OS24B-01 INVITED 16:00h
Recent observations of warming in the North Pacific: Is this evidence of anthropogenic climate change?
A transbasin survey of the North Pacific Ocean in 2004 reveals sizable warming of the upper ocean. Temperature observations from the 2004 CLIVAR P2 occupation along 30 degrees north are compared with 1994 WOCE occupation of the same line. The greatest observed warming (about 0.5 C) is observed in the lower ventilated layers of the subtropical thermocline. Analysis of the transient tracers concentrations offers a temporal diagnostic of the characteristic renewal time of the ventilated water masses. Chloroflourocarbon measurements collected on the 2004 CLIVAR P2 cruise show that the water masses with the greatest observed temperature increases are those with ventilation time scales of a decade to a century.
OS24B-02 16:15h
CLIVAR/CO2 Repeat Hydrography Program: Initial Carbon Results from the North Pacific Ocean
The primary goal of the CLIVAR/CO$_2$ Repeat Hydrography Program is to quantify the role of the ocean in sequestering anthropogenic CO$_2$. Information on shorter timescales is essential to determine any feedbacks of oceanic carbon system due to climate change, and to determine the impacts of natural variability. The North Pacific Ocean plays a unique role in controlling the long-term fate of anthropogenic CO$_2$ because: (1) the North Pacific is the final destination of circulation of the deep water that contains a high level of preformed nutrients and dissolved inorganic carbon (DIC); and (2) the North Pacific Intermediate Water stores dissolved CO$_2$ for more than a few tens of years. Discrete high-quality dissolved inorganic carbon and total alkalinity data were acquired as part of the WOCE/JGOFS Global CO$_2$ survey in the Pacific Ocean between 1991 and 1999 followed by repeat surveys in 2001 and 2004 as part of the Sub-arctic Gyre Experiment (SAGE) along the P17N line in the eastern North Pacific and the CLIVAR/CO$_2$ Repeat Hydrography Program east-west P2 cruise along 30$^\circ$N. The DIC increases range from 5--35 $\mu$mol kg$^{-1}$ at intermediate depths from about 100--800~m depth along the 30$^\circ$N section reflect the rapid uptake, transport, and transformation of CO$_2$ in these waters. The DIC changes are consistent with the corresponding changes in AOU and other water mass properties, suggesting that decadal changes in the local circulation, invasion of anthropogenic CO$_2$ into the interior North Pacific, and/or changes in new production and remineralization of organic matter along the flow path are all possible causes for the observed variations. The results of these research studies suggest an annual CO$_2$ uptake of 1.0--1.4 $\mu$mol kg$^{-1}$ yr$^{-1}$ in the mixed layer, based on direct observations and multiple linear regression approaches. Water column integrated uptake rates ranged from 0.25 to 1.3 mol m$^{-2}$ yr$^{-1}$, depending on location and/or approach used. Deep ventilation within the Kuroshio Extension and the subsequent circulation in the subtropical gyre generates a strong east-west gradient in the anthropogenic CO$_2$ penetration depth. The combined effect of the tilted density surfaces and the younger waters with higher anthropogenic CO$_2$ concentrations leads to higher total column inventories in the western North Pacific. The integrated amount of anthropogenic CO$_2$ in the North Pacific is estimated to be 16.5 Pg C through 1994 north of the equator but not including the marginal seas. This estimate is approximately 16% of the amount of anthropogenic CO$_2$ taken by the global oceans.
http://www.pmel.noaa.gov/co2/co2-home.html
OS24B-03 INVITED 16:30h
Temporal Trends In Apparent Oxygen Utilization In The Upper Pycnocline Of The North Pacific
We present a compilation of apparent oxygen utilization (AOU) changes observed in the upper pycnocline of the north Pacific Ocean over the last several decades. The general trend along repeat cross sections of the eastern and western subtropical ocean and the subarctic ocean is an increase in AOU from the mid 1980s to the mid 1990s. AOU has also been increasing in a time-series study in the northwest subarctic ocean off of Japan since the late 1960s. Observed AOU changes south of 35$\deg$N in the subtropical ocean are 10 to 20 $\micron$ol kg$^{-1}$, with much greater changes, reaching 60 to 80 $\micron$ol kg$^{-1}$ in isolated areas, in the subtropical/subarctic boundary and the subarctic ocean. Analysis of changes in both AOU and salinity on isopycnals suggests that there are significant salinity-normalized increases that must be due to alteration in the rate of ventilation, circulation or organic matter degradation. A common feature in the data is that the maximum increase in AOU is centered near the density horizon $\sigma_{\theta}$ = 26.6. Time series results from the Oyashio Current region near the winter outcrop area of this density horizon indicate that surface waters there have become fresher with time, which may mean this density surface has ceased to outcrop in the latter decades of the 20th century. A model description of the cause of these changes is described in a companion abstract in this session. Future determinations of the oxygen concentrations along these transects will be necessary to verify the suggested causes and help determine the role of anthropogenic influences.
OS24B-04 16:45h
Attributing the causes of North Pacific oxygen change
We investigate the variability of dissolved oxygen in the upper water column of the North Pacific using a hind cast model simulation. The model applies the biogeochemical framework of the Ocean Carbon Model Intercomparison Project (OCMIP) to an isopycnal General Circulation Model (GCM) whose circulation is forced at the surface by historical atmospheric conditions from 1948-2000. Simulated O2 changes from the 1980's to the 1990's include O2 decreases in the subpolar region, especially in the lower ventilated thermocline, and O2 increases over much of the subtropics. Both of these features are similar in pattern and magnitude to those observed along repeat transects through the subtropical and subpolar gyres [Emerson et al. 2004]. We perform a set of additional simulations designed to separate the contributions of changes in biology, ventilation, and circulation to O2 variability. We find that the direct effect of circulation variability, including a southward expansion of the subtropical gyre, is the dominant cause of O2 changes over much of the North Pacific. Variations in thermocline ventilation lead to significant O2 decreases in the Subarctic Northwest Pacific, and the impact of changes in biological export production are confined to the upper thermocline. Simulated O2 changes involve both long-term trends, such as the decreased Subarctic O2 due to reduced ventilation, as well as transient responses to short-term perturbations, many of which occurred in the 1970's.
OS24B-05 INVITED 17:00h
Preliminary transport analysis for P06, A10 and I4+I3 revisits (BEAGLE2003)
Mass and heat transports across all P06, A10 and I4+I3 were analyzed together based on both revisit data prepared by BEAGLE2003 Expedition and original data during WOCE period. Transports were estimated using an inverse model with 25 neutral density categories. Constraints for calculations were rather simple, i.e., the mass conservation for whole water column, no net mass flux into a basin at deeper layers than the deepest shill located in the north of the observation line, the salt conservation for whole water column and 4Sv of the northward transport between the Rio Grande Rise and the S. American continent. Initial `levels of no motion' were given a priori from vertical profiles of salt and DO. Ekman transports were estimated using NCEP re-analysis data both for the revisit and the original WOCE periods, respectively. Finally, overturn structures especially in the Indian and the Pacific were examined by applying several mass transport values of ITF and we selected solutions which included ITF transport of 15Sv as basic ones for revisit and original WOCE data. Results from present analysis for original WOCE data show good agreements with those from published inverse analyses in the mass and the heat transports. As for results from the analysis with revisit data, a 5 Sv larger (3 Sv smaller) deep overturn is estimated for the Indian (the Pacific) than those with original WOCE data. On the other hand, 5 Sv smaller overturns are estimated both for intermediate and shallower ones in the Pacific-Indian system. As the result, the geostrophic heat flux during the revisit period is estimated to be 0.08 PW which is significantly smaller than that of 0.20 PW during WOCE period. However, the Ekman heat transports compensate geostrophic fluxes, and total heat transports during the revisit and WOCE period turn out to be almost the same value as -0.84+/-0.32 PW and -0.82+/- 0.31 PW, respectively.
OS24B-06 17:15h
Temporal evolution of the CFC distributions along the SR3 section between Tasmania and Antarctica
The dissolved CFCs were measured as part of a 2001 Australian CLIVAR repeat hydrography expedition between Tasmania and Antarctica on the SR3 section. The CFCs were measured along this section previously in 1991 and 1995, providing an opportunity for determining the penetration of these transient tracers into the water column in this region of the Southern Ocean. The largest changes in CFC concentrations are in the Adelie Land Bottom Water and in the waters with neutral density less than 27.7. The CFC concentrations within Circumpolar Deep Waters are only slightly changed over the 10 years except adjacent to the Antarctic continental slope. When pCFC values are compared on isopycnal surfaces, the waters within the pycnocline between the Subantarctic Front and the southern expression of the Polar Front are less ventilated in 2001 than in previous occupations. Changes in AOU and DIC concentrations are consistent with the changes in pCFC ages within this feature. These changes in properties appear to be the result of a much weaker temperature minimum accompanied by an expansion of the underlying temperature maximum waters. The changes in the CFC concentrations in ALBW provide limits for the calculation of bottom water formation in the Southern Ocean. CFC saturations relative to equilibrium with the atmosphere vary between 30% and 42% in the recently ventilated bottom water. The relationship between CFCs and DIC in these waters will also be examined.
OS24B-07 17:30h
Westerly Winds over the Southern Ocean Determine the Partition of Carbon Dioxide Between Atmosphere and Ocean
The recently documented trend in the position and strength of the Westerly Winds in the Southern Hemisphere may be having profound impacts on the fate of anthropogenic carbon dioxide in the ocean. Previous transitions in atmospheric CO2 between low-CO2 periods in Earth's climate (glacial) and high-CO2 periods (interglacial) are usually attributed to changes in the biogeochemistry of the ocean. We propose a new mechanism for determining the partition of CO2 between the atmosphere and ocean. This mechanism depends on the position of the mid-latitude westerlies in relation to the Antarctic Circumpolar Current (ACC) in the Southern Hemisphere. In the past, westerlies that were shifted equatorward of the ACC weakened the divergence around Antarctica allowing biologically-cycled CO2 to accumulate in the deep ocean. Poleward-shifted westerlies that were aligned with the ACC enhanced the divergence around Antarctica, flushing organically cycled CO2 out of the deep ocean and into the atmosphere. Today, the shift in the Westerly Winds associated with the trend in the Southern Annular Mode increases the exposure of Southern Ocean deep waters to the anthropogenically enriched high CO2 atmosphere. We will present results from an atmosphere-ocean MOM4 simulation of the recent changes in the ocean's circulation and carbon chemistry associated with changes in the strength and position of the westerlies.
OS24B-08 17:45h
Aragonite Undersaturation in the High-Latitude Surface Ocean Within the 21st Century
The surface ocean is everywhere saturated with calcium carbonate (CaCO$_3$). Yet increasing atmospheric CO$_2$ reduces ocean pH and carbonate ion concentration and thus the level of saturation. Here we show with ocean data and models that due to this anthropogenic acidification, some surface waters will become undersaturated within decades. When atmospheric CO$_2$ reaches 550 ppmv, in year 2050 under the IS92a business-as-usual scenario, Southern Ocean surface waters begin to become undersaturated with respect to aragonite, a metastable form of CaCO$_3$. By 2100 as atmospheric CO$_2$ reaches 788 ppmv, undersaturation extends throughout the entire Southern Ocean ($<$60$^\circ$S) and into the surbarctic Pacific. Meanwhile, Weddell Sea surface waters also become undersaturated with respect to calcite, the stable form of CaCO$_3$. These transient changes are much larger than seasonal, interannual, and decadal variability. They threaten high-latitude aragonite secreting organisms including cold-water corals, which provide essential fish habitat, and shelled pteropods, i.e., zooplankton that serve as an abundant food source for marine predators.
http://www.ipsl.jussieu.fr/OCMIP