Marine carbon sources and sinks assessment

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 VOS line mission

Atmospheric inversions
Atmospheric transport models using CO2 distribution measurements indicate that there is a large sink of CO2 at mid to high northern latitudes. However, the locations and mechanisms for this CO2 sink remain difficult to identify. In addition to a large oceanic sink, centred in the temperate and sub-polar North Atlantic, it is recognised that there must be large mid-latitude sinks in the terrestrial biosphere. The carbon budgets of Europe and North America cannot however at present be reliably separated by atmospheric inversions. An important reason contributing to this uncertainty is the fact that the North Atlantic sink must be accurately known to partition the land sinks with precision. Better determination and monitoring of these sinks is therefore important for the Kyoto emission reduction targets to be successfully implemented and verified. Theme 1 of CARBOOCEAN will reduce the uncertainty in the North Atlantic sink by providing a seasonally resolved estimates if fluxes into the North Atlantic. Theme 1 will include atmospheric inversion modelling using the fluxes inferred from the observing system, and these studies will be integrated with those to be done under Theme 3 (coastal ocean) CarboEurope IP.
North Atlantic Ocean carbon fluxes
During the FP5 CAVASSOO project, we tested elements of an observing system designed to deliver accurate estimates of the flux of CO2 into the North Atlantic. To be useful to constrain atmospheric inversions, such a system must have basin-wide coverage and a time resolution approaching one month – since it is known that the sink will be highly variable on from one season to the next. The backbone of such a system must be the use of “voluntary observing ships” (VOS) instrumented to measure surface pCO2 and ancillary measurements, including nutrients, temperature and salinity. During CAVASSOO, we have shown that a network of such ships can produce accurate and complete data using automated instruments with no scientist on board, We have been able to produce maps of CO2 in much better detail than ever before, for limited areas of the North Atlantic. We have additionally shown during CAVASSOO that the North Atlantic sink appears to be changing – decreasing slightly, rather than increasing rapidly as ocean carbon model studies suggest. Such studies have shown us how to design an observing system that can deliver the required information needed to accurately specify the North Atlantic. The CARBOOCEAN system will have a better coverage of VOS and will supplement them. It will consist of:
• Increased coverage by 6 VOS lines funded from Europe, in combination with VOS lines of our US colleagues
• Time series at selected sites
• High accuracy atmospheric sampling for CO2 and O2/N2 ratios from selected vessels and sites, as an added constraint on atmospheric inversion studies.
In addition, in recognition that time series of the surface at remote locations would be very helpful in such a system, we will study a modification to an ANIMATE-style mooring to enable surface-layer measurements.
We include in the North Atlantic the equatorial Atlantic: We expect the tropical regions to be variable. For example, the Amazon influences the carbon budget in the western equatorial zone, but there are comparatively few measurements there. New VOS lines going into the tropics will be commissioned, and we will take advantage of a collaboration to put CO2 measuring equipment on PIRATA moorings in the equatorial region to produce time series here.
Interpolation, data assimilation
It will be necessary to interpolate the data from the network to the entire North Atlantic. During CAVASSOO we have investigated how to do this, and have tried both regression techniques and neural networks to attempt the interpolation in time and space, with encouraging results. We expect that by including remotely sensed parameters such as chlorophyll and other ocean pigments we will be able to improve on our present methods and produce more detailed estimates of the surface pCO2. Remote sensing will also be used to derive estimates of the gas transfer velocity, and hence CO2 fluxes.
Longer-term, a better method for interpolating the data would be to assimilate it into a model of the CO2 fluxes for the regions under study. Several modelling groups will be investigating the route of data assimilation using different ocean carbon models having different strengths. In addition, we will investigate the use of very high resolution models of the North Atlantic, and how assimilating physical data into models in the EU-MERSEA IP on operational ocean models.
Southern ocean biogeochemistry
The Southern ocean is the least well measured region for air-sea fluxes. It is the region of most disagreement between models, such that it is not even clear whether it is a source or a sink for atmospheric CO2. Such measurements as we do have, suggest that the sub-Antarctic, where mode waters are formed, are a relatively strong sink region. This seems to be equally true for the Atlantic and Indian sectors of the Southern Ocean.
Because of the its remoteness, the Southern Ocean cannot be studied using the same methods as the Atlantic. We will use the new European CARIOCA buoys to obtain data over long periods from this remote area. We will contribute a new set of observations for the Southern Ocean, concentrating on the sub-antarctic in the Indian and Atlantic sectors. We will attempt to define the net CO2 flux there, and understand the processes responsible for it. The CARIOCA data will be supplemented by those from Antarctic supply vessels. Remote sensing data will be used to interpolate and extend the data, in a similar manner to that described above for the North Atlantic.


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