On the seasonal development of mesoscale variability: the influence of the seasonal pycnocline formation

Abstract

The hypothesis is raised and tested that the formation of the seasonal pycnocline in a baroclinic field leads to a temporal increase of mesoscale spatial variability during the heating season; the reasoning is that 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 by analysis of hydrographic data collected repeatedly in high horizontal and vertical resolution along a section running from the Azores towards Greenland; the analysis reveals a temporal increase during the heating season of (1) mesoscale horizontal variability—most prominent in the region of the North Atlantic Current eddy field; and (2) the tendency of the IPVG to change sign vertically. Comparison with altimeter sea surface height data indicates that an increase of mesoscale variability during summer is an overall feature of the mid-latitude surface-intensified western boundary currents.