CTD=Conductivity, Temperature, Depth
Electronic CTD instruments examine water properties to detect how conductivity and temperature of the water column change relative to depth. The instruments used by AAOKH observers also measure the concentration of chlorophyll in the water column.
Conductivity is a measure of how well a solution conducts electricity, and is directly related to the salinity of the water.
Salinity is the concentration of salts and other inorganic compounds in seawater. When combined with the temperature of the water, the salinity can be used to determine the density of the seawater, which is a primary driving force for major ocean currents.
Chlorophyll represents primary productivity from phytoplankton, indicating biological productivity at the base of the marine food web.
Why these data are useful
Scientists analyze the CTD data to determine changes in water properties at the measurement location, giving insight to origins of a water parcel (such as Atlantic or Pacific origins) and variations in oceanic current regimes.
CTD data collected through a hole in sea ice, or near the ice edge, give indications on the state of the seasonal ice cycle. For example, an increase in salinity in the fall would indicate freezing sea water during freeze up, while a decrease in salinity in spring would be melting ice at the onset of break up. Chlorophyll concentration measurements allow scientists to estimate the potential primary biological production for a parcel of water.
Repeated CTD measurements at a given location shows how water properties vary over time, while measurements along different parts of Alaska’s Arctic and subarctic coast reveals geographic differences in seawater properties.
Prudhoe Bay CTD Example
CTD cast was made near North Star Island in Prudhoe Bay (70.3875N, 148.5167W) on June 14, 2018.
In the plot, the thermocline and halocline can be easily seen around 3 meters depth. There is a warm, relatively fresh water layer on top of a colder, saltier layer of water.
Note the presence of chlorophyll in only the top layer of water, indicating possible primary biological production in this portion of the water column.
This data was collected in the melting period of the annual sea ice cycle, when fresh, relatively warm, and less dense melt water forms a "lens" on the ocean surface. The melt water could be from sources such as melting snow from the land or sea ice surface, runoff from breaking up rivers or streams, or the melting of sea ice itself.