Variability of Deep Water Formation and Convection in the North Atlantic: A Model Study

The thermohaline circulation in the Atlantic Ocean responds to variations in large-scale atmospheric features. Examples are the observed correlations between the North Atlantic Oscillation (NAO) and the properties of the Labrador Sea Water (LSW) or variations in the meridional heat fluxes. Moreover, the NAO triggers the relative importance of the deep water formation sites to the south (Labrador Sea, LS) and to the north (Greenland-Iceland-Norwegian Sea, GIN) of the Greenland-Scotland Ridge (GSR). The C-Grid Hamburg Ocean Primitive Equation model (C-HOPE) is used to study the ocean's response to realistic atmospheric forcing. The global ocean model features an embedded dynamic-thermodynamic sea ice model and, owing to the conformal mapping technique, allows for varying horizontal resolution. The model is forced with daily data from the NCEP reanalysis for the period 1948-1999. As well as in the LS the model is able to produce a substantial portion of the deep water in the GIN sea. In particular, a sensitivity of the convective activity in the respective deep water production areas to the NAO phase was found on decadal time scales. During NAO - high phases (e.g. mid 1970s to 1990s) the convection is intensified in the LS and weakened in the GIN sea, vice versa during NAO - low periods (1950s to 1960s). The formation rate of LSW is strongly related to the intensity of wintertime deep convection and determined by the local surface conditions. The simulated LS convection reproduces the main characteristics of the observations. Anomalies spread out into the North Atlantic and modulate the upper North Atlantic Deep Water (NADW). The deeper layers of the NADW are fed by the dense overflows over the GSR. These overflows represent a blend of water masses formed by air-sea interaction and deep convection in the GIN sea and the Arctic Ocean. We investigate the temporal and spatial variability of the deep water formation and their relation to atmospheric forcing and the presence of fresh water anomalies.