Suppression of CO₂ Outgassing by Gas Bubbles Under a Hurricane

Abstract

The role of gas bubbles on the air-sea CO 2 flux during Hurricane Frances (2004) is studied using a large-eddy simulation model that couples ocean surface boundary layer turbulence, gas bubbles, and dissolved gases. In the subtropical surface ocean where gases are slightly supersaturated, gases in bubbles can still dissolve due to hydrostatic pressure and surface tension exerted on bubbles. Under the simulated conditions, the CO 2 efflux with an explicit bubble effect is less than 2% of that calculated using a gas flux formula without explicit inclusion of bubble effect. The use of a gas flux parameterization without bubble-induced supersaturation contributes to uncertainty in the global carbon budget. The results highlight the importance of bubbles under high winds even for soluble gases such as CO 2 and demonstrate that gas flux parameterization derived from gases of certain solubility may not be accurate for gases of very different solubility. Plain Language Summary Carbon dioxide (CO 2) is the primary anthropogenic greenhouse gases in the atmosphere and is the gas most responsible for global warming. The ocean is an important sink of anthropogenic atmospheric CO 2 , yet the exchange of CO 2 between the ocean and the atmosphere is not fully understood. This study reexamines the exchange of CO 2 under hurricanes over the low-latitude ocean, using hurricane Frances (2004) as an example. Previous studies show that hurricanes significantly facilitate the outgassing of CO 2 due to the extreme wind. Those studies, however, do not explicitly consider gas bubbles. Gas bubbles, entrained into the ocean when ocean wave breaks, are ubiquitous under hurricanes also due to the extreme wind. While in the ocean, gas bubbles not only move around under the influence of the chaotic wind-driven currents and their own buoyancy, but they also exchange gases with the water. Our study, using state-of-the-art computer models that concurrently simulate the chaotic ocean currents, gas bubbles, and dissolved gases, demonstrates that hurricane plays a significantly smaller role in the ocean-atmosphere transfer of CO 2 than previously estimated.

Mesoscale Temporal Wind Variability Biases Global Air–Sea Gas Transfer Velocity of CO2 and Other Slightly Soluble Gases

“…With constant I = 0.15, Equation ( 14) and Equation ( 16) can be used to approximate unbiased f(U), and k considering differences in Sc for CO2 and other gases (see [33] for Sc of other gases). The same f(U) parameterization for all slightly soluble gases may not be realistic for gases with differing solubilities [55][56][57], but this inquiry is better kept for the future. The newly proposed corrections were compared to three commonly used methods (listed in Table 2) when adjusting wind variability-induced bias in k for wind speeds sampled at a resolution of 0.5° × 0.5° spatially and temporally monthly.…”

Gas Fluxes and Steady State Saturation Anomalies at Very High Wind Speeds

“…However, the flattening of the fluxes and steady state saturation anomalies at very high wind speeds might not occur if deep bubbles were present and could transfer large amounts of gas since deep bubbles provide a mechanism for enhanced bubble injection when shallow bubble injection reaches a maximum. Thus, it is imperative for more field observations of gas fluxes and steady state saturation anomalies in high wind conditions (such as work by Liang et al [2020] and McNeil and D'Asaro [2007] on measurements collected autonomously in typhoons). Thus, the main power of this study is that it points to intriguing behavior that should be tested in the real ocean (i.e., the flattening of the fluxes and steady state saturation anomalies) as well as key variables (significant wave height, wave steepness, and wave dependent Reynolds number) that should be further studied.…”

The Intensifying Role of High Wind Speeds on Air‐Sea Carbon Dioxide Exchange

“…Furthermore, the gas transfer velocities estimated using k D18 are in good agreement with eddy covariance field measurements for U in the range of 0–25 m s −1 . Hence, the k D18 may indirectly include bubble‐induced supersaturation effect though it remains unclear whether the equation derived in CO 2 sink regions can be applied in CO 2 source regions (Liang et al., 2020). Second, time‐averaged U products can introduce some biases in all k due to the non‐linear relation between U and k (Wanninkhof, 2002), a topic that is better kept for a future inquiry.…”

“…First, the k D18 and k W14 expressions do not explicitly consider bubble-induced supersaturation. Using Large Eddy Simulations coupled with bubble dynamics and a transport model, Liang et al (2020) showed that bubble supersaturation under hurricane conditions can substantially suppress CO 2 outgassing. It is difficult to extrapolate their results here as hurricane winds commence at 33 m s −1 , while 99% of the HW considered here are in the range of 15-18 m s −1 (Figure S5a).…”

“…Over tropical oceans, HW occur infrequently (<1%) and are predominantly associated with sporadic tropical cyclones (Sampe & Xie, 2007). While the impact of such extreme wind events (U > 33 m s −1 ) on k has been explored in earlier studies (see references above and Lévy et al, 2012;Liang et al, 2020), the broader implications of HW on k is unclear.…”