Sea Change: The World Ocean Circulation Experiment and the Productive Limits of Ocean Variability

The ability to quantify the relationship between the ocean and the atmosphere is an enduring challenge for global-scale science. This paper analyzes the World Ocean Circulation Experiment (WOCE, 1990–2002), an international oceanographic program that aimed to provide data for decadal-scale climate modeling and for the first time produce a “snapshot” of ocean circulation against which future change could be measured. WOCE was an ambitious project that drew on extensive international collaboration and emerging technologies that continue to play a significant role in how the global environment is known and governed. However, a main outcome of WOCE was an encounter with ocean variability: the notion that the ocean is governed not by the circular currents shown in the popular “conveyor-belt” diagram but by eddies, filaments, jets, and other nonlinear forces. This paper suggests the concept of “productive limits” as an analytic for understanding how ocean variability both prompted new forms of knowledge and the development of a global knowledge infrastructure that is contingent, uneven, and fully entwined with geopolitical dynamics.

World Ocean Circulation Experiment – Argo Global Hydrographic Climatology

The paper describes the new gridded World Ocean Circulation Experiment-Argo Global Hydrographic Climatology (WAGHC). The climatology has a 1∕4∘ spatial resolution resolving the annual cycle of temperature and salinity on a monthly basis. Two versions of the climatology were produced and differ with respect to whether the spatial interpolation was performed on isobaric or isopycnal surfaces, respectively. The WAGHC climatology is based on the quality controlled temperature and salinity profiles obtained before January 2016, and the average climatological year is in the range from 2008 to 2012. To avoid biases due to the significant step-like decrease of the data below 2 km, the profile extrapolation procedure is implemented. We compare the WAGHC climatology to the 1∕4∘ resolution isobarically averaged WOA13 climatology, produced by the NOAA Ocean Climate Laboratory (Locarnini et al., 2013) and diagnose a generally good agreement between these two gridded products. The differences between the two climatologies are basically attributed to the interpolation method and the considerably extended data basis. Specifically, the WAGHC climatology improved the representation of the thermohaline structure, in both the data poor polar regions and several data abundant regions like the Baltic Sea, the Caspian sea, the Gulf of California, the Caribbean Sea, and the Weddell Sea. Further, the dependence of the ocean heat content anomaly (OHCA) time series on the baseline climatology was tested. Since the 1950s, both of the baseline climatologies produce almost identical OHCA time series. The gridded dataset can be found at https://doi.org/10.1594/WDCC/WAGHC_V1.0 (Gouretski, 2018).

A novel inter-comparison of nutrient analysis at sea: recommendations to enhance comparability of open ocean nutrient data

An inter-comparison study has been carried out on the analysis of inorganic nutrients at sea following the operation of two nutrient analysers simultaneously on the GO-SHIP A02 trans-Atlantic survey in May 2017. Both instruments were Skalar San++ Continuous Flow Analysers, one from the Marine Institute, Ireland and the other from Dalhousie University, Canada, each operated by their own laboratory analysts following GO-SHIP guidelines, while adopting their existing laboratory methods. High quality control of the nutrient analysis was achieved on both instruments and there was high comparability between the two datasets. Vertical profiles of nutrients also compared well with those collected in 1997 along the same A02 transect by the World Ocean Circulation Experiment. The comparison of the two 2017 datasets and individual laboratory methods, did however raise some interesting questions on the comparison of nutrients analysed from different systems, in particular the calibration range of daily standards and its influence on low nutrient samples, and the importance of using certified reference materials of high and low concentrations to identify bias in the data. Based on the results from this inter-comparison, a number of recommendations have been suggested that we feel will enhance the existing GO-SHIP guidelines to improve the comparability of global nutrient datasets. The A02 nutrient dataset is currently available at the National Oceanographic Data Centre of Ireland; http://dx.doi.org/10.20393/CE49BC4C-91CC-41B9-A07F-D4E36B18B26F.

Transport across 48°N in the Atlantic Ocean

Transports across 48°N in the Atlantic Ocean are estimated from five repeat World Ocean Circulation Experiment (WOCE) hydrographic lines collected in this region in 1993–2000, from time-varying air–sea heat and freshwater fluxes north of 48°N, and from a synthesis of these two data sources using inverse box model methods. Results from hydrography and air–sea fluxes treated separately are analogous to recently published transport variation studies and demonstrate the sensitivity of the results to either the choice of reference level and reference velocities for thermal wind calculations or the specific flux dataset chosen. In addition, flux-based calculations do not include the effects of subsurface mixing on overturning and transports of specific water masses. The inverse model approach was used to find unknown depth-independent velocities, interior diapycnal fluxes, and adjustments to air–sea fluxes subject to various constraints on the system. Various model choices were made to focus on annually averaged results, as opposed to instantaneous values during the occupation of the hydrographic lines. The results reflect the constraints and choices made in the construction of the model. The inverse model solutions show only marginal, not significantly different temporal changes in the net overturning cell strength and heat transport across 48°N. These small changes are similar to seasonally or annually averaged numerical model simulations of overturning. Significant variability is found for deep transports and air–sea flux quantities in density layers. Put another way, if one ignores the details of layer exchanges, the model can be constrained to produce the same net overturning for each repeat line; however, constraining individual layers to have the same transport for each line fails. Diapycnal fluxes are found to be important in the mean but are relatively constant from one repeat line to the next. Mean air–sea fluxes are modified slightly but are still essentially consistent with either the NCEP data or the National Oceanography Centre, Southampton (NOC) Comprehensive Ocean–Atmosphere Data Set (COADS) within error. Modest reductions in air–sea flux uncertainties would give these constraints a much greater impact. Direct transport estimates over broader regions than the western boundary North Atlantic Current are needed to help resolve circulation structure that is important for variability in net overturning.