Ballooning of River-Plume Bulge and Its Stabilization by Tidal Currents

When freshwater debouches into an adjacent ocean, an anticyclonic eddy (bulge) is formed in front of the river mouth. It is well known that a bulge growing offshore (ballooning) hardly reaches a steady state in the absence of either ambient currents or wind forcing. This study provides a physical interpretation for the ballooning of river-plume bulges by conducting numerical experiments in which a river plume is induced by a coastal freshwater source. Part of the freshwater released to the model ocean undergoes inertial instability. Near-inertial oscillations are predominant when disturbances are not forced in ambient waters of the river plume. These isotropic disturbances are amplified by inertial instability, so that unstabilized freshwater can move in arbitrary directions. Thus, unstabilized freshwater does not need to move toward the coastal boundary current on the right-hand side of the river mouth. Freshwater unstabilized for a long time can stay in the bulge for a long time. Unstabilized freshwater accumulates gradually in the bulge, and so ballooning occurs. When the direction of disturbances is prescribed in ambient waters, unstabilized freshwater is forced to move in the same direction. Thereby, the motion of unstabilized freshwater is restricted in the alongshore direction when background disturbances are induced by alongshore tidal currents. It is therefore concluded that tidal currents play a role in stabilizing the offshore growth of river-plume bulges in coastal and shelf waters.

[1]  M. Ikeda.  Coastal flows driven by a local density flux , 1984 .

[2]  D. Chapman,et al.  Trapping of a Coastal Density Front by the Bottom Boundary Layer , 1994 .

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

[4]  S. Chao,et al.  Onset of Estuarine Plumes , 1986 .

[5]  A. J. Clarke,et al.  A Simple Method for Estimating Barotropic Tidal Currents on Continental Margins with Specific Application to the M2 Tide off the Atlantic and Pacific Coasts of the United States , 1982 .

[6]  A. Isobe,et al.  Seasonal Variability in Circulation Pattern and Residence Time of Suo-Nada , 2003 .

[7]  R. Garvine.  Penetration of Buoyant Coastal Discharge onto the Continental Shelf: A Numerical Model Experiment , 1999 .

[8]  P. Huq,et al.  The characteristics of the recirculating bulge region in coastal buoyant outflows , 2003 .

[9]  J. Holton.  An introduction to dynamic meteorology , 2004 .

[10]  Doron Nof,et al.  The ballooning of outflows , 2001 .

[11]  D. Nof,et al.  The momentum imbalance paradox , 1997 .

[12]  Pil-Hun Chang,et al.  A numerical study on the Changjiang diluted water in the Yellow and East China Seas , 2003 .

[13]  John H. Simpson,et al.  Physical processes in the ROFI regime , 1997 .

[14]  T. Yanagi.  Fundamental study on the tidal residual circulation—I , 1976 .

[15]  L. Oey,et al.  The fate of river discharge on the continental shelf 1 , 1996 .

[16]  George L. Mellor,et al.  Subtidal Variability of Estuarine Outflow, Plume, and Coastal Current: A Model Study , 1993 .

[17]  L. Pietrafesa,et al.  The Columbia River plume study : Subtidal variability in the velocity and salinity fields , 1998 .

[18]  A. M. Davies,et al.  The effect of wind direction and mixing upon the spreading of a buoyant plume in a non-tidal regime , 1999 .

[19]  Shinya Minato.  Geostrophic response near the coast , 1983 .

[20]  C. Garrett,et al.  A simple criterion for gyre formation by the surface outflow from a strait, with application to the Alboran Sea , 1989 .

[21]  A. Yankovsky.  The cyclonic turning and propagation of buoyant coastal discharge along the shelf , 2000 .

[22]  R. Beardsley,et al.  A simple theoretical model for the flow of an estuary onto a continental shelf , 1978 .

[23]  D. Chapman,et al.  A Simple Theory for the Fate of Buoyant Coastal Discharges , 1997 .

[24]  R. Garvine.  The impact of model configuration in studies of buoyant coastal discharge , 2001 .

[25]  W. Rockwell Geyer,et al.  The Alongshore Transport of Freshwater in a Surface-Trapped River Plume , 2002 .

[26]  A. Isobe,et al.  Current Structure and Behavior of the River Plume in Suo-Nada , 2003 .

[27]  P. Huq,et al.  The role of outflow geometry in the formation of the recirculating bulge region in coastal buoyant outflows , 2003 .

[28]  R. Garvine,et al.  Wind influence on a coastal buoyant outflow , 2005 .