| Summary: | In this dissertation work, the processes responsible for the observed interannual and near-decadal variability in the mode waters of the North Atlantic Ocean are investigated, in particular those related to the renewal of Subtropical Mode Water (STMW) and Labrador Sea Water (LSW). This study is carried out within the framework of numerical simulations performed with an Ocean General Circulation Model (the Miami Isopycnic Coordinate Ocean Model - MICOM), under realistic boundary forcing.First, the constraints imposed upon the model thermodynamic adjustment by the formulation of the surface and lateral forcing are explored in detail, in a series of experiments under climatological boundary conditions. Relaxation of model properties to climatology at the oceanic boundaries (buffer zones) is found to be a robust approach in MICOM. Different surface heat and fresh water flux parameterizations, which include restoring and flux boundary conditions, are shown to not only impose different constraints on the modeled sea surface temperature and salinity, but also on the subsurface adjustment, affecting the model's ability to simulate the production of water masses and the associated meridional transports of heat and fresh water.The atmospherically forced subsurface variability is then investigated, focusing on the impact of surface heat flux anomalies upon the convective activity in the water mass formation regions. Realistic monthly atmospheric anomalies for the period 1947--1988 are included in the thermodynamical and in the mechanical surface forcing, and the model's response to the anomalous forcing is analyzed. Anomalous surface heat fluxes are able to bring about changes in the potential vorticity (PV) of STMW that agree quite well with observations at Bermuda. PV anomalies are generated to the northeast of Bermuda and are advected within a large area in the western subtropics. Persistence of cold or warm conditions, associated with anomalous heat loss over the western subtropics, is more significant for the generation of the simulated STMW variability than are strong anomalous events in isolated years. In the Labrador Sea, the phase and order of magnitude of the observed near-decadal variability in the convective activity is relatively well simulated under heat flux anomalies, despite the absence in the model of fresh water anomalies. The simulated (as well as observed) PV anomalies in the Labrador Sea are out-of-phase with those in the western subtropics, with the variability in both sites being related to changes in the pattern of the large scale atmospheric circulation associated with the North Atlantic Oscillation.
|
|---|