World Ocean Circulation Experiment Cruises Research Articles

Anthropogenic CO2 accumulation and uptake rates in the Pacific Ocean based on changes in the 13C/12C of dissolved inorganic carbon

AbstractThe anthropogenic CO2 accumulation rate for the Pacific Ocean was estimated from the decrease in δ13C of the dissolved inorganic carbon measured on six World Ocean Circulation Experiment cruises during the 1990s and repeated during Climate Variability and Predictability in the 2000s. A mean depth‐integrated anthropogenic δ13C change of −83 ± 20‰ m decade−1 was estimated for the basin by using the multiple linear regression approach. The largest anthropogenic δ13C decreases occurred between 40°S and 60°S, whereas the smallest decreases occurred in the Southern Ocean and subpolar North Pacific. A mean anthropogenic CO2 accumulation rate of 0.41 ± 0.13 mol C m−2 yr−1 (0.82 ± 0.26 Pg C yr−1) was determined based on observed δ13C changes and is in agreement with previous observation‐ and model‐based estimates. The mean dissolved inorganic carbon DIC13 inventory change of −178 ± 43‰ mol m−2 decade−1 was primarily the result of air‐sea CO2 exchange acting on the measured air‐sea δ13C disequilibrium of ~ −1.2 ± 0.1‰. Regional differences between the DIC13 inventory change and air‐sea 13CO2 flux yielded net anthropogenic CO2 uptake rates (independent of ΔpCO2) that ranged from ~0 to 1 mol m−2 yr−1 and basin‐wide mean of 1.2 ± 1.5 Pg C yr−1. High rates of surface ocean DIC increase and δ13C decrease observed in the Drake Passage (53°S–60°S) support above average anthropogenic CO2 accumulation since 2005. Observed δ13C changes in the Pacific Ocean indicate that ocean transport significantly impacted the anthropogenic CO2 distribution and illustrate the utility of δ13C as a tracer to unravel the processes controlling the present and future accumulation of anthropogenic CO2 in the ocean.

A comparison of hydrographically and optically derived mixed layer depths

Efforts to understand and model the dynamics of the upper ocean would be significantly advanced given the ability to rapidly determine mixed layer depths (MLDs) over large regions. Remote sensing technologies are an ideal choice for achieving this goal. This study addresses the feasibility of estimating MLDs from optical properties. These properties are strongly influenced by suspended particle concentrations, which generally reach a maximum at pycnoclines. The premise therefore is to use a gradient in beam attenuation at 660 nm (c660) as a proxy for the depth of a particle‐scattering layer. Using a global data set collected during World Ocean Circulation Experiment cruises from 1988–1997, six algorithms were employed to compute MLDs from either density or temperature profiles. Given the absence of published optically based MLD algorithms, two new methods were developed that use c660 profiles to estimate the MLD. Intercomparison of the six hydrographically based algorithms revealed some significant disparities among the resulting MLD values. Comparisons between the hydrographical and optical approaches indicated a first‐order agreement between the MLDs based on the depths of gradient maxima for density and c660. When comparing various hydrographically based algorithms, other investigators reported that inherent fluctuations of the mixed layer depth limit the accuracy of its determination to 20 m. Using this benchmark, we found a ∼70% agreement between the best hydrographical‐optical algorithm pairings.

Unaccounted role of Mediterranean Water in the drawdown of anthropogenic carbon

A three‐sided box, MedBox, comprising the Strait of Gibraltar, 24°–41°N east of 22°W, was constructed using hydrographic and chemical data from three World Ocean Circulation Experiment cruises in 1997–1998. An extended optimum multiparameter (OMP) analysis resolves the water mass structure in the MedBox. The mass transport is estimated from data and models warranting mass conservation and no deep water formation within the region. Combining the mass transport, the OMP analysis, and the spatial distribution of anthropogenic carbon (CANT), some relevant questions about the Mediterranean Water (MW) formation, transport, and role in the global CO2 cycle are resolved. MW is produced at a rate of 2.7 ± 0.2 Sv in the eastern North Atlantic. It entrains 2.3 Sv of central waters during its formation, including also remnants of Antarctic Intermediate Water. The main advection route of MW is northward, close to the Iberian margin (2.2 Sv), while about 0.5 Sv are exported into the subtropical eastern North Atlantic. The eastern North Atlantic within the MedBox is a strong region of air‐sea CANT uptake, 0.7 ± 0.7 mmol m−2 d−1, where CANT is accumulated at a rate of 66 ± 21 kmol s−1 (0.025 GtC yr−1), 17% are taken up through the air‐sea interface, and the rest is advected into the area by the circulation. The entrainment of central waters to form MW drives a drawdown of 151 ± 14 kmol s−1 (0.06 GtC yr−1) from the surface to intermediate levels, and 88 ± 8 kmol s−1 (0.03 GtC yr−1) are exported with MW into the North Atlantic, mainly by the horizontal/eddy circulation.

Redfield stoichiometry in Arabian Sea subsurface waters

A linear inverse mixing model is applied to hydrographic, nutrient, and carbon data collected during Joint Global Ocean Flux Study and World Ocean Circulation Experiment cruises in 1995 to estimate the ΔCorg/ΔN/ΔP/ΔSi/‐ΔO2 remineralization ratios within the Arabian Sea between 550 and 4500 m. The observed concentrations are separated into mixing fractions of source water masses and changes caused by remineralization processes, while the effect of denitrification is considered. In contrast to earlier investigations, diapycnal mixing, which plays an important role in dissolved matter fluxes in the Arabian Sea, is accounted for. The ratios are found to be variable with depth, especially in the upper 2000 m of the water column. We suppose that in general nutrients are released faster than carbon dioxide during remineralization. The ΔCorg/ΔCinorg decrease from ∼4 ± 1 at 550 m to 2 ± 0.2 at 2000 m and 1.2 ± 0.3 at 4000 m, suggesting that the dissolution of calcium carbonate above the calcite lysocline is a potentially important process within the Arabian Sea.

Tracer distributions and deep circulation in the western tropical Atlantic during CITHER 1 and ETAMBOT cruises, 1993–1996

This paper presents CFC and nontransient tracer observations in the western equatorial Atlantic Ocean on repeated sections along 7°30′N, the 35°W meridian, and a transect crossing the Ceara Rise. Three World Ocean Circulation Experiment cruises have been carried out in this area, in February‐March 1993 (CITHER 1) and September‐October 1995 and April‐May 1996 (ETAMBOT 1 and 2). Together with the tracer data, the direct current measurements are used to deduce the circulation pathways. The data confirm the principal circulation features of the Upper North Atlantic Deep Water within the area. The Deep Western Boundary Current flows southward along the continental slope, while, adjacent to the DWBC, there is a northward flow corresponding to the DWBC recirculation whose origin appears variable. The largest variability is observed along the 35°W meridional section, where the DWBC bifurcates eastward north of 3°S but the eastward equatorial flow does not appear to be permanent. At the Middle North Atlantic Deep Water level, CFC distributions exhibit the most important contrast between waters of northern and southern origin. The circulation at the Lower North Atlantic Deep Water level, mainly controlled by the topography, looks more permanent because the DWBC is regularly observed during the three cruises, and there is no evidence for a northwestward recirculation along the Mid‐Atlantic Ridge. The transient behavior of CFC distributions, which is superimposed on local circulation effects, can be seen clearly through their temporal evolution within the DWBC. The variability of the deep circulation observed during the 1993–1996 interval rules out a dominant variability in response to semi annual forcing.

On the circulation in the upper layer of the western equatorial Atlantic

Hydrographic observations of pressure, temperature, salinity, dissolved oxygen, and shipboard acoustic Doppler current profiler measurements are used to study the upper layer circulation in the western equatorial Atlantic Ocean, limited to the region bounded by the 10°S and 14°N latitudes between the longitudes 30°W and 52°W. Data were obtained during four World Ocean Circulation Experiment cruises, carried out in January‐March 1993, January‐March 1994, September‐October 1995, and April‐May 1996. In the upper layer, the continuity of the northwestward flowing North Brazil Current along the American continent toward the Caribbean Sea is confirmed in boreal spring. Furthermore, part of the North Brazil Current also continues northwestward in the subthermocline layer during short periods in boreal spring, contrary to previous estimates. The North Equatorial Countercurrent (NECC) is present in boreal spring west of 40°W, fed with water of Northern Hemisphere origin only. The southeastward flowing current observed at 3°N–44°W is fed by the North Brazil Current retroflection and by a cyclonic recirculation of the southern edge of the North Equatorial Current. The upper layer of this current, at 3°N–44°W, feeds the Equatorial Undercurrent (EUC) and the NECC, when its subthermocline layer feeds the EUC and the North Equatorial Undercurrent. Near‐surface eastward flow is present above the EUC during all cruises, yielding to a strong increase of the eastward warm water transport.

AMS measurements of the 14C distribution in the Pacific Ocean

In the framework of the World Ocean Circulation Experiment (WOCE), 4250 (out of about 8000) Pacific sea water samples have been processed and analyzed for the carbon isotopes during the last five years, representing about 70% of the total number of samples measured at NOSAMS. The majority of these samples (3180) are from the surface layers of the ocean (depth: 0–1200 m). During the last 2 years the NOSAMS precision has reached the level required to measure the small Δ14C gradients in the deeper water column (< 0.4%). Using only the data for 0–1200 m depth, some spatial and time aspects of the distribution of nuclear-bomb-produced radiocarbon are discussed here. Principal investigators for the small sample AMS 14C program on the 18 WOCE cruises were Robert Key (Princeton University), Peter Schlosser (Lamont-Doherty Earth Observatory), and Paul Quay (University of Washington).

The water masses between Mindanao and New Guinea

Water masses between Mindanao and New Guinea are investigated, using hydrographic data, during two R/V Kaiyo World Ocean Circulation Experiment cruises conducted in October 1992 and February 1994. The analysis of this data indicates that the South Pacific Tropical Water (SPTW), which is formed in the southern hemisphere, reached the area of Morotai Island in 1992 and farther north in 1994. The Antarctic Intermediate Water (AAIW) exists in the southwestern area of the southernmost Philippine Sea. There is northeastward flow between Talaud Islands and Morotai Island, which is probably a part of the retroflection of the Mindanao Current. The remnant of the North Pacific Intermediate Water, advected by the flow from the Celebes Sea, was found in 1992. The New Guinea Coastal Undercurrent, transporting SPTW and AAIW from the southern hemisphere, is divided into at least two parts because the retroflection of the Mindanao Current prevents its shallow part from reaching farther north; the shallow part of SPTW turns eastward as a source water of North Equatorial Countercurrent and retroflects toward the southeast. AAIW and the lower part of SPTW flow northward and seem to be linked with the Mindanao Undercurrent.

Development of improved space sampling strategies for ocean chemical properties: Total carbon dioxide and dissolved nitrate

Large‐scale ocean observing programs such as the Joint Global Ocean Flux Study (JGOFS) and the World Ocean Circulation Experiment (WOCE) today, must face the problem of designing an adequate sampling strategy. For ocean chemical variables, the goals and observing technologies are quite different from ocean physical variables (temperature, salinity, pressure). We have recently acquired data on the ocean CO2 properties on WOCE cruises P16c and P17c that are sufficiently dense to test for sampling redundancy. We use linear and quadratic interpolation methods on the sampled field to investigate what is the minimum number of samples required to define the deep ocean total inorganic carbon (TCO2) field within the limits of experimental accuracy (±4 µmol/kg). Within the limits of current measurements, these lines were oversampled in the deep ocean. Should the precision of the measurement be improved, then a denser sampling pattern may be desirable in the future. This approach rationalizes the efficient use of resources for field work and for estimating gridded TCO2 fields needed to constrain geochemical models.