Previous
and Current Research
The impact of two parameterizations using ocean color for penetrating shortwave radiation on the ocean circulation is studied using the GFDL Modular Ocean Model. We find that parameterizations which increase the penetration of shortwave radiation increases the mixed layer depths, and decreases the export of heat from the tropics. We show that within deeper mixed layers a larger fraction of the Ekman transport is compensated by geostrophic transport. We demonstrate that the distribution of heat within the upper part of the mixed layer, rather than the penetration of heat below the mixed layer, is most important in changing the heat transport.
To increase spatial, seasonal and inter-annual observations of surface CO2, I have been funded by the National Foundation of Science to install, maintain, and analyze continuous surface water partial pressure of CO2 (pCO2) measurements along with a sparse sampling of discrete surface TCO2 on the R/VIB Gould which will provide an opportunity for an unparalleled time series of data across in the Drake Passage. It is my hope to supplement my insitu observations with surface chlorophyll concentrations using the SeaWiFS, MODIS and HPLC analysis taken in the Drake Passage. The goal of this study is to increase our understanding of the role that primary productivity plays in the variability of surface CO2 in Southern Ocean. This study, will be done over the next three years in cooperation with Dr. Taro Takahashi (Lamont-Doherty Earth Observatory).
2001-2002)
A time series of the surface partial pressure of CO2 (pCO2) made from underway samples on the R/V N. B.
Palmer from the austral spring of 1996 through mid-summer of 1999 in the Ross
Sea, Antarctica are compared with the predicted pCO2 derived from climatological mixed layer
depths and estimated primary productivity using observations of ocean color
(OCTS and SeaWiFS). Both insitu measurements and predicted values of pCO2
indicate a large net biological drawdown in CO2 which is responsible for pCO2 values of more than 200 matm below saturation. In addition, both measured
and predicted values of pCO2 demonstrate large inter annual variability in the biological draw down
of CO2 in the Ross
Sea. While it is clear that remotely sensed ocean color data may be essential
tool for monitoring inter annual variability in surface pCO2 throughout the world oceans,
parameterization of mixed layer depths, upwelling and diapycnal diffusion from
below the mixed layer and biological precipitation and dissolution of calcium
carbonate are essential. This work is done being done jointly with Dr. Kevin
Arrigo (Stanford University).
Presented:
Sweeney, C. K. Arrigo, and G. van Gijken (2001).
Prediction of seasonal changes in surface pCO2 in the Ross Sea,
Antarctica using ocean color satellite data. 2001 Annual AGU meeting, San
Fransisco, CA Dec. 10-15.
The annual cycle of NO3 + NO2 + NH4, CO2 and O2 in the surface waters of the southwestern Ross Sea along 76.5oS is presented in this study. From the surface data and sea ice concentrations annual sea-air fluxes of CO2 (-1.5±1.5 mol C m-2) and O2 (-3.7±3.0 mol C m-2) are calculated and confirmed by a mass balance approach which accounts for the total flux of CO2 (0.16±0.13) and O2 (-5.2±0.2 mol C m-2) entering the Ross Sea from off the shelf. The mass balance approach assumes that a negligible amount of carbon and oxygen accumulates in the sediments and that all of the gas that ventilates to the atmosphere must be replaced by fresher waters entering the Ross Sea. Based on this study, a combination of winter sea ice cover and summer primary productivity prevent any significant change in the CO2 inventory due to gas exchange despite the high partial pressure of CO2 surface waters (425 matm) during the winter. Oxygen inventories in the Ross Sea, on the other hand, are significantly increased as a result of gas exchange with the atmosphere due to low winter time O2 concentrations in the Ross Sea which are 90 mmol kg-1 below saturation at sea surface temperatures of –1.89 C. The high flux associated with large sea surface gradient in O2 is the source of high PO4* found in deep waters formed along the Antarctic continental shelf.
Based on stability of wintertime CO2
concentrations and the “ice rectification” hypothesis introduced by Yager et
al. (1995), it is projected that with increases in atmospheric pCO2
and greater seasonal ice cover, the Ross Sea will become a greater CO2
sink with time. This analysis also supports the hypothesis that winter ice
cover and summer primary productivity at the polar front may have been an
important factor in the decrease in CO2 during the last glacial
maximum.
Refereed Journal Articles:
Sweeney, C. 2002. The annual cycle of surface CO2 and O2 in the Ross Sea: A model for gas exchange on the continental shelves of Antarctica. Antarctic Research Series (submitted).
Using a statistical analysis technique that I developed from analysis of meso-scale variability in my thesis work, I have shown how often the sea-air pCO2 difference in the regions of temperate North Pacific, tropical Pacific, polar South Pacific and temperate North Atlantic need to be sampled using evenly spaced sampling to estimate regional fluxes to better than 0.1 Pg-C yr-1. The results assumed that the uncertainty in the estimated fluxes were due entirely to the precision of the sea-air pCO2 difference and did not include the errors from the sea-air gas transfer coefficient. Basin-scale mean annual estimates for net sea-air CO2 flux may be achieved annually by making 6-15 evenly spaced measurements of pCO2 in time 200- 1500 km apart. This analysis also pointed out the advantage of evenly spaced sampling over randomly spaced sampling in time and space. This work has been done with the assistance of Dr. Takahashi (Lamont-Doherty Earth Observatory), Rik Wanninkhof (NOAA, AMOL), Anand Gnanadesikan (Princeton University)
Presented:
Sweeney, C., T. Takahashi, R. Wanninkhof, 2000. Spatial and temporal variability of surface water pCO2 and sampling strategies. Report prepared for the NOAA Advisory Meeting for Sea-air CO2 Flux Program, October 8-10, 2000, Boulder, CO.
The meso-scale variability of temperature, salinity, biofluoresence, pCO2 and net community production (NCP) was measured from high-resolution data collected using the Lamont Pumping Seasoar (LPSS), which has a mean horizontal resolution of about 4.5 km. Three 300 km E-W transects along 76.5 °S in the southwestern section of the Ross Sea show that conventional hydrographic sample spacing of 50 km miss up to 60% of the variance in NCP. These results also indicate that, although the mean value of the NCP and pCO2 obtained from the hydrocast data may be within 15% of that using closely spaced LPSS data, predicted values between measurement points have an average error which is 35% of the mean NCP using conventional hydrocast sample spacing. Significant correlations between surface density and fluorescence, pCO2 and NCP were the result of large differences in both productivity and surface density of the three biogeochemical regimes. Mechanistic conclusions can not be drawn from these correlations. These observations lead us to conclude that the high horizontal variability at length scales of 8-20 km in carbon measurements are significant and can not be predicted with observations of variability in other measured parameters such as temperature, salinity, biofluorescence or density fields in the Ross Sea. This work was the last chapter in my thesis and was done with the assistance of Dr. Takahashi (Lamont-Doherty Earth Observatory), and Dr. Burke Hales (Oregon State University).
Presented:
Sweeney,
C., B. Hales, and T. Takahashi (2000). The importance of mesoscale variability
in estimates of net community production and mapping of physical and
biochemical features in the Ross Sea, Antarctica 2000 AGU, Ocean Sciences
Meeting, San Antonio, TX, Jan. 24-28.
Inorganic carbon, nutrient
and organic matter data were obtained during four cruises in the Ross Sea,
Antarctica in 1996 and 1997 during the JGOFS Antarctic Environment and Southern
Ocean Process Study (AESOPS). Analysis of these data showed that Diatom
dominated regimes in the Ross Sea were distinguished by significantly lower N/P
and C/P removal ratios and were located in areas of shallower mixed layers
depths relative to the Pheaocystis antarctica dominated regimes. While sediment
traps indicated only a small fraction (<3%) of the carbon taken up in the
surface was export to depth greater than 206m, an independent estimate
indicated that more than half (53 +/- 38%) of the carbon was exported. This was done with the help of may investigators:
most notably Dr. Robert Anderson (Lamont-Doherty Earth Observatory), Dr. Dennis
A. Hansell (University of Miami), Dr. Walker O. Smith (Virginia Institute of
Marine Science) and Dr. Taro Takahashi (Lamont-Doherty Earth Observatory)
Refereed Journal Articles:
Sweeney, C., Hansell, D. A., Carlson, C. A., Codispoti, L. A., Gordon, L. I., Marra, J., Millero, F. J., Smith, W. O. and Takahashi, T., 2000. Biogeochemical regimes, net community production and carbon export in the Ross Sea, Antarctica, Deep Sea Research, 47, 3369-3394.
Sweeney, C., Smith, W. O., Hales, B., Bidigare, R. R., Carlson, C. A., Codispoti, L. A., Gordon, L. I., Hansell, D. A., Millero, F. J., Park, M.-O. and Takahashi, T., 2000. Nutrient and carbon removal ratios and fluxes in the Ross Sea, Antarctica, Deep-Sea Research, 47, 3395-3421.
Gordon, L. I., Codispoti, L. A., Morrison, J. M., Millero, F. J., Jennings, J. C. and Sweeney, C., 2000. Seasonal evolution of hydrographic properties during the US JGOFS/AESOPS expedition to the Ross Sea, 1996-1997, Deep-Sea Research II, 47, 3095-3117.
Presentations:
Sweeney, C., Hansell, D. A., F. J. Millero, T. Takahashi, L. I. Gordon, C. A. Carlson, L. A. Codispoti, W. O. Smith and J. Marra. (1999) The biogeochemical regimes of the Ross Sea indicated by net utilization of carbon and nutrients and net production of organic carbon. 1999 ASLO Aquatic Sciences Meeting, Sante Fe, NM, Feb. 1-5, published program, p. 175.
Sweeney, C., B. Hales, D. A. Hansell, C. A. Carlson, L. A. Codispoti, L. I. Gordon, F. J. Millero W. O. Smith, and T. Takahashi. Utilization of nutrients and carbon in the pelagic waters of the Ross Sea.1999 ASLO Aquatic Sciences Meeting, Sante Fe, NM, Feb. 1-5, published program, p. 175.
The purpose of this study was to find the dominant processes affecting the diel carbon cycle of the Biophere 2 ocean. As with many natural coral reefs, the Biosphere 2 ocean was extremely depleted in nutrients; yet, a large daily change in surface water concentrations oxygen and CO2 indicated that photosynthesis was taking place despite the low nutrient values. A model constructed to estimate surface pCO2 using measured O2 concentrations suggested that CaCO3 and organic carbon production were dominating the pCO2 signal while gas exchange was relatively insignificant.
Refereed Journal Articles:
Langdon, C., T. Takahashi, C. Sweeney, D. Chipman, J. Goddard, F. Marubini, J. Aceves, H. Barnett, M. Atkinson, 2000. Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef. Global Biogeochemical Cycles 14(2), 639-654.
Sweeney, C. The Diel Carbon Cycle of the Biosphere II Ocean. Ecological Engineering 13, 235-247.
Created: November 5, 2002