1. Useful texts: Mann & Lazier, The Dynamics of Marine Ecosystems, Blackwell Science. Simpson, In: The Sea, vol 10, chapter 5. Simpson, In: The Sea, vol 11, chapter 23. The importance of stratification. Consider how much energy is required to lift 1 m 3 of water from just below a pycnocline to a height  h above a pycnocline. 1. 2. The Physics of Shelf Seas

2. Energy is required to mix water from below to above a density discontinuity. The source of this energy is turbulence. In shelf seas the dominant generating mechanism for turbulence is stress at the seabed (tidally-dominated) and stress at the sea surface (wind stress). Internal waves are also thought to be important. We quantify the stability of a stratified water column using the gradient Richardson number: There is a limited amount of turbulence available, so we could eventually reach a state where the pycnocline represents an effective block to transport between the two layers of water. We can define a critical Ri, Ri c, which is often taken to be about 0.25 - 1 (see Canuto et al., J. Phys. Oceanogr., 31, 1413-1426, 2001). The vertical density gradient (stronger stratification  larger Ri) The vertical shear in the current speed (stronger shear  more turbulence  mixing and reduced Ri)

3. Important consequences of this: Prevention of momentum transfer through the pycnocline (e.g. Sherwin, Cont. Shelf Res., 7, 191-211, 1987). Inhibition of oxygen mixing from the surface layer to below the pycnocline (e.g. Fujiwara et al., Estuar. Coast. Shelf Sci., 54,19-31 2002). Inhibition of nutrient mixing from below the pycnocline into the surface layer (e.g. Pingree, & Pennycuick, J. Mar. Biol. Assoc. U. K., 55, 261-274, 1975). Ri Ri c mixing We expect that the mixing through a density gradient will be described by some decreasing function of Ri.

4. Stratification sources: Surface heating (net heat input) Freshwater Stratification sinks: Tidal mixing (turbulence mixes up from seabed) Wind mixing (turbulence mixes down from sea surface) Convective overturning Internal waves

5. ROFIs The open shelf sea Shelf edge See: Simpson, The Sea, vol. 10, chapter 5. Simpson, The Sea, vol. 11, chapter 23. NOTE: The Sea vol. 11 has information on all the world’s coastal/shelf regions.

6. References: Simpson et al., 1990. Estuaries, 13(2), 125-132. Nunes & Lennon, 1987. Jour. Geophys. Res., 92, 5465-5480. Sharples & Simpson, 1995. Cont. Shelf Res., 15(2/3), 295-314. Linden & Simpson, 1988. Cont. Shelf Res., 8(10), 1107-1127. Simpson, et al. 1993. Oceanologica acta. 16(1), 23-32. All of Journal of Marine Systems, vol 12, 1997. Regions of Freshwater Influence (ROFIs) freshsalty Freshwater as a source of buoyancy (the “lock-exchange” experiment)

7. Example: Liverpool Bay. The horizontal density gradient is the result of the freshwater input from several rivers along the coast. From: Sharples & Simpson, Cont. Shelf Res., 15(2/3), 295- 314, 1995. The density gradient always acts to try to stratify the system, and tidal mixing acts to prevent this stratification.

8. Mooring results from position LB3 in Liverpool Bay. Salinity difference between bottom and surface Currents

9. Notice that there are 2 very clear stratification-mixing cycles in the mooring time series. 1. A spring-neap cycle. Mixing at spring tide is able to prevent stratification by the density-driven flow. The reduction of mixing at neap tide leads to frontogenesis, and large scale stratification over the whole of Liverpool Bay. Thus, a large region of shelf sea undergoes mixing-stratification every 2 (or 4) weeks.

10. 2. Semi-diurnal stratification - tidal straining. Surface flow is stronger than nearbed flow. So, ebb currents drag water of lower density on top of denser bottom water. This generates stratification during the ebb. Flood currents re-form the initial density profile. RiverSea HW Mixed density profile Ebb flow: Current shear strains the density profile During ebb flow: Tidal mixing acts against straining and produces a bottom mixed layer Surface winds can also produce a surface mixed layer Stronger tidal mixing

11. A tidal-cycle “anchor station” in Liverpool Bay. CTD + current measurements Estimates of the gradient Richardson number