Lab 7.3

Lab 7.3 – TSD DIAGRAMS: HOW DO WE USE DATA TO IDENTIFY DIFFERENT WATER MASSES?

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

Both temperature and salinity contribute to density of seawater (see Lab 6).  Just as differences in density can lead to distinct layering of seawater with depth in the ocean, variations in density can also lead to the formation of distinct water masses.  A water mass can be defined as a body of ocean water with a distinctive narrow range of temperature and salinity and a particular density resulting from these two parameters (Britannica.com). Labs 7.1 and 7.2 introduced the Gulf Stream as an important surface current.  But we also learned that the Gulf Stream is a distinct water mass that can be identified by its temperature and salinity.

Changes in temperature and salinity occur at the surface of the ocean, not in the deep sea.  Water masses take on their characteristic salinity and temperature, and therefore their density, at the surface where they form.  In fact, the American Meteorological Society defines a water mass as a body of water with a common formation history (https://glossarytest.ametsoc.net/wiki/Water_mass).

To understand why water masses form at the ocean surface, let’s think about what causes ocean water to change temperature.  What causes ocean water to increase in temperature or heat up?  If you are thinking “the sun”, you are correct.  As the sun shines down on the ocean, it transmits energy that warms the ocean and this happens, of course, at the surface of the ocean.  Similarly, less solar radiation, shorter days and therefore less hours of sunlight each day, or the sun being low in the sky (rather than more directly overhead), all would cause the ocean to be cooler.

Now think about what processes would cause a change in the salinity of the ocean. Remember that salinity is measured as how much salt is in the water relative to the total amount of water, like a ratio or percentage.  So, although it is true that any process that adds salt to the ocean would increase salinity, so would any process that causes there to be relatively less water.  Similarly, any process that removes salt from the ocean would decrease salinity, but so would any process that causes there to be relatively more water.  Removing water concentrates seawater while adding water dilutes ocean water. Think about what you just read and answer the following questions.

 

 

Now think about what you know about the impact of salinity and temperature on density and answer the remaining questions:

 

 

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

6. What is the range of salinity values for the first week in June? What is the range of temperature values for that week?
7. What is the range of salinity values for the last week in June? What is the range of temperature values for that week?
8. Which varies more throughout the month of June, salinity or temperature?
9. During which week did salinity values change the most?
10. 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.