The climatology of the North Atlantic

Data from approximately 144,000 hydrographic stations in the North Atlantic have been retrieved from the national Oceanic Data Centre and analysed to produce maps of mean pressure, temperature, salinity and oxygen on selected potential density surfaces for the domain bounded by 0°–85°W and 0°–65°N. The data span the period from 1904 to 1990 with the majority of the data from the last four decades. The data set for this region is 60% larger than that used in the production of Levitus' Climatological World Atlas. This increase in stations, coupled with smoothing scales specific to the North Atlantic rather than the global ocean, considerably improves the resolution of the basin's features. The mean property fields and their associated standard deviations are resolved on a one-degree grid with little smoothing, contrasting with the Levitus Atlas where properties, although presented on a one-degree grid, have been smoothed on the order of 1000km. Another important feature of this database is the process by which irregularly spaced data are averaged onto a regular grid. In a significant departure from the Levitus analysis, which averaged on depth surfaces, these data were averaged on potential density surfaces, thus eliminating an artificial mixing of water mass properties. The database is used to describe the baroclinicity of well-known features such as the Gulf Stream, the North Atlantic Current and the Deep Western Boundary Current, and to elucidate the recirculations associated with these currents. It additionally resolves several new features in the intermediate and deep North Atlantic. These features include the signature of a large scale deep recirculation that extends southwestward from the eastward extension of the North Atlantic Current to the separation point of the Gulf Stream near Cape Hatteras. This recirculation, which spans approximately 2000m of the water column, encompasses more local recirculations and potentially mixes subpolar and subtropical waters. Furthermore, in the upper thermocline, this database reveals a coherent Azores Current that stretches from the Gulf Stream system south of the Tail of the Grand Banks to Madeira. This flow is marked by divergences to the south and convergences from the north such that its downstream transport is not much changed.