How do sensors reveal hidden aspects of the world’s oceans?

Oceans comprise about 96.5% of the water-covered surface (71%) of the Earth. Surprisingly, the salinity of these huge bodies of water varies considerably. Interconnected masses and layers mix together and split apart through currents, eddies, and changes to temperature and salinity, where salinity is measured in practical salinity units or PSUs.

The Practical Salinity Scale 1978, or PSS-78, an international standard, uses a non-linear equation for computing salinity from conductivity. It defines a reference seawater composition with a conductivity of 42.9 milliSiemens (mS)/cm at a temperature of 15°C and a pressure of 1 atmosphere. The salinity of a seawater sample is then calculated relative to this reference with a set of empirical relationships and correction factors to account for the impact of temperature and pressure.

Using salinity measurements, it has been found that the Pacific and Indian oceans mix at specific points. While these equatorial waters were discovered in 1942, the mixing in the Atlantic Ocean was not.

Sensing conductivity, temperature, and depth

Today’s tool to measure key ocean data is a conductivity, temperature, depth (CTD) sensor.

Conductivity sensors typically consist of two or 4 electrodes or probes and a reference electrode, and can achieve an accuracy of ±0.0003 S/m. A thermistor, a platinum thermometer, or a combination of the two usually measures temperature in a CTD. Measurements occur within an accuracy of 0.001°C. Pressure measurements (to determine depth) are typically obtained using either a strain gauge pressure sensor or a quartz crystal-based digital pressure gauge, and occur with an accuracy of 0.1 dbar.

With conductivity, temperature, and pressure data, salinity can be calculated with an accuracy of 0.001 PSU using the PSS-78 equation.

Figure 1. The CTD is mounted in an Argo SOLO-II float. (Image: Argo)

Floats

CTD sensors, installed in robotic, self-submerging floats as part of the Argo program (a component of the Global Ocean Observing System, GOOS), began being deployed in 1999 (see Figure 1). Now, over 3,000 floats have been deployed to continuously provide data on the upper 2,000 m of the world’s oceans (see Figure 2). In addition to the CTD mounted on the top of every Argo float, other sensors may include a pH meter, dissolved oxygen sensors, transmissometers (to measure light transmission), fluorometers (to measure the amount of chlorophyll in the water), and more.

Figure 2. The autonomous process for measurements down to ocean depths of 2000m. (Image: Argo)

Data collected from the sensors installed in the floats also provides information for determining the impact of climate change, weather and ocean forecasting, and predicting hurricanes.

Recent discovery

Using data from Argos, scientists recently discovered a previously undetected water mass in the middle of the Atlantic Ocean. This body of water stretches across the Atlantic from the tip of Brazil to the Gulf of Guinea, near West Africa. Called the Atlantic Equatorial Water, it forms along the equator as ocean currents mix separate bodies of water to the north and south.

Now that the water mass has been identified, it provides scientists a better understanding of the ocean’s mixing processes, which are vital to the oceans’ transport of heat, oxygen, and nutrients around the globe.