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Air-Sea Carbon Dioxide Fluxes and Surface Physical Processes

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Final Summary Report CICOR Cooperative Agreement 1998-2402

Wade McGillis, James Edson, and Eugene Terray
Woods Hole Oceanographic Institution
Woods Hole, MA 02543

Program Manager: James Todd, Office of Global Programs

C02 exchange across the air-sea interface is an important mechanism in modulating global climate and the absorption of anthropogenically produced C02 (Siegenthaler and Sarmiento, 1993). Depending on the time of year, different regions of the ocean can be sources or sinks for atmospheric C02. Currently, it is estimated that the ocean as a whole acts as a sink for C02, taking up about 2 gigatons per year of the approximately 5.5 gigatons of carbon dioxide produced by industrial and agricultural activity. However, there is significant uncertainty in this estimate, partly because the kinetics of ocean-air C02 transfer are not well understood.

The GasEx-2001 study took place aboard the NOAA Ship RONALD H. BROWN (RHB) in the Eastern Equatorial Pacific along 3°S between 125°W-130°W. The primary.objective was to use direct gas flux measurements to improve our understanding of the forcing functions on the kinetics of air-sea gas exchange. A second focus was to determine the physical, chemical, and biological factors controlling pC02 in the surface water (Figure 1 }. The region is a CO2 source with relatively low wind speeds offering a strong contrast with the first 1998 Gas-Ex study conducted in the North Atlantic in an area of high winds and large C02 sink.

The Equatorial Pacific has been a focal point for chemical and physical studies such as JGOFS and TOGA because it has a major influence on climate variability through the
ENSO cycle. The questions about mesoscale C02 dynamics in this region relate to biological versus physical control, and remote versus local influences. Near the upwelling center it seems that the patterns in pC02 are dominated by physics while further off axis biological control becomes more important . The pC02 in the surface
water relates directly to upwelling strength , but regional fluxes are strongly influenced by remote factors such as the capping off of the upwelling system by the low salinity water advecting from the West. Diurnal heating , tropical instability waves, variations in biological productivity, and trace metal limitations on productivity are also important.

The main objective of the February/March GasEx 2001 process study was to determine the magnitude and controls on the CO2 gas transfer velocity in the Equatorial Pacific. This region is the largest oceanic source of C02 and shows large interannual variability caused by the ENSO cycle. The area experiences low wind speeds relative to most other ocean regions. Since the area is of such importance to the global carbon cycle, and because of its unique conditions, the region warrants direct determination of fluxes and gas transfer velocities, rather than using parameterizations developed for other environments. The study was performed in a similar fashion as the study in the North Atlantic, GasEx-98, where direct flux measurements (eddy co-covariance and gradient measurements) could be reconciled with fluxes inferred from mass balances of CO2 with the addition of surface forcing characteristics. The GasEx-98 study was done in a Lagrangian frame of a warm core eddy with a relatively homogeneous water mass such that the air-water difference in partial pressure of C02 (ΔpC02), was large and remained relatively uniform.

Goals during the GasEx-2001 air-sea gas transfer study were to:
• Use the Air-Sea Interaction Spar buoy (ASIS), to explore the role of physics and biogeochemistry in the lower atmosphere and surface ocean.
• Perform continuous measurements of the air-sea fluxes of momentum, heat, water vapor, and C02, surface wave characteristics, profiles of currents, TKE dissipation rate, temperature, salinity, 02 and C02 in the oceanic boundary layer, and mean atmospheric properties and boundary layer stability.
• Contribute to better parameterization of gas exchange velocities in the study region.

GasEx-2001 consisted of cruise legs 1 A, 1 B, and 1 C. Leg 1 A was a 1-1 /2 day transit between Charleston and Miami, and was used as training for both ETL and UW
personnel. Most Leg 1 A scientists disembarked in Miami. Leg 1 B began in Miami and ended in Panama. Most scientists participating in the GasEx-2001 cruise embarked in Miami for the duration of the cruise to Honolulu; however, several scientists were aboard for testing and training purposes and disembarked in Panama.

Leg 1 C was the primary leg of GasEx-2001. The operations during the GasEx-2001 cruise was multi-faceted with high demands on ship s operations. A wide range of
intensive over-the-side measurements were performed including : the LADAS catamaran, the ASIS platform, CARIOCAISAMI buoy, and FSTP buoys, zodiac SMS and SPIP operations, CTD/Niskin sampling , underway seawater surface measurements and SPMR biological profiler measurements . In addition , atmospheric measurements were made using equipment mounted on the RHB bow tower, which is aft of the jackstaff; additional atmospheric measurements were made from a bow boom.

The Air-Sea Interactions Spar buoy, shown in Figure 2, was instrumented by RSMAS (PIs: Donelan and Drennan} and WHOI (PIs: McGillis , Edson, and Terray). This work was also conducted by Mike Rebozo (RSMAS), Joe Gabriele (CCIW), Sean McKenna (WHOI), Neil McPhee (WH4I), Tito Collasius (WHDI), and Ed Hobart (WHOI). The Air-Sea Interaction Spar (ASIS) is a hybrid spar buoy designed to provide a stable platform for near-surface measurements of air - and water-side fluxes . During GasEx-2001 it was deployed on four separate occasions -the table below lists the deployment times and locations . The top of the meteorological tower on the spar is roughly 5.5 meters above the mean water level (MWL), while the base of the spar is approximately 6.5 m below MWL . The buoy was equipped along its length with a variety of meteorological and oceanographic sensors , including sonic anemometers, C02 and water vapor sensors, and sensors to measure air temperature, relative humidity, barometric pressure, short- and long-wave radiation , surface waves (having wavelengths greater than 2 m), and near-surface profiles of temperature and current along the spar. In addition, we deployed adownward-looking ADCP at the base of the spar. This instrument measured current profiles to a depth of approximately 40 m. An inventory of the data collected is given in summary form in the second table below.

During the experiment, a surface platform train consisting of the ASISp s arY boy Figure 3), CARIOCA pCO2 buoy , and SAMI/YSI chain was constructed and de to ed. The platform aassembly stayed on the desired track line. Daily  rgos transmissions showed that ASIS was taking continuous , good quality measurements. CARIOCA drifted down current of ASIS, then the drogue . CARIOCA contained a single SAMI and a YSI at about 1.5 m. ASIS had 2 SAMIs and 1 YSI at about 1.5m, and a SAMI at about 5 m. The drogue had 2 SAMIs with YSIs (at 4 and 30 m) and 2 Langdon probes( at 10 and )15 m .  Two GPS/Argos trackers were mounted on CARIOCA, one on ASIS, and one on the GPS buoy on the drogue.

The Surface Processes Instrument Platform -- SPIP (Figure 4) is a 15-foot remotely - operated Hobie Wave catamaran and was used to measure the atmospheric radients of C02, temperature , water vapo r, and momgentum very close to the air-water interface. During this study, SPIP was deployed. as a self contained unit to determine thep rocesses that effect these air-sea exchanges and has supporting measurements of the water-side forcing . SPIP has the advantage of measuring gradients right at the surface that the Brown mast may miss and with potentially less flow distortion than the Brown mast and ASIS. Operationally , SPIP has a mast with fixed and traveling atmosp heric sensors. The fixed atmospheric sensors are located at the top of the mast while a second set of identical sensors are mounted to a motorized traveler with 3 meters of range on the mast track from the top of the mast to 30 cm above the water surface. From the measured gradients, we calculate the C02, latent, sensible, and momentum fluxes as well as the appropriate transfer coefficients for comparison to the direct covariance measurements and bulk formulae.

The ASIS/CARIOCAISAMI buoy train continuously measured surface forcing (Figures 5 and 6), air-sea fluxes , and vertical profiles of currents, temperature , pC02, and salinity. An intercomparison ofthe meteorological measurements made from the ship systems and ASIS MET systems is shown in Figure 7. The agreement between the systems is good. The steady winds found in the equatorial Pacific provide the capability to understand the role that surface diurnal heating , instabilities in the lower marine boundary layer, and stability in the upper surface ocean have on air-sea fluxes. Under these conditions, we will reduce our measurement uncertainty and
provide an accurate understanding of the air-sea C02 flux in this region.

During GasEx-2001, the ASIS, CARIOCA, and drifter buoy train were a central focus of surface process measurements. The measurements from this system are being used for collaborations with many interdisciplinary studies performed during GasEx-2001. Progress will continue on ASIS/BPIF data synthesis and analysis. The data has been analyzed for quality assurance and instrument calibrations.

The mean oceanic and atmospheric measurements (U, Ta, SST, q, C02), along with momentum and latent heat fluxes have been calculated. Inter-comparisons with other underway systems measurements have been performed. Corrections for surface drift have been applied to wind speed and momentum calculations. Alll-D and partial 2-D wave parameters (Hs, fp) and spectra have been calculated for the full ASIS deployment period (shown in Measurement Tables).

The physical response of the oceanic mixed layer to atmospheric forcing is shown in Figure 8. Stratification from solar heating causes an enhanced shear. The magnitude and dynamics of the shear is a balance between wind and wave forcing and stratification. We are now in the processes of correlating the surface ocean dynamics with atmospheric forcing and the subsequent gas exchange. Results were presented during the February 2002 AGU National Meeting in Honlolulu, and are being compiled in a manuscript to be submitted to a special issue of Journal of Geophysical Research.


Last updated: August 19, 2008

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