Analysis of the presence of internal waves on the Gulf of Valencia continental slope and their role with sediment transport

Abstract

The Gulf of Valencia (GoV) is located in the northwestern Mediterranean, between the Ebro Delta and the promontory Cap La Nao. This area receives most of the sediment transported in a southwesterly direction along the Ebro margin and it is characterized by water mass mixing between southwestwards flowing Northern Current, carrying old Atlantic Waters (AW), and the northward intrusions of recent AW through the Ibiza Channel. Previous studies have evidenced the presence of several intermediate and bottom nepheloid layers along the central and southern part of the GoV continental slope, mainly between 400 and 600 m, where the Levantine Intermediate Water (LIW) interacts with the seafloor. Such nepheloid layers have been related to the presence of internal waves resuspension processes on the mid-slope region. To investigate the presence of internal waves in the GoV slope, and to analyze their role in the suspended sediment transport, a mooring line was deployed in the GoV at 578 m, between the 3th of October 2011 and the 3th of March 2012. This mooring was equipped with a 300 kHz four-beam RDI Acoustic Doppler Current Profiler (ADCP) at 80 m above bottom (mab) and a thermistor string with 120 high resolution thermistors, between 5 and 70 mab, and 10 turbidity Optical Backscatter Sensors (OBS) in between them. Results of this deployment showed the main current on the GoV slope was ~ 20 cm/s towards the SSE, and clearly affected by the local bathymetry. Shear turbulence was detected at the boundary layer, generated by the interaction between the warmer (and saltier) LIW and the colder (and fresher) Western Mediterranean Deep Waters (WMDW). Continuous water overturning could be recorded in the thermistor time series. However, changes in the velocity direction, towards the north, with an inertial periodicity, were observed and were directly related with LIW warm water intrusions (increases of 0.1 ºC). A shear-induced turbulence was registered during these periods and also high frequency internal waves were identified in the current velocity. During eastward and upslope phase of the inertial motions a much larger convective turbulent boundary layer was observed. Although a good relation was found between velocity variations and temperature oscillations, sediment transport was barely related with these changes when low turbidity was recorded. However, during the periods of relatively high suspended sediment, high frequency oscillations were correlated with temperature and current variations