Predictions in the North Sea
263
Appendix D: Vertical Exchange
In general vertical velocities are small and the effect of vertical advection can be
accounted for within the vertical dispersion coefficient Kz. The time, T y, for complete vertical mixing of a tracer introduced at the sea surface or seabed is H 2 /K.z.
For the North Sea, T y varies from a few hours in shalIow strongly tidal seas to the
order of a year in the deep sluggish waters of the Northem North Sea. Vertical
stratification induced by solar heating exists over much of the deeper regions from
May until October. In the region of the thermocline, such stratification can reduce
Kz to close to the kinematic viscosity level.
In practice Kz varies continuously with depth and time over both the ti dai and
seasonal cycle with significant input from surface waves and surge currents. As
indicated in Table 13.2 , the various models for simulating these variations in Kz
are now well developed (Burchard and Baumert 1995).
For sediment modelling the falI velocity can be regarded as an effective advective term - alIowing semi-analytical solutions (Prandle 1997b).
Prandle and Lane (1995) show how temperatures in the North Sea are determined by the dynamics of vertical exchange assuming a localised balance. Prandle
(1998) extends this analysis to incorporate a coupled ocean-sea thermal model to
provide global expressions for the seasonal cycle of both sea surf ace and ambient
air temperature. This study shows that, in general away from the immediate influence of pronounced ocean current systems, the temperatures of the ambient air is
also determined by these same localised vertical exchange rates.
263
Appendix D: Vertical Exchange
In general vertical velocities are small and the effect of vertical advection can be
accounted for within the vertical dispersion coefficient Kz. The time, T y, for complete vertical mixing of a tracer introduced at the sea surface or seabed is H 2 /K.z.
For the North Sea, T y varies from a few hours in shalIow strongly tidal seas to the
order of a year in the deep sluggish waters of the Northem North Sea. Vertical
stratification induced by solar heating exists over much of the deeper regions from
May until October. In the region of the thermocline, such stratification can reduce
Kz to close to the kinematic viscosity level.
In practice Kz varies continuously with depth and time over both the ti dai and
seasonal cycle with significant input from surface waves and surge currents. As
indicated in Table 13.2 , the various models for simulating these variations in Kz
are now well developed (Burchard and Baumert 1995).
For sediment modelling the falI velocity can be regarded as an effective advective term - alIowing semi-analytical solutions (Prandle 1997b).
Prandle and Lane (1995) show how temperatures in the North Sea are determined by the dynamics of vertical exchange assuming a localised balance. Prandle
(1998) extends this analysis to incorporate a coupled ocean-sea thermal model to
provide global expressions for the seasonal cycle of both sea surf ace and ambient
air temperature. This study shows that, in general away from the immediate influence of pronounced ocean current systems, the temperatures of the ambient air is
also determined by these same localised vertical exchange rates.