Two 3.5-month profile time series of temperature, salinity, and current in the seasonal thermocline of the northwest Sargasso Sea are used to estimate the contributions of density advection by geostrophic currents and diapycnal mixing to the observed density changes. The two records were collected in opposite seasons (spring-summer and fall-winter) at 34¿N,70¿W as part of the Long-Term Upper Ocean Study (LOTUS). These records show the formation and decay of the seasonal pycnocline and the accompanying changes in upper ocean temperature-salinity relationships. Geostrophic advection is an important contributor to density variability at vertical wavelengths of 100 m and greater at subinertial frequencies. Diapycnal diffusion accounts for observed cooling over time scales of a week and longer when currents are sufficiently weak that advective effects do not overwhelm the mixing signal. ¿American Geophysical Union 1987