WOCE Hydrographic Program Research Articles

Influence of Ross Sea Bottom Water changes on the warming and freshening of the Antarctic Bottom Water in the Australian-Antarctic Basin

Abstract. Changes to the properties of Antarctic Bottom Water in the Australian-Antarctic Basin (AA-AABW) between the 1990s and 2000s are documented using data from the WOCE Hydrographic Program (WHP) and repeated hydrographic surveys. Strong cooling and freshening are observed on isopycnal layers denser than γn = 28.30 kg m−3. Changes in the average salinity and potential temperature below this isopycnal correspond to a basin-wide warming of 1300 ± 200 GW and freshening of 24 ± 3 Gt year−1. Recent changes to dense shelf water in the source regions in the Ross Sea and George V Land can explain the freshening of AA-AABW but not its extensive warming. An alternative mechanism for this warming is a decrease in the supply of AABW from the Ross Sea (RSBW). Hydrographic profiles between the western Ross Sea and George V Land (171–158° E) were analyzed with a simple advective-diffusive model to assess the causes of the observed changes. The model suggests that the warming of RSBW observed between the 1970s and 2000s can be explained by a 21 ± 23% reduction in RSBW transport and the enhancement of the vertical diffusion of heat resulting from a 30 ± 7% weakening of the abyssal stratification. The documented freshening of Ross Sea dense shelf water leads to a reduction in both salinity and density stratification. Therefore the direct freshening of RSBW at its source also produces an indirect warming of the RSBW. A simple box model suggests that the changes in RSBW properties and volume transport (a decrease of 6.7% is assumed between the year 1995 and 2005) can explain 51 ± 6% of the warming and 84 ± 10% of the freshening observed in AA-AABW.

CARINA TCO<sub>2</sub> data in the Atlantic Ocean

Abstract. Water column data of carbon and carbon-relevant hydrographic and hydrochemical parameters from 188 cruises in the Arctic Mediterranean Seas, Atlantic and Southern Ocean have been retrieved and merged in a new data base: the CARINA (CARbon IN the Atlantic) Project. These data have gone through rigorous quality control (QC) procedures so as to improve the quality and consistency of the data as much as possible. Secondary quality control, which involved objective study of data in order to quantify systematic differences in the reported values, was performed for the pertinent parameters in the CARINA data base. Systematic biases in the data have been tentatively corrected in the data products. The products are three merged data files with measured, adjusted and interpolated data of all cruises for each of the three CARINA regions (Arctic Mediterranean Seas, Atlantic and Southern Ocean). Ninety-eight cruises were conducted in the "Atlantic" defined as the region south of the Greenland-Iceland-Scotland Ridge and north of about 30° S. Here we report the details of the secondary QC which was done on the total dissolved inorganic carbon (TCO2) data and the adjustments that were applied to yield the final data product in the Atlantic. Procedures of quality control – including crossover analysis between stations and inversion analysis of all crossover data – are briefly described. Adjustments were applied to TCO2 measurements for 17 of the cruises in the Atlantic Ocean region. With these adjustments, the CARINA data base is consistent both internally as well as with GLODAP data, an oceanographic data set based on the WOCE Hydrographic Program in the 1990s, and is now suitable for accurate assessments of, for example, regional oceanic carbon inventories, uptake rates and model validation.

Observations of decadal time scale salinity changes in the subtropical thermocline of the North Pacific Ocean

Data from Argo floats indicate that significant salinity changes have occurred in the North Pacific thermocline relative to data collected in the previous two decades, including observations obtained as part of the WOCE hydrographic program. Such a salinity decrease on both isopyncals and isobars implies a freshening scenario in the near-surface source region of this water mass. The frequently repeated meridional section P16 supports this inference. The subsurface salinity freshening likely began in the early 1990s, strengthened through 1997, and continued into the 2000s; the surface salinity freshening had commenced by 1984 and continued through the first decade of the 21st century. The spatial distribution of salinity change on the density surface σθ=25.5 is examined through comparisons of Argo and most of the North Pacific WOCE sections (1985-1994) and between Argo and the Hydrobase climatology, largely composed of data from the late 1970s through the mid-1980s. Both comparisons show a large-scale, basin-wide decrease in subsurface salinity through the Argo time period used in this analysis (2003-2006). The salinity difference is maximum in the northeast area and spreads southward and westward, approximately following geostrophic streamlines.

Variability of water properties, heat and salt fluxes in the Arabian Sea, between the onset and wane of the 1995 southwest monsoon

Abstract We investigate the variability of the circulation, water masses, heat and salt fluxes in the Arabian Sea over the course of the southwest monsoon. Two zonal sections taken along 8°30′N in 1995 as part of the Indian Ocean WOCE hydrographic program are used. The first was occupied in early June at the onset of the southwest monsoon winds, the second in late September, at the wane of the monsoon. The September section was found to be generally warmer (+0.32°C) and saltier (+0.04) than in June, despite a 50 mm drop in mean sea level. Therefore, the common assumption that an increase in sea-surface height follows an increase in heat content (the hydrostatic response) does not hold. Instead, we conclude that the heat content increases due to the advection of Arabian Sea Surface Water and Red Sea Water onto the section from the north, and the drop in sea level is due to a loss of mass, rather than heat, from the water column. There are large uncertainties involved in diagnosing the heat-flux divergence across the Arabian Sea, because the seasonal variability of the water masses and circulation in the basin mean that our data are not representative of a steady state. We treat each section separately and find an oceanic heat export of −0.72 PW in June and −0.19 PW in September, implying a basin cooling rate of about −0.36 PW in June and a slight heating of 0.12 PW in September. In June the mass and heat balances are dominated by the Ekman transport and the Somali Current, with very flat density surfaces resulting in a small interior geostrophic transport. By September the Ekman transport has reduced, and it is primarily the interior transport that balances a strong Somali Current. There are two main overturning cells in June and September: A shallow one of approximate magnitude 15 Sv in June and 0 Sv in September, which reaches depths of no more than 500 m and is driven by Ekman divergence at the surface; and a deep cell of magnitude 1 Sv representing a weak inflow and subsequent upwelling of Circumpolar Deep water. The deep cell implies a basin-averaged upwelling velocity of 3.2×10−5 cm s−1 through 2200 m.

The overflows across the Ninetyeast Ridge

Abstract Water deeper than, say, 3800 m in the Central Indian Basin comes from the West Australian Basin across deep saddles on the Ninetyeast Ridge near Lats. 5°S, 10°S, and 28°S. The WOCE Hydrographic Program in the Indian Ocean (1994–1996) provided opportunities for more detailed measurements of water properties in these overflows than any made earlier. With zero-velocity surfaces inferred from contrasts in oxygen and silicic-acid concentrations and from distributions of specific-volume anomaly, geostrophic estimates of these overflow rates are, respectively, 1.0, 1.1, and 0.1×10 6 m 3 s −1 (westward). These constructions also imply strong zonal flows of mid-depth water (between 2000 db and the zero-velocity surfaces) through the passages: 2.2 and 6.7×10 6 m 3 s −1 westward at Lats. 5°S and 10°S, respectively, and 1.8×10 6 m 3 s −1 eastward at Lat. 28°S. As in the southward flow through Denmark Strait, density stratification explains the splitting of westward currents at 5°S and 10°S into mid-depth and deep flows as they pass over their saddles (but not why such vertically extended flows should exist in the first place). The cause of opposed eastward flow at 28°S is less certain, but might be related to the particular geometry of the West Australian Basin.

Effect of changes in the composition of seawater on the density–salinity relationship

Over the next 5–10 years, the WOCE hydrographic program will generate reliable hydrographic data for the world oceans. The resultant conductivity salinity, temperature, and pressure data will generate calculated densities that will be used to examine mixing along constant density surfaces. Changes in the composition of deep waters due to the breakdown of plant material can effect the calculated densities at a given conductivity salinity. The increases in SiO 2, nitrate, alkalinity, and TCO 2 (or pH) can change the density of seawater as well as the conductivity. For studies of the salinity and density fields over small spatial scales, these changes will be small, but for large scale and ocean to ocean studies the differences can be significant. The density calculations based on the salinity determined from conductivity need to be adjusted for the offsets due to changes in the composition of seawater. This report describes how this correction should be made using existing information.

Assessment of the quality of the shipboard measurements of total alkalinity on the WOCE Hydrographic Program Indian Ocean CO2 survey cruises 1994–1996

In 1995, we participated in a number of WOCE Hydrographic Program cruises in the Indian Ocean as part of the Joint Global Ocean Flux Study (JGOFS) CO2 Survey sponsored by the Department of Energy (DOE). Two titration systems were used throughout this study to determine the pH, total alkalinity (TA) and total inorganic carbon dioxide (TCO2) of the samples collected during these cruises. The performance of these systems was monitored by making closed cell titration measurements on Certified Reference Materials (CRMs). A total of 962 titrations were made on six batches of CRMs during the cruises. The reproducibility calculated from these titrations was ±0.007 in pH, ±4.2 μmol kg−1 in TA, and ±4.1 μmol kg−1 in TCO2. The at-sea measurements on the CRMs were in reasonable agreement with laboratory measurements made on the same batches. These results demonstrate that the CRMs can be used as a reference standard for TA and to monitor the performance of titration systems at sea. Measurements made on the various legs of the cruise agreed to within 6 μmol kg−1 at the 15 crossover points. The overall mean and standard deviation of the differences at all the crossovers are 2.1±2.1 μmol kg−1. These crossover results are quite consistent with the overall reproducibility of the CRM analyses for TA (±4 μmol kg−1) over the duration of the entire survey. The TA results for the Indian Ocean cruises provide a reliable data set that when combined with TCO2 data can completely characterize the carbonate system.

Northern and southern water masses in the equatorial Atlantic: distribution of nutrients on the WOCE A6 and A7 lines

In the framework of the WOCE Hydrographic Program, two trans-Atlantic CTDO/tracer sections with closely-spaced stations, along 7°30′N and 4°30′S (WHP Lines A6 and A7), and two meridional sections, along 3°50′W and 35°W joining the two zonal sections, were occupied in January–March 1993 (CITHER 1 cruise on board the N/OL′ATALANTE). CTD profiles and nutrient (silicate, phosphate and nitrate) data at 32 depths between surface and bottom were obtained at each station. The distributions on vertical sections, and on isopycnal surfaces, of these three chemical tracers are presented and discussed in the context of large-scale circulation in the equatorial Atlantic Ocean.The nutrient fields are used to show the main components of the circulation on four main levels: near-surface, intermediate, deep and bottom layers. Near the surface the nutrient distribution pattern is dominated by westward advective flows on either side of the equator from the eastern regions enriched by coastal upwellings. Beneath the lower thermocline water, high silicate concentrations (at about 1000m depth), at a larger depth than that of the salinity minimum of the Antarctic Intermediate Water, enable a differentiation of the Upper Circumpolar Water (UCPW) from the former. In the deeper layers, the nutrient distribution confirms the bifurcation of the Deep Western Boundary Current (DWBC), carrying the North Atlantic Deep Water, at the equator into an eastward flow and another one continuing southward along the western boundary. The eastward flow of the UNADW along the equator can be traced as far as 3°50′W. The analysis of nutrient distribution on isopycnal surfaces also shows the existence of recirculation components of the DWBC in the western equatorial region. The northward succession of bottom concentrations of silicate and phosphate indicates that the flow of the Antarctic Bottom Water (AABW) exiting the Brazil Basin is topographically constrained along the equator. The higher silicate and phosphate concentrations in the near-bottom waters west of the Mid-Atlantic Ridge (MAR) and north of the equator indicate that the main flow of bottom waters is northwestward rather than through the equatorial fracture zones into the eastern Atlantic. Finally, the distributions of silicate and phosphate on near-bottom and isopycnal surfaces suggests a recirculation of AABW in the northern part of the Brazil Basin.

Deep upwelling and diffusivity in the southern central Indian Basin

Transport of the deepest water westward through a gap at 28°S in the Ninetyeast Ridge between the Central Indian Basin and the West Australia Basin is calculated from hydrographic data collected as part of WOCE Hydrographic Program section I8N. Zero reference velocity levels at mid‐depth were chosen through consideration of water masses. The small transport of 1.0 Sv westward of water denser than σ4 = 45.92 kg m−3 through the gap must all upwell in the southern Central Indian Basin. Of this, 0.7 Sv upwells between the central and western sill sections, that is, close to the sill itself. Using the areas covered by the isopycnal, we calculate an average vertical velocity of 3.3 · 10−3 cm s−1 close to the sill and of 4.2 · 10−4 cm s−1 west of the sill. Associated average vertical diffusivities are 105 cm² s−1 close to the sill and 13 cm² s−1 west of the sill, in this bottom layer.

WOCE ancillary projects, cruise schedule

The U.S. WOCE Hydrographic Program Implementation Panel (WHPIP) and the U.S. WOCE Science Steering Committee (SSC) on January 26 established the procedure for selecting ancillary projects on U.S. WOCE Hydrographic Program (WHP) cruises.WOCE is the World Ocean Circulation Experiment, an international effort to understand how the ocean circulation affects climate, to help make it possible to predict climate changes over periods of decades and to discover how human activity is influencing climate patterns (“The WOCE Hydrographic Project,” Eos, 69, 68, 1988, and “U.S. WOCE Program Ready to Begin,” Eos, 70, 874, 1989).