ABSTRACT:The hypothesis is raised and tested that the formation of the seasonal pycnocline in a baroclinic fieldleads to a temporal increase of mesoscale spatial variability during the heating season; the reasoning isthat the process of seasonal pycnocline formation sets up a necessary condition for baroclinic instability,the vertical reversal of the isopycnic potential vorticity gradient (IPVG). This hypothesis is confirmed byanalysis of hydrographic data collected repeatedly in high horizontal and vertical resolution along asection running from the Azores towards Greenland; the analysis reveals a temporal increase during theheating season of (1) mesoscale horizontal variability - most prominent in the region of the North AtlanticCurrent eddy field - and of (2) the tendency of the IPVG to change sign vertically. Comparison withaltimeter sea surface height data indicates that an increase of mesoscale variability during summer is anoverall feature of the mid-latitude surface-intensified western boundary currents.