Steric height variability in the Nordic seas

The variability of steric height in the Nordic Seas is analyzed on seasonal, interannual, and decadal timescales using a comprehensive data set of temperature and salinity observations for the second half of the twentieth century. Results from a regional Ocean General Circulation Model (OGCM) are used to assess the reliability of the averaging and the temporal interpolation of the inhomogeneous distributed observation data. The annual cycle explains only a minor part of the monthly variability for most of the region. The analysis on interannual to decadal timescales confined to the Norwegian Sea displays a clear rising trend starting at the end of the 1960s with particularly strong changes along the Barents Sea opening (6 to 7 cm). Moreover, a general freshening is found for the entire Norwegian Sea. In addition, a north-south dipole of the thermal component of the steric height variability is identified. This dipole elevates the general rising trend along the Barents Sea opening and reduces it in the southern Norwegian Sea. The bulk of the interannual variability in steric height is governed by variations in the local meridional wind stress that determine the relative distribution of Atlantic and Arctic waters in the Norwegian Sea. In addition, reduced heat loss to the atmosphere strongly correlates with the North Atlantic Oscillation winter index. This, in turn, may in particular explain the large steric height increase found at the Barents Sea opening.