Thursday, March 10, 2016

How Argo floats work: words plus an animated explanation

Now that all of our floats are in the water, what happens next? Throughout this blog, I have described bits and pieces of how Argo floats work but I think we're a bit overdue for a complete explanation.

Kaia drifting away.
Just to recap, Argo floats are free drifting, battery operated, observational platforms that sample the upper 2000m of the ocean. On these platforms, we mount instruments or sensors that measure different properties of the ocean. All Argo platforms have a sensor called a CTD, which stands for conductivity, temperature and depth. From conductivity we can infer salinity, so a CTD is able to measure temperature and salinity at different depths of the ocean.

Schematic explaining how Argo floats cycle through the ocean. Source:
Argo floats are programed to sample the ocean on a 10 day cycle. The floats spend most of their time "parked" at a depth of 1000m. In this state, they floats are in hibernation mode and simply drift with the prevailing currents. However, every 10 days or so the floats wake from their slumber to begin sampling.

To initiate sampling, the float first descends to its maximum depth of 2000m. From there, the CTD switches on and begins recording temperature and salinity. The float ascends to the surface while measuring temperature and salinity along the way. When the float reaches the surface, it transmits its most recent data to land via satellite. The engineers at the UW lab receives this data instantly. When the data transfer is complete, the floats descends back to its parking depth to repeat the entire process. The entire cycle is depicted in the schematic above. Each float is designed to last 5-7 years, so they produce about 200 ocean profiles in their lifetimes.

This is the standard operation of a normal Argo float. One thing to note is that these floats have no propellors. They rise and sink through the ocean by changing their density. Each float is equipped with a bladder (a thick balloon) that is connected to a pump inside the float. The bladder is located in a chamber at the bottom of the float, which has a small opening to allow water flood in. By inflating its bladder, the float effectively expands its volume, becomes less dense and moves up through the ocean. When it deflates its bladder, the float decreases its volume, becomes less dense and sinks.

That's basically how a float works. If that wasn't clear enough, I think a certain video will do the trick. Michelle Weirathmueller is a talented graduate student in our department who makes amazing animated videos about oceanography. Not too long ago, she did an animated short explaining how Argo floats work. Please checkout her blog post!

My explanation and Michelle's video only discuss regular Argo floats. The floats we deploy for SOCCOM are a bit different. The first difference is that most SOCCOM floats have ice avoidance capability. Normal Argo floats need to transfer their data every 10 days, but due to the presence of sea ice, this is not always possible in the Southern Ocean. The float cannot transmit its data through ice and if it tries to penetrate its way to the surface, it will likely damage itself.

If the water temperature is very close to the freezing point (approximately -2 degrees celsius), it is usually a good indicator of sea ice. In this scenario, the float retains its data and returns to its parking depth before reaching the surface; they usually turn back at a depth of about 15 meters. Currently, these ice-enabled floats are able to store a year's worth of profiling data before running out of storage space. Whereas normal Argo floats report their data every day, we sometimes don't hear back from SOCCOM floats for several months.

Rick showcasing the difference between the internal electronics of a SOCCOM "bio-Argo" float (left) and a regular Argo float that only measures temperature and salinity (right).
Another special feature of SOCCOM floats is that they have additional sensors to measure biological activity in the ocean. These extra sensors measure oxygen concentration, nitrate concentration, pH, chlorophyll and backscatter.

One of the ultimate goals of SOCCOM is to further our understanding of the ocean's biological pump. The ocean's biological pump is an immensely important concept, but I won't get into its details here. In short, the biological pump refers to the cycling of carbon between the atmosphere and the deep ocean. Since carbon dioxide is involved, this process has a huge impact on the global climate.

It just so happens that a huge portion of this ocean-atmosphere carbon exchange occurs through the Southern Ocean. This is linked to the large scale upwelling of nutrients and deep water formation I mentioned a couple blog posts ago. Direct observations of the ocean's biological pump is very sparse, but these ice-enabled, bio-Argo floats will allow us to observe these processes in near-real time, throughout the year. This is a huge step forward.

That's all for now! At some point, I will share some of the data we have received from the floats we just deployed.



  1. How many people are on board? What is the ratio of women to men?

    1. I don't know the exact number but I would guess between 40-50. There were about 30 scientists and the rest were ship crew. Among the science party, the gender ratio was about 50:50. The crew were mostly men.