Article: Carranza, M.M., Gille, S.T., Franks, P. J., Johnson, K. S., Pinkel, R., & Girton, J. B. (2018). When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean. Journal of Geophysical Research: Oceans, 123, 7264–7289. https://doi.org/10.1029/2018JC014416.
Since I began studying oceanography, I’ve heard the Southern Ocean described as the “wild west” of the world’s oceans. Located in the Southern Hemisphere, it surrounds the distant, isolated continent of Antarctica, and serves as a passageway between the Pacific, Atlantic, and Indian Oceans through the Antarctic Circumpolar Current (ACC).
Unlike other ocean currents, the ACC is the only one to encircle the globe without being bounded by a continent. This unique feature gives the Southern Ocean its “wild west” feel – vast swaths of emptiness are accompanied by bands of chaotic motion in the ocean – or turbulence – as the wind-driven ACC circles around Antarctica, flowing over underwater mountains along the way. Boundaries between distinct masses of water set up mixing that promotes large-scale algae (phytoplankton) blooms. Frequent storms produce waves and mixing that complicate dynamics at small scales. Combined, these processes enable widespread uptake of atmospheric carbon (CO2) and heat, making the Southern Ocean a primary region of interest for understanding the how oceans regulate our climate.
Entering the wild west
In 2014, a program called SOCCOM (Southern Ocean Carbon and Climate Observations Modeling) was launched to shed light on the significance of this region through in situ (observational) initiatives – specifically by using free-drifting floats that profile ocean properties. Since 2004, the Argo program has released nearly 4000 such floats around the world. These floats follow surface currents, profiling ocean properties down to 2000 meters depth (about 1.2 miles). The Southern Ocean’s frequent storms and frigid icy weather make it largely inaccessible to humans, which is why Argo floats – along with satellite data – have dominated recent observational studies in the region. In addition, elephant seals have helped us out by retrieving ocean data via CTD (conductivity, temperature, depth) sensors attached to their heads as they migrate and dive to locations inaccessible to humans or floats, such as under sea ice and continental shelves. These seals are particularly useful to scientists because they continuously dive deep and can stay below water for up to 80 minutes.
The aim of SOCCOM is to learn about physical and biological patterns in the Southern Ocean. The study outlined below uses these in situ methods to provide some of the first physical evidence of the resilience and adaptability of phytoplankton residing there.
Will phytoplankton weather the storm?
This study, led by Dr. Magdalena Carranza at Scripps Institution of Oceanography at UC San Diego, aims to elucidate the effect of continuous storms on mixing in the Southern Ocean’s surface mixed layer. The mixed layer is a biologically productive layer of well-mixed (uniform density) water at the top of the ocean, separated from the deep ocean by a strong gradient of temperature and salinity. Wind blends the upper ocean’s temperature and salinity, deepening the mixed layer and drawing up nutrients from below. These conditions set the stage for phytoplankton blooms to occur near the surface.
In recent years, researchers have found that phytoplankton are driven by more than just the temperature and salinity conditions in the mixed layer. For instance, phytoplankton abundance will decrease if there aren’t enough nutrients, or if the wind is too strong for them to survive and reproduce.
The researchers wanted to find out how these algae are responding to the continuous stream of storms that churn and homogenize the upper layer of the Southern Ocean. With such frequent mixing by storms, how could they possibly have time to set up a unique existence in the mixed layer?
Floats and seals
Argo floats measured fluorescence (light emitted by photosynthetic activity) and backscatter (light reflection from particles, such as phytoplankton) to quantify the abundance of phytoplankton in the mixed layer. Data from 51 floats in the Southern Ocean were retrieved every 5-10 days from 2007 to 2016, including temperature, salinity, backscatter, and chlorophyll-a (chl-a) fluorescence. In addition, 27 elephant seals equipped with CTDs provided data in more remote areas from 2007 to 2011 (see Fig. 1).
These data were organized to form a clear picture of physical and biological properties over a nine-year period. Wind conditions were found using satellite data, with stormy conditions classified as any wind speed above 10 meters per second (about 22 miles per hour). The mixed layer was characterized by its depth from the surface, which was defined by the depth at which temperature and salinity were no longer uniform.
Harsh environments breed rugged phytoplankton
Chl-a fluorescence and backscatter, both indicators of biological productivity (i.e. how much phytoplankton are able to stay healthy and photosynthesize), were commonly found to have vertical structure in the Southern Ocean mixed layer where temperature and salinity had none. Maximum amounts of chl-a typically occurred near the bottom depth of the mixed layer, meaning phytoplankton were more keen to dwell there than at the surface, despite the latter providing more sunlight. Although this depth changed with the season and wind, it always remained calmer than the surface.
This result highlights two important findings: 1.) wind mixing is often not uniform within the mixed layer, being stronger near the surface than at the calmer bottom that phytoplankton prefer, and 2.) phytoplankton are able to reproduce faster than the wind is able to mix and homogenize them, allowing us to detect where they grow most abundant.
Therefore, we see that phytoplankton can maintain structure in the mixed layer distinct from the physical mixing processes that would be expected to suppress them. The researchers suggest that this structure is driven by biological (rather than physical) factors – such as the ability to adapt to different light conditions. To determine exactly how they adapt would require a closer look than the current study is capable of (such as higher resolution observations).
Calm after the storm
To tackle how much faster phytoplankton are reproducing than the wind is mixing, the researchers used wind data to estimate storm time scales. They estimated intervals between storms to be about 3-5 days on average, depending on the season. Phytoplankton doubling – the time it takes a population of phytoplankton to double in abundance – was estimated to be about 2.2 days, well under the average between-storm interval (see Fig. 2). This indicated that phytoplankton are capable of rebuilding their populations after storms pass and conditions return to calm.
Dr. Carranza and her team emphasize that Southern Ocean phytoplankton are well-adapted to low temperatures, lack of sunlight, and low iron (a nutrient needed for growth). While the physical conditions do have the power to suppress phytoplankton, they still find ways to bloom during calm conditions. The ways that these phytoplankton are able to adapt and how it might affect the export of carbon to the deep ocean remain mysteries. Luckily, the SOCCOM program is just getting started.
I’m a PhD student at the University of Rhode Island’s Graduate School of Oceanography. I use a small-scale computer model to study how physical features like surface waves at the air-sea interface produce friction for the wind that can limit momentum, energy, gas, and heat exchange between the ocean and atmosphere. In the future, I hope to learn more about the role waves play in different parts of the world as weather and climate patterns evolve. Also, I love to write.