The relationship between the North and South Atlantic bidecadal sea level (SL) oscillations during the twentieth century is investigated for the first time using Simple Ocean Data Assimilation reanalysis. Complex empirical orthogonal function analysis of the bidecadal band gives two dominant modes: the first dominating from 1915 to 1965 and the second from 1970 onward. The long-term time-dependent change of mode dominance suggests a structural-type regime shift. The first mode is characterized by states with North and South Atlantic subtropical gyres in phase, while tropical and subpolar regions are in opposite phase relative to them. The second mode is characterized by the subpolar gyre and North subtropical gyre almost in quadrature, with North and South subtropical gyres out of phase. Thermal Rossby waves are very clear only in the second mode. These waves seem to be influenced by bottom topography. Known Atlantic meridional overturning circulation (AMOC) sea level fingerprints and AMOC strength indices are discussed in the bidecadal oscillations context.