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Anthropogenically-induced global warming is expected to decrease primary productivity in the subtropical oceans by strengthening stratification of the water column and reducing the flux of nutrients from deep-waters to the sunlit surface layers. Identification of such changes is hindered by a paucity of long-term, spatially-resolved, biological time-series data at the basin scale. This paper exploits Atlantic Meridional Transect (AMT) data on physical and biogeochemical properties (1995–2014) in synergy with a wide range of remote-sensing (RS) observations from ocean colour, Sea Surface Temperature (SST), Sea Surface Salinity (SSS) and altimetry (surface currents), combined with different modelling approaches (both empirical and a coupled 1-D Ecosystem model), to produce a synthesis of the seasonal functioning of the North and South Atlantic Sub-Tropical Gyres (STGs), and assess their response to longer-term changes in climate. We explore definitive characteristics of the STGs using data of physical (SST, SSS and peripheral current systems) and biogeochemical variables (chlorophyll and nitrate), with inherent criteria (permanent thermal stratification and oligotrophy), and define the gyre boundary from a sharp gradient in these physical and biogeochemical properties. From RS data, the seasonal cycles for the period 1998–2012 show significant relationships between physical properties (SST and PAR) and gyre area. In contrast to expectations, the surface layer chlorophyll concentration from RS data (CHL) shows an upward trend for the mean values in both subtropical gyres. Furthermore, trends in physical properties (SST, PAR, gyre area) differ between the North and South STGs, suggesting the processes responsible for an upward trend in CHL may vary between gyres. There are significant anomalies in CHL and SST that are associated with El Nino events. These conclusions are drawn cautiously considering the short length of the time-series (1998–2012), emphasising the need to sustain spatially-extensive surveys such as AMT and integrate such observations with models, autonomous observations and RS data, to help address fundamental questions about how our planet is responding to climate change. A small number of dedicated AMT cruises in the keystone months of January and July would complement our understanding of seasonal cycles in the STGs.

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.

The Atlantic Meridional Transect programme uses the twice-annual passage of the RRS James Clark Ross between the UK and the Falkland Islands, before and after the Antarctic research programme in the Austral Summer (see Aiken, J., & Bale, A. J. (2000). An introduction to the Atlantic Meridional Transect (AMT) Programme. Progress in Oceanography, this issue). This paper examines the scientific rationale for a spatially-extensive time and space series programme and reviews the relevant physical and biological oceanography of the Atlantic Ocean. The main scientific observations from the research programme are reported. These are set in the context of historical and contemporary observations pertinent to the principal objectives of the cruise, notably the satellite remotely sensed observations of ocean properties. The extent to which the programme goals have been realised by the research to date is assessed and discussed. New bio-optical signatures, which can be related to productivity parameters, have been derived. These can be used to interpret remotely sensed observations of ocean colour in terms of productivity and production processes such as the air/sea exchange of biogenic gases, which relate to the issues of climate change and the sustainability of marine ecosystems.