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Najjar, RG, Keeling RF.  1997.  Analysis of the mean annual cycle of the dissolved oxygen anomaly in the World Ocean. Journal of Marine Research. 55:117-151.   10.1357/0022240973224481   AbstractWebsite

A global climatology of the dissolved oxygen anomaly (the excess over saturation) is created with monthly resolution in the upper 500 m of the ocean. The climatology is based on dissolved oxygen, temperature and salinity data archived at the National Oceanographic Data Center. Examination of this climatology reveals statistically significant annual cycles throughout the upper 500 m of the World Ocean, though seasonal variations are most coherent in the North Atlantic, where data density is greatest. Vertical trends in the phase and amplitude of the annual cycle are noted. The cycle in surface waters is characterized by a summer maximum and a winter minimum, consistent with warming and high rates of photosynthesis during the summer, and cooling and entrainment of oxygen-depleted water during the winter. In low and middle latitudes, the amplitude increases with depth and the maximum occurs later in the year, a trend consistent with the seasonal accumulation of oxygen associated with the shallow oxygen maximum. At a depth that varies between about 30 and 130 m, the phase of the annual cycle undergoes an abrupt shift. We call this depth the oxygen nodal depth. Below the nodal depth, the annual cycle is characterized by an early-spring maximum and a late-fall minimum, consistent with a cycle dominated by respiration during the spring and summer and replenishment of oxygen from the atmosphere by ventilation during the fall and winter. Below the nodal depth, the amplitude of the annual cycle generally decreases with depth, indicative of decreasing respiration and ventilation rates, or less seasonality in both processes. We postulate that the nodal depth in middle and high latitudes corresponds closely to the summertime compensation depth, where photosynthesis and net community respiration are equal. With this interpretation of the nodal depth and a simple model of the penetration of light in the water column, a compensation light intensity of 1 W m(-2) (4 mu E m(-2) s(-1)) is deduced, at the low end of independent estimates. Horizontal trends in the phase and amplitude of the annual cycle are also noted. We find that the nodal depth decreases toward the poles in both hemispheres and is generally greater in the Southern Hemisphere, patterns found to be consistent with light-based estimates of the compensation depth. The amplitude of the annual cycle in the oxygen anomaly increases monotonically with latitude, and higher latitudes lag lower latitudes. In the North Atlantic and North Pacific, the amplitude of the annual cycle tends to increase from east to west at all depths and latitudes, as expected considering that physical forcing has greater seasonal variability in the west. The tropics and the North Indian Ocean have features that distinguish them from other regions. Below about 75 m, these waters have pronounced annual cycles of the oxygen anomaly that areshown to be caused mainly by wind-driven adiabatic displacements of the thermocline. A semiannual cycle of the oxygen anomaly is found in the surface waters of the North Indian Ocean, consistent with the known semiannual cycle of surface heat flux in this region.

Najjar, RG, Keeling RF.  2000.  Mean annual cycle of the air-sea oxygen flux: A global view. Global Biogeochemical Cycles. 14:573-584.   10.1029/1999gb900086   AbstractWebsite

A global monthly-mean climatology of the air-sea oxygen flux is presented and discussed. The climatology is based on the ocean oxygen climatology of Najjar and Keeling [1997] and wind speeds derived from a meteorological analysis center. Seasonal variations are characterized by outgassing of oxygen during spring and summer and ingassing of oxygen during fall and winter, a pattern consistent with thermal and biological forcing of the air-sea oxygen flux. The annual mean flux pattern is characterized by ingassing at high latitudes and the tropics and outgassing in middle latitudes. The air-sea oxygen flux is shown to exhibit patterns that agree well with patterns seen in a marine primary productivity climatology, in model generated air-sea O-2 fluxes, in estimates of remineralization in the shallow aphotic zone based on seasonal oxygen variations, in observed seasonal nutrient-temperature relationships, and in independent estimates of meridional oxygen transport in the Atlantic ocean. We also find that extratropical mixed layer new production during the spring-summer period, computed from biological seasonal net outgassing of oxygen, is equivalent to the production of 4.5-5.6 Gt C, much lower than previous estimates based on atmospheric O-2/N-2 measurements.

Nevison, CD, Manizza M, Keeling RF, Stephens BB, Bent JD, Dunne J, Ilyina T, Long M, Resplandy L, Tjiputra J, Yukimoto S.  2016.  Evaluating CMIP5 ocean biogeochemistry and Southern Ocean carbon uptake using atmospheric potential oxygen: Present-day performance and future projection. Geophysical Research Letters. 43:2077-2085.   10.1002/2015gl067584   AbstractWebsite

Observed seasonal cycles in atmospheric potential oxygen (APO similar to O-2+1.1 CO2) were used to evaluate eight ocean biogeochemistry models from the Coupled Model Intercomparison Project (CMIP5). Model APO seasonal cycles were computed from the CMIP5 air-sea O-2 and CO2 fluxes and compared to observations at three Southern Hemisphere monitoring sites. Four of the models captured either the observed APO seasonal amplitude or phasing relatively well, while the other four did not. Many models had an unrealistic seasonal phasing or amplitude of the CO2 flux, which in turn influenced APO. By 2100 under RCP8.5, the models projected little change in the O-2 component of APO but large changes in the seasonality of the CO2 component associated with ocean acidification. The models with poorer performance on present-day APO tended to project larger net carbon uptake in the Southern Ocean, both today and in 2100.

Nevison, CD, Keeling RF, Weiss RF, Popp BN, Jin X, Fraser PJ, Porter LW, Hess PG.  2005.  Southern Ocean ventilation inferred from seasonal cycles of atmospheric N2O and O2/N2 at Cape Grim, Tasmania. Tellus Series B-Chemical and Physical Meteorology. 57:218-229.   10.1111/j.1600-0889.2005.00143.x   AbstractWebsite

The seasonal cycle of atmospheric N(2)O is derived from a 10-yr observational record at Cape Grim, Tasmania (41 degrees S, 145 degrees E). After correcting for thermal and stratospheric influences, the observed atmospheric seasonal cycle is consistent with the seasonal outgassing of microbially produced N(2)O from the Southern Ocean, as predicted by an ocean biogeochemistry model coupled to an atmospheric transport model (ATM). The model-observation comparison suggests a Southern Ocean N(2)O source of similar to 0.9 Tg N yr(-1) and is the first study to reproduce observed atmospheric seasonal cycles in N(2)O using specified surface sources in forward ATM runs. However, these results are sensitive to the thermal and stratospheric corrections applied to the atmospheric N(2)O data. The correlation in subsurface waters between apparent oxygen utilization (AOU) and N(2)O production (approximated as the concentration in excess of atmospheric equilibrium Delta N(2)O) is exploited to infer the atmospheric seasonal cycle in O(2)/N(2) due to ventilation of O(2)-depleted subsurface waters. Subtracting this cycle from the observed, thermally corrected seasonal cycle in atmospheric O(2)/N(2) allows the residual O(2)/N(2) signal from surface net community production to be inferred. Because N(2)O is only produced in subsurface ocean waters, where it is correlated to O(2) consumption, atmospheric N(2)O observations provide a methodology for distinguishing the surface production and subsurface ventilation signals in atmospheric O(2)/N(2), which have previously been inseparable.

Nevison, CD, Keeling RF, Kahru M, Manizza M, Mitchell BG, Cassar N.  2012.  Estimating net community production in the Southern Ocean based on atmospheric potential oxygen and satellite ocean color data. Global Biogeochemical Cycles. 26   10.1029/2011gb004040   AbstractWebsite

The seasonal cycle of atmospheric potential oxygen (APO similar to O-2 + 1.1 CO2) reflects three seasonally varying ocean processes: 1) thermal in- and outgassing, 2) mixed layer net community production (NCP) and 3) deep water ventilation. Previous studies have isolated the net biological seasonal signal (i.e., the sum of NCP and ventilation), after using air-sea heat flux data to estimate the thermal signal. In this study, we resolve all three components of the APO seasonal cycle using a methodology in which the ventilation signal is estimated based on atmospheric N2O data, the thermal signal is estimated based on heat flux or atmospheric Ar/N-2 data, and the production signal is inferred as a residual. The isolation of the NCP signal in APO allows for direct comparison to estimates of NCP based on satellite ocean color data, after translating the latter into an atmospheric signal using an atmospheric transport model. When applied to ocean color data using algorithms specially adapted to the Southern Ocean and APO data at three southern monitoring sites, these two independent methods converge on a similar phase and amplitude of the seasonal NCP signal in APO and yield an estimate of annual mean NCP south of 50 degrees S of 0.8-1.2 Pg C/yr, with corresponding annual mean NPP of similar to 3 Pg C/yr and a mean growing season f ratio of similar to 0.33. These results are supported by ocean biogeochemistry model simulations, in which air-sea O-2 and N2O fluxes are resolved into component thermal, ventilation and (for O-2) NCP contributions.

Nevison, C, Munro D, Lovenduski N, Cassar N, Keeling R, Krummel P, Tjiputra J.  2018.  Net community production in the Southern Ocean: Insights from comparing atmospheric potential oxygen to satellite ocean color algorithms and ocean models. Geophysical Research Letters. 45:10549-10559.   10.1029/2018gl079575   AbstractWebsite

The contribution of oceanic net community production (NCP) to the observed seasonal cycle in atmospheric potential oxygen (APO) is estimated at Cape Grim, Tasmania. The resulting APO(NCP) signal is compared to satellite and ocean model-based estimates of POC export and NCP across the Southern Ocean. The satellite products underestimate the amplitude of the observed APONCP seasonal cycle by more than a factor of 2. Ocean models suggest two reasons for this underestimate: (1) Current satellite products substantially underestimate the magnitude of NCP in early spring. (2) Seasonal O-2 outgassing is supported in large part by storage of carbon in DOC and living biomass. More DOC observations are needed to help evaluate this latter model prediction. Satellite products could be improved by developing seasonally dependent relationships between remote sensing chlorophyll data and in situ NCP, recognizing that the former is a measure of mass, the latter of flux. Plain Language Summary Phytoplankton in the surface ocean transform carbon dioxide into organic carbon while also producing oxygen. A fraction of the carbon is exported into the deep ocean, while the oxygen is emitted to the atmosphere. The carbon export rate influences how much carbon dioxide the ocean can absorb. The rate is commonly estimated using satellite-based phytoplankton color measured in the surface ocean, but such estimates involve many uncertain steps and assumptions. Small but detectible seasonal cycles in atmospheric oxygen have been used as an independent method for evaluating satellite-based estimates of organic carbon export. In this study, we evaluate eight satellite-derived carbon export estimates based on their ability to reproduce the observed seasonal cycle of atmospheric oxygen measured at a southeastern Australia site. All underpredict the seasonal oxygen cycle by at least a factor of 2, in part because they fail to capture the carbon and oxygen produced in early springtime and also because they focus on large particles of carbon that are heavy enough to sink while neglecting the dissolved fraction of organic carbon. Our study suggests that satellite estimates could be improved by a better understanding of seasonal variations in the relationship between phytoplankton productivity and carbon export.

Newman, S, Xu XM, Gurney KR, Hsu YK, Li KF, Jiang X, Keeling R, Feng S, O'Keefe D, Patarasuk R, Wong KW, Rao P, Fischer ML, Yung YL.  2016.  Toward consistency between trends in bottom-up CO2 emissions and top-down atmospheric measurements in the Los Angeles megacity. Atmospheric Chemistry and Physics. 16:3843-3863. AbstractWebsite

Large urban emissions of greenhouse gases result in large atmospheric enhancements relative to background that are easily measured. Using CO2 mole fractions and delta C-14 and delta C-13 values of CO2 in the Los Angeles megacity observed in inland Pasadena (2006-2013) and coastal Palos Verdes peninsula (autumn 2009-2013), we have determined time series for CO2 contributions from fossil fuel combustion (C-ff) for both sites and broken those down into contributions from petroleum and/or gasoline and natural gas burning for Pasadena. We find a 10 % reduction in Pasadena C-ff during the Great Recession of 2008-2010, which is consistent with the bottom-up inventory determined by the California Air Resources Board. The isotopic variations and total atmospheric CO2 from our observations are used to infer seasonality of natural gas and petroleum combustion. The trend of CO2 contributions to the atmosphere from natural gas combustion is out of phase with the seasonal cycle of total natural gas combustion seasonal patterns in bottom-up inventories but is consistent with the seasonality of natural gas usage by the area's electricity generating power plants. For petroleum, the inferred seasonality of CO2 contributions from burning petroleum is delayed by several months relative to usage indicated by statewide gasoline taxes. Using the high-resolution Hestia-LA data product to compare C-ff from parts of the basin sampled by winds at different times of year, we find that variations in observed fossil fuel CO2 reflect seasonal variations in wind direction. The seasonality of the local CO2 excess from fossil fuel combustion along the coast, on Palos Verdes peninsula, is higher in autumn and winter than spring and summer, almost completely out of phase with that from Pasadena, also because of the annual variations of winds in the region. Variations in fossil fuel CO2 signals are consistent with sampling the bottom-up Hestia-LA fossil CO2 emissions product for sub-city source regions in the LA megacity domain when wind directions are considered.

Nottrott, A, Kleissl J, Keeling R.  2014.  Modeling passive scalar dispersion in the atmospheric boundary layer with WRF large-eddy simulation. Atmospheric Environment. 82:172-182.   10.1016/j.atmosenv.2013.10.026   AbstractWebsite

The ability of the Weather Research and Forecasting, large-eddy simulation model (WRF-LES) to model passive scalar dispersion from continuous sources in convective and neutral atmospheric boundary layers was investigated. WRF-LES accurately modeled mean plume trajectories and concentration fields. WRF-LES statistics of concentration fluctuations in the daytime convective boundary layer were similar to data obtained from laboratory experiments and other LES models. However, poor turbulence resolution near the surface in neutral boundary layer simulations caused under prediction of mean dispersion in the crosswind horizontal direction and over prediction of concentration variance in the neutral surface layer. A gradient in the intermittency factor for concentration fluctuations was observed near the surface, downwind of ground-level sources in the daytime boundary layer. That observation suggests that the intermittency factor is a promising metric for estimating source-sensor distance in source determination applications. (C) 2013 Elsevier Ltd. All rights reserved.