OS31A-0544 0800h
Variability of Antarctic Bottom Water Flow into the North Atlantic
The flow of Antarctic Bottom Water from the Brazil Basin in the South Atlantic into the Guiana Basin in the western North Atlantic has been measured by current meters for 44 months. Flow is confined by a relatively flat 4500 m deep passage between 1$^\circ$ S and 4$^\circ$ N with complex sidewall shape. At the southern end is a 300 km wide zonal channel straddling the equator between 32$^\circ$ W and 38$^\circ$ W with approximately a 500 m deep layer of bottom water. The deep Antarctic Bottom Water current into the North Atlantic is confined to a region south of the equator and the direction is westward. Previous measurements of this deep current over 600 days in duration (Hall et al., 1997) indicated both a small warming trend and a decreasing volume flux. New data presented in this paper of 1388 days duration show no evidence for continuation of such trends. The average temperature and yearly averaged velocities are about the same as earlier at revisited locations. The long-term drift of the deep Antarctic Bottom Water temperature is of the order of -0.002 $^\circ$C/ year. Average velocity is 0.0474 m/s westward and slope is 0.0015 m/s/y. The greatest variability is found near the top of the deep Antarctic Bottom Water at the northern edge of the current. The spectrum of the new 44-month data reveals a distinct yearly peak. We conclude that there has been no change in Antarctic Bottom Water flux into the North Atlantic during the past decade.
OS31A-0545 0800h
An Updated Anthropogenic CO$_{2}$Inventory in the Atlantic Ocean
We present a comprehensive analysis of the basin-wide inventory of anthropogenic CO$_{2}$ in the Atlantic Ocean based on high-quality inorganic carbon, alkalinity, chlorofluorocarbon, and nutrient data collected during the WOCE/JGOFS/OACES surveys of the Atlantic Ocean between 1990 and 1998. Anthropogenic CO$_{2}$ was separated from the large pool of dissolved inorganic carbon using an extended version of the $\Delta$C$^{*}$ method originally developed by Gruber et al. [1996]. The extension of the method includes the use of an optimum multiparameter analysis to determine the relative contributions from various source water types to the sample on an isopycnal surface. The anthropogenic CO$_{2}$ distribution in the Atlantic Ocean determined in the present analysis displays various features that are consistent with those found in the Indian Ocean and Pacific Ocean, most notably high column inventories in the mid-latitude regions and lower column inventories in the Southern Ocean. However, in contrast to other basins, the Atlantic Ocean was found to have high column inventories at latitudes $>$$40\deg$N, largely due to the effective transport of anthropogenic CO$_{2}$ by newly formed deep water transported southward in the Deep Western Boundary Current. Analysis of the data collected during the 1990-1998 period yielded a total anthropogenic CO$_{2}$ inventory of $28.4 \pm$4.7 Pg C in the North Atlantic (equator-$70\deg$N) and of $18.5 \pm$3.9 Pg C in the South Atlantic (equator-$70\deg$S).
OS31A-0546 0800h
Increase of Anthropogenic CO$_2$ in the Atlantic Ocean in the Last Two Decades
The inorganic carbon data collected during the GEOSECS program in early 1970s represent results of the first systematic survey of total dissolved inorganic carbon (DIC) and alkalinity on a global scale. These data sets have become a critical component of studies of decadal variations of geochemical properties over the last thirty years. In particular, the oceanic uptake of anthropogenic CO$_2$ can be quantified from the increase in concentration of total dissolved inorganic carbon. It is essential that the historic data set be devoid of bias compared to recent high quality carbon data. The recent data has benefited from improvements in measuring techniques and the use of reference material. Results of re-evaluation of GEOSECS carbon data in the Atlantic Ocean by examining deep water properties at crossover stations between GEOSECS and WOCE cruises show that DIC measurements made in GEOSECS program are systematically higher than those made during recent WOCE/DOE global CO$_2$ survey. In the North Atlantic, the overall average offset is estimated to be 27 $\pm$ 9 umol/kg for DIC north of 15$^o$N. In the equatorial region between 15$^o$N and 15$^o$S, the mean DIC offset is 9 $\pm$ 11 umol/kg, and in the South Atlantic is 5 $\pm$ 5 umol/kg south of 15$^o$S. Applying the Multi-parameter Linear Regression method (MLR) to WOCE carbon data, DIC in the three latitudinal regions and separating the Western and Eastern Basins in the Atlantic Ocean is derived as a function of potential temperature, salinity, AOU, Silica and PO$_4$. After correcting GEOSECS DIC data, a residual DIC is computed between adjusted DIC and predicted DIC using the DIC equations derived from WOCE carbon data. The estimates of the increase in anthropogenic CO$_2$ based on these residual DIC indicates that the mean CO$_2$ uptake rate in the west basin is 0.70 mol/m$^2$/yr for the region north of 15$^o$N, 0.53 mol/m$^2$/yr for the equatorial region, and 0.83 mol/m$^2$/yr in the south Atlantic. In the east basin, the mean CO2 uptake rate is estimated to be 0.97 mol/m2/yr for the north Atlantic north of 15$^o$N, 0.57 mol/m$^2$/yr for the equatorial region, and 0.28 mol/m$^2$/yr for the south Atlantic south of 15$^o$S. The uncertainty of these estimates is high due to insufficient GEOSECS data and the quality of these background data is not as good as recent WOCE data.
OS31A-0547 0800h
Decadal Variations of a Long-Term Carbon Sink in the Labrador Sea
Deep convection in the Labrador Sea ventilates the water column every winter. Depths of deep convection are influenced by the North Atlantic Oscillation (NAO). During the NAO positive phase, convection reaches over 2000m as observed in early 1990's, while during the NAO negative phase, convection becomes shallower to 500-1000m. Convection regimes in the Labrador Sea are, therefore, in the opposite phase from deep the convection in the Nordic Seas and the formation of subtropical mode water. Carbon dioxide sequestered during the winter convection in the Labrador Sea is partly transported to the south as Deep Western Boundary Current, while some spreads at the intermediate depths into the North Atlantic sub-polar gyre. Thus the Labrador Sea provides a conduit for a long-term storage of atmospheric carbon dioxide on time scales of centuries to millennium. We will present a time series study of Dissolved Inorganic Carbon (DIC), Alkalinity (Alk) and auxiliary measurements along the Labrador Sea repeat section from 1993 to 2004. Water masses in the Labrador Sea can be described by four layers from the surface to the bottom: namely, (1) new Labrador Sea Water (LSWnew) ventilated in the previous winter, (2) Labrador Sea Water produced during the exceptionally deep convection period of 1993-1994 (LSWold), (3) North East Atlantic Deep Water (NEADW) and (4) Denmark Strait Overflow Water (DSOW). All these water masses were formed in deep convection regions, LSWnew and LSWold in the Labrador Sea and NEADW and DSOW in the Nordic Seas. DIC has increased in all four water masses since 1993 with the highest rate of increase in LSWnew, corresponding to the atmospheric CO2 increase. LSWold has been isolated from the atmosphere since 1994 and accumulating DIC from the respired organic carbon. In the latter half of 1990's, during the period of shallow convection (<1000m), DIC concentrations were constantly higher in LSWold than LSWnew. However, in 2000, convection reached 1500m and DIC concentrations in LSWnew increased to the same levels as in LSWold. The inventory of DIC in the Labrador Sea was estimated to be 106 (S.D.=14) PgC from the basin wide survey of over 100 stations in 1996. The average inventory increase is 0.03 PgC/year for the period from 1993 to 2004. A sharp inventory increase in 2000 suggests the higher uptake of atmospheric CO2 during the deep convection period despite the entrainment of abundant DIC from the depth to the surface.
OS31A-0548 0800h
The Advective Nutricline: The Impact of Subtropical Mode Water on the North Atlantic Subsurface Nutrient Gradient and Primary Productivity
The ocean's ability to act as a sink for atmospheric carbon dioxide is subject to considerable spatial and temporal variability. Not least of the causes of this variability is the strength of the biological pump, which is critically dependent on the physical supply of nutrients from the subsurface ocean to the euphotic zone. Using hydrographic data from the World Ocean Circulation Experiment (WOCE), the NODC historical data, and time series data from Hydrostation S and Bermuda Atlantic Time Series, we have found that the North Atlantic Subtropical Mode Water (STMW) is a source of spatial and temporal variability in the vertical nutrient gradient in the subtropical gyre. The STMW essentially injects low nutrient water between the seasonal and permanent pycnocline. STMW is likely depleted in nutrients by biological uptake and the convective mixing of zero-nutrient surface water with subsurface water during formation. After formation, the water mass is advected around the subtropical gyre and spread along isopycnals. It can be characterized by low potential vorticity and low nutrient concentrations up to 1500 km from the formation region, suggesting that in the region of tight subtropical recirculation, the advective time scale is faster than the remineralization time scale. The weak vertical nutrient gradient imposed by the STMW impacts all vertical processes that supply nutrients to the euphotic zone. Using ship based primary productivity data and satellite observations of sea surface chlorophyll, we also examine the hypothesis that the presence of STMW influences rates of primary productivity and the size and location of the low productivity region in the subtropical North Atlantic.
OS31A-0549 0800h
Stocks and Distribution of Carbohydrates and the Bioreactivity of DOC Along Meridional Transects in the North Atlantic Basin
Oceanic dissolved organic carbon (DOC) demonstrates a broad range of bioreactivity with turnover rates ranging from seconds to millennia. While the majority of dissolved organic matter remains uncharacterized through current chemical analyses, the ratio of total hydrolyzable carbohydrate (TCHO) contribution to bulk DOC does provide insight into the bioreactivity of DOC (Cowie and Hedges, 1994). Here we present a high spatial resolution data set of TCHO, dissolved combined neutral sugars (DCNS) and the ratio of TCHO and DCNS to the bulk DOC pool over the surface 1000 m. The samples were collected during two meridional transects of the U.S. Repeat Hydrography Program conducted in the North Atlantic during the autumn of 2003. Vertically, the TCHO concentrations were highest in the surface 50 m and decreased over 1000 m. Meridionally, TCHO were most concentrated in subtropical and tropical latitude surface waters (15-19 $\mu$M C) yet the ratio of TCHO:DOC in these waters decreased from 26% in the subtropical latitudes to $<$ 21% in more permanently stratified regions. Such a trend indicates an overall decrease in the degree of bioreactivity of bulk DOC from 35$\deg$ N to the south concomitant with increased stratification. Interestingly, the highest levels of bacterial productivity did not coincide with the regions of high TCHO:DOC. Because TCHO pool consists of hydrolyzed material that is both labile and recalcitrant in nature, use of the TCHO:DOC as an index bioreactivity reveals that the TCHO portion of the DOC pool is reactive on longer time scales than those used to support rapid and instantaneous heterotrophic bacterial production.
OS31A-0550 0800h
Dissolved Organic Carbon and Nitrogen Distributions in the North Atlantic Basin
The U.S. Repeat Hydrography program was instituted to sustain sampling of ocean transports and inventories of climatically significant parameters on decadal time scales. Each ocean basin will be occupied with at least one meridional section to determine inventories and gradients of carbon and tracers. In 2003 DOM was included as a level one measurement for this set of cruises. Paucity of high spatial resolution DOM data has resulted in a limited understanding of the stocks, distributions and processes that control these patterns throughout the World oceans. Here we report DOC and DON data collected from three meridional transects (A16N, A20 and A22 cruises) in the North Atlantic in 2003. The data set displays a first ever high-resolution, basin scale view DOC and DON distribution in the North Atlantic. The findings are at once both consistent and inconsistent with our expectations. While the data displayed incredible information on the export of DOC with the formation of North Atlantic Deep Water, we also observe a larger but systematic variability in DOM concentrations. This DOM variability is consistent with other tracers within the interior of the North Atlantic. The use of certified reference material allowed us to confidently assess low-level variability in DOC concentrations. At depths deeper than 2000 m, DOC concentrations ranged from 48 $\mu$M in waters characteristic of NADW to as low as 39 $\mu$M in waters characteristic of AABW. We also observed a low DOC tongue (i.e. 39 to 42 $\mu$m) extending from South America to 30$\deg$N along 51$\deg$ W, consistent with observations of AAIW. These expansive data sets are already changing our understanding DOM distribution and the processes that control DOM transport in the ocean interior. These DOM data will serve as a base line to be compared with future measurements and will be used to assess temporal as well as spatial trends of this important carbon variable.
OS31A-0551 0800h
Distribution of chromophoric dissolved organic matter in the North Atlantic: Implications for biogeochemistry and tracer studies
Chromophoric dissolved organic material (CDOM), the colored fraction of the dissolved organic material (DOM) pool, is a highly dynamic property found throughout the open ocean. Most CDOM found away from continental margins is of open ocean origin and is not derived from riverine input or coastal runoff. CDOM is an optical property; hence, its concentration can be easily detected over large spatial scales using (for example) satellite-borne sensors. Net CDOM production is related to heterotrophic bacterial cycling processes while its losses are due to photobleaching. Vertical distributions of CDOM and in particular its surface signature are therefore regulated by vertical mixing processes in conjunction with these basic CDOM cycling processes. These properties make CDOM a potential ocean circulation tracer whose surface expression can be constrained globally by ocean color data. We are participating in selected CO2/CLIVAR Repeat Hydrography program cruises in order to collect an unprecedented baseline ocean CDOM dataset, to evaluate biological and dynamic processes controlling CDOM, and to evaluate the use of CDOM as an ocean tracer. Initial results from the North Atlantic A16N, A20, and A22 sections show that CDOM abundance (evaluated as optical absorption coefficient) tracks major oceanographic features such as the subtropical mode water and Gulf Stream recirculation. CDOM also has a slight inverse relationship with overall dissolved organic carbon (DOC) concentration. CDOM abundances also appear to correlate to distributions of transient and conserved tracers. These results are consistent with our findings concerning CDOM cycling from our ongoing study at the Bermuda Atlantic Time-series Study (BATS) station, and encourage further application of CDOM to tracer problems.
http://www.icess.ucsb.edu/GlobalCDOM/
OS31A-0552 0800h
The 1997-99 abrupt change of the upper ocean temperature in the northcentral Pacific
The abrupt warming of the northcentral Pacific Ocean from 1997 to 1999 is studied using an ocean data assimilation product. During this period, the average mixed-layer temperature in the region of 170-210E, 25-40N rises by 1.8K. The major contributors to the warming are surface heat flux (1.3K), geostrophic advection (0.7K) and entrainment (0.7K). For the geostrophic advection, the contributions by the zonal, meridional, and vertical components are 0.4, -0.1 and 0.3K, respectively. Mixing and meridional Ekman advection cool the domain. The significance of the geostrophic advection indicates the importance of ocean dynamics in controlling the abrupt warming tendency during the 1997-99 period and the inadequacy of a slab mixed-layer model in simulating such warming tendency. To the extent that this event reflects a phase switch of Pacific decadal variability, ocean dynamics cannot be ignored in studying such variability.
OS31A-0553 0800h
Interannual Variability of Chlorofluorocarbons, pCFC Ages and Ideal Ages in the North Pacific from 1958-2000 as Simulated by an Ocean General Circulation Model
Chlorofluorocarbon (CFCs) concentrations and ideal age are simulated in a global integration of the Parallel Ocean Program (POP) with realistic surface forcing derived from the NCEP/NCAR reanalysis (1958-2000). The present study focuses on the subtropical and subpolar thermocline of the North Pacific where the simulated CFC concentration fields are in reasonably good agreement with WOCE observations. Tracer ventilation ages are computed from the simulation using the CFC partial pressure (pCFC) and CFC-11/CFC-12 ratio approaches. Biases arise in pCFC ages due to the interaction of mixing with the non-linear CFC atmospheric time-histories, and the model tracer ages significantly underestimate ideal age in the mid to lower thermocline and in the shadow zone of the eastern tropics/subtropics. The spatial patterns in the pCFC age errors are consistent with earlier studies using more idealized models, but the magnitude of the error is considerably larger, with the tracer/ideal age ratio approaching a factor of 5-7 compared to 1.5-2 as estimated perviously. Variability in pCFC ages is well correlated with variability of ideal age on interannual timescales, though the amplitude of the pCFC variability may be substantially less due to the offsets in the tracer/ideal age. For ideal ages greater than 15-20 years, age variability is also well correlated with that of potential vorticity. The significant interannual variability in tracer ages and longer-term temporal trends in the tracer age biases complicate the interpretation of CFC data collected from repeat occupations on 5 to 10 year intervals as currently envisioned by CLIVAR. In particular, the tracer age differences from the instantaneous samples are likely not representative, at least directly, of interdecadal variability in ocean ventilation.
OS31A-0554 0800h
Revisit of WHP P17N in 2001
During the WHP P17N revisit cruise by R/V Mirai in 2001 summer, we carried out hydrographic observations in the eastern North Pacific from 30 to 55 N degrees. A total of 78 CTDO/rosette stations were conducted and more than 3,500 seawater samples for salinity, oxygen, phosphate, silicate, nitrate, nitrite, dissolved inorganic carbon (DIC), pH, and alkalinity were measured on board. Additionally, vertical profiles of radiocarbon and carbon-13 of DIC were also measured at 21 stations (about 700 samples). Comparison of hydrographic data between the first visit in the early 1990s (P17N in 1993 and P17C in 1991) and the re-visit elucidated temporal changes of intermediate water properties in the eastern North Pacific in the 1990s. From the subtropical region to the Alaskan Gyre, apparent oxygen utilization (AOU) in intermediate layers (26 - 27 sigma-theta) increased by up to 60 micro-mol/kg with increased nutrients and DIC concentrations. Patchy AOU increases in the subtropical region (30 - 40 N degrees) suggest that high AOU water was brought by mesoscale eddies from the east, off of California, where oxygen concentration is low. Between 41 and 53 N degrees, a tongue of AOU increase was measured around 26.6 sigma-theta isopycnal. This AOU increase is in agreement with previous studies showing AOU and CFC age increases along 152 W degrees from 30 to 45 N degrees between 1991 and 1997 (Emerson et al., 2001) and along 47 N degrees from 160 E to 145 W degrees between 1985 and 1999 (Watanabe et al., 2001). Furthermore, the AOU increase was larger in the Alaskan Gyre from 44 to 53 N degrees and was associated with intensification of stratification. These results suggest that ventilation rate of intermediate water slowed down in the subpolar and the subpolar/subtropical transition regions of the eastern North Pacific during the 1990s. Measured changes of the carbon-13 and radiocarbon of DIC also imply stagnation of the subpolar intermediate water.
OS31A-0555 0800h
Nutrients Variability in the Subtropical Gyres in the Southern Hemisphere
Nutrients analyses were accomplished on 16,000 samples together with the reference material of nutrients in seawater (RMNS) during R/V Mirai cruise which revisited WHP P06, A10, I03 and I04 in the Southern Hemisphere in 2003/2004. The target nutrients are nitrate, nitrite, phosphate and silicate, respectively. To establish the traceability of nutrients concentrations and to get higher quality data, we used 500 bottles of RMNS in a same lot and 150 sets of RMNSs involving 6 levels of RMNSs of which nutrients concentrations ranged from lower concentration as surface water to higher concentrations exceed the maximum concentrations of nutrients. The precisions of phosphate, nitrate and silicate measurements were 0.18%, 0.17% and 0.16% in terms of median of those at 493 stations, respectively. The nutrients concentrations could be expressed with uncertainties explicitly as a function of nutrients concentration because of the repeated runs of RMNSs. Preliminary comparisons with previous WOCE cruises in 1990s revealed that systematic decrease of nutrients concentrations occupies a layer 100-1000 m in the wide area in the subtropical gyres, in which lower upwelling rate of nutrients rich water are suggested. Although lower lateral transport of the nutrients should be also considered, lower nutrients flux into nutrient-depleted layers might occurs in the regions anyway. In the deeper layers below approximately 3000 m, nutrients data also suggest significant variations at the western part of the Atlantic Ocean. In this talk, we will focus on the global view of the changes of nutrients in the subtropical gyres in the Southern Hemisphere and discuss them with the analytical uncertainty of nutrient concentrations.
OS31A-0556 0800h
CLIVAR/CO2 Repeat Hydrography Program: Carbon Measurements along 30°N in the North Pacific during June-August 2004
Distributions of dissolved inorganic carbon (DIC) were determined throughout the water column at 190 stations during the CLIVAR/CO$_2$ Repeat Hydrography Program 30$^\circ$N P2 Cruise between June and August of 2004 onboard the R/V {\it Melville}. The DIC measurements were made with an overall accuracy and precision of approximately 1.5 $\mu$moles kg$^{-1}$. We compared our cruise results with the previous WOCE cruise data collected along the same cruise track in 1994 using a number of different fitting routines. Based on our results, we recommend that a 4 $\mu$mol kg$^{-1}$ downward adjustment be made to the 1994 P2 DIC data. The 2004 DIC results show evidence for deep mixing in the western subtropical gyre, particularly within the region of Kuroshio Extension. The combined effect of the tilted density surfaces and the deeper ventilation leads to lower DIC concentrations at intermediate depths between 200 and 700~m west of 180$^\circ$. DIC values east of 180$^\circ$ in the upper 1000~m shoal due to upwelling processes. As we continue to process the physical and biogeochemical results from these and future cruises, we will be able to determine the large-scale changes in the carbon content of the Pacific Ocean.
http://www.pmel.noaa.gov/co2/co2-home.html
OS31A-0557 0800h
Decadal Variations in Ecosystem, Biogeochemistry and Carbon Cycle in the Equatorial Pacific: A Model Study
The equatorial Pacific Ocean is known to undergo significant physical, biological, and chemical changes on seasonal (e.g., upwelling), interannual (e.g., El Ni\~{n}o and La Ni\~{n}a), and decadal time scales (e.g., the Pacific Decadal Oscillation). We undertook a study to investigate special and temporal variations in ecosystem, biogeochemistry, and carbon cycle using a basin-scale 3-dimensional physical-ecosystem-carbon model forced by 6-day mean surface wind-stresses from the NCEP. The model is able to capture the surface chlorophyll variability observed in the SeaWiFS, showing significant seasonal-to-interannual variability in biogeochemical fields. Moreover, simulations indicate pronounced decadal variations and regime shift in the high-nitrate low-chlorophyll (HNLC) region of the equatorial Pacific over the past 50 years. The largest decadal variations in the mixed layer depth (MLD) were found in the central equatorial Pacific where mid-year MLDs vary from approximately 60 m prior to 1970 to about 40 m post 1980, which results in weakening of nutrient supply through entrainment during the period of 1980-2000. The model simulated remarkable increase in zooplankton biomass but slight decrease in phytoplankton biomass in surface waters of the central/eastern equatorial Pacific after the 1976/7 phase shift. Modeled primary productivity and new production declined by approximately 5 mmol C m$^{-2}$ d$^{-1}$ during 1980-2000 relative to 1955-1975 in the HNLC. While delta-pCO$_{2}$ increased in most parts of region after the shift, sea-air CO$_{2}$ flux dropped in the east but increased in the west of $150\deg$W after the shift.
OS31A-0558 0800h
The warming of the California Current System: Dynamics and ecosystem implications
Long-term changes in the observed temperature and salinity along the Southern California coast are studied using a four-dimensional space-time analysis of the 52-year (1949-2000) California Cooperative Oceanic Fisheries Investigations (CalCOFI) hydrography combined with a sensitivity analysis of an eddy permitting primitive equation ocean model under various forcing scenarios. An overall warming trend of 1.3 C in the ocean surface, a deepening in the depth of the mean thermocline (18 m) and increased stratification between 1950 and 1999 are found to be primarily forced by large-scale decadal fluctuations in surface heat fluxes combined with horizontal advection by the mean currents. After 1998 the surface heat fluxes suggest the beginning of a period of cooling, consistent with colder observed ocean temperatures. Salinity changes are decoupled from temperature and appear to be controlled locally in the coastal ocean by horizontal advection by anomalous currents. A cooling trend of -0.5C in SST is driven in the ocean model by the 50 year NCEP wind reanalysis, which contains a positive trend in upwelling favorable winds along the Southern California Coast. A net warming trend of +1C in SST occurs, however, when the effects of observed surface heat fluxes are included as forcing functions in the model. Within 50 to 100 km of the coast, the ocean model simulations show that increased stratification/deepening of the thermocline associated with the warming reduces the efficiency of coastal upwelling in advecting subsurface waters to the ocean surface, counteracting any effects of the increased strength of the upwelling winds. Such a reduction in upwelling efficiency leads in the model to a freshening of surface coastal waters. Because salinity and nutrients at the coast have similar distributions this must reflect a reduction of the nutrient supply at the coast, which is manifestly important in explaining the observed decline in zooplankton concentration. The increased winds also drive an intensification of the mean currents of the Southern California Current System (SCCS). Model mesoscale eddy variance significantly increases in recent decades in response to both the stronger upwelling winds and the warmer upper ocean temperatures, suggesting that the stability properties of the SCCS have also changed.
OS31A-0559 0800h
Decadal weakening of the shallow overturning circulation in the South Indian Ocean
Analysis of satellite measurements of wind stress and sea level suggests (1) a near-decadal slowdown of the shallow overturning cell in the South Indian Ocean (SIO) associated with a significant weakening of the southeasterly trade during 1992-2000, and (2) a relatively steady cross-equatorial shallow overturning cell. The slowdown of the SIO cell, estimated to be 6.8 Sv over the 9 year period, is about 70% its averaged strength (about 10 Sv). The results have important implications to upper-ocean heat content, decadal climate variability, and biochemistry in the region.
OS31A-0560 0800h
Transport and high silica of the South Equatorial Current in the Indian Ocean: the deep Indonesian throughflow
In the Indian Ocean, the narrow westward flow of the South Equatorial Current (SEC) carries waters from the Pacific modified through mixing in the Indonesian Seas. The maximum density of these waters in the eastern Indian Ocean corresponds to the sill density at Leti Strait and Ombai Straits and spreads to higher densities as it is diluted towards the west. The high silica of the Indonesian Intermediate Water is traced to high silica within the Banda Sea. High silica in the Banda Sea is mainly due to diapycnal mixing of Pacific waters that enter through Lifamatola Strait and possible local sources that are assumed to be hydrothermal in origin. The transport of the SEC between densities sigma1 = 31.5 to 32.2 (neutral density 27.12 to 27.76) is 3 to 7 Sv westward, yielding a total ITF transport of about 15 Sv, which corresponds to the Pacific inflow to the Indonesian seas at Makassar and Lifamatola Straits.
OS31A-0561 0800h
Extending Observational Volume and Heat Transport Records of the Indonesian Throughflow Using Repeat Expendable Bathythermograph (XBT) Lines
The Indonesian Throughflow (ITF) is the only tropical link between ocean basins on the planet, carrying roughly 10\times 10$^{6}$ m$^{3}$/s (10 Sv) of thermocline water from the Pacific Ocean to the Indian Ocean. The ITF varies on interannual and decadal time scales, forced by ENSO, the Asian monsoons and interannual climate variability in the Indian Ocean basin. To date, direct current measurements of the ITF have been limited to two years duration, which is not enough to fully describe this Pacific-to-Indian connection on long-period scales. Recent measurements taken from December 1996 to July 1998 in the Makassar Strait of the Indonesian seas provide a detailed vertical and temporal picture of the ITF through its main channel, but only provide 1.7 years of record. Temperature data from a repeat expendable bathythermograph (XBT) program have been collected between Shark Bay, Australia and Sunda Strait between Sumatra and Java, Indonesia since 1982. This repeat line is perpendicular to the path of ITF outflow into the Indian Ocean South Equatorial Current at 12°S, allowing for geostrophic calculations of ITF volume transport into the Indian Ocean. A geostrophic transport time series is constructed from the Australia-Indonesia repeat XBT line and compared to the Makassar time series, finding excellent correlations for the 1.7-year duration of the Makassar series. It is inferred that the repeat XBT line may serve as an appropriate proxy record for interior ITF flow, extending the observational record of the ITF for at least two decades. The twenty-year record of ITF flow is most strongly influenced by ENSO, but is significantly influenced by other climatic process.
http://www.earthinstitute.columbia.edu/flash/itf2004.html
OS31A-0562 0800h
Interannual Variability in Upper-Ocean Heat Content, Temperature and Thermosteric Expansion on Global Scales
Satellite altimetric height was combined with approximately 1,000,000 in situ temperature profiles to produce global estimates of upper-ocean heat content, temperature and thermosteric sea-level variability on interannual time scales. Maps of these quantities from mid-1993 through mid-2003 were calculated using the technique developed by {\it Willis et al.} [2003]. The time series of globally averaged heat content contains some interannual variability and implies an oceanic warming rate of 0.86 $\pm$ 0.12 Watts per square meter of ocean (0.29 $\pm$ 0.04 pW) from 1993 to 2003 for the upper 750 m of the water column. As a result of the warming, thermosteric sea level rose at a rate of 1.6 $\pm$ 0.3 mm/yr over the same time period. Maps of yearly heat content anomaly show patterns of warming commensurate with ENSO variability in the tropics, but also show that a large part of the trend in global, oceanic heat content is caused by regional warming at mid-latitudes in the Southern Hemisphere. In addition to quantifying interannual variability on a global scale, this work illustrates the importance of maintaining continuously updated monitoring systems that provide global coverage of the worlds oceans. Ongoing projects, such as the Jason/TOPEX series of satellite altimeters and the Argo float program provide a critical foundation for characterizing variability on regional, basin, and global scales and quantifying the ocean's role as part of the climate system.
OS31A-0563 0800h
Cruise Summary of WHP P6, A10, I3 and I4 Revisits in 2003
Japan Agency for Marin-Earth Science and Technology (JAMSTEC) conducted a research cruise to round in the southern hemisphere by R/V Mirai. In this presentation, we introduce an outline of the cruise and data quality obtained during the cruise. The cruise started on Aug. 3, 2003 in Brisbane, Australia and sailed eastward until it reached Fremantle, Australia on Feb. 19, 2004. It contained six legs and legs 1, 2, 4 and 5 were revisits of WOCE Hydrographic Program (WHP) sections P6W, P6E, A10 and I3/I4, respectively. The sections consisted of about 500 hydrographic stations in total. On each station, CTD profiles and up to 36 water samples by 12L Niskin-X bottles were taken from the surface to within 10 m of the bottom. Water samples were analyzed at every station for salinity, dissolved oxygen (DO), and nutrients and at alternate stations for concentration of freons, dissolved inorganic carbon (CT), total alkalinity (AT), pH, and so on. Approximately 17,000 samples were obtained for salinity. The standard seawater was measured repeatedly to estimate the uncertainty caused by the setting and stability of the salinometer. The standard deviation of 699 repeated runs of standard seawater was 0.0002 in salinity. Replicate samples, which are a pair of samples drawn from the same Niskin bottle to different sample bottles, were taken to evaluate the overall uncertainty. The standard deviation of absolute differences of 2,769 replicates was also 0.0002 in salinity. For DO, about 13,400 samples were obtained. The analysis was made by a photometric titration technique. The reproducibility estimated from the absolute standard deviation of 1,625 replicates was about 0.09 umol/kg. CTD temperature was calibrated against a deep ocean standards thermometer (SBE35) which was attached to the CTD using a polynomial expression Tcal = T - (a +b*P + c*t), where Tcal is calibrated temperature, T is CTD temperature, P is CTD pressure and t is time. Calibration coefficients, a, b and c, were determined for each station by minimizing the sum of absolute deviation from SBE35 temperature below 2,000dbar. CTD salinity and DO were fitted to values obtained by sampled water analysis using similar polynomials. These corrections yielded deviations of about 0.0002 K in temperature, 0.0003 in salinity and 0.6 umol/kg in DO. Nutrients analyses were accomplished on 16,000 samples using the reference material of nutrients in seawater (RMNS). To establish the traceability and to get higher quality data, 500 bottles of RMNS from the same lot and 150 sets of RMNSs were used. The precisions of phosphate, nitrate and silicate measurements were 0.18 %, 0.17 % and 0.16 % in terms of median of those at 493 stations, respectively. The nutrients concentrations could be expressed with uncertainties explicitly because of the repeated runs of RMNSs. All the analyses for the CO${2}$-system parameters in water columns were finished onboard. Analytical precisions of CT, AT and pH were estimated to be \sim1.0 umol/kg, \sim2.0 umol/kg, and \sim7*10$^{-4}$ pH unit, respectively. Approximately 6,300 samples were obtained for CFC-11 and CFC-12. The concentrations were determined with an electron capture detector - gas chromatograph (ECD-GC) attached the purge and trapping system. The reproducibility estimated from the absolute standard deviation of 365 replicates was less than 1% with respect to the surface concentrations.
OS31A-0564 0800h
Descriptions for CFCs distributions observed on the revisit of WHP A10
Dissolved chlorofluorocarbons (here after CFCs) were measured along the WHP A10 in November 2003 by R/V Mirai cruise (MR03-K04 Leg 4). We describe the feature of these distributions and compare these in 1993 and in 2003. Maximum concentrations of CFC-11 and 12 were found around 300 - 400 m depth and were up to 3.7 pmol/kg and 1.8 pmol/kg, respectively. The maxima increase around 1.3 times of that of the previous results and deepen around 200 m deeper. These would reflect the increase in the mixing ratios of CFCs in the Atmosphere. However, the penetration depths of the CFCs were approximately 1500 m depth which is close to previous data. We detected the significant concentrations of CFCs in the deep water (below the 4000 db) at Hunter channel and at Cape Basin in our observation but could not found them in previous data. The concentrations were around 0.06 and 0.03 pmol/kg for CFC-11 and CFC-12, respectively. These mean the water that the age is younger than 1950s has reached these areas within recent 10 years.
OS31A-0565 0800h
Deacadal variations in hydrographic properties of Circumpolar Deep Water: revisit of WHP P6, A10, I3 and I4.
We compare potential temperature, salinity and dissolved oxygen (DO) along WHP-P6, A10, I3 and I4 sections observed in 2003 (BEAGLE2003) with those in the early 1990s (WOCE), focusing on inflows of the Circumpolar Deep Water (CDW) to the Pacific, Atlantic and Indian Oceans. Potential temperatures in the bottom and/or deep layers became higher by $0.01\deg$C or more in 2003 in the Brazil, Wharton and Southwest Basins, where the CDW enters each ocean without major modification. The near bottom salinities did not show common changes among the basins. Intensive analyses in the Southwest Pacific Basin revealed that potential temperatures in CDW increased by 0.01-$0.02\deg$C, while salinity and DO decreased by 0.004 and 3 $\mu$mol/kg, respectively, at maximum. The NADW salinity maximum core of S $>$ 34.72, which was lying at 3000 - 4000 m from the deep western boundary to the east of the Louisville Seamount Chain previously (1991-1992), shrank by half to the west of the chain. To clarify the cause of these changes, we compared the $\theta$-S and $\theta$-DO plots of P6-rivisit with those of P15S (in 1996). The P15S is a meridional section crossing P6 at $32.5\deg$S, $170\deg$W and covers a northward flow of the CDW toward the central Pacific Ocean. It was found that around $32.5\deg$S, $170\deg$W in 2003, the $\theta$-S and $\theta$-DO plots have shifted to the low-salinity and low-oxygen sides in $\theta$ $<$ $1.2\deg$C, but they were almost identical to the plots at $29\deg$S of P15S in 1996. This suggests that the meridional property distributions characterizing the northward intrusion and dissipation of CDW receded southward by 400 km in 2003. Meridional geostrophic transport integrated over the Southwest Pacific Basin beneath the reference level of 3500 dbar is 10.8 Sv (northward), which is 30% smaller than that in 1992. The similar result is obtained with an alternative reference level at $\theta$=$1.2\deg$C or $\gamma$$^{n}$=28.1, which corresponds to the upper boundary of the CDW. Thus, it is reasonable to suppose that the CDW inflow actually decreased during this decade. The time scale of the variations in properties and geostrophic flow should be clarified in further study. At the time of presentation, we will also show the results in other oceans.
OS31A-0566 0800h
Decadal Changes in Distributions of CO2-system Properties in the Southern Hemisphere Subtropical Oceans
From August, 2003 to January, 2004, we conducted hydrographic observations along the WHP P6, A10, I4 and I3 lines in the Southern Hemisphere subtropical oceans as part of the CLIVAR/CO$_{2}$ Repeat Hydrography Program. The main target of the observations was to quantify uptake, transport and storage of anthropogenic CO$_{2}$ by Southern Ocean-origin waters. On board the ship (R/V Mirai), we measured pCO$_{2}$ in surface seawater, and dissolved inorganic carbon (C$_{T}$), total alkalinity (A$_{T}$) and pH in water columns. Precision was calculated to be 1.5-1.0 $\mu$mol/kg, 2.0 $\mu$mol/kg and 0.0017-0.0008 pH unit for C$_{T}$, A$_{T}$ and pH, respectively. Both C$_{T}$ and A$_{T}$ were set to the certified values of CRM (Bach 65) provided by Dr. A.G. Dickson of Scripps Institution of Oceanography. The accuracy was estimated to be approx. 3 $\mu$mol/kg, 3 $\mu$mol/kg and 0.008 pH unit for C$_{T}$, A$_{T}$ and pH, respectively, by comparing measured and calculated values, which were based on the thermodynamic relationship of oceanic carbonate system. We intend to detect decadal changes of transport and storage of CO$_{2}$ in the ocean's interior by calculating the differences of CO$_{2}$-system properties and anthropogenic CO$_{2}$ tracers between 1990s and 2003/2004. At the time of presentation, we will discuss the decadal changes from the viewpoint of global climate change.
OS31A-0567 0800h
An Implicit Biogeochemistry Ocean Model
Because of the long ventilation time-scales for the deep ocean, the required spin-up for global ocean carbon-cycle models can be several thousand model years. This makes it difficult to explore the sensitivity of the models to parameters that control the deep biogeochemistry. To overcome this difficulty we have implemented a fully implicit solver for an OCMIP 2 -type biogeochemistry model. The offline model uses the 3-D flow and eddy-diffusion fields obtained from a global OGCM. The equilibrium solutions are solved using an iterative approach based on Newton's method. Computations that would normally take several days using an explicit time-stepping scheme now only take minutes or hours. Here we present some preliminary results obtained using a continuation approach in which model parameters are varied continuously to explore the model sensitivity to those parameters.
OS31A-0568 0800h
The CLIVAR and Carbon Hydrographic Data Office at UCSD/SIO
At the CLIVAR and Carbon Hydrographic Data Office (CCHDO) at the UCSD Scripps Institution of Oceanography, also known as the WOCE Hydrographic Program Office (WHPO), the CTD, hydrographic, and tracer data used in large scale ocean circulation studies are brought together, verified, corrected for content and format errors, assembled with relevant documentation, and carefully prepared for dissemination and archive. In addition the CCHDO/WHPO works to promote appropriate methodology, applicable community standards, communications, and data compatibility. The WHPO at UCSD/SIO supported these functions for WOCE Hydrographic Program data from 1997-present. The office, with the new name CCHDO, has been invited to continue these functions for CLIVAR hydrography, global ocean carbon hydrography, and similar programs which make use of high quality ocean profile data. Data of the type dealt with by the CCHDO/WHPO are created by >100 data originators worldwide, sometimes 5 or more contributing to one file. All data users must cope with the temporal-, content-, and format-related file diversity these different originators engender. It is the enormous advantage of bringing data sets together to a common content and readability standard that remains the strongest rationale for the CCHDO/WHPO, with the principal additional advantage that the documentation associated with the data are collected, reorganized to a common standard (where possible), and preserved with the data. The CCHDO/WHPO disseminates data via the internet and on CD-ROMs and data DVDs. It also provides its total public holdings, including documentation, to NODC/WDC-A for archive and further distribution. Preliminary CTD/hydrographic/carbon/tracer data from the 2003-2004 cruises for the US Global Ocean Carbon and Repeat Hydrography program are already on line and available from the CCHDO.