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Ocean Surface Topography from Space
TripleA — PML's planned use of altimetry in the Arctic, Atlantic and Agulhas regions


Graham Quartly - (Plymouth Marine Laboratory)

  Andrey Kurekin
Ben Loveday
Peter Miller
Francesco Nencioli
(Plymouth Marine Laboratory)
(Plymouth Marine Laboratory))
(Plymouth Marine Laboratory)
(Plymouth Marine Laboratory)

TripleA — PML's planned use of altimetry in the Arctic, Atlantic and Agulhas regions
One aspect of TripleA is to examine scales of variability in the Atlantic, and their effect on productivity, making use of the annual AMT (Atlantic Meridional Transect) cruise.. (left) Chlorophyll distribution, with the 25 different cruise tracks overlaid (from Rees, A.P. et al., 2015). (right) R.m.s. of total current (m/s) from AVISO data.
This proposal details the breadth of altimetry investigations planned to be carried out at Plymouth Marine Laboratory (PML) over the next 4 years.  It has a global calibration/validation component (hereafter 'cal/val') relating to sigma0, wind speed and rain-flagging, plus three regional foci - the Arctic, the Atlantic and the Agulhas. 

The activities brought together in this proposal include some aspects that have already commenced, some that are financially supported, plus some for which the necessary funding will be sought at the due time.  Similarly some components will focus on altimetry data collected in 2016 onwards (the Jason-3/Sentinel-3 era), whilst others will utilise the longer extent of altimeter data, combining them with different satellites or in situ measurements.

The global cal/val will focus on the metocean data from Jason-3: significant wave height (SWH), normalised backscatter strength (σ0) and further derived products such as rain rate.  A strong strand of the work will concentrate on measuring and understanding the differences between records for Jason-2 and Jason-3 during the tandem phase (February-September 2016), but will also look at longer time changes for the Jason series of altimeters over 2002-2020.  Improvements to the operational rain-flagging algorithm will be produced that can be implemented by the data distribution services.

Investigation in the Arctic will build on recent developments of a new classification and retracking algorithm that allow a better recovery of sea level information in leads (sea-ice fractures).  This will primarily focus on the ERS, Envisat, Sentinel-3 and AltiKa instruments that operate north of 66˚N, with the aim of understanding the seasonal and interannual changes in sea level and thus, via the geostrophic assumption, in circulation.

Our work in the Atlantic will address a number of issues.  Near the UK, we will expand on our work on the European Shelf Current to look at techniques for combining altimetry with in situ measurements from floats, gliders and ships.  We will also be looking at the prevalence of frontal structures further offshore, comparing features noted in infra-red and ocean colour imagery with Lagrangian Coherent Structures derived from merged maps of altimetry-based surface geostrophic currents.  On a larger scale we will investigate the spectral characteristics of currents from shipborne ADCP along the Atlantic Meridional Transect, and contrast that with the view from near-instantaneous altimeter transits.

Part of the Agulhas work will look at the challenge of combining an altimeter's measurement of across-track currents with the directional information that may be gained from optical or thermal imagery.  We will also assess the reliability of the directional information obtained by standard mapping techniques.  This research is concentrated on the area south of Madagascar, where we have a 14-month current meter record for three locations under a Jason track.  This work may later be expanded to analyse the whole of the Agulhas region.  Finally, the connection between the Agulhas and the Atlantic will be studied using specialised eddy-tracking software to follow eddies from the Agulhas Retroflection across the South Atlantic, combined with Argo and Bio-Argo observations to identify and quantify the main transport and exchange patterns of heat, salt and, potentially, biogeochemical variables such as oxygen and nutrients associated with Agulhas Rings.

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