An offline tracer transport model transport is used to simulate chlorofluorocarbon (CFCs), sulfur hexafluoride (SF₆), oxygen, ideal age, and model transit time distributions (TTDs) to evaluate how well tracers can be used to constrain both the mean state and variability of oceanic ventilation. Using climatological transports, the two-parameter 1-D inverse Gaussian approximation of the model TTD is found to be an adequate representation of ventilation pathways within the parts of the subtropical gyres with simple ventilation dynamics, but a poor approximation for regions with large gradients in ideal age (i.e., near the base of the thermocline and the continental boundaries). TTDs inferred from CFC-12 and SF₆ using a Peclet number-based lookup table approach yield poor representations of the model TTD with a consistent bias toward ventilation being strongly dominated by along-isopycnal diffusion. In a run with variable circulation, ideal age is used to track changes in thermocline ventilation. Variability in both apparent oxygen utilization (AOU) and tracer-inferred TTD mean ages inferred using CFC-12 (assuming fixed Peclet number) and dual tracers (SF₆ and CFC-12) are well-correlated to ideal age variability in most of the thermocline. Changes in AOU are correlated with ideal age variability in even more regions compared to the TTD ages both horizontally and vertically down to intermediate depths. Generally, when changes in TTD mean age and AOU agreed in sign, correlations of both with ideal age changes were positive indicating the usefulness of tracers in diagnosing ventilation changes.

An offline tracer transport model transport is used to simulate chlorofluorocarbon (CFCs), sulfur hexafluoride (SF₆), oxygen, ideal age, and model transit time distributions (TTDs) to evaluate how well tracers can be used to constrain both the mean state and variability of oceanic ventilation. Using climatological transports, the two-parameter 1-D inverse Gaussian approximation of the model TTD is found to be an adequate representation of ventilation pathways within the parts of the subtropical gyres with simple ventilation dynamics, but a poor approximation for regions with large gradients in ideal age (i.e., near the base of the thermocline and the continental boundaries). TTDs inferred from CFC-12 and SF₆ using a Peclet number-based lookup table approach yield poor representations of the model TTD with a consistent bias toward ventilation being strongly dominated by along-isopycnal diffusion. In a run with variable circulation, ideal age is used to track changes in thermocline ventilation. Variability in both apparent oxygen utilization (AOU) and tracer-inferred TTD mean ages inferred using CFC-12 (assuming fixed Peclet number) and dual tracers (SF₆ and CFC-12) are well-correlated to ideal age variability in most of the thermocline. Changes in AOU are correlated with ideal age variability in even more regions compared to the TTD ages both horizontally and vertically down to intermediate depths. Generally, when changes in TTD mean age and AOU agreed in sign, correlations of both with ideal age changes were positive indicating the usefulness of tracers in diagnosing ventilation changes.

We employ profiling floats with dissolved oxygen sensors to observe in situ temporal oxygen evolution below the mixed layer, allowing us to characterize net respiration of organic carbon in eight distinct regions over the globe. Export and export efficiency are generally high in locations with strong seasonal variability and low in locations of weak seasonality. Vertically integrated respiration is weakly, yet significantly, correlated with remote observations of chlorophyll, net primary production, and planktonic community size structure. These correlations suggest that regimes of high net primary production and large phytoplankton fuel elevated respiration at depth. Several regions of float-based observations intersect with sites of other detailed observations (e.g., Hawaii and Sargasso Sea), which allows us to compare our results to independent studies. We find that there is good agreement among export production estimates at highly seasonal locations, and that float-based observations may be biased low at weakly seasonal locations. We posit that the reason for the low-latitude discrepancy is the relative steady state of oxygen concentration caused by weak seasonality and shallow wintertime mixed layer depths.

Global ship-based programs, with highly accurate, full water column physical and biogeochemical observations repeated decadally since the 1970s, provide a crucial resource for documenting ocean change. The ocean, a central component of Earth's climate system, is taking up most of Earth's excess anthropogenic heat, with about 19% of this excess in the abyssal ocean beneath 2,000 m, dominated by Southern Ocean warming. The ocean also has taken up about 27% of anthropogenic carbon, resulting in acidification of the upper ocean. Increased stratification has resulted in a decline in oxygen and increase in nutrients in the Northern Hemisphere thermocline and an expansion of tropical oxygen minimum zones. Southern Hemisphere thermocline oxygen increased in the 2000s owing to stronger wind forcingand ventilation. The most recent decade of global hydrography has mapped dissolved organic carbon, a large, bioactive reservoir, for the first time and quantified its contribution to export production (∼20%) and deep-ocean oxygen utilization. Ship-based measurements also show that vertical diffusivity increases from a minimum in the thermocline to a maximum within the bottom 1,500 m, shifting our physical paradigm of the ocean's overturning circulation.

The region of the Southern Ocean that encompasses the Subantarctic Front (SAF) to the north and the Polar Front (PF) to the south contains most of the transport of the Antarctic Circumpolar Current. Here skewness of sea level anomaly (SLA) from 1992 to 2013 is coupled with a meandering Gaussian jetw model to estimate the mean position, meridional width, and the percent variance that each front contributes to total SLA variability. The SAF and PF have comparable widths (85 km) in the circumpolar average, but their widths differ significantly in the East Pacific Basin (85 and 60 km, respectively). Interannual variability in the positions of the SAF and PF are also estimated using annual subsets of the SLA data from 1993 to 2012. The PF position has enhanced variability near strong topographic features such as the Kerguelen Plateau, the Campbell Plateau east of New Zealand, and downstream of Drake Passage. Neither the SAF nor the PF showed a robust meridional trend over the 20 year period. The Southern Annular Mode was significantly correlated with basin-averaged SAF and PF positions in the East Pacific and with the PF south of Australia. A correlation between the PF and the basin-scale wind stress curl anomaly was also found in the western extratropical Pacific but not in other basins.

Chlorofluorocarbons-11 (CFC-11), CFC-12, and sulfur hexafluoride (SF₆) were measured during the December 2007 to February 2008 CLIVAR/Repeat Hydrography (RH) P18 section along ~103°W in the Southeast Pacific Ocean. Transit-time distributions (TTDs) of 1-D transport that matched all three tracers were consistent with high Peclet number flow ventilating the subtropical mode water and the main subtropical thermocline (30°S–42°S, 200–800 m). In the subtropics, TTDs with predominantly advective transport predicted decadal increases in CFC-12 and CFC-11 consistent with those observed comparing 1994 WOCE with 2007/2008 CLIVAR/RH data, indicating steady ventilation in this region, and consistent with the near-zero changes observed in dissolved oxygen. The mean transport timescales from the tracer-tuned TTDs were used to estimate apparent oxygen utilization rates (OURs) on the order of 8–20 &181;mol kg^-1 yr^-1 at ~200 m depth, attenuating to ~2 &181;mol kg^-1 yr^-1 typically by 500 m depth in this region. Depth-integrated over the thermocline, these OURs implied carbon export rates from the overlying sea surface on the order of ~1.8 moles C m^-2 yr^-1 from 30°S to 45°S, 2–2.5 moles C m^-2 yr^-1 from 45°S to 52°S, and 2.5–3.5 moles C m^-2 yr^-1 from 52°S to 60°S.

A method is presented to find the age distribution of ocean waters, the transit-time distribution (TTD), by combining an eddying global ocean model's estimate of the TTD with hydrographic observations of CFC-11, temperature, and salinity. The method uses a mixture model of an assumed form of the TTD, an inverse Gaussian (IG), and an established Bayesian statistical method. All known significant sources of uncertainty are propagated to arrive at estimates of two oceanic transport parameters associated with the IG TTD, the mean age and either the half-variance or the Peclet number. It is found that the uncertainties on mean age do not overlap zero in most locations using only CFC-11, temperature, and salinity. However, the uncertainty on the other IG parameter does not overlap zero in only a few locations. With the inclusion of another transient tracer (³He/³H), the uncertainty on this other IG parameter does not overlap zero in just a few additional locations in the deep North Atlantic Ocean. Neither a single- nor mixture-IG representation is adequate for representing the full TTD in the ocean, particularly in the Southern Ocean.

Differences between the IG parameters estimated using the model's tracers as data (BayesPOP) and those estimated using tracer observations as data (BayesObs) provide information about the sources of model biases, and give a more nuanced picture than can be found by comparing the simulated CFCs with observed CFCs. Using the differences between each of the oceanic transport parameters from BayesObs and those from BayesPOP with and without a constant Pe assumption along each of the hydrographic cross-sections considered here, it is found that the model's eddy mixing biases often lead to larger model errors than the model's mean advection time biases. It is also found that mean advection time biases in the model can be statistically significant at the 95% level where mode water is found in the Southern Ocean.

The use of CFC-11, CFC-12, and SF₆ to quantify oceanic ventilation rates, interior water age, and formation rates requires knowledge of the saturation levels at the sea surface. While their atmospheric histories are relatively well known, physical processes in the mixed layer in conjunction with limited air-sea gas exchange can cause surface concentrations to be in disequilibrium with the atmosphere. We use an offline tracer advection-diffusion code that evolves tracers using along-isopycnal and cross-isopycnal mass fluxes from a global, climatological run of the Hallberg Isopycnal Model to reconstruct the saturation level of all three tracers over the entirety of their atmospheric histories. Disequilibria on a global scale occur in regions associated with deep winter mixed layers and are found throughout the time period of the release of these chemicals into the atmosphere. Sensitivity studies using targeted model simulations, focusing on the North Pacific, show that seasonal cycles in temperature and salinity that affect gas solubility as well as entrainment of water containing low concentration of tracers during mixed layer deepening are the dominant causes of undersaturation. When using the transit time distribution method, our results show that these undersaturations introduce a significant bias toward older ages for North Pacific Central Mode Water but do not significantly affect estimates of anthropogenic carbon inventory.

Depth profiles of dissolved chlorofluorocarbon-11 (CFC-11) and sulfur hexafluoride (SF6) were measured during a September 2008 cruise in the Northeast Pacific Ocean. For each water sample, the two tracers were used in concert to estimate likely mean ages and widths of parameterized 1-D transit time distributions (TTDs). In shallow waters (<250 m), the TTDs' mean ages were relatively loosely constrained due to the slow decrease of atmospheric CFC-11 since 1994. In the main thermocline (25.0–26.6 σ_θ, ~300–550 m), the CFC-11/SF₆ tracer pair constrained TTDs' mean ages to within ±10%. Deeper than 26.8 σ_θ (~600 m), SF₆ levels in 2008 were too low for the CFC-11/SF₆ tracer pair to constrain the TTDs' mean ages. Within the main thermocline of the subtropical North Pacific Ocean (20°–37°N along 152°W), the TTDs'9 mean ages were used to estimate Oxygen Utilization Rates (OURs) of ~11 μmol kg^-1 yr^-1 on 25.0–25.5 σ_θ (~160 m), attenuating to very low rates (0.12 μmol kg^-1 yr^-1) by 26.8–27.0 σ_θ (~600 m). Depth integration of the in-situ OURs implied an average carbon remineralization rate of 1.7±0.3 mol C m^-2 yr^-1 in this region and depth range, somewhat lower than other independent estimates. Along the 152°W section, depth integrating the apparent OURs implied carbon remineralization rates of 2.5–3.5 mol C m^-2 yr^-1 from 20°N to 30°N, 3.5–4.0 mol C m^-2 yr^-1 from 30°N to 40°N, and 2–2.7 mol C m^-2 yr^-1 north of 45°N.

The mean residence times, subduction rates, and formation rates of Subtropical Mode Water (STMW) and Subpolar Mode Water (SPMW) in the North Atlantic and Subantarctic Mode Water (SAMW) in the Southern Ocean are estimated by combining a model and observations of chlorofluorocarbon-11 (CFC-11) via Bayesian Model Averaging (BMA), a statistical technique that weights model estimates according to how close they agree with observations. Subduction rates are estimated in two different ways to investigate the non-advective contribution to thermocline ventilation, which in turn are compared to formation rate estimates. One subduction rate estimate is based on entrainment/detrainment velocities and the other subduction rate estimate allows ventilation to be both an advective and diffusive process instead of a purely advective one by using transit-time distributions (TTDs). It is found that the subduction of all three mode waters is mostly an advective process, but up to about one-third of STMW subduction likely owes to non-advective processes. Also, while the formation of STMW is mostly due to subduction, the formation of SPMW is mostly due to other processes. About half of the formation of SAMW is due to subduction and half is due to other processes. A combination of air-sea flux, acting on relatively short timescales, and turbulent mixing, acting on a wide range of timescales, is likely the dominant SPMW erosion mechanism. Air-sea flux is likely responsible for most STMW erosion, and turbulent mixing is likely responsible for most SAMW erosion.

Global ocean acidification is a prominent, inexorable change associated with rising levels of atmospheric CO₂. Here we present the first basin-wide direct observations of recently declining pH, along with estimates of anthropogenic and non-anthropogenic contributions to that signal. Along 152 deg W in the North Pacific Ocean (22–56 deg N), pH changes between 1991 and 2006 were essentially zero below about 800 m depth. However, in the upper 500 m, significant pH changes, as large as –0.06, were observed. Anthropogenic and non-anthropogenic contributions over the upper 800 m are estimated to be of similar magnitude. In the surface mixed layer (depths to ~100 m), the extent of pH change is consistent with that expected under conditions of seawater/atmosphere equilibration, with an average rate of change of –0.0017/yr. Future mixed layer changes can be expected to closely mirror changes in atmospheric CO₂, with surface seawater pH continuing to fall as atmospheric CO₂ rises.

To address questions concerning the intensity and spatial structure of the three-dimensional circulation within the Pacific Ocean and the associated advective and diffusive property flux divergences, data from approximately 3000 high-quality hydrographic stations collected on 40 zonal and meridional cruises have been merged into a physically consistent model. The majority of the stations were occupied as part of the World Ocean Circulation Experiment (WOCE), which took place in the 1990s. These data are supplemented by a few pre-WOCE surveys of similar quality, and time-averaged direct-velocity and historical hydrographic measurements about the equator.

An inverse box model formalism is employed to estimate the absolute along-isopycnal velocity field, the magnitude and spatial distribution of the associated diapycnal flow and the corresponding diapycnal advective and diffusive property flux divergences. The resulting large-scale WOCE Pacific circulation can be described as two shallow overturning cells at mid- to low latitudes, one in each hemisphere, and a single deep cell which brings abyssal waters from the Southern Ocean into the Pacific where they upwell across isopycnals and are returned south as deep waters. Upwelling is seen to occur throughout most of the basin with generally larger dianeutral transport and greater mixing occurring at depth. The derived pattern of ocean heat transport divergence is compared to published results based on air-sea flux estimates. The synthesis suggests a strongly east/west oriented pattern of air-sea heat flux with heat loss to the atmosphere throughout most of the western basins, and a gain of heat throughout the tropics extending poleward through the eastern basins. The calculated meridional heat transport agrees well with previous hydrographic estimates. Consistent with many of the climatologies at a variety of latitudes as well, our meridional heat transport estimates tend toward lower values in both hemispheres.

New observations of oxygen variability in the North Pacific Ocean are reported on the basis of comparison of the U.S. Climate Variability and Predictability and Carbon (CLIVAR/CO2) Repeat Hydrography sections conducted along 30°N (2004) and 152–W (2006) with the earlier World Ocean Circulation Experiment (WOCE) data and other cruises along these sections. The largest changes in apparent oxygen utilization (AOU) continue to occur, as found in earlier North Pacific repeat section analyses, within the thermocline on σΘ = 26.6 kg m^-3, which is the densest isopycnal to outcrop in the open North Pacific in climatological data. In the northeastern North Pacific along 152°W, where a total of five cruises (1980, 1984, 1991, 1997, and 2006) spanning a period of 26 years are available, the AOU changes correspond to an overall increase in AOU on σΘ = 26.6 kg m^-3 from the 1980s/early 1990s to 2006. However, from 1997 to 2006 a decrease in AOU is observed within the boundary region between the subtropical and subpolar gyres at 40–45°N. Along the center axis of the subtropical gyre at 30°N, where two cruises are available (1994 and 2004), AOU has also substantially increased on σΘ = 26.6 kg m^-3 from 1994 to 2004 in the eastern part of the section. The repeat section data along 152°W and 30°N are consistent with a pattern of decadal-scale ventilation anomalies that originate in the northwestern Pacific, possibly through variability (including cessation) of the σΘ = 26.6 kg m^-3 outcrop, travel eastward along the subtropical-subpolar gyre boundary, and enter the northern portion of the subtropical gyre along the way. For the 152°W AOU data within the gyre boundary region (40–45°N), good agreement exists with the close-by time series data from Ocean Station P (50°N, 145°W) where a bidecadal cycle in AOU has been observed. In contrast, a sensible correlation with the Pacific Decadal Oscillation could not be found.

This paper uses the extended multiple linear regression (eMLR) technique to investigate changes over the last decade in dissolved inorganic carbon (DIC) inventories on a meridional line (P16 along 152°W) up the central Pacific and on a zonal line (P02 along 30°N) across the North Pacific. Maximum changes in the total DIC concentrations along P02 are 15–20 µmol kg^-1 over 10 years, somewhat higher than the ~1 µmol kg^-1 a^-1 increase in DIC expected based on the rate of atmospheric CO₂ increase. The maximum changes of 15–20 µmol kg^-1 along the P16 line over the 14/15-year time frame fit with the expected magnitude of the anthropogenic signal, but there is a deeper than expected penetration of the signal in the North Pacific compared to the South Pacific. The effect of varying circulation on the total DIC change, based on decadal alterations of the apparent oxygen utilization rate, is estimated to be greater than 10 µmol kg^-1 in the North Pacific, accounting for as much as 80% of the total DIC change in that region. The average anthropogenic CO₂ inventory increase along 30µN between 1994 and 2004 was 0.43 mol m^-2 a^-1, with much higher inventories in the western Pacific. Along P16, the average Northern Hemisphere increase was 0.25 mol m^-2 a^-1 between 1991/1992 and 2006 compared to an average Southern Hemisphere anthropogenic CO₂ inventory increase between 1991 and 2005 of 0.41 mol m^-2 a^-1.

Decadal changes of abyssal temperature in the Pacific Ocean are analyzed using high-quality, full-depth hydrographic sections, each occupied at least twice between 1984 and 2006. The deep warming found over this time period agrees with previous analyses. The analysis presented here suggests it may have occurred after 1991, at least in the North Pacific. Mean temperature changes for the three zonal and three meridional hydrographic sections analyzed here exhibit abyssal warming often significantly different from zero at 95% confidence limits for this time period. Warming rates are generally larger to the south, and smaller to the north. This pattern is consistent with changes being attenuated with distance from the source of bottom water for the Pacific Ocean, which enters the main deep basins of this ocean southeast of New Zealand. Rough estimates of the change in ocean heat content suggest that the abyssal warming may amount to a significant fraction of upper World Ocean heat gain over the past few decades.

A space-time regression analysis of chlorofluorocarbon (CFC) data from the eastern North Pacific Ocean's subtropical thermocline (25.6–26.6 ) is used to extract age and velocity changes from a collection of 1980s and 1990s hydrographic sections. Results indicate substantial increases of CFC ages, from 0.2 yr yr^-1 on 25.8 σ θ increasing with depth up to 0.4 yr yr^-1 on 26.6 σ θ, and increases of meridional age gradients, implying a circulation slowdown. Using the regression-derived spatial and temporal derivatives, an isopycnal balance of advection and diffusion, including the effects of mixing on CFC ages, was applied to estimate changes in flow. Southward velocities at 20°N, 145°W generally decreased by ~ 0.03 ± 0.02 cm s^-1 yr^-1 on isopycnals 25.6 to 26.4 σ θ during the 1980s and 1990s, consistent with overturning changes from historical hydrographic data and with concurrent increases in AOU. The deeper (26.6 σ θ) age increase was consistent with mixing effects.