Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 290

In spite of the problems here, 3D models have been applied, with some success, to
test our understanding of the way ROFIs work (Whitney and Garvine, 2006 ) .
A detailed model of the Rhine region (de Boer et al., 2006 ) , for example, gives a
convincing demonstration of ROFI behaviour including the development of cross-
shore straining, periodic stratification and upwelling along the coast during periods
of reduced vertical mixing.

Physics summary box

ROFIs are distinctive regions of the shelf seas where the freshwater input from
rivers constitutes a buoyancy source which is comparable in magnitude to that of
surface heating and cooling further offshore.

In contrast to surface heating, this buoyancy input is not distributed uniformly
over the surface but enters at one or more localised estuarine sources at the coast.

The buoyancy input drives a circulation in which the low salinity water emerging
from the estuary is deflected by the Coriolis force to the right/left in the northern/
southern hemisphere and, in the absence of friction, forms a current flowing
parallel to the coast.

The flow structure is complicated by frictional effects which induce an additional
two-layer flow normal to the coast and parallel to the density gradient.

This estuarine-type flow moves low salinity water over heavier water further
offshore and thus tends to stratify the water column in competition with stirring by
the tides and winds.

Variations in freshwater input, tidal stirring and wind stirring shift the balance in
this competition so that ROFIs experience large changes in stratification on a
variety of time scales in contrast to other areas of the shelf seas.

When tidal stirring predominates, its regular variation over the springs-neaps cycle
may induce a fortnightly switching between stratified and fully mixed conditions.

The interaction between shear in the tidal currents and the density gradient leads
to periodic variations in stratification at the M 2 tidal frequency through the
process of tidal straining.

The effect of tidal straining is subtly different for ROFIs with progressive and
standing wave tidal regimes.

Tidal straining can act to augment density-driven circulation and sediment
transport in estuaries and ROFIs.

9.8 Biological responses in estuaries and ROFIs

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There are three aspects of ROFIs that we will consider when we look for biological
responses. First, the horizontal density gradient in a ROFI tends to establish a
vertically sheared mean flow. We might expect any near-surface material in the water