I have calculated zonal heat advection at depth 60 m as follow:-

• let za1 = current[z=60,gxy=za2]
• let za2 = temp[x=@ddc,z=60]
• let za = za1*za2

where, current is zonal (vertical) velocity at depth 60m, temp is temperature at 60m.

But while calculating vertical heat advection at depth 60m, it was showing following error.

**ERROR: command syntax: multiple specifications on Z axis

Z=30