Lab 7.3 – Introduction to TSD diagrams

Fundamental concept: TBD
Estimated time to complete: 45 minutes
Data skills preparation: TBD
Materials needed: None

This lab has focused on using salinity and temperature data to define and identify water masses.  You have viewed time series graphs showing salinity or temperature at one depth over time.  You have viewed color coded profile graphs that show salinity or temperature variations by color with depth over time. In Lab 6, you viewed vertical profiles that showed salinity or temperature variations with depth at one time.

Hopefully you are realizing that there are many ways to visualize oceanographic data; even the same data may be shown in multiple formats. Let’s go back to the May 2024 salinity intrusion event that you analyzed in the last section of this lab (Lab 7.2).  That same event can be visualized using maps as shown below. These two maps use color to display the variation in sea surface salinity predicted by an ocean model from May 7th, 2024 (left figure), before the event, and May 12th, 2024 (right figure), during the event, from MARACOOS (Mid-Atlantic Regional Association Coastal Ocean Observing System) Ocean Map (https://oceansmap.maracoos.org)

 

 

 

The black marker on the maps indicates the location of the Northern Profiler Mooring of the Coastal Pioneer MAB array.  Observe the maps to note that on May 7th, the water over the array had a salinity between 27-33 or an average of 30 PSU while on May 12th, the Gulf Stream intruded over the shelf and onto the location of the array as evidenced by the darker colors indicating a higher salinity value in the range of 33-36 PSU.

Another way to visualize salinity and temperature data that is frequently used to identify water masses in the ocean is called a TSD Diagram (TSD stands for Temperature, Salinity, Density).  These graphs plot salinity versus temperature, rather than plotting either one versus time or location (depth or distance).  As you learned in Lab 6, salinity and temperature determine the density of seawater, so if you know the salinity and temperature of a water mass, you can determine its density.  Let’s practice.

Let’s go back to the May 2024 salinity intrusion event that you analyzed in the last section of this lab (Lab 7.2). The graph below is a TSD diagram.  We can plot the maximum surface temperature and salinity during the few days before the gulf stream intrusion, which were 8.4oC and 32.0 PSU. The red dot labeled “Point A” plotted on the TSD diagram represents this water mass.  You can see that this point is close to being on the curved line that represents a density of 1025 kg/m3.  This tells us that seawater with a temperature of 8.4°C and a salinity of 32.0 PSU has a density of slightly less than 1025 kg/m3.

[open in new window]

 

During the Gulf Stream intrusion, the maximum sea surface temperature was 23.8°C and the maximum surface salinity was 36.5 PSU. You can plot this point on the same graph; this is the red dot labeled “Point B”. You can see that this point is also close to being on the curved line that represents 1025 kg/m3 density!  This tells us that seawater with a temperature of 23.8°C and a salinity of 36.5 PSU also has a density of slightly less than 1025 kg/m3.  How can this be?  Salinity and temperature changes could have conflicting effects on the overall density of seawater.  Let’s review:

Quick Check Questions

 

So, the decrease in density caused by the warmer temperature of the Gulf Stream water was offset by the increase in density due to the higher salinity of the Gulf Stream water and overall density did not change.

This is not always the case, sometimes water masses with different temperatures and salinities do have different densities.  OOI data from the Pioneer MAB location detected another shift in the Gulf Stream in the middle of June 2024.  The following TSD diagram shows the temperature and salinity values from each instance data was recorded across the month of June.  You can interact with the graph by toggling labeled density lines on and off, toggling color-coding that represents dates on and off, or toggling on and off data points for each of four weeks (June 1-8, June 9-16, June 17-23, June 24-30).  Analyze the graph and answer the questions that follow.

[open in new window]

Orientation Questions

  1. What variable is shown on the x-axis of the graph? What are the units?
  2. What variable is shown on the y-axis of the graph? What are the units?
  3. What variable is plotted and shown as the diagonal lines? What are the units?
  4. What do the colors of the plotted circles represent?
  5. What time period does the graph cover? What is the starting date? What is the ending date?

Interpretation Questions

  1. What is the range of salinity values for the first week in June? What is the range of temperature values for that week?
  2. What is the range of salinity values for the last week in June? What is the range of temperature values for that week?
  3. Which varies more throughout the month of June, salinity or temperature?
  4. During which week did salinity values change the most?
  5. What dates show evidence that there was Gulf Stream water over the continental shelf where this data was collected? Provided evidence to support your answer.

Let’s finish this lab activity by taking it full circle back to thermohaline circulation from Lab 7.1; back to the Irminger Sea and the formation of NADW (North Atlantic Deep Water).  If you look at those graphs, you will see that in March of 2022, the temperature of the water was about 3.8°C and the density was 1027.75 which is close to 1028 kg/m3.  We can plot this point on a TSD diagram by extrapolating and realizing that a density of 1027.75 would be just above (to the left of) the density line labeled “1028” (which represents 1028 kg/m3).  Then follow the density line to where it crosses 3.8°C.  Once we have plotted the point, we can determine the salinity of that water by reading down from the dot to the salinity scale.

Try it out by clicking on the TSD diagram below at the appropriate spot; a dot will appear on the graph where you click.  You can then read down to the salinity scale.

Application Question

  1. What is the salinity of the water that is sinking at the Irminger Sea?

[open in new window]

Your textbook likely has a figure like the one below which represents the water masses that are part of thermohaline circulation in the Atlantic Ocean. If we plot the same point of 3.8oC, 35 PSU, and 1.02775 kg/L on the graph below, we can see that the water mass at the surface of the Irminger Sea is consistent with the characteristics of NADW.  Similar processes occur at other locations at the surface of the ocean (like off the coast of Antarctica) that lead to changes in temperature and salinity which give water masses their characteristic density and cause them to sink to depths based on this density and contribute to deep water circulation.

Example of a T-S diagram showing the locations of certain water masses: North Atlantic Central Surface Water (NACSW), Mediterranean Intermediate Water (MIW), Antarctic Intermediate Water (AIW), North Atlantic Deep Water (NADW), and Antarctic Bottom Water (AABW).

T-S Diagram showing Water Mass Location by Christina Cardona. Original Source.

Reflection Questions

  1. Recall that the “average” salinity and temperature surface values for the Coastal Pioneer in May were 32 PSU and 8.4C.  
      1. Would this data plot on the “textbook” diagram of North Atlantic Water Masses?
      2. Which variable is outside of the range on this graph?
      3. Why do you think that might be?  Consider the location of the Coastal Pioneer Array.
  2. Recall that the Gulf Stream salinity and temperature surface values measured at the Coastal Pioneer in May were 36.5 PSU and 23.8C.  
      1. Would this data plot on the “textbook” diagram of North Atlantic Water Masses?
      2. Which variable is outside of the range on this graph?

 

⇐ Previous LAB 7.2

Table of contents