to this research. The project received support from the National Science Foundation and the National Natural Science Foundation of China, as well as a seed grant from the Penn State Institute for Computational and Data Sciences.