The Atlantic Multidecadal Variability in a pre-industrial EC-Earth3 run

Abstract

The Atlantic Multidecadal Variability (AMV) is a variation in sea surface temperature (SST) in the North Atlantic with a period of about 60-90 years. The drivers of this mode of variability are largely unknown as observational data only go back 150 years. Global climate models can help us better understand the AMV and its drivers and this thesis investigates how the AMV behaves in the Earth system model EC-Earth3. The thesis examines a 1000-year-long pre-industrial run and evaluates potential driving variables for the AMV. Specifically, the Atlantic Meridional Overturning Circulation (AMOC), which supplies the North Atlantic with warm, salty waters of tropical origin, as well as total cloud cover that affects the incoming solar radiation to the sea surface. The thesis finds that the AMOC is well correlated with the AMV with a lead of 4 years, suggesting that the AMOC is a key factor in modulating the AMV. It also finds that the AMV in the model experiences a period of around 128 years, longer than in observations, and that the subtropical arm of the AMV is weaker than in observations. Finally, the thesis finds positive cloud feedback in the subtropical arm of the AMV that enhance and propagate the changes in SSTs southward. Since the subtropical AMV arm is not as strong as in observations, the feedbacks are either not strong enough, or there are missing feedbacks needed to fully form the subtropical arm of the AMV in EC-Earth3.

Popular Abstract

The ocean affects our climate to a great extent. Here in Scandinavia we have the Gulf Stream to thank for our warm climate as it transports warm water from the south up to the western coast of Scandinavia. The warm surface water heats up the atmosphere and brings us our pleasant Swedish summers, and relatively mild winters. If something were to happen to the Gulf Stream our climate would drastically change. The ocean is a massive interconnected web of different processes that all intertwine with each other. Pull one string and it can have unforeseen consequences in other parts of the ocean. One such string is the Atlantic Multidecadal Variability (AMV).

The AMV is a variation in sea surface temperature (SST) in the North Atlantic. It changes between a warm period and a cold period on a time-scale of around 60-90 years. While it is not a very large variation it still packs enough power to affect the climate of the entire Northern Hemisphere by a significant amount. The AMV moved into its warm phase in the 1990s, resulting in warming and a change in weather patterns across the entire Northern Hemisphere. If we do not take the AMV into account in our global climate models the observed climate change will be larger than our predictions, as models underestimate the warming. It is thus important to know how the AMV functions and how it interacts with the rest of the North Atlantic.

Scientists argue over what the main power source of the AMV is, and part of the reason why it is hard to fully understand the dynamics of the AMV is because of its long period. We only have observational data going back around 150 years, which is only enough to maybe cover one full cycle of the AMV. We therefore have to turn to climate models to investigate it properly. One such model is the relatively new global climate model, EC-Earth3. This thesis investigates the AMV through a model run of EC-Earth3 and finds that in the model, the AMV is in part caused by variations in oceanic circulation like the Gulf Stream. If the Gulf Stream transports more warm surface water to the North Atlantic, then the temperatures across the North Atlantic will increase. The AMV is then amplified by changes in cloud cover as warm SSTs reduce cloud cover, which further increases SSTs as the sun heats the surface of the ocean during clear skies. This means there is a positive feedback between SSTs and clouds, that changes in one enhances changes in the other. Therefore, the dynamics behind the AMV in EC-Earth3 indicate that both ocean and atmospheric interactions are likely needed to fully form the AMV. With a greater understanding of the AMV we gain insight into one part of the massive web that makes up the oceans. Processes like the AMV rarely occur in isolation, but are instead connected to all the surrounding oscillations and changes in nature. In the end, this thesis has gathered new insight into how EC-Earth3 reproduces the AMV, and the surrounding dynamics that modulate it.

• author: Magndal, Stina
• supervisor: Moa Sporre; Annika Drews
• organization: Nuclear physics; Department of Physics
• course: FYSK03 20231
• year: 2024
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy; Earth and Environmental Sciences
• keywords: Atlantic Multidecadal Variability, Earth System Model, Coupled General Circulation Model, EC-Earth 3, North Atlantic, Meteorology, Oceanography
• language: English
• id: 9150976
• date added to LUP: 2024-04-29 08:35:24
• date last changed: 2024-04-29 08:35:24