Hi Xiaoyu,
`@DIN` is just a numerical integral - multiply the cell width (which in your case is a difference in pressure rather than distance in meters) by the data value in that cell and sum these up. As always, be conscious of the units of the computed value. There is the special case if you are integrating over both longitude and latitude (Ferret computes an area value of the cell assuming a spherical earth), but you are not doing that here.
If your pressure axis is marked as a depth-type axis then positive is down. Use a `show grid` for the variable, and if axis is positive-down, there will be a minus sign after the number of points; for example (from levitus_climatology):
ZAXLEVITR DEPTH (m) 20 i- 0 5000 full
You can explicitly specify the range for the integration to make it clear what region you are integrating over:
SALT[Z=0:5000@DIN]
Karl
Karl M. Smith, Ph.D.
JISAO Univ. Wash. and PMEL NOAA
"The contents of this message are mine personally and do
not necessarily reflect any position of the Government
or the National Oceanic and Atmospheric Administration."