CDO

With the last release 1.7.2, there was a new operator introduced called 'smooth'. It performs a smoothing on a local area for each gridpoint. By default, this area has a radius of one degree and is executed only once.

The following images are based on a 1 degree ocean salinity field. I used it, because it has missing values on land points, which can be filled by smooth. Of cause field values are changed - if you want to avoid this, consider the operators used for last image: setmisstonn.