However, the network card frequently stalled and provided transmission rates of only max. 30-40 kB/second. Copying data by scp via ssh would be way too slow. Initial attempts indicated that the file transfer could take several weeks.
Rsync could be an option, but it was not installed on the AIX machine. Installing rsync was not considered, because no one really dared to touch the AIX machine with root rights.
The solution was to pipe the data via a compressed ssh tunnel. Most data on the AIX machine were CT scans, which can provide a significant compression ratio.
The command line is slightly cryptic though: (it is a one-liner, ignore the line wrapping)
tar cvf - /tp_data | gzip -9c | ssh root@gorm 'cd /target_dir; gunzip - | tar xpf -'
which will place the /tp_data directory inside the /target directory on the gorm computer, running linux.
Result: all data were transferred in less than a week.