Mean CO2 concentration profiles i. The vertical distribution of stress in the atmospheric layer is shown; the green profile is the turbulent stress, the red profile is the viscous stress, and the orange line represents the wave-induced stress. Within the bubble plume, bubbles will either completely dissolve leading to a full exchange of gas or they will expand and contract while ascending to the surface giving a partial exchange of gas.
With an encompassing interfacial and bubble gas exchange model in place, interesting questions can begin to be answered.
Near Surface Turbulence and Gas Exchange Across the Air-Sea Interface
The obvious question pertains to high-wind events; to what extent does bubble mediated gas flux play a role and furthermore, under which sea states does bubble mediated flux dominate if at all? To what extent do isolated large wind and fetch event impact in overall gas exchange; is there a plateau in the total air-gas fluxes and under which sea-state conditions? Finally, how robust are these results under changing saturation and atmospheric gas concentrations.
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Sea spray and spray fluxes. Role of rain in upper ocean mixing and gas exchange. Gas flux through the bubbly upper ocean layer. Handler, B. While this bulk formulation is frequently used in this form, there are known issues with these approximations that are discussed elsewhere [26, 29].
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The issue of applying gas transfer velocities derived from other trace gases to CO2 exchange is presented. The impact of the recent reassessment of the inventory of excess C in the ocean is assessed. Excess14 C is the 14 C produced by nuclear bomb tests corrected for dilution by 14 C-free fossil fuel emissions. Henceforth, the excess C is referred to as bomb C. Over smooth surfaces there is a weak dependence of gas exchange with wind that closely follows theoretical considerations of transfer across a smooth wall .
Once capillary and capillary-gravity waves form, the linear dependency strengthens appreciably. The onset of breaking waves enhances the gas transfer and gas transfer shows a solubility dependence with gases of lower solubility, showing a stronger enhancement. Liss et al. Because of limitations of wind-wave tank studies, most empirical gas exchange-wind speed relationships are either derived from observations over the ocean or scaled to such studies. Initial studies over the ocean were performed using the Rn disequilibrium method.
The results showed no discernable trend with wind . Several, but not all, of the limitations of the Rn are circumvented using injected tracers into the surface mixed layer. Wanninkhof While these dependencies are well established based on theoretical and experimental considerations [10, 20], it is important to consider that the interrelationships will break down under conditions of bubble entrainment.
This is of particular concern when the results of the dual tracer technique using the gases 3 He and SF6 that have very low solubilities are used to estimate the exchange of CO2 which has a higher solubility. For example, Ho et al. The k is the gas transfer velocity, k, normalized to a Schmidt number of according to Eq. As shown in Fig. The adjustment procedures are strictly only applicable for situations where the gases are far from equilibrium. This method takes advantage of the rapid increase of 14 CO2 in the atmosphere in the s due to testing of thermo-nuclear devices.
The atmospheric 14 C anomaly is followed as it penetrates into the ocean. Wanninkhof  used this estimate, along with an inferred 1 Impact of Gas Exchange Formulations and Wind Speed on Global CO2 5 quadratic functional dependence with wind, to obtain a global parameterization of gas exchange with wind speed. In this case, the invasion rate of CO2 was assumed equivalent to that of 14 CO2 , and the average mixing ratio of CO2 in the atmosphere in , at the peak of nuclear bomb testing, was used. A global average wind speed normalized to 10m height U10 of 7. This is, in part, due to the fact that many of the older general circulation models are tuned to or validated with the same bomb C inventories in the ocean.
Therefore, while the global gas transfer velocity could be estimated from the invasion rate , the functional form of the relationship between gas exchange and wind had to be obtained by other means. A quadratic dependence was suggested since this was the approximate dependence observed in wind-wave tanks . Moreover, wind stress 2 scales with U10 , and some theories suggest that gas exchange scales with stress. Monahan was one of the original proposers of a cubic dependence of gas exchange and wind speed .source link
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In this formulation, it is implicitly assumed that bubbles have a controlling role on air-sea gas transfer. Several improvements of these global empirical parameterizations have been 6 R. Wanninkhof developed that include boundary layer stability criteria [12, 15], and both bubble-mediated exchange and exchange over the air-water interface . An important advance over the last decade has been the improved wind speed measurements over the ocean from active and passive microwave sensors on earth-orbiting satellites.
These measurements provide coverage of much of the ocean surface, once or twice a day, at a resolution of 25 km. It was known that wind speed distributions vary by location and by averaging time, but lack of winds at high resolution prevented an exact solution. With the remotely sensed winds it is now possible to determine gas transfer velocities without needing to assume a particular wind speed distribution curve. The results of these changes are shown in Table 1. It is also of note that while many of the proposed relationships have a zero intercept, there is little evidence to support this premise.
McGillis et al. Separation of the bomb C contribution from the natural background was problematic . A simple box model used in the original analysis  could roughly reproduce the observed surface values and basin inventories obtained dur- 8 R.
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Wanninkhof Table 1. The resulting global average winds Uav are listed in parentheses. The change in a for a quadratic or cubic dependence, respectively, is listed in parentheses. The controversy about the bomb C inventory in the ocean and resulting global 14 C constraint started when the inventory values  were put in question by an independent stratospheric 14 C constraint and a global mass balance .
Change in bomb C inventory over time. The solid line is the result of the box model optimized for basin-wide 14 C inventories and surface concentrations  as recently rerun by Peng pers. The points are the model and data-based estimates listed in Table 1. This was followed by an analysis which suggested the results could be reconciled if a more sophisticated ocean model was used . The comparison of estimates based on data from the s is complicated by the rapid rise of 14 C in the ocean during this time Fig.
Of note is that the original optimized ocean model results  fall below the estimates of global inventory see Fig. Currently, inventories are estimated for two time periods from large hydrographic surveys that were conducted in the s the Geochemical 10 R. A summary of the estimates is listed in Table 1. The observational bomb C inventory is 3.
As indicated in Table 1. Also, general uncertainties in ocean transport preclude determining a robust functional dependence. This approach yielded a gas transfer velocity of The estimate of Krakauer et al. In Krakauer et al. An important point that has been largely ignored in many estimates of global CO2 uptake by the ocean is that the relationships of gas exchange and wind speed should be scaled to the global average squared wind of the wind speed product used . For instance, if a global wind estimate of 7. If an average global wind of 6.
Estimates of global bomb C inventories. Source Estimate [ atoms] Broecker et al.
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Key et al. Comparison of inventories Table 1. While important improvements have been made in 14 C inventory estimates, separation of bomb 14 C from natural 14 C, and modeling of 14 C in the ocean, the recent analyses bear more scrutiny before considering them as conclusive. Based on this work, it appears that the popular estimate developed in the s  is on the high side of the envelope but well within the uncertainty of believable estimates. Global gas transfer rates estimated from bomb C.
High-resolution data are critical for non-linear relationships between wind speed and gas exchange in which the distribution of winds is important, in addition to the mean wind. The average number of observations per pixel per month is but at high latitudes this value can be as low as This hybrid dataset has shortcomings with regard to possible biases, particularly as this dataset encompasses four years before QuikSCAT data were available. Also, algorithms for satellite winds get improved as well, and the data undergo periodic reprocessing. Both factors have been taken into consideration in previous estimates of interannual variability for .
Here we look at variability from and utilize more comprehensive wind statistics that are available for the more recent data. The QuikSCAT data came online in mid such that the datasets before and after are disparate. Any changes should be interpreted with caution. The results in Fig. Similar year-to-year global average winds do not exclude regional variability.
The resulting zonal averages are shown in Fig. The average standard deviation for all pixels over the last decade is 5. With a non-linear dependence on gas exchange, it will be the high latitudes with higher winds that will experience the greatest year-to-year absolute variability in gas transfer velocity.