this height, when multiplied by the area fraction of the grid cell covered by
ocean (or sea ice), yields the volume of sea water above the geoid. As
defined here, "the geoid" is a surface of constant geopotential that, if the
ocean were at rest, would coincide with mean sea level. Under this
definition, the geoid changes as the mean volume of the ocean changes
(e.g., due to glacial melt, or global warming of the ocean). Report zos as
"missing" over grid cells that are entirely land. There are a couple of
acceptable options for reporting this field: 1) if the geoid is defined to
relate to the instantaneous volume of the ocean, the global mean of zos will
always be zero, and 2) if the geoid is defined relative to a time-mean sea
level over some period, then the global mean of zos will be timedependent.
In either case a global mean time-series of sea level should also
be reported as described in the next two table entries immediately below.
In general IPCC analysis of global mean sea level changes will not rely on
zos. It is recommended that in reporting zos, the atmospheric "inverted
barometer" effect be omitted, since it can be easily calculated from the
reported mean sea level pressure field. The "comment" attribute associated
with zos should indicate whether or not the atmospheric "inverted
barometer" influence on zos has been included. Additionally, it should be
noted in the "comment" attribute whether zos is obtained directly, as in a
free-surface model, or has been derived, for example, from geostrophy
using diagnosed velocities at some level or from geostrophy relative to an
assumed level of quiescence.
