Sentinel-3A satellite

Discrepancies in satellite chlorophyll are half what we previously thought

European Space Agency (ESA) 

Accurate chlorophyll measurements in the ocean are crucial for understanding environmental change.  

It is impossible to take enough samples of the global ocean in situ to obtain a comprehensive map of chlorophyll measurements for a given week or month, therefore satellites are vitally important because they can observe vast areas of the ocean in a matter of days that are difficult to access and sample using traditional methods. Satellite data therefore offers a cost-effective means of providing a global overview of oceanic conditions. However, in order for these observations to be meaningful they need to be corroborated by measurements taken in the field.

Our current understanding of the accuracy and precision of satellite ocean-colour chlorophyll data is impeded by the limited number and geographical coverage of in situ data coincident with the satellite data.  In addition the distribution of existing observations is often biased towards coastal waters, with limited data in the remote oligotrophic waters, despite these constituting the majority of the surface ocean.

Recent work by AMT4SentinelFRM scientists measured chlorophyll on two Atlantic Meridional Transect (AMT) cruises using a relatively new technique that uses spectrophotometry to measure  the optical properties of particles sampled by the flow-through system of the moving ship.  This provides continuous highly accurate measurements of chlorophyll meaning that there are more match-ups with the satellite overhead, giving multiple in situ reference measurements matched to each satellite pixel.  Compared to previous work where the difference in scale from a single in situ measurement to a 4km square pixel is immense, this method has proved to be better suited for validating satellite data.

Bob Brewin, the lead author on the paper, said “What is really important to note about this work is that by using this new technique, we found the errors in satellite chlorophyll measurements are half of what we previously thought, so the data can be used with greater confidence to help understand the dynamics of the ocean and predict how it may change in the future.”

The AMT transect is of particular value as it covers a vast range of environments from the productive coastal regions to the desert-like gyres in the centre of the ocean, which are rarely accessed by research ships, thus enabling the satellite data to be validated under a wide range of conditions.

The paper highlights the benefits of using these underway spectrophotometric systems for evaluating satellite ocean colour data and underlines the importance of in situ observatories, particularly in the oligotrophic gyres.  This method will be utilised as part of the AMT4SentinelFRM project to validate satellite data during the AMT26 cruise.

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