New techniques to help understand the ocean

The primary objective of the project was to obtain accurate measurements of the air-sea flux of CO₂, which are essential for understanding the role of the oceans in mitigating anthropogenic climate change. To achieve this, AMT4OceanSatFlux used a new technique, deploying cutting edge instrumentation to directly measure CO₂ flux, alongside state of the art calculations from satellite products simultaneously captured over the Atlantic Ocean. Assessing the performance of satellite measurements of CO₂ flux with this high quality in situ data allows us to use these satellite estimates with greater confidence. A global perspective of ocean sources and sinks of CO₂ that can be obtained from satellite data, can be used to address large scale issues and scientific questions on the effects of climate change.

Measurements of CO₂ in air and seawater have been made on Atlantic Meridional Transect (AMT) field campaigns for over a decade, however direct estimates of CO₂ flux have not been possible until now. Using an eddy covariance technique, where high frequency measurements of wind velocity are used to determine the vertical flux of gases, AMT4OceanSatFlux was able to directly measure the air-sea flux of CO₂. 

To date the oceans have absorbed around 25% of all human generated CO₂, which has helped to slow the impact of climate change on land, but is fundamentally changing the chemistry of the ocean. Long-term absorption of atmospheric CO₂ is causing a gradual decrease in seawater pH, a phenomenon called ocean acidification. This is having an adverse effect on many important marine species such as corals, oysters, crabs and plankton. AMT4OceanSatFlux have developed and evaluated algorithms that estimate pH, CO₂ levels in the water and aragonite saturation state - the chemical that many organisms need to build their skeletons and shells. In this project, Satellite data is being used to identify areas of the ocean that are most at risk from acidification.

Another major strength of the project is that measurements of gas exchange are complemented by a suite of underway measurements including sea surface temperature, ocean surface roughness, and phytoplankton chlorophyll to quantify the biological component of the ocean carbonate chemistry and cycling of CO₂. Phytoplankton production provides direct linkage to the oceans capturing and locking up CO₂. This parameter can also be derived from satellite measurements of ocean colour. There are many ocean colour products available. However, the most accurate of these need to be selected for use in linking to the CO₂ flux, to and from the ocean. AMT4OceanSatFlux has undertaken a rigorous performance assessment to select the most suitable phytoplankton product for different waters. This utilised in situ data from three AMT cruises, equating to over one million data points with over 800 match-ups with satellite observations. AMT is an ideal platform as the ship passes through a huge range of environmental conditions and water types, including the infrequently sampled oligotrophic gyres. This work represents a significant advance in data from the oligotrophic regions and has resulted in recommendations on which Copernicus Sentinel 3A and 3B OLCI (Ocean and Land Colour Instrument) products perform best in the sparsely sampled Atlantic gyres.

The techniques and algorithms developed by AMT4OceanSatFlux will continue to be used into the future to provide high quality in situ and satellite data to help understand the cycling of carbon in the ocean and atmosphere.