1. Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde hans.burchard@io-warnemuende.de Periodic straining, a process which lakes (due to seiches) and estuaries (due to tides) have in common.

2. Principle of estuarine circulation MacCready and Geyer (2010)

3. The principle of tidal straining MacCready and Geyer (2010), after Simpson et al., 1990

4. Observations of tidal straining in Liverpool Bay MacCready and Geyer (2010), after Simpson et al., 1990 Stratifying during ebb, destratitifying during flood Bottom-top salinity difference

5. GOTM application: Tidal straining in Liverpool Bay Simpson et al., 2002

6. GOTM application: Tidal straining in Liverpool Bay Observations Salinity Longitudinal vel. Transverse vel. Temperature Simulation Observed dissipation rate Simulated dissipation rate Simpson et al., 2002

7. MacCready & Geyer (2010) after Jay & Musiak (1994) Tidal straining as driver of estuarine circulation 75% level

8. Result: Tidal straining makes about 2/3 of estuarine circulation. With full-scale 1D model (GOTM): Gravitational circulation and tidal straining profiles Burchard and Hetland (JPO 2010) Estuarine circ. Straining Gravitational Tidally-averaged currents for typical Wadden Sea conditions

9. Non-dimensional parameters of this problem 1.Non-dimensional bed roughness: 2.Strouhal number: 3.Inverse Ekman number: 4.Simpson number:

10. Estuarine circulation and SPM* transport Analytical solution for constant eddy viscosity/diffusivity: * SPM = Suspended Particulate Matter Analytical solution for parabolic viscosity/diffusivity, see Burchard et al. (JPO, 2013)

11. Enhancement of estuarine circulation in channelised tidal flow (2D slice modelling with GETM) Burchard et al. (JPO 2011) www.getm.eu

12. Circulation in transverse estuary

13. Transverse structure of estuarine circulation Burchard et al. (JPO 2011 ) Tidal straining circulation Gravitational circulation Advective circulationBarotropic circulation

14. Does this all happen in nature? Ok, let’s go out to the Wadden Sea and measure:

15. Campaign in Lister Deep (April 2008) Becherer et al. (GRL 2011) shoals

16. Becherer et al. (GRL 2011)

17. Puzzling however: Water column stability Tidal phase Near lateral shoals, stratification kicks in already during flood... This is topic of another study... Becherer et al. (GRL 2011)

18. Do similar things also happen in this lake (Constance) … … or in this lake (Alpnach)?

19. Simulation Lake Alpnach (Switzerland) Becherer & Umlauf (2011)

20. Upslope currents: Destabilisation Downslope currents: Stabilisation stable mixing unstable mixing GOTM 1D simulation Periodic straining in lakes and non-tidal basins? down-slope up-slope Umlauf and Burchard (2011)

21. Observations in Lake Constance Lorke et al. (2008)

22. Take home: Tidal straining is the interaction between tidal oscillations and lateral buoyancy gradients as they typically occur in coastal areas. Tidal straining drives estuarine residual circulation in a comparable magnitude as gravitational circulation. Cross-sectional straining due to lateral buoyancy gradients may be much more effective than longitudinal straining. In lakes, the same mechanisms work, with seiches providing the oscillating forcing, and vertical stratification along sloping beds providing the along-flow buoyancy gradients. Question: do these seiches also cause near-bottom up-slope residual circulation? If so, this may have significant consequences for lake ecosystems.