Dataset: mixed_layer

  PI:             Wilford Gardner (Texas A&M University),
                  Jan Gundersen (Texas A&M University)
                  John Morrison (North Carolina University)
  dataset:        Mixed layer depths
  dates:          January 08, 1995 to December 26, 1995
  location:       N: 22.5  S: 10  W: 57.3  E: 68.75
  project/cruise: Arabian Sea - All Process Cruises
  ship:           Thomas Thompson

  Wilford Gardner - based on temperature changes
  John Morrison - based on density changes						
  Arabian Sea Mixed Layer Depths - all process cruises

  PI Notes
  PI Notes on density vs. temperature calculation

 

Mixed layer depths are based on a temperature increase of 0.1 and 0.5
degrees C from the second temperature value listed in the CTD files, which
corresponds roughly to a density increase of 0.03 and 0.125 density units.

The second TEMPERATURE value was used because there occasionally appeared to
be questionable numbers as the first value.  The temperature change of 0.1
degree C was recommended by Dr. Craig Lee (WHOI) after looking at the data
and the 0.5 degree C value is the Levitus standard used in his NOAA global
atlases.  These are also the values used in the US JGOFS EqPac program.

Please note variable initial (starting) depths.

Calculations were made by Jan Gundersen. (TAMU).

Wilford D. Gardner
Dept. of Oceanography
Texas A&M University
College Station, TX 77843-3146

From Wilf Gardner, Texas A & M University, August 8, 1997

to Arabian Sea investigators and all interested others

Since the New Hampshire meeting [July, 1997], we have compared the MLD
based on the temperature criteria of 0.1 and 0.5 degrees C compared with
the density differences of 0.03 and 0.125 density units.  About 75% of
the time the values are identical.  There are other occasions, however,
where there are significant differences, especially for the 0.1 degrees
C/0.03 comparison. When there is a difference, the temperature
calculations generally give deeper MLDs than density calculations.
Differences most often occur when there is a salinity increase below
the surface.  Temperature criteria are often used for MLDs on moored
data as salinity sensors are sometimes subject to greater drift than
temperature sensors.

None of the above criteria identify the "mixed-layer depth" perfectly in
all cases, but examination of depth plots where there are differences
suggest that the density criteria more reliably identify the depth of most
recent mixing.  For consistency, we recommend that the MLDs based on
density be used when you are trying to calculate mass budgets.  In some
situations, such as sometimes during the TN045 intermonsoon period, there
is no true mixed layer - just a depth at which the MLD criteria are finally
exceeded.  We suggest that you look at an expanded plot of the density
profile if your calculations are critical.