Cold Tongues and FastCTDs

In November 2024, our team of scientists, students, and engineers from the Multiscale Ocean Dynamics (MOD) group at UC San Diego’s Scripps Institution of Oceanography departed from Honolulu on R/V Sikuliaq for  a research cruise to the equatorial Pacific.  

Led by principal investigators Caitlin Whalen, a Scripps Oceanography graduate now at the University of Washington, and Scripps oceanographer Gunnar Voet, and joined by researchers and students from Seattle, Boulder, Liverpool and Tel Aviv, we set out to study how internal waves below the ocean surface may break, mix water, and thereby affect climate patterns on a global scale. The research cruise was funded by the National Science Foundation.

The equatorial Pacific Ocean is a key region for the El Niño - Southern Oscillation (ENSO) mode of climate variability. More commonly known as the two siblings El Niño and La Niña, ENSO is a complex interplay of oceanic and atmospheric dynamics that impact communities surrounding the Pacific Ocean and beyond. While there has been tremendous progress in the understanding of ENSO over the past few decades, enabling forecasts of conditions over the time span of several months, there are still key processes driving and modifying ENSO that are not very well understood yet.

In this equatorial area, the westward trade winds transport warm surface water toward Asia and Micronesia. These warm waters are replaced by upwelled cold water from the deeper ocean, creating a cold tongue at the surface of the equatorial ocean observable from satellites. The cold tongue of the eastern equatorial Pacific Ocean plays a critical role in ENSO which, to a large degree, is driven by heat fluxes between the ocean and atmosphere. 

Past studies at the equator have shown that ocean mixing and associated heat fluxes are vital for the heat budget of the cold tongue and have important implications for ENSO. As the cold tongue carries relatively cool waters westward in the upper equatorial Pacific, it becomes unstable and forms so-called tropical instability waves (TIWs) that can extend several hundred kilometers away from the equator and are visible in satellite images of sea surface temperature as westward moving cusps of cold water. Turbulent mixing in the cold tongue near the ocean surface has recently been found to be strongly modulated by TIWs.

TIWs are much more prevalent during La Niña conditions when the cold tongue is more pronounced than during El Niño or neutral ENSO years, and we had been anxiously following the switch from El Niño in 2023 to a mild La Niña in 2024. By summer 2024, temperatures in the tropical central Pacific had cooled down substantially compared to the previous year and TIWs were clearly visible in satellite sea-surface temperature images when R/VSikuliaq left Honolulu for its seven-day transit to our remote study region near 140°W and just north of the equator.

The goal of our team on R/V Sikuliaq, the University of Alaska's research vessel, was to study how westward propagating fronts of TIWs generate internal waves traveling into the deeper ocean, where they may break and cause mixing and vertical heat transports. If found to be true, this process would extend the depth-reach of ocean-atmosphere heat exchanges and thus potentially be important for the ENSO cycle.  

Preliminary results based on data from the large fleet of Argo floats have shown a correlation between elevated turbulent mixing in the deeper ocean and strong TIW activity, indicating the generation of internal waves. Internal wave dynamics in the vicinity of the equator, however, are not well understood as the Coriolis force, which forms ocean eddies and atmospheric storm systems across most of the globe, differs drastically here. Our team was eager to collect data with a number of unique observational systems to gain more insight into these dynamics.

Using the images of sea-surface temperature taken from space, we identified the edge of a TIW cusp as a sudden increase in surface temperature, also known as a "front," where computer simulations have shown waves to be generated and to travel downwards into the ocean interior. Led by Arnaud Le Boyer and Nicole Couto of Scripps Oceanography, we used the FastCTD and Epsifish systems with a custom electrically powered winch, all unique systems developed by the MOD group here at Scripps Oceanography, to record rapid profiles of ocean temperature, salinity, and turbulence down to depths of about 1,000 meters (3,280 feet) while moving slowly across the surface fronts. The measurements help map internal waves and associate them with ocean turbulence where these waves break.  

With shore support from principal investigator and Scripps physical oceanographer Amy Waterhouse, Scripps graduate student Caique Dias Luko oversaw the deployment of an array of freely drifting WireWalkers, instruments also developed in-house at Scripps, in the vicinity of the shipboard measurements. Equipped with a host of sensors to measure physical and biogeochemical ocean parameters, the WireWalkers profiled the upper 750 meters (2,460 feet) of the ocean with their vertical motion driven by the mechanical power of ocean surface waves.  Initial analysis of the measurements shows indications of internal wave generation and propagation to greater depths. More analysis in the coming months will disentangle complex signals from the unique equatorial environment.

To gather data beyond the duration of the cruise, our team deployed three subsurface moorings spanning close to the full ocean depth of more than 4,000 meters (13,120 feet). Each of the three moorings is equipped with about 100 internally recording sensors that will provide data capturing many TIWs passing the array. Through collaboration with Oregon State University oceanographer Jim Moum, sensors directly resolving turbulence scales the size of a few inches were added to the moorings to capture the processes cascading the energy from the TIWs to turbulent mixing. The moorings will be recovered during a cruise planned for December 2025.

More than five weeks at sea is a long time. While most of it was spent preparing instruments and running observational systems around the clock, life also happened and the beauty of the ocean environment did not disappoint. AM watchstanders marveled at purple skies during beautiful sunrises and confirmed with the PM watches that sunsets were similarly spectacular with a flurry of green flashes. Several birthdays were celebrated and on Thanksgiving Day, scientists and crew dressed up in their best shirts for an afternoon Thanksgiving dinner enjoyed by everyone on board. As more and more space cleared up after mooring deployments—the moorings now in the water filled up almost two 20-foot shipping containers—a ping-pong table was set up in the lab and wire baskets, usually used for shipping gear, were filled with ocean water for a makeshift pool. A ceremony was held for those crossing the equator at sea for the first time, which not only included most of the crew and science party but also R/VSikuliaq itself, which usually sails in Arctic waters.  

We felt a range of emotions as the Hawaiian Islands came back into sight and the cruise neared its end: exhaustion from hard work at sea, excitement about the unique dataset we collected, deep appreciation for a wonderful and diverse research team, anticipation for seeing loved ones at home or a vacation on the Hawaiian islands, gratefulness for an incredibly welcoming and well-run research environment run by the captain and crew of R/V Sikuliaq, and many more.

Gunnar Voet is an associate researcher in the Marine Physical Laboratory at Scripps Institution of Oceanography at UC San Diego. 

Read more about the cruise in Anna Deppenmeier’s blog.

View additional photos from the expedition in the gallery below.