An objective methodology for identifying oceanic provinces

An objective methodology for identifying oceanic provinces in hydrographic data is presented, although, the technique is sufficiently general so as to be applicable to a variety of data sets. The sub- and near-surface temperature (T) and salinity (S) of the Atlantic Ocean were measured on two Atlantic Meridional Transect (AMT) cruises covering approximately 100° of latitude and 50° of longitude. The cruises covered the boreal autumn and austral spring (AMT-1) and the boreal spring and austral autumn (AMT-2) during 1995 and 1996, respectively. There are distinct patterns of change in T-S from 50°N to 50°S for both sub- and near-surface measurements, although this study concentrates primarily on the latter. Near-surface density (σt) encapsulates the variations in T-S. The position of localized extrema in the σt first spatial derivative locates the change in bulk T-S properties and, hence, the extent of a physical province. The province identification methodology is validated by comparing the results obtained using in situ density, a mixed layer depth space series constructed from expendable bathythermograph profiles, and climatological density. Additional validation of the methodology is achieved by intercomparing different in situ data sources between the two cruise periods. The comparisons show the near-surface measurements are indicative of the mixed layer and that the AMT-1 and AMT-2 cruises are representative of the corresponding climatological months. A simple scheme for investigating how far a province extends away from the cruise track uses the climatological data in concert with a T-S description of each province from the AMT along-track observations to produce a two-dimensional map of the geographical extent of each province. Although exceptions between the province extents and the basic circulation of the Atlantic Ocean occur, the majority of the provinces have the correct shape and extent, that is, their edges follow the flow fields associated with the currents that bound them and they extend over an appropriate area. The province identification methodology is sensitive and reliable enough to investigate temporal differences in oceanic provinces, which means it might be useful for parameterizing global budget calculations in numerical models.