image: During the AMO+, the meridional gradient of the surface temperatures decreases, triggering more atmospheric blockings over Greenland (red rings with clockwise arrows). This high-pressure system weakens south-to-north meridional winds, reducing the FSSIE (dashed dark gray arrow). Consequently, this maintains the high salinity of the Labrador Sea and the strength of the AMOC (red shaded area), thus extending the AMO+ (double thick solid red arrows). Conversely, during the negative phase of the AMO (AMO–), the meridional gradient of surface temperatures increases. In the absence of atmospheric blocking, the low-pressure system (blue rings with anticlockwise arrows) facilitates greater FSSIE into the Labrador Sea (solid dark gray arrow), maintaining its low salinity and the weakened state of the AMOC (blue shaded area), thereby prolonging the AMO– (double thick solid blue arrows). Background of the image depicts the phase spatial pattern of the AMO. Dark gray area covering the Arctic represents the Arctic sea ice extent. Black arrows on the sea ice indicate the direction of sea ice movement. Curved arrows represent the influence of the ocean on the atmosphere.
Credit: Figure from Modeling the Atlantic Multidecadal Oscillation: The High-Resolution Ocean Brings the Timescale; the Atmosphere, the Amplitude, created by Xiaojie Hao.
The Atlantic Multidecadal Oscillation (AMO) — a long-term cycle in Atlantic Ocean temperatures that repeats roughly every 40 to 80 years — influences everything from the frequency of hurricanes to heatwaves in Europe, North America and Asia, and even affect where species like Atlantic bluefin tuna migrate.
Scientists knew that high-resolution models can improve simulations of AMO, but did not understand how. Now, an international team has figured out why more detailed models can simulate the AMO more realistically.
This new study, led by Xiaojie Hao, a doctoral student at the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research in Germany, was published on March 21 in Ocean-Land-Atmosphere Research.
“Given its inherent scientific value, along with its enormous socioeconomic implications, a realistic representation of the AMO, which relies on a thorough understanding of the mechanisms driving AMO formation, is in high demand in numerical climate models,” said Hao, who is also affiliated with the Frontiers Science Center for Deep Ocean Multispheres and Earth System and Key Laboratory of Physical Oceanography at the Ocean University of China.
Using the Alfred Wegener Institute Climate Model (https://fesom.de/models/awi-cm/), the team ran four experiments, investigating how the following resolution combinations contribute to simulations: low-resolution atmosphere and low-resolution ocean; high-resolution atmosphere and low-resolution ocean; low-resolution atmosphere and high-resolution ocean; and high-resolution atmosphere and high-resolution ocean.
They discovered that increasing the ocean’s resolution is key to accurately simulating AMO variability, particularly at multidecadal timescales. Low-resolution ocean experiments showed unrealistic cycles every 10 to 20 years, while high-resolution ocean experiments correctly showed the AMO lasting 40 to 80 years. Increasing the atmosphere’s resolution also helped, especially in bringing the simulated amplitude of the AMO closer to that of the observations
But the real breakthrough was in uncovering how this improved resolution made such a difference. A key revelation, according to Hao, was how AMO interacted with the Fram Strait sea ice export (FSSIE) that serves to move ice from the Arctic to the North Atlantic Ocean, as well as the high-pressure atmospheric system over Greenland — known as atmospheric blocking.
By increasing the ocean resolution of the model, the circulation structure in the North Atlantic was more accurately simulated, which allowed the model to capture AMO-FSSIE positive feedback via the Atlantic Ocean circulations. Higher atmospheric resolution further improved AMO simulations by strengthening the connection between atmospheric blocking and Arctic sea ice. These highlight the role of resolution in capturing weather-scale processes and long-term ocean interactions as key for AMO modeling.
These findings demonstrate both the potential and the necessity of developing high-resolution models, according to Hao. She also said that future research should further explore high-resolution simulations to better understand the physical mechanisms underlying low-frequency climate variability and their impacts on past, present and future climate.
Other collaborators include Dimitry V. Sein, Tobias Spiegl, Lu Niu and Gerrit Lohmann, Alfred Wegener Institute Helmholtz Center for Polar and Marine Research; and Xianyao Chen, Frontiers Science Center for Deep Ocean Multispheres and Earth System and Key Laboratory of Physical Oceanography at the Ocean University of China. Sein is also affiliated with the Shirshov Institute of Oceanology, Russian Academy of Sciences, and the Moscow Institute of Physics and Technology. Lohmann is also affiliated with the University of Bremen in Germany, and Chen is also affiliated with the Laoshan Laboratory in China.
The Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities, the Germany-Sino Joint Project, the MHESRF Scientific Task and the Moscow Institute of Physics and Technology Development Program supported this research.
Journal
Ocean-Land-Atmosphere Research
Method of Research
Experimental study
Subject of Research
Not applicable
Article Publication Date
21-Mar-2025
COI Statement
There are no conflicts of interest to declare.