Seasonal variability in the intermediate water of the eastern North Atlantic

Abstract

Observational evidence of seasonal variability below the main thermocline in the eastern North Atlantic is described, and a theoretical model of oceanic response to seasonally varying windstress forcing is constructed to assist in the interpretation of the observations. The observations are historical conductivity-temperature-depth data from the Bay of Biscay region (2° to 20°W, 42° to 52°N), a series of eleven cruises over the three years 1972 through 1974, spaced approximately three months apart. The analysis of the observations utilizes a new technique for identifying the adiabatically leveled density field corresponding to the observed density field. The distribution of salinity anomaly along the leveled surfaces is examined, as are the vertical displacements of observed density surfaces from the leveled reference surfaces, and the available potential energy. Seasonal variations in salinity anomaly and vertical displacement occur as westward propagating disturbances with zonal wavelength 390 (±50) km, phase 71 (±30) days from 1 January, and maximum amplitudes of ±30 ppm and ±20 db respectively. The leveled density field varies seasonally with an amplitude corresponding to a thermocline displacement of ±15 db. The observations are consistent with the predictions of a model in which an ocean of variable stratification with a surface mixed layer and an eastern boundary is forced by seasonal changes in a sinusoidal windstress pattern, when windstress parameters calculated from the observations of Bunker and Worthington (1976) are applied.