How to find yearly mean for multiple netcdf files having monthly means?

I have era 20cm datasets. Each subdataset has monthly means of 10 variables of a particular region of interest. the datasets are 3 dimensional - latitude, longitude and time. the dimensions of latitude and longitude are different. i have gone through the following question:

https://in.mathworks.com/matlabcentral/answers/371225-calculate-daily-monthly-annual-averages-from-hourly-data%20

used "accumarray"..commands are as shown below

 precJanuary = netcdf.getVar(ncid1,3,[0 0 0],[225 82 1]);
 yearlyaverage = accumarray(id, precJanuary, [], @mean);

Getting the following error: Second input VAL must be a vector with one element for each row in SUBS, or a scalar.

I have to calculate yearly means for all variables. Any suggestions in this regard will be helpful. A sample dataset can be found at:

https://drive.google.com/open?id=1OXPjG8pb9Yslqz_NvILLsoEq9VhaNR3u