OS23F-01 13:40h
Circulation and Property Variability at 66$^{o}$W in the North Atlantic Ocean
One of the meridional lines occupied in 1997 as part of WOCE was 66 $^{o}$W (A22) in the North Atlantic. This section, which was the only one including measurements in the Caribbean, was again repeated in 2003 as part of a global NSF/NOAA Repeat Hydrography/Carbon resurvey of the oceans. Comparison of the recent cruise with that in 1997 shows most change in the upper ocean, with reduced dissolved oxygen and thickness of the subtropical mode water: Eighteen Degree Water. Further property changes can be seen to the north of Puerto Rico in the Deep Western Boundary Current. We will also compare the zonal circulation estimated from direct velocity measurements and an inverse model conserving mass and silica. The total mass transport and that of the North Atlantic Deep Water (NADW) is less "energetic" in 2003, with smaller transports in the major currents and their recirculations, although the total southward flow of NADW is ca. 16 Sv (1 Sv=10 $^{6}$ m $^{3}$/s) during both periods. Some interesting new features of the 2003 section are discussed, including upper level flows and deep ventilation of the Caribbean, and the horizontal structure of the deep-reaching (800m) direct flow measured in the Gulf Stream from the vessel-mounted ADCP.
OS23F-02 13:55h
Water Mass Property Variability in the Eastern Basin of the North Atlantic Ocean
A meridional hydrographic section has been occupied from south of Iceland across the equator at least three times over the past two decades. The first occupation was WOCE section A16N in July - August 1988, the second was a NOAA OACES section in July - August 1993, and the third was the first section of the Repeat Hydrography Program in support of the Carbon Cycle and CLIVAR Programs in June - August 2003. Some of the differences between the 2003 and 1988 occupations include: 1. Colder, fresher, denser, more oxygen-rich Labrador Sea Water (LSW) in 2003 compared with 1988. This change, visible between 40$\deg$N and Iceland from 1500 to 3000 dbar, reveals variability in LSW reaching to the eastern basins of the North Atlantic. LSW was more vigorously ventilated in the years leading up to 2003 than in the years prior to 1988, consistent with the water mass property changes. 2. Regional mode waters (indicated by widely spaced isotherms in the vertical starting near 7$\deg$C off Iceland and ending near 14$\deg$C by 32$\deg$N) are warmer and lighter in 2003 compared with 1988, and their associated potential vorticity minimum shifts to lighter, warmer, and shallower horizons north of about 42$\deg$N. The pattern suggests that over a few years prior to 2003 conditions were warmer, and these waters less vigorously ventilated, than prior to 1988. 3. Substantial decreases in oxygen concentration are observed from the base of these mode waters down to the oxygen minimum in 2003 compared with 1988. These changes within the oxycline, strongest from about 40$\deg$N to Iceland, are again consistent with less vigorous ventilation of the local mode waters just above in the years prior to 2003 compared with 1988.
OS23F-03 14:10h
Decadal Changes in Dissolved CFCs in the Eastern Basin of the North Atlantic
As part of the CLIVAR Global Repeat Hydrography Program, a hydrographic section was occupied in the eastern basin of North Atlantic in 2003, nominally along 20$\deg$W from 67$\deg$N to 5$\deg$S. The 2003 A16N section repeated 20$\deg$W sections occupied in 1988 and 1993. A full suite of physical and chemical measurements were made along this section. Dissolved CFC concentrations generally increased along the section between 1988 and 2003, with the largest increase occurring in Labrador Sea Water (LSW). The leading edge of the CFC transient moved progressively deeper in the water column. CFC concentrations in maxima along the equator at mid-($\sim$1800 m) and abyssal depths showed large relative increases between 1988 and 2003. Between the 2003 section and earlier occupations, significant changes in water mass properties, including temperature, salinity and dissolved oxygen, were observed in subpolar mode waters and LSW. The changes in apparent oxygen utilization (AOU) observed may be due to changes in the strength of ventilation processes in the region during the decade prior to the 2003 occupation compared to the years prior to the earlier samplings, as well as to possible changes in biological production, export and remineralization rates. Regions in the water column with large increases in apparent oxygen utilization (AOU) tended to have relatively large increases in pCFC derived apparent ages, indicating that the AOU changes may be driven in part by a slowdown in the strength of the ventilation processes. Simple numerical model simulations are used to estimate the contribution of steady-state mixing processes to the temporal trends in pCFC derived apparent age fields observed in this region.
OS23F-04 14:25h
Variable Subduction Rates of North Atlantic Salinity Maximum Water
Data from programs back to 1982, and most recently the 2003-04 Repeat Hydrography, Tracer, CO$_{2}$ program are used to highlight the role tracer data can have in monitoring decadal variability of ocean circulation. The CFC data are used to document changes in thermocline ventilation, and specifically the affect increased thermocline salinity is having, if any, on Salinity Maximum Water (SMW) subduction rates in the North Atlantic Ocean. Hydrobase data show a long term trend of increasing salinity in the North Atlantic thermocline since the 1950s, and in particular in SMW. SMW is located in the subtropical/tropical north Atlantic under the evaporation minus precipitation maximum. Since SMW are the most saline of the thermocline waters, no source for additional salt exists within the ocean. Subduction rates have been calculated for SMW from 1982-2004 CFC data at stations where there was a subsurface salinity maximum present. The subduction rate for an isopycnal projected back to its outcrop, is calculated from the inverse tracer age gradient corrected for vortex stretching. Subduction rates vary from 10 to 40 m/year. The results show a pattern of changing subduction rates that correlate well with the NAO index. Since salinity seems to have been steadily increasing, though at different rates, it appears that local fresh water variability is not the major forcing effect on the subduction process. The 1990s rise in evaporation rate coincided with a prolonged high state of the North Atlantic Oscillation index (NAO). The associated increase in trade winds will have enhanced both evaporation and Ekman pumping of waters into the upper ventilated thermocline. Consistent with model results, the observational based calculations suggest that increased (decreased) wind stress curl accompanying an increased (decreased) NAO has the major forcing effect on SMW subduction variability.
OS23F-05 14:40h
CO2/CLIVAR Repeat Hydrography Program: Decadal Changes in the Inorganic Carbon Cycle in the North Atlantic Ocean CO2/CLIVAR Repeat Hydrography Program: Decadal Changes in the Inorganic Carbon Cycle in the North Atlantic Ocean
During the summer and fall of 2003 the first three cruises of the CO2/CLIVAR Repeat Hydrography Program took place in the North Atlantic along the meridional lines A22 (65?W), A20 (50?W), and A16N (20?W). The primary objectives of the cruises were to quantify changes in ventilation and carbon inventories, including anthropogenic CO2, (Canthro) in the North Atlantic basin on a decadal timescale. The decadal evaluation is performed by comparison with the WOCE/WHP A20 and A22 cruises occupied in July/August 1997 and the NOAA/OACES cruise along 20 ?W occupied during July/August 1993. The most striking aspects of the comparisons are the significant changes in apparent oxygen utilization (AOU) and dissolved inorganic carbon (DIC) at mid-depth between 200 and 1000 m. The increases in AOU are due to a either changes in organic matter remineralization rate or changes in ventilation or a combination thereof. The locations of large AOU changes correspond to regions of large DIC increases as expected from remineralization processes. These observed changes challenge some of the assumptions used to separate the Canthro increase from natural CO2 changes in the ocean. Changes in DIC and Canthro are determined for the western basin of the North Atlantic using different calculation methods to illustrate the sensitivity of the methods to determine Canthro to natural changes in background DIC. The observations will be put in context with recent published results suggesting that the uptake of CO2 in the North Atlantic is greater than expected from the current atmospheric CO2 perturbation.
OS23F-06 14:55h
Changes in the $^{13}$C/$^{12}$C of Dissolved Inorganic Carbon in the N. Atlantic along A16N between 1993 and 2003
The $^{13}$C/$^{12}$C of dissolved inorganic carbon ($\delta$$^{13}$C-DIC) was measured at 12 stations during the Repeat Hydrography A16N section (between 6$\deg$S and 61$\deg$N along 20-25$\deg$W) in summer 2003 and compared to $\delta$$^{13}$C-DIC measurements in summer 1993 along the same cruise track. In the surface layer, the $\delta$$^{13}$C-DIC in 2003 is consistently lower by up to 0.5 $\permil$ compared to 1993, with the largest decrease occurring north of 40$\deg$N. After normalizing for changes in phosphate, there remains a consistent surface $\delta$$^{13}$C-DIC decrease of up to 0.3 $\permil$. The depth profiles of $\delta$$^{13}$C-DIC show a decrease between 1993 and 2003, typically of $<$0.2 $\permil$ in the upper 1000m, except in the tropical latitudes (south of 12$\deg$N), where the $\delta$$^{13}$C-DIC decrease is restricted to $\sim$100m. For several depth profiles, especially in the tropics and subtropics, the $\delta$$^{13}$C-DIC decrease was reduced after normalizing for changes in phosphate. The observed $\delta$$^{13}$C-DIC changes will be discussed in terms of natural variability and uptake of anthropogenic CO$_{2}$.
OS23F-07 15:10h
Decreasing trends in the dissolved oxygen content of North Atlantic thermocline waters: Possible causes and implications for the ocean carbon cycle
Investigations of thermocline waters in the Pacific and Southern Ocean have revealed substantial recent decreases in their dissolved oxygen content. Simulations with ocean biogeochemistry models suggest that some of these decreases are consistent with expected trends arising from the impact of global climate change on ocean biogeochemistry. These models simulated similar changes in the North Atlantic. We investigate this model prediction by using results from three CO$_2$/CLIVAR repeat hydrography cruises in the North Atlantic (A16N, A20, A22) that permit us to investigate changes in thermocline oxygen concentrations between the early to mid 1990-ties and 2004 in the entire North Atlantic basin. Preliminary analyses in the eastern basin show a strong decrease of the oxygen concentration and corresponding increase in the apparent oxygen utilization (AOU) in the lower thermocline north of about 35$^{\circ}$N (sigma-theta about 27.0 to 27.5), with changes of up to 40 $\mu$mol kg$^{-1}$ within 10 years. The affected regions are not associated with a particular water mass, but extend from just below recently ventilated waters towards the interior ocean oxgyen minimum. We propose that the primary reason for this synchronous increase in AOU across a wide density range is a recent change in ocean circulation that has decreased the ventilation of all isopycnals outcropping in the northern North Atlantic with densities between about sigma-theta 27.0 and 27.5. Adopting an oxygen utilization rate of about 5 to 10 $\mu$mol kg$^{-1}$ yr$^{-1}$, we estimate an increase in the apparent ventilation age of several years, amounting in some case to a near doubling of this age. We suspect that a large-scale change in North Atlantic climate, perhaps associated with the decadal trend in the North Atlantic Oscillation, must have been at the root of these changes in thermocline oxygen. We will present results from an extended study that includes changes in the western basin and discuss the impact of these findings on our understanding of ocean biogeochemistry and the carbon cycle in greater detail during the presentation.
http://ushydro.ucsd.edu/
OS23F-08 INVITED 15:25h
Sections of Dissolved Iron and Aluminum in the North Atlantic and the North Pacific: Results from the CLIVAR Repeat Hydrography A16N and P02 Expeditions
Vertical profiles for dissolved Fe and Al were collected from the 2003 A16N and 2004 P02 expeditions (62 profiles and 76 profiles, respectively). Dissolved Al and Fe were determined by shipboard FIA (and isotope dilution ICP-MS for Fe) on samples collected from 12 depths in the upper 1000 m at each station. Contour plots of the distributions reveal new hydrographic features for both elements that are well resolved by the approximately 1 degree station spacing. Surface water distributions in the North Atlantic show significant inputs of both elements from the deposition of Saharan dust in the equatorial Atlantic. Sub-surface Al enhancement is observed in the North Atlantic "mode" water between 20N and 35N and in the Mediterranean outflow water. Elevated dissolved Fe concentrations (greater than 2nM) are observed between 20N and 5N beneath the Saharan dust plume and are coincident with the nitrate maximum underlying the high productivity upwelling zone off NW Africa. The North Pacific "mode" water is also labeled with dissolved Al, while dissolved Fe concentrations are highest in the western North Pacific as a result of Asian dust input. Surface water dissolved Fe is slightly elevated across most of the P02 section, but decreases to less than 50 pM from 60-150 meters before increasing again with depth due to nutrient-type regeneration. Nitrate:Fe ratios are low in surface waters across most of both sections, but increase to greater than 15,000 mol-N/mol-Fe below 200 meters. When these waters return to the surface via upwelling, Fe should be the limiting nutrient relative to nitrate.