An estimate of the climatology and variability of Eighteen Degree Water potential vorticity forcing

Abstract

The input of potential vorticity (PV) over the oceans is estimated from observations to produce climatological maps and study interannual forcing variability. The estimate is obtained using a potential vorticity like variable employing potential temperature in place of potential density, and thus we refer to it as pseudo-potential vorticity (PPV). This choice allows us to examine the effects of storms and high frequency events as sources of PPV. We postulate the characteristics of the PPV fluxes reflect the properties of PV flux. Particular attention is paid to the North Atlantic subtropical mode water potential temperature range from 17 to 19°C. The sea surface PPV flux is estimated through buoyancy and wind stress contributions and using a climatological mixed layer depth product. Wind forcing of PPV is strong in frontal regions and in the Antarctic Circumpolar current. When averaged following outcrops, however, buoyant forcing emerges as dominant. A major observational subtropical mode water program named CLIMODE was conducted during the winters from 2004/5 to 2006/7. Summer months during these years are very much in agreement with climatology; the winter months are slightly more variable. Attempts are made to relate PPV forcing variability to the NAO, a major mode of North Atlantic atmospheric variability. Correlation between the flux and the NAO is significant, and increases in amplitude if analysis is restricted to the winter time frame. There is also a weaker correlation between the NAO and the net EDW flux the following year. Considerable variability of the flux is unaccounted for, however, and we speculate that this is a signal of intrinsic ocean variability.