Shelf response to intense offshore wind

Abstract

Cross and along‐shelf winds drive cross‐shelf transport that promotes the exchange of tracers and nutrients to the open sea. The shelf response to cross‐shelf winds is studied in the north shelf of the Ebro Delta (Mediterranean Sea), where those winds are prevalent and intense. Offshore winds in the region exhibit strong intensities (wind stress larger than 0.8 Pa) during winter and fall. The monthly average flow observed in a 1 year current meter record at 43.5 m was polarized following the isobaths with the along‐shelf variability being larger than the cross‐shelf. Prevalent southwestward along‐shelf flow was induced by the three‐dimensional regional response to cross‐shelf winds and the coastal constraint. Seaward near‐surface velocities occurred predominantly during offshore wind events. During intense wind periods, the surface cross‐shelf water transport exceeded the net along‐shelf transport. During typically stratified seasons, the intense cross‐shelf winds resulted in a well‐defined two‐layer flow and were more effective at driving offshore transport than during unstratified conditions. While transfer coefficients between wind and currents were generally around 1%, higher cross‐shelf transfer coefficients were observed in the near‐inertial band. The regional extent of the resulting surface cold water during energetic cross‐shelf winds events was concentrated around the region of the wind jet. Cross‐shelf transport due to along‐shelf winds was only effective during northeast wind events. During along‐shelf wind conditions, the transport was estimated to be between 10 and 50% of the theoretical Ekman transport.

Ocean–atmosphere–wave characterisation of a wind jet (Ebro shelf, NW Mediterranean Sea)

“…A channelisation effect associated with the Ebro valley triggers north-westerly winds (called Mestral: Catalan for Mistral wind), resulting in a wind jet. Previous studies based on long-term wind measurements in the proximity of the region showed that winds have a persistent seasonal pattern (Font, 1990;Cerralbo et al, 2015;Grifoll et al, 2015). During winter and autumn, a dominant northwesterly component caused by wind channelisation was observed.…”

“…Observational analyses have revealed that the inner and midshelf (less than 50 m water depth) dynamics in the Ebro shelf are characterised by a strong influence of the frictional component of the flow (Jordà, 2005;Grifoll et al, 2015). Furthermore, the regional response to wind jets is not clear due to the complex bathymetry and the spatial variability of the wind jet.…”

Wind-Wave Characterization in a Wind-Jet Region: The Ebro Delta Case

“…This wind jet is dry, cold, perpendicular to the coastline and associated with high wind intensities. It is more usual and intense in autumn and winter [13], when a larger atmospheric pressure gradient takes place, but a small pressure difference along the Ebro Valley is enough to initiate wind during any season [14].…”

Shelf Circulation Induced by an Orographic Wind Jet

“…Regions with coastal wind jets induced by orographic effects are areas with complex dynamics in terms of water circulation response (Grifoll et al, ; Jordà, ; Trasviña et al, ), spatial and temporal wind variability (Jiang et al, ), and sharp wave height variations (Shimada & Kawamura, ). In this sense, several researchers have focused on the description of different aspects such as the wave generation/decaying process (Shimada & Kawamura, ), the surface water cooling effect due to upwelling (Trasviña et al, ), or the influence of the sea surface roughness variation due to the wave climate (Grifoll et al, ).…”

“…Thus, regions where the circulation pattern may be rapidly modified under the presence of a wind jet deserve special attention. Grifoll et al () performed an observational study and found a seasonal response of the water circulation due to cross‐shelf winds. They also highlighted the presence of cold water at the surface and attempted to discern between an upwelling process and a cooling effect due to mixing.…”