We can express the term 
, entering
in Eq. (46b) in terms of T, S, p according to the preceding
development, as
The equations of the isotherms and isohalines in the x-z plane are
thus the slopes of the isothermal and isohaline surfaces in the x directin are
Using these relations together with Eq. (44), we can express Eq. (58) in the form
Therefore, Eq. (46b) can be written as
For all practical purposes, the variation of 
with
z compared with the variation of 
can be ignored
so that Eq. (62) can be written approximately as
The magnitude of the coefficient 
is about
per 100 meters.
The quantity
where 
is a constant (isobaric slope at 
),
is of the order of magnitude of 1 per 200 meters in the upper
layers of the ocean in the vicinity of the thermocline.
Consequently, is negligible compared with this
fractional rate of change of and Eq. (63) can
be approximated by
Integration of Eq. (64) from level 
to 
yields
If level is selected such that 
is zero
then
