This paper is from an open-access journal, yet it's a mind blower.
GPS signals have enough information content that they can pick up sensitive measures related to earth deformation, so the paper describes the results.
That strict biennial signal is readily apparent along even years and it likely has everything to do with the biennial-modulated ENSO behavior. Have to remember that a thermocline separating two-slightly different density liquids acts as a highly sensitive sensor -- any slight forcing will get the sloshing in motion, and the GPS is likely isolating the biennial component.
Also some indication that the measure is picking up the 28 month QBO signal. See Table 1 in the paper, which shows an average period of 2.3 years for the quasi-biennial oscillation.