Time series of temperature and salinity from Ocean Weather Station M are analysed for periodic cycles of interannual to decadal scale. Time evolutions of the spectra show various spectral peaks at all depths, but none of these cycles show persistence throughout the 50 years. In addition isopycnal surfaces and temperature and salinity values on these surfaces, are estimated and studied in terms of the relative influence of horizontal advection and vertical movement on the observed changes of water properties.