Ocean colour remote sensing in the southern Laptev Sea: evaluation and applications

[摘要] Enhanced permafrost warming and increased Arctic river discharges haveheightened concern about the input of terrigenous matter into Arcticcoastal waters. We used optical operational satellite data from the oceancolour sensor MERIS (Medium-Resolution Imaging Spectrometer) aboard the ENVISAT satellite mission for synopticmonitoring of the pathways of terrigenous matter on the shallow Laptev Seashelf. Despite the high cloud coverage in summer that is inherent to thisArctic region, time series from MERIS satellite data from 2006 on to 2011could be acquired and were processed using the Case-2 Regional Processor(C2R) for optically complex surface waters installed in the open-source software ESA BEAM-VISAT.

Since optical remote sensing using ocean colour satellite data has seenlittle application in Siberian Arctic coastal and shelf waters, we assessthe applicability of the calculated MERIS C2R parameters with surface watersampling data from the Russian–German ship expeditions LENA2008, LENA2010and TRANSDRIFT-XVII taking place in August 2008 and August and September2010 in the southern Laptev Sea. The shallow Siberian shelf waters areoptically not comparable to the deeper, more transparent waters of the ArcticOcean. The inner-shelf waters are characterized by low transparencies, dueto turbid river water input, terrestrial input by coastal erosion,resuspension events and, therefore, high background concentrations ofsuspended particulate matter and coloured dissolved organic matter.

We compared the field-based measurements with the satellite data that areclosest in time. The match-up analyses related to LENA2008 and LENA2010expedition data show the technical limits of matching in optically highlyheterogeneous and dynamic shallow inner-shelf waters. The match-up analysesusing the data from the marine TRANSDRIFT expedition were constrained byseveral days' difference between a match-up pair of satellite-derived andin situ parameters but are also based on the more stable hydrodynamicconditions of the deeper inner- and the outer-shelf waters. The relationshipof satellite-derived turbidity-related parameters versus in situ suspended matter fromTRANSDRIFT data shows that the backscattering coefficient C2R_bb_spm can beused to derive a Laptev-Sea-adapted SPM algorithm. Satellite-derived Chl aestimates are highly overestimated by a minimum factor of 10 if applied tothe inner-shelf region due to elevated concentrations of terrestrial organicmatter.

To evaluate the applicability of ocean colour remote sensing, we include thevisual analysis of lateral hydrographical features. The mappedturbidity-related MERIS C2R parameters show that the Laptev Sea is dominatedby resuspension above submarine shallow banks and by frontal instabilitiessuch as frontal meanders with amplitudes up to 30 km and eddies andfilaments with horizontal scales up to 100 km that prevail throughout thesea-ice-free season. The widespread turbidity above submarine shallow banksindicates inner-shelf vertical mixing that seems frequently to reach down tosubmarine depths of a minimum of 10 m. The resuspension events and thefrontal meanders, filaments and eddies indicate enhanced vertical mixingbeing widespread on the inner shelf.

It is a new finding for the Laptev Sea that numerous frontal instabilitiesare made visible, and how highly time-dependent and turbulent the Laptev Seashelf is. The meanders, filaments and eddies revealed by the ocean colourparameters indicate the lateral transportation pathways of terrestrial andliving biological material in surface waters.

[学科分类] 地球化学与岩石