Numerical study of a coastal current on a steep slope in presence of a cape: The case of the Promontorio di Portofino

[1] A process study aimed at investigating the winter circulation in the area of the Promontorio of Portofino (northwestern Mediterranean) is presented, motivated by historical current measurements suggesting the presence of an attached recirculating eddy in the lee of the Promontorio (cape). A sensitivity study is first performed in an idealized setting, considering the interaction of a steady incoming current with the cape. Numerical experiments are performed using both two-dimensional (2-D) (vertically integrated) and 3-D versions of the POM model. From the 2-D results, the main controlling parameter appears to be the equivalent Reynolds number Ref, in agreement with previous historical results. In the 3-D case, the dependence on the vertical Ekman number Ekv is investigated at fixed Ref, and a significant intensification of the attached eddy with respect to the 2-D solutions is obtained. The main difference between the 2-D and 3-D dynamics is the presence of a resolved bottom Ekman layer in 3-D, introducing a vertical shear in the incoming current, particularly noticeable in the region of the shelf break. This shear is likely to be responsible for a “secondary circulation” (dominated by Coriolis effect), inducing an inshore surface current and associated upwelling in the lee of the cape, and causing the observed eddy intensification. Experiments with realistic bathymetry are performed, and the 3-D results are found to be in good qualitative agreement with the measurements. The results are expected to be relevant for biological transport, pointing out to a mechanism for eddy intensification connected with surface inshore transport and upwelling.

[1]  K. Black.  Eddy formation in unsteady flows , 1987 .

[2]  G. Batchelor,et al.  An Introduction to Fluid Dynamics , 1968 .

[3]  J. Pedlosky.  Geophysical Fluid Dynamics , 1979 .

[4]  M. Collins,et al.  Water and sediment movement around a coastal headland: Portland Bill, southern UK , 2003 .

[5]  M. Collins,et al.  Sedimentary processes, bedforms and facies, associated with a coastal headland: Portland Bill, Southern UK , 2002 .

[6]  R. Booij,et al.  Adaptation of secondary flow in nearly-horizontal flow , 1986 .

[7]  M. Collins,et al.  Island wakes and headland eddies: A comparison between remotely sensed data and laboratory experiments , 1987 .

[8]  P. Davies,et al.  Boundary currents over shelf and slope topography , 1999 .

[9]  D. Haidvogel,et al.  Numerical models of the coastal ocean , 1998 .

[10]  D. Chapman,et al.  Generation of mesoscale flows over the shelf and slope by shelf wave scattering in the presence of a stable, sheared mean current , 1995 .

[11]  J. Smagorinsky,et al.  GENERAL CIRCULATION EXPERIMENTS WITH THE PRIMITIVE EQUATIONS , 1963 .

[12]  S. Chiswell,et al.  The East Cape Current and two eddies: A mechanism for larval retention? , 1998 .

[13]  D. Renouard,et al.  Experimental study of the stability of an intermediate current and its interaction with a cape , 2000 .

[14]  P. Sigray,et al.  Slope‐dependent friction in a barotropic model , 2004 .

[15]  M. Collins,et al.  Effects of shoreline irregularities on a rectilinear tidal current and their significance in sedimentation processes , 1980 .

[16]  David P. Marshall,et al.  Flow past a Cylinder on a β Plane, with Application to Gulf Stream Separation and the Antarctic Circumpolar Current , 2001 .

[17]  D. Chapman,et al.  Anticyclonic eddies trapped on the continental shelf by topographic irregularities , 1997 .

[18]  C. Garrett,et al.  Upwelling Along the Yarmouth Shore of Nova Scotia , 1976 .

[19]  W. Geyer.  Three-dimensional tidal flow around headlands , 1993 .

[20]  J. Imberger,et al.  Island wakes in shallow coastal waters , 1984 .

[21]  P. Salmona,et al.  The marine protected area of Portofino, Italy : a difficult balance , 2001 .

[22]  Peter Coutis,et al.  Flow-topography interaction in the vicinity of an isolated, deep ocean island , 1999 .

[23]  D. Chapman,et al.  Scattering of shelf waves by a spatially varying mean current , 1996 .

[24]  George L. Mellor,et al.  The Pressure Gradient Conundrum of Sigma Coordinate Ocean Models , 1994 .

[25]  L. Oey,et al.  Sigma Coordinate Pressure Gradient Errors and the Seamount Problem , 1998 .

[26]  P. Wang,et al.  Water Quality and Eutrophication in Tampa Bay, Florida , 1999 .

[27]  Peter Coutis,et al.  The physical and biological impact of a small island wake in the deep ocean , 2002 .

[28]  A. Esposito,et al.  Current Circulation in the Ligurian Sea , 1982 .

[29]  Caskey,et al.  GENERAL CIRCULATION EXPERIMENTS WITH THE PRIMITIVE EQUATIONS I . THE BASIC EXPERIMENT , 1962 .

[30]  R. Manasseh,et al.  Recirculation in the lee of complicated headlands: A case study of Bass Point , 1995 .

[31]  Dale B. Haidvogel,et al.  Wind driven general circulation of the Mediterranean Sea simulated with a Spectral Element Ocean Model , 2002 .

[32]  H. Freeland.  The flow of a coastal current past a blunt headland , 1990 .

[33]  Scott Glenn,et al.  Vertical mixing schemes in the coastal ocean: Comparison of the level 2.5 Mellor‐Yamada scheme with an enhanced version of the K profile parameterization , 2004 .

[34]  I. Orlanski.  A Simple Boundary Condition for Unbounded Hyperbolic Flows , 1976 .

[35]  E. Zambianchi,et al.  Dispersion processes and residence times in a semi‐enclosed basin with recirculating gyres: An application to the Tyrrhenian Sea , 1997 .

[36]  Andrew M. Moore,et al.  The Nonnormality of Coastal Ocean Flows around Obstacles, and Their Response to Stochastic Forcing , 2002 .

[37]  M. Astraldi,et al.  Temporal variability of currents in the eastern Ligurian Sea , 1990 .

[38]  R. Falconer,et al.  Eddy Formation Behind a Coastal Headland , 1995 .

[39]  Hemantha W. Wijesekera,et al.  Modeling study of turbulent mixing over the continental shelf: Comparison of turbulent closure schemes , 2003 .

[40]  R. Pingree,et al.  The Formation Of The Shambles And Other Banks By Tidal Stirring Of The Seas , 1978, Journal of the Marine Biological Association of the United Kingdom.

[41]  M. Astraldi,et al.  Some observations on current measurements on the East Ligurian Shelf, Mediterranean Sea , 1983 .

[42]  木村 竜治,et al.  J. Pedlosky: Geophysical Fluid Dynamics, Springer-Verlag, New York and Heidelberg, 1979, xii+624ページ, 23.5×15.5cm, $39.8. , 1981 .

[43]  David C. Smith,et al.  Impulsively Started Flow in a Submarine Canyon: Comparison of Results from Laboratory and Numerical Models* , 2001 .

[44]  P. Davies,et al.  Quasigeostrophic flow past a cape in a homogeneous fluid , 1991 .

[45]  G. Mellor.  USERS GUIDE for A THREE-DIMENSIONAL, PRIMITIVE EQUATION, NUMERICAL OCEAN MODEL , 1998 .

[46]  R. Flather,et al.  Results from a storm surge prediction model of the north-west European continental shelf for April, November and December, 1973 , 1976 .

[47]  R. Signell,et al.  Transient eddy formation around headlands , 1991 .

[48]  David M. Farmer,et al.  Tilting separation flows: a mechanism for intense vertical mixing in the coastal ocean , 2002 .

[49]  E. Palma,et al.  On the implementation of passive open boundary conditions for a general circulation model: The barotropic mode , 1998 .

[50]  S. Castellari,et al.  Simulation of water mass formation processes in the Mediterranean Sea: Influence of the time frequency of the atmospheric forcing , 2000 .

[51]  E. Palma,et al.  On the implementation of open boundary conditions for a general circulation model: The three-dimensional case , 2000 .

[52]  Currents at Lindenkohl Sill in the southern Gulf of Maine , 1990 .

[53]  A. Solan,et al.  The separation of flow past a cylinder in a rotating system , 1979, Journal of Fluid Mechanics.

[54]  A. Molcard,et al.  Floating debris in the Ligurian Sea, north-western Mediterranean. , 2003, Marine pollution bulletin.

[55]  L. Merkine.  Flow separation on a β-plane , 1980, Journal of Fluid Mechanics.

[56]  D. Chapman.  Numerical Treatment of Cross-Shelf Open Boundaries in a Barotropic Coastal Ocean Model , 1985 .

[57]  Andrea Doglioli,et al.  Development of a numerical model to study the dispersion of wastes coming from a marine fish farm in the Ligurian Sea (Western Mediterranean) , 2004 .