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The input of potential vorticity (PV) over the Oceans is estimated from observations to produce a climatological map of PV flux and to study the interannual forcing variability. Particular attention is paid to the North Atlantic subtropical mode water potential temperature range from 17 to 19 ºC. The sea surface PV flux is estimated through buoyancy and wind stress contributions and using a climatological mixed layer depth product. Wind forcing of PV is found to be weak. A major observational subtropical mode water program named CLIMODE was conducted during the winters of 2004 to 2006. These years appear to be slightly stronger in their forcing than climatology, although the difference is comparable to the uncertainty. Attempts are made to relate the fluctuations to the NAO, a major mode of North Atlantic atmospheric variability, showing that there is a strong correlation between March values of the NAO and the PV fluctuation