A model of the subpolar North Atlantic Ocean is used to study different aspects of ventilation and water mass transformation during a year with moderate convection intensity in the Labrador Sea. The model realistically describes the salient features of the observed hydrographic structure and current system, including boundary currents and recirculations. Ventilation and transformation rates are defined and compared. The transformation rate of Labrador Sea Water (LSW), defined in analogy to several observational studies, is 6.3 Sv (Sv ϵ 10 6 m 3 s Ϫ1) in the model. Using an idealized ventilation tracer, mimicking analyses based on chlorofluorocarbon inventories, an LSW ventilation rate of 10 Sv is found. Differences between both rates are particularly significant for those water masses that are partially transformed into denser water masses during winter. The main export route of the ventilated LSW is the deep Labrador Current (LC). Backward calculation of particle trajectories demonstrates that about one-half of the LSW leaving the Labrador Sea within the deep LC originates in the mixed layer during that same year. Near the offshore flank of the deep LC at about 55°W, the transformation of LSW begins in January and is at a maximum in February/March. While the export of transformed LSW out of the central Labrador Sea continues for several months, LSW generated near the boundary current is exported more rapidly, with maximum transport rates during March/April within the deep LC.

The importance of the Gulf Stream Extension region in climate and seasonal prediction research is being increasingly recognised. Here we use satellite‐derived eddy momentum fluxes to drive a shallow water model for the North Atlantic Ocean that includes the realistic ocean bottom topography. The results show that the eddy momentum fluxes can drive significant transport, sufficient to explain the observed increase in transport of the Gulf Stream following its separation from the coast at Cape Hatteras, as well as the observed recirculation gyres. The model also captures recirculating gyres seen in the mean sea surface height field within the North Atlantic Current system east of the Grand Banks of Newfoundland, including a representation of the Mann Eddy.

[1] The Atlantic Meridional Overturning Circulation (AMOC) carries warm upper waters into northern highlatitudes and returns cold deep waters southward. Under anthropogenic greenhouse gas forcing the AMOC is expected to weaken due to high-latitude warming and freshening. Here, we show that the sensitivity of the AMOC to an impulsive forcing at high latitudes is an oscillatory function of forcing lead time. This leads to the counter-intuitive result that a stronger AMOC can emerge as a result of, although some years after, anomalous warming at high latitudes. In our model study, there is no simple one-to-one correspondence between buoyancy forcing anomalies and AMOC variations, which retain memory of surface buoyancy fluxes in the subpolar gyre for 15-20 years. These results make it challenging to detect secular change from short observational time series.