SUNRISE in the Gulf of Mexico

What happens when you have fresh river water meeting salty ocean water while the winds keep shifting and the earth keeps spinning? The answer: lots of interesting things!



Researchers and graduate students of the Multiscale Ocean Dynamics (MOD) lab at Scripps Institution of Oceanography at UC San Diego recently took part in the latest cruise of the Submesoscales Under Near-Resonant Inertial Shear Experiment (SUNRISE) in the Gulf of Mexico. Together with researchers from Oregon State University, Stanford University, Texas A&M University, and Cambridge University, and with the help of Louisiana Universities Marine Consortium-(LUMCON) based crews aboard the research vessels Point Sur and R/V Pelican, we spent three weeks at the end of June 2022 chasing ocean fronts and using unconventional sampling methods with a smorgasbord of oceanographic instruments.



The SUNRISE project looks at mixing and water exchanges as a result of various physical processes in a complex coastal environment in the northern Gulf of Mexico. The aim of the project is to map how wind-driven, near-inertial motions affect and modify fronts and eddies that are anywhere from hundreds of meters to nearly 10 kilometers wide (the “submesoscales”) near where the Mississippi and Atchafalaya rivers meet the ocean. This region has a rich field of eddies and fronts this size and is thus a perfect place to study this type of interaction.

Our team’s larger goal is to help answer some of today’s most interesting questions on where energy comes from and goes to and the interactions between large-scale and small-scale motion in the ocean. A better understanding of what drives the mixing of different water masses with different properties, such as temperature, salinity, oxygen, and various biological properties, will help protect this region as a dynamic and productive environment, critical for the success of fisheries, tourism, and local economies.



Inertial oscillations are phenomena that happen in fluids that are in motion in a rotating frame of reference like the earth. Because our planet is spinning around its own axis, objects that are in motion will turn to the right in the northern hemisphere and to the left in the southern hemisphere. This is called the Coriolis effect.

In the ocean, a water parcel in motion, such as a surface region of ocean water being pushed in a certain direction by the wind blowing over it, will continuously turn to the right (or left) tracing out circles. These are called inertial oscillations. If you look at the meridional (along longitudes) and zonal (along latitudes) movements separately, an object going in a circle will continuously alternate, or oscillate, between going north and south meridionally and east and west zonally.

The frequency with which the water parcels oscillate is determined by the Coriolis parameter which is a function of latitude. The closer one is to the equator the smaller the Coriolis parameter and the slower the motion (to the point where at the equator there is no inertial motion at all in the ocean), and the higher the latitude the greater the Coriolis parameter and the shorter the inertial frequency (faster circles).

The location of the SUNRISE project was chosen because at a latitude of about 28-29°N, the summertime land-sea breeze happens at the same frequency as the inertial oscillations and reinforces them which gives rise to all sorts of interesting phenomena. 

 R/V Pelican and R/V Point Sur were both equipped with an instrument called a vertical microstructure profiler (VMP) to measure the turbulence often associated with mixing, and acoustic Doppler current profilers (ADCP) to measure ocean currents and temperature. We also used salinity profilers equipped with additional biological sensors to measure things like chlorophyll, dissolved organic matter, and oxygen.

To be able to capture the full water column in the shallow waters just off the coast of Louisiana, our instruments were dropped all the way down to the bottom and then pulled back up. Each round trip was one profile. Between the two ships, almost 25,000 profiles were collected during the experiment, with teams often working in shifts 24 hours a day. The instruments were fitted with crash guards to protect the sensors on the bottom-facing end of the instrument from slamming into the seafloor, with profiles every 100 to 200 meters (328 feet to 656 feet).

We also deployed two Wirewalkers, which are autonomous profilers developed by MOD, as well as two remotely-controlled small boats from Oregon State equipped with very high-resolution thermistor chains. We used these assets in various combinations together with the two ships to do different transect shapes, trying to cross the ever-changing fronts and filaments developing between the inshore fresher water and offshore saltier ocean water over multiple internal oscillations. Capturing a wide range of spatial and time scales, we have already seen all sorts of interesting phenomena in the data. There are signs of everything from trapped internal waves to curious layers of shear and subduction, symmetric instabilities and remarkable patterns of biological variables such as chlorophyll, oxygen, and organic matter.

A highlight of the 2022 cruise was the last two days of sampling, which were dedicated to students practicing being chief scientists. Coordinating between the two ships, the students managed to find a front to sample and then orchestrated a 24-hour continuous survey that had two ships and two small boats running parallel transects, something that took a bit of coordination. The students agreed that it was a great experience and learned about everything from leadership to communication while gaining a  new respect for the complex process of collecting good data to do great science.


In addition to doing science, the team also took the time to enjoy the many beautiful sunrises and sunsets that the Gulf served up and marveled at the local wildlife – everything from dolphins and spinner sharks to flying fish and pelicans. The science parties onboard R/V Pelican and R/V Point Sur also represented upwards of a dozen countries so we honored various traditions and holidays, celebrating everything from Canada Day to the 4th of July and Swedish Midsummer.   

After 19 days at sea, the SUNRISE science party arrived back in Louisiana with lots of data that will keep us busy for a long time, cameras full of sunrise and sunset photos, and many good memories. A massive thank you goes out to the phenomenal crews aboard R/V Pelican and R/V Point Sur – without their help, the cruise would not have been such a succes! Equally, a big thank you to everyone who supported the cruise from land and to LUMCON for all their great work.


There is more from the cruise on the Multiscale Ocean Dynamics website.
 

Kerstin Bergentz is a third-year student in the Multiscale Ocean Dynamics laboratory at Scripps Institution of Oceanography. Her research focuses on small-scale energetic processes in the ocean.