Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 288

(a) (b)

39°N

Delaware
Bay

39°N

38°N

NW Atlantic
Ocean

37°N

38°N

75°30 W 75°W

75°30 W 75°W

Figure 9.13 Surface salinity in the ROFI outside the Delaware River. (a) Following light
winds from the northeast (downwelling favourable); (b) Following strong winds from the
southwest (upwelling favourable). Figure provided by Rich Garvine, based on Munchow
and R.W.Garvine, 1993 .

acts to reduce the circulation and diminish stratification. This 'wind straining'
mechanism (Scully et al., 2005 ) also operates in ROFIs (e.g. Verspecht et al., 2009 ) ,
but in a rather different way since the flow is constrained horizontally only by the
coast. In deep water at some distance from the coast, we know that the Ekman
transport ( Section 3.5.1 ) will be directed at right angles to the wind stress, so that it is
the alongshore wind component which is responsible for generating the coast-normal
transport and the consequent effects on the structure of the ROFI.
As an example, Fig. 9.13 shows the influence of wind stress on the ROFI
generated by the Delaware River on the east coast of the United States. Observa-
tions (Munchow and Garvine, 1993 ) shown in Fig. 9.13a have revealed that coast-
parallel winds which are downwelling favourable (i.e. in a direction of the coastal
current) constrain the low salinity waters close to the coast and reinforce the along
coast jet. Conversely upwelling favourable winds tend to arrest the alongshore
flow and force offshore flow with a consequent increase in the width of the ROFI,
asshowninFig 9.13b . Similar responses to wind direction have been demon-
strated for the Columbia River discharge onto the NW shelf of the United States
(Hickey et al., 2005 ). In shallow water, the cross-shore component of wind
may also drive upwelling and downwelling motions and hence influence the
structure of the coastal jet as has been shown for the Rhine ROFI (Souza and
Simpson, 1997 ) .