Coastal Water Masses: Maine Intermediate Water

The core of a water mass is the point at which its defining characteristics are the strongest.  For example, since Antarctic Intermediate Water (AAIW) in the open ocean is defined as a salinity minimum layer below the permanent thermocline, the core of AAIW is where the salinity reaches its lowest value within that layer.  Many water masses are defined in terms of just their temperature and salinity, but others, such as tropical Atlantic water, are defined in terms of other properties, such as the concentration of dissolved oxygen.  Once the core has been located, its other associated values (such as its temperature) can be identified, and the core depth and the total vertical extent of the layer can be located in the original station data.

The method by which the core of a water mass is identified is through the use of  "property-property" plots, a term indicating that two different chemical or physical properties are charted against one another in a Cartesian (x-y) plot.The most common plot for identifying water mass cores is the T-S plot, which has Salinity as the X-axis and (potential) Temperature as the Y-axis.  "T-S space" refers to the visualization of hydrographic station data on such a T-S plot. Also relatively common are T-O (temperature-oxygen) plots.

Please note that when physical oceanographers use the term "temperature", they usually mean potential temperature unless specifically noted otherwise. Similarly, when the term "depth" is used, oceanographers are typically referring to pressure, the directly measured quantity.

Maine Intermediate Water (MIW) forms when winter cooling mixes the water column in the Gulf of Maine very deeply. This cold, layer of water is capped in summer with a heated surface layer. The core of MIW is the temperature minimum found between this surface layer and the remnant of Slope Water, which is found at the bottom of the water column in the Gulf of Maine with a salinity and temperature of approximately 33 psu and 6 C.

  1. Print out the T-S diagram provided below and plot the following station data in T-S space.  Identify the MIW core and the remnant slope water on your plot.
  2. Between what depths is MIW found in the following station?
  3. At what depth is the remnant Slope Water found?
  4. What is the density of the surface water?  Of the MIW? Of the slope water?  Is the water column stably stratified here or not?  How do you know that?