A new study was published this week in Frontiers in Earth Science, “Ocean Mesoscale Variability: A Case Study on the Mediterranean Sea From a Re-Analysis Perspective”. The study under the lead of Antonio Bonaduce includes Johnny A. Johannessen and Roshin P. Raj from NERSC, as well as colleagues from Italy and Germany.

What are mesoscale eddies?

Example of eddies in the Mediterranean off the Italian coast.: Natural-colour satellite image by Landsat 8. Sediments are transported by eddies and create the swirl patterns typical for these. Source: NASA Earth Observatory, https://earthobservatory.nasa.gov/images/146231/eddy-extravaganza-off-th...An eddy is basically a swirl. When you stir your milk into your morning coffee, you are creating an eddy. In this example, it is caused by the spoon moving through the coffee. That movement promotes mixing, and it transports energy. You can see the swirl because the cup is sitting right in front of you, and it isn’t very big. On larger spatial scales – imagine the Mediterranean Sea instead of a coffee cup – it is not a spoon creating swirls in the sea water, but currents, winds, temperature changes, and geographic constraints. Here, those swirls can be up to 100 km in diameter (“mesoscale”) – and just like in your coffee cup – not just the surface is swirling, but the liquid below does, too. In oceans, this movement can reach down to several km, it is a 3D-structure. In the Mediterranean Sea, they “only” reach down several hundred meters. Regarding the duration of eddies: In your coffee cup the swirling stops after a short while, but mesoscale eddies in oceans can exist for about a month or even longer.

How do we observe mesoscale eddies?

Such large swirls in oceans, mesoscale eddies, cannot be seen with the naked eye from a ship, so we need tools to observe them from far above the ocean surface. Satellites with special instruments onboard are the perfect tool, and satellite observations of the ocean surface have been made for decades now. In addition, measurements from the ocean itself (in-situ measurements of temperature, salinity, etc. for example from ARGO profiling floats) help complete the picture. Satellite data and in-situ data can also be combined with ocean models to create a look back in time, a reanalysis. 

Why do we need to know about eddies in oceans?

The mixing that is inadvertently caused by eddies is relevant for our global climate: It contributes to the ocean circulation, both horizontal and vertical, and during this process, heat, salt, and nutrients are moved around and dispersed. The process also carries a lot of energy that is being distributed.

What did Bonaduce and his colleagues investigate?

Bonaduce and his colleagues set out to examine 24 years of regional ocean reanalysis data (1993-2016) from MED-REA (Mediterranean Re-analysis) with an eddy-detection system that they improved. The system cannot just interpret occurrences on the surface, but also below, it makes it possible to make 3D analyses. They investigated how eddies in the Mediterranean Sea contribute to ocean dynamics, and how they influence the thermodynamics in the area. 

Eddy contribution to ocean dynamics

Ocean dynamics describe how water moves within oceans. There are several ways of determining eddy contribution to ocean dynamics, and Bonaduce and his colleagues looked both at sea surface height and eddy kinetic energy. Where eddies were present, they found anomalies in sea surface height – elevations and depressions – dependent on the rotation direction of the eddy creating the respective anomaly. In this study they found that mesoscale eddies drive a large portion of the kinetic energy in the Mediterranean Sea. Figure 7 from the publication (see below) shows that up to 50-60% of the kinetic energy in areas with eddies present, actually stems from the eddies themselves, both at the sea surface and down to 500 m water depth! This means that mesoscale eddies are a significant energy source in the Mediterranean Sea and thus contribute significantly to the ocean dynamics, also by affecting the sea surface height.Relative eddy kinetic energy at the surface (a) and down to 500 m depth, in percentage. Figure 7 from Bonaduce et al., 2021

Eddy contribution to ocean thermodynamics

To figure out to what extent eddies contribute to ocean thermodynamics, Bonaduce and his colleagues looked at the ocean heat content. Ocean heat content varies due to several reasons over time. Eddies influence the temperature in any ocean: Depending on whether an eddy swirls clockwise or counter-clockwise, the core of the eddy is typically warm (clockwise) or cold (counter-clockwise). This phenomenon has been observed worldwide and is also present in the Mediterranean Sea eddies. Bonaduce and his colleagues found that mesoscale eddies account for up to 15-25% of the monthly variability in ocean heat content in the study area!

Lead author Antonio Bonaduce on the study:

“This work shows the importance of characterizing the 3D mesoscale eddy field both for ocean dynamics and thermodynamics.” Knowing that we can quantify to what extent these eddies affect the movement and temperature in oceans, and quite robustly thanks to this new study, will benefit future eddy investigations in other areas than the Mediterranean Sea.

Reference:

Bonaduce, A, Cipollone, A, Johannessen, JA, Staneva, J, Raj, RP & A Aydogdu. Ocean Mesoscale Variability: A Case Study on the Mediterranean Sea From a Re-Analysis Perspective. Frontiers in Earth Science 2021. 9:724879, https://doi.org/10.3389/feart.2021.724879