Oceanographic conditions influencing the surface mixed layer at OWS Lima, N.E. Atlantic

• Main Author: Osborne, Michael John
• Format: Thesis
• Language: English
• Published: University of Southampton 1990
• Subjects: Arctic; Hatton Bank; North Atlantic; Northeast Atlantic
• Online Access: https://eprints.soton.ac.uk/462629/

An investigation into the oceanographic influences affecting the surface mixed layer and underlying water column at Ocean Weather Station Lima (57 o N, 20 o W) in the Northeast Atlantic is described. Data from an intensive field programme in 1987 are presented. The seasonal thermocline and structure of the underlying water column was influenced by the presence and admixture of various water masses. North Atlantic Water (NAW) was observed to enter the region from the south. Modified NAW (fresher and cooler) entered the region at the surface from the north, probably due to Ekman drift. During the summer months, this fresher water was `trapped' in the surface mixed layer - thus increasing the stability of the upper water column. Below a depth of 450 m, Sub-Arctic Intermediate Water entered the region from the west. Distributions of density and dynamic height indicate the presence of baroclinic instability and the considerable local influence of the Hatton Bank on circulation and hydrography of the region. Calculation of the annual heat budget indicates an annual loss of around 80 Wm -1 equivalent to an overall heat input by advection of 0.02 o C per day and to a spatial meridional temperature gradient of -1.2 x 10 -6 o Cm -1 . The open nature of the heat budget has serious consequences for the application of one-dimensional mixed layer models. Comparisons between observed and modelled sea surface temperature, mixed layer depths and heat content show good agreement at times of positive net surface heat flux, when there is little or no input of heat by advection. However, during a period of cooling, the model grossly over-predicted the depth of the surface mixed layer.