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Internal waves effect on near-surface chlorophyll concentration: remote sensing and modelling

Sérgio Muacho(1), José da Silva(1) and Vanda Brotas(1)

(1) Institute of Oceanography - University of Lisbon, Instituto de Oceanografia, FCUL, Campo Grande, 1749-016 Lisboa, Portugal

Abstract

This paper reports on the effects of internal tidal waves on near-surface chlorophyll concentration in the central region of the Biscay Bay (47º:45ºN, -8º:-6ºW), using synergy of ENVISAT MERIS, ASAR and MODIS image data. The observations cover the summer periods from 2002 to 2007.

Internal wave propagation introduces vertical motion within the water column (especially near the pycnocline), forcing water particles to undergo upward and downward motions. Since neutrally buoyant phytoplankton cells are usually passive, these cells can be significantly displaced in the vertical. Because light intensity decreases non-linearly (exponentially) with depth, internal waves will expose phytoplankton cells to a larger average light intensity. As a consequence, it is expected that internal waves affect primary production, producing an increase in biomass.

Ocean colour signatures of internal waves of tidal period have been identified in MERIS and MODIS images in the Bay of Biscay. A simple model has been proposed to explain these signatures based on the uplifting of a subsurface chlorophyll maximum by the passing internal tides. Furthermore, new observations suggest that high values of near-surface chlorophyll concentration can remain for several days after the peak of internal wave activity (spring tides). This may be the result of an effective increase of biomass. A simple numerical model has been developed to account for the effect of light variability in phytoplankton during internal wave events. The model describes the most relevant bio-physical processes, and successful explains and quantifies the biomass increase due to internal wave activity.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry