How get mean value from 4d dimension in oscar data

Hi all.

i have oscar data, which one of variable "u" contains 4 dimensions: lon*lat*depth*time

"u" is current sea speed in m/s

this is my problem:

How i get the mean value for "u" every month in one year, example:

mean januari = --this is result--

mean february = --this is result--

.

.

.

etc until december month

i'm very appreciate your time to help me

tks

link download data

<ftp://podaac-ftp.jpl.nasa.gov/allData/oscar/preview/L4/oscar_third_deg/oscar_vel2017.nc.gz>

(sorry i can't attach here because it's huge size file)