By Peter Shipton, Mooring Technician, University of Alaska Fairbanks
July 24, 2016
A CTD is an essential tool used in all disciplines of oceanography. CTD stands for Conductivity, Temperature, and Depth. By measuring the conductivity of seawater, the salinity can be derived from the temperature and pressure of the same water. The depth is then derived from the pressure measurement by calculating the density of water from the temperature and the salinity.
In addition to these standard measurements, most CTDs are outfitted with additional sensors to measure the amount of chlorophyll (phytoplankton) present, the turbidity (the amount of particles in the water), the irradiance (amount of light from the sun that penetrates the water), and the amount of dissolved oxygen in the water. All of these measurements are looked at side by side in relation to depth.
By comparing the data in situ at each depth, the physical characteristics of the water can be described. These characteristics drive the mechanisms that cause the formation of what are described as “different water masses.”
These water masses form quite distinct layers in the ocean, in particular the Arctic Ocean. The layers of water that are prevalent in the Arctic Ocean are the “Pacific Water,” “Atlantic Water,” “Arctic bottom water,” and the “melt layer.” These layers are driven by differences in density (pycnocline), differences in temperature (thermocline), and differences in salinity (halocline). The strength of the boundaries between these layers can be strong or weak, depending on the variety of two adjacent water masses.
Sea ice begins to form in the Arctic winters. As the sea water freezes, the salt in the ice is removed. This creates ice made of fresh water, and saltier water below it. Because the saltier, cold water is denser, it will sink. This process causes the formation of “Arctic Bottom Water” deep in the Arctic Ocean.
Atlantic water is also salty, but has been warmed at lower latitudes and carried north by the Gulf Stream. Currents carry this Atlantic water into the Arctic Ocean where, due to its density, it slides underneath the “Pacific Water,” which is fresher, and sits atop the “Arctic Bottom Water,” which is colder.
In the Arctic spring, the long periods of sunlight warm the sea ice and cause melting. The melted sea ice produces very fresh water. This fresh water is less dense than the rest of the water column. The melt layer forms a thin layer that stays at the surface. This melt layer persists in the Arctic Ocean, sitting on top of the Pacific Water as a distinct surface later. Because this layer is closest to the surface, it is where the phytoplankton bloom.
As the phytoplankton settle down in the water, they concentrate at the boundary layer between the fresh melt water and the saltier water below, building what is called the chlorophyll maximum. The boundary between the fresh melt water and the saltier water persists until external forces, such as wind and currents, cause mixing.
This detailed understanding of the ocean water’s characteristic through the entire water column is crucial for understanding the physics involved. The physics in turn allow biologists to understand why the biology is present or not present at different depths and why the chemical make-up of the water changes over depth. The CTD is the key to understanding the physics, chemistry, and biology of the water column.