The article was originally published on the NERSC website. Written by Henrike Wilborn, communication manager at NERSC.
Fabio Mangini, a PhD candidate at NERSC and UiB, and affiliated with the Bjerknes Centre, recently published his first paper. He worked on it together with Léon Chafik (Stockholm University), Erica Madonna (previously UiB/GFI), Camille Li (UiB/GFI, Mangini’s UiB supervisor), Laurent Bertino (NERSC, Mangini’s NERSC supervisor), and Jan Even Øie Nilsen (IMR).
They set out to investigate how sea level variability in the North Sea and the jet stream over the North Atlantic are related and have found interesting connections.
You surely know that the sea level in oceans all over the globe varies due to the tide, driven by the gravitational pull of the moon. You might also know that storms can affect the sea level, too. But did you know that certain wind patterns also have a strong effect on how high the sea level is?
How high or low the sea level is, is described as sea level variability.
Winds and atmospheric changes over the North Atlantic cause sea level variability
In winter, the variability of the sea level over the northern European continental shelf is mainly influenced by winds. These winds are regulated by the conditions in the atmosphere over the North Atlantic. This means that changes in these atmospheric conditions contribute to sea level variability in Northern Europe.
A portion of the changes can be related to the weather phenomenon called North Atlantic Oscillation (NAO). But the NAO gives a too rough picture of all the changes in the atmosphere over the North Atlantic.
What else is going on in the atmosphere over the North Atlantic?
Mangini and his co-authors investigated the jet stream over the North Atlantic as source of change. They found that it does have distinct effects on the sea level variability over the northern European continental shelf.
But wait, what is this jet stream over the North Atlantic?
The jet stream is a narrow band of very strong winds in the atmosphere, always blowing eastwards. The wind strength increases with height, from the Earth’s surface to the tropopause in several kilometers' height. It is strongest in winter, because it occurs due to the temperature differences between the Arctic and the Tropics. It meanders – sometimes further south, sometimes further north – over the North Atlantic, forming an atmospheric pattern that is quite interesting to study in the context of sea level variability.
Why is the jet stream interesting?
The way the jet stream blows over the northern European continental shelf during wintertime takes four preferential configurations. Each configuration is unique and happens on a somewhat regular basis. They affect the weather over the Atlantic and Europe during winter. But not just the weather, as Mangini and his co-authors found out.
The four jet stream configurations can be matched with four distinctly different changes in sea level, as seen below. This is pretty exciting, as it explains the sea level variability over the northern European continental shelf in more details than previous studies.
Three main points of the study
- The sea level variability in the western part of the North Sea seems to be mainly affected by wind direction, rather than wind speeds.
- Previous studies have been using empirical orthogonal functions, such as the NAO and the like, to investigate the northern European sea level variability. This approach has nice mathematical properties, but also a limitation since the empirical orthogonal functions might not represent atmospheric patterns that exists in nature. In contrast, the jet stream patterns that Mangini and his colleagues utilized to investigate the sea level variability, do represent actual weather situations. Using these jet stream patterns, Mangini and his colleagues show that the sea level variability is influenced strongly by these winds. They are much better suited to capture large-scale atmospheric configuration patterns over the North Atlantic than empirical orthogonal functions.
- Another approach (composite analysis) previously used in several studies depends on the choice of a target location and therefore lacks universality.
In short, this paper describes an interesting and novel approach to investigate and classify sea level variability over the northern European continental shelf, caused by jet stream patterns over the North Atlantic.
Mangini, F, Chafik, L, Madonna, E, Li, C, Bertino, L & JEØ Nilsen. The relationship between the eddy-driven jet stream and northern European sea level variability. Tellus A: Dynamic Meteorology and Oceanography 2021. 73(1), 1-15 https://doi.org/10.1080/16000870.2021.1886419