3D models help researchers understand the climate impact of eddies

Mesoscale eddies are ocean vortices less than 100 kilometers in diameter that are responsible for the localized “weather” of the oceans. Because of the large amount of mass and energy movement associated with these currents, mesoscale eddies play an integral role in determining Earth’s climate.

In order to better understand the influence mesoscale eddies have on the global climate system, these large water vortices must be rendered in three dimensions to model how eddy heat and mass interacts with other climate processes over time. To address this issue, a group of oceanographers from Ocean University in Qingdao, China and Fudan University in Shanghai, China wrote a review outlining how to best model mesoscale eddies to better predict future changes in climate.

The group published their review online on July 15 in the journal Ocean-Land-Atmosphere Research.

“[T]he complexity of [mesoscale eddy] structures, including their horizontal and vertical structures, temporal evolution and fine structures, has posed challenges [in] fully capturing their influence on climate-related processes. This paper addresses these challenges by reviewing the development of mesoscale edd[y] structures, bringing up potential topics and unresolved difficulties for further research and future development,” said Zhengguang Zhang, professor in the Physical Oceanography Laboratory at Ocean University of China in Qingdao, China.

The team noted that the horizontal structures of mesoscale eddies, because of their sheer size and ability to be observed, are much better understood and more easily modeled in laboratory experiments and numerical models than vertical structures. As an ocean vortex, the horizontal structures of mesoscale eddies automatically form a circular shape; much like galaxies, Jupiter’s Great Red Spot or hurricanes; as a minimum-energy state.

This spontaneous process, called axisymmetrization, occurs when an elliptical vortex rotates around a central axis and loses smaller filaments to create a horizontally circular eddy. But in the crowded ocean environment, this circular form is only temporary and is often deformed by other eddies, changes in water density or seafloor topography, or strong atmospheric processes, such as hurricanes.

But mesoscale eddies don’t exist in only two dimensions. Modeling the 3D shape of these vortices also requires accurately predicting the vertical axis, which is much more difficult to measure. Mesoscale eddies can reach the ocean floor, accounting for the majority of kinetic energy in the ocean. The complex motions occurring in the ocean also make a perfectly vertical axis in a mesoscale eddy very unlikely, as changes in water density and large-scale currents can easily tilt the vertical axis.

The eddy lifespan consists of three different stages: formation, maintenance and destruction, and better characterization of these dynamics over time can help researchers do a better job modeling these structures. From the start of an eddy as an ocean instability to the growth of the vortex as it absorbs energy, intensifies and forms an axisymmetric circle, the 3D structure of the eddy is constantly influenced by dynamic oceanic and atmospheric forces that change the state and shape of the vortex.

All of these factors can make it very difficult for researchers to effectively model eddies and their effect on global energy dynamics. This work emphasizes the importance of incorporating the vertical and horizontal axes, their associated structures and the evolution of the vortex over time into a unified framework.

“Based on this unified framework of mesoscale eddy structures, [it is easier to] understand… [the] impacts [of eddies] on oceanic energy cascades, heat and water mass transport, as well as nutrient and chlorophyll distribution. Subsequently, mesoscale eddies should be placed into an even broader integrated framework to comprehend their role in the overall climate system and biogeochemical cycles,” said Zhang.

The research team acknowledges that more work is needed to accurately represent the effects of mesoscale eddies in climate models.

“Future research will focus on refining the 3D structural models of mesoscale eddies and integrating these insights into climate models. The ultimate goal is to enhance predictive climate models by accurately representing mesoscale eddies’ dynamic role in global heat and mass circulation, leading to more reliable climate change projections,” said Zhang.