CDO

Hi,
I have below netcdf file dimension which is formed by using CDO mergetime files to out.nc. I see that after merging, the depth dimension has attribute as depth:positive = "up" ;. Is there a way to change this using CDO to "down" so that the graphs will appear in normal direction as expected..

ncdump -h EN4P2_2015.nc
netcdf EN4P2_2015 {
dimensions:
lon = 360 ;
lat = 173 ;
depth = 42 ;
bnds = 2 ;
time = UNLIMITED ; // (12 currently)

variables:
float lon(lon) ;
lon:standard_name = "longitude" ;
lon:long_name = "longitude" ;
lon:units = "degrees_east" ;
lon:axis = "X" ;
float lat(lat) ;
lat:standard_name = "latitude" ;
lat:long_name = "latitude" ;
lat:units = "degrees_north" ;
lat:axis = "Y" ;
float depth(depth) ;
depth:long_name = "depth" ;
depth:units = "metres" ;
depth:positive = "up" ;
depth:axis = "Z" ;
depth:bounds = "depth_bnds" ;
float depth_bnds(depth, bnds) ;
double time(time) ;
time:standard_name = "time" ;
time:bounds = "time_bnds" ;
time:units = "days since 1800-1-1 00:00:00" ;
time:calendar = "standard" ;
time:axis = "T" ;