Early white paper draft version- October 2013
This preliminary draft is intended as a consultation document.
Comments/edits welcome. Please do not use or cite.
Theme 2: The air-sea interface and fluxes of mass, energy
Co-authors: Brian Ward and Christoph Garbe
1. Brief statement defining the theme:
The transfer of material across the air-sea interface is controlled by several processes including wind, waves, turbulence, bubbles, sea spray, rain, and surface films. Air-sea transfer is mostly modelled by various functional dependencies of the wind speed, but more sophisticated measurement techniques and physically based parameterisations are required to adequately describe air-sea fluxes.
2. The scientific and societal basis justifying research on this issue. Why is it critical and why does it need to be done now? What is the end goal? Why is international coordination required?
The scientific and societal basis justifying research on this issue: Atmospheric greenhouse gas (GHG) concentrations (CO
2
and non-CO
2
) are about 40% higher today than they were in 1850 and surpass all levels in the past 2M years. It is estimated that a business-as-usual scenario could cost at least 5% of annual global gross domestic product (GDP). Reduction in GHG emissions to 80-95% of 1990 levels could limit this economic cost to 1% of annual GDP.
Ocean-atmosphere fluxes are some of the most critical factors affecting climate. However, these fluxes remain poorly quantified. For this reason, there is an urgent requirement to generate more scientific expertise for the quantification of air-sea fluxes.
On regional scales, air-sea fluxes are a driving factor of air-quality and ecological communities. On these scales, fluxes are even less well constrained than on global ones. This is mainly due to the inhomogeneous nature of processes and their complex interactions on a wide range of scales.
There is also a strong policy requirement: nothing in the current international agreements on climate specifically addresses the effects of GHG absorption in the oceans. But it is likely that future treaties will require accurate quantification of the transport of GHGs across the air-sea interface. The scientific community therefore needs to act now to develop skills required to address this.
Why is it critical and why does it need to be done now? Recent evidence from observations of air-sea fluxes has shown that the North Atlantic efficiency in CO
2
uptake is declining (Watson et al., 2009). The question as to whether this is a trend or whether it will fluctuate remains to be answered. At the same time, regional and mesoscale numerical models have been making significant progress in these areas, but are currently limited by our current inability to produce an accurate parameterization of fluxes.
What is the end goal? The end goal is to parameterise air-sea exchange with processes that more completely describe the transport, and to accurately predict the transfer over the full range of conditions. This prediction will not be limited to global scales, but also include meso- and regional scales.
Why is international coordination required? This challenge can only be addressed at the international level, as individual countries have neither the means nor the expertise to address all the scientific issues. In order to make progress, a strong interlink between measurement facilities and techniques as well as modelling capabilities are required. Particularly for regional studies, expertise in the uniqueness of the region at hand is also required.
The challenge of Future SOLAS is a global one, and meeting these challenges cannot be at the national level.
3. Background – major scientific concepts, key prior work defining the issues:
The oceans are the biggest reservoirs for heat, freshwater, and CO
2
, and distribute these constituents globally.
Ocean-atmosphere fluxes are the exchanges of these quantities at the air-sea interface. Uncertainty in the quantification of these fluxes inhibits our ability to model a changing climate. Accurate knowledge of air-sea
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fluxes is essential for assessing the ocean’s role in climate variability, understanding climate dynamics, for forcing ocean models for predictions from weather to climate timescales.
It is well recognised that the ocean and atmosphere are a coupled system which affects and is affected by climate and environmental change. Understanding the air-sea interconnection requires that the processes that control the transport across the interface be accurately quantified. The primary forcing factor for air-sea exchange is wind speed, which induces near-surface turbulence, causing the molecular boundary layer to be eroded thereby enhancing transport across the interface. Traditionally the air-sea exchange of the physical parameters (momentum, sensible & latent heat) are the best understood as they have been the most widely measured, and advances in understanding the fluxes of gas and aerosols are largely based on heat fluxes. Of the GHGs, we are primarily concerned with CO
2
as this contributes to radiative forcing more than the others. The air-sea exchange of aerosols is largely driven by wave breaking at the ocean surface. Turbulence in the ocean surface controls the air-sea exchange of heat, GHGs, and momentum. These exchanges heavily control the ability of the oceans to regulate climate. The ocean surface boundary layer (OSBL) also controls the subduction of atmospheric material into the ocean interior through the boundary conditions to deep-ocean stratification and dynamics. The surface ocean is therefore critical in determining the role of global ocean circulation on climate.
Quantitative understanding of the turbulent processes in the OSBL is likely to be the key to understanding airsea exchange and the role in affecting climate.
Figure 1: Processes controlling air-sea fluxes of heat, gas, momentum, and aerosols.
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Figure 2: Simplified schematic of factors influencing air-sea CO
2
fluxes.
4. Approaches – what will it take to make substantive progress on the issue? What will be achieved in the 10 years of Future SOLAS?
Approaches what will it take to make substantive progress on the issue?
• Technology development for new observations: improved sensitivity and accuracy for sensors for direct gas and particle fluxes; longer battery life; deployment of microstructure sensors on platforms such as autonomous gliders and floats.
• More realistic, multidisciplinary experiments, e.g. wind/wave channel experiments with full saline water, including biology, surface films
• Linking measurements and models through advances in coupled models, simulating gas transfer over a wide range of Schmidt numbers.
What will be achieved in the 10 years of Future SOLAS?
Ability to accurately quantify air-sea fluxes of GHG’s and particles through new observations and to better parameterize these fluxes for inclusion into Earth-System models. Significant progress will be made particularly on regional and meso-scales.
5. Community readiness – is there an existing community engaged on this issue? Are there institutional or other barriers to progress? Is infrastructure or human capacity building required in order to achieve the goals?
The need for progress in understanding air-sea fluxes has been widely recognized. The community is strongly pushing forward. The questions of air-sea fluxes are major themes in national and international programs, such as the German SOPRAN or the international CARBOOCEAN initiative. Recently, there has also been a push from the earth observation community through ESA’s Oceanflux projects. Currently available satellite products have been merged to refine a more physically based parameterization and improving the accuracy of flux estimates.
Is infrastructure or human capacity building required in order to achieve the goals?
Capacity building is strongly needed, in order to gain expertise in the complex processes involved and their influence ranging from small-scale processes all the way to ecological impact and regional effects. Even though a strong infrastructure is in place for global assessments, this is still lacking for a wide range of very important regional studies.
One of the fundamental goals for FE is to bring capacity building to developing countries. Future SOLAS should consider establishing capacity building programmes in developing countries whereby SOLAS scientists and PhD students would spend short periods training students in SOLAS science.
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6. External connections – what partnerships are required in order to achieve the goals? What mechanisms will be used to accomplish the interactions?
What partnerships are required in order to achieve the goals?
Strong international partnerships between observationalists (linked studies in the laboratory and in the field), theoreticians, modellers and earth observationalists are required in order to achieve the set goals. Through the multifaceted and highly complex processes, strong links between disciplines such as physicists, chemists, biologist and ecologists are required. Particularly on regional scales, on which air-sea fluxes have a significant impact on ecosystems and human activities, research of this theme needs to be put in a wider context.
7. Sustainability – articulate relationship (if any) between this project and the FE goals of Global
Development and Transformation Towards Sustainability.
Transport of heat and mass across the air-water interface directly impacts ecosystems and is central to a number of different services important to achieve FE's goals of global development and transformation towards sustainability. Regulating services such as air quality, waste treatment, climate regulation and nutrient cycles are becoming increasingly more important, also for policy makers. Only through a better understanding of fluxes and relevant processes at the interface can predictions be made and future impacts be assessed. This theme is central to FE and might also be cross cutting to other projects such as IGAC, IMBER, iLEAPS, PAGES and
CLIVAR.
References:
Watson, A. J., et al. (2009), Tracking the variable north Atlantic sink for atmospheric CO
2
,
Science , 326, doi:10.1126/science.1177394.
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