Creating an Ocean Eddy Map to Understand Global Climate Change from a Statistical Perspective

This year’s Climate and Society class is out in the field (or lab or office) completing a summer internship or thesis. They’ll be documenting their experiences one blog post at a time. Read on to see what they’re up to.

Ci Zhang, C+S ’16

Mesoscale eddies, known as the weather of the ocean, are masses of spinning water. The radical scale of an eddy ranges from 15 miles to more than 150 miles wide, with a lifetime of 10–100 days. Eddies are present almost everywhere in the world ocean, transporting heat, salt- and freshwater, dissolved carbon dioxide, and other tracers all around the globe.

Studies of ocean mesoscale eddies are therefore of great significance and may lead to better understandings of climate change. The goal of my internship has been to produce an interactive website in which the positions, sizes, and tracks of ocean mesoscale eddies that can be viewed on a global map.

I selected courses Introduction to Physical Oceanography and Geophysical Fluid Dynamics in the fall semester and the following spring semester, respectively. Through these courses, I gradually found my own interests in oceanography. I then joined an ocean and climate physics research group and worked on a project regarding ocean mesoscale eddies at Lamont-Doherty Earth Observatory. As a graduate intern, I focused on this project for several months, and acquired some preliminary results before the summer.

Comprehensive Maps of Ocean Eddy Origins (Left Panel) and Terminations (Right Panel).
Data Source: Mesoscale Eddies in Altimeter Observation of SSH

Eddy origins and terminations are the respective start and end points of eddy tracks. Before the generation of the final interactive maps, I created two comprehensive maps for eddy origins and terminations, respectively, in one of my oceanography course projects. As shown in the first image, those maps consist of both probability distribution histograms and hexagonal density bins. These types of maps could convey two aspects of valuable information at the same time, making it more convenient to view the global distribution patterns of ocean mesoscale eddies. I also produced high-resolution global density maps, which you can see in the other image.

Density Maps of Ocean Eddy Origins (Upper Panel) and Terminations (Lower Panel). Data Source: Mesoscale Eddies in Altimeter Observations of SSH

From maps shown above, the amounts of eddy origins and terminations in the southern hemisphere are much larger than that in the northern hemisphere due to the difference in total ocean area between two hemispheres. In addition, eddies are evenly distributed with respect to latitude in the northern hemisphere and become denser as latitude increases in the southern hemisphere.

Oceans around the Antarctic Circumpolar Current and off North America are two major hotspots of both eddy origins and terminations over the globe. In general, eastern boundaries have a higher density of eddy origins, while western boundaries have higher density of eddy terminations.

During the summer internship period, I studied the physical properties of ocean mesoscale eddies through previous course materials and scientific publications, processed oceanic data using open-source document database MongoDB, and developed the final interactive website in programming languages HTML/CSS, Python, and JavaScript. Those works have a firm basis in climate and society, and are not only substantive but also challenging and illuminating. I learned a lot through printed and online resources, meetings and discussions with my supervisor, and figuring out complicated problems independently.

The basic functions of the website work well after the summer. However, there’s more work to be done to finish it up. One of next steps should be creating an option for using sea surface temperature and sea surface height as map backgrounds, which would require more knowledge of map design.

A research article regarding recent trends in the Southern Ocean eddy field suggests that a decadal increase in eddy kinetic energy is most likely due to continuing increases in the wind stress over the Southern Ocean. This change may create cooling effects in the southern hemisphere. To further this study, interactive ocean maps developed by my internship could serve as a practical tool to investigate the ocean energy budget in the Southern Hemisphere from a statistical perspective and provide some implications for overturning circulation, carbon cycle, sea ice extent, and global climate change.