Mixed-layer simulations at OWS Bravo: the role of salinity in interannual variability of the upper ocean at high latitude

The role of salinity in mixed layer dynamics is investigated at the high latitude location of ocean weathership station (OWS) Bravo. To accomplish this two common types of onedimensional mixed-layer models were employed during two periods exhibiting significantly different oceanographic conditions. In the mixed-layer simulations the salinity is prescribed according to existing data. Depending how the salinity is specified, good agreement between simulated and observed SST can be achieved. A comparison of the different methods used to specify salinity reveals that the simulated SST and mixedlayer depths are particularly sensitive to changes in salinity, especially during the cooling phase. This is because density changes are generally more sensitive to salinity at high latitudes. Model simulations are carried out during the period 1964–1965 as well as during the period 1969–1971. The 1964–1965 period is one having normal salinity while the 19691971 period is characterized as having abnormally low salinity and mild winters. A thorough comparison of the model simulations during these two periods indicates that the low surface salinity, and not the reduced surface cooling during 1969–1971, is mainly responsible for the shallow depth of the mixed-layer during that period. Furthermore, the smaller surface heat loss during 1969–1971 does not result in a higher winter sea-surface temperature as expected due to the low entrainment rate during this period. The model results suggest that salinity can be as important as the surface heat flux in controlling the interannual variation of the mixed layer depth at OWS Bravo.