Will the Gulf Stream stop in our time?

1. Will the Gulf Stream stop in our time? About the dynamics of the Gulf Stream system. Ole Anders Nøst Norwegian Polar Institute

2. Outline of the talkThe Gulf Stream system. • A description • The dynamics. Factors controlling strength and direction. • Will it stop???

3. The bottom topography and the main circulation pattern Fratantoni, 2001

4. Western Intensification Velocity from a global simulation (taken from NASA web page).

5. Topographic steering and the flow along the Norwegian Coast Jakobsen et al., 2003

6. Sinking This circulation pattern involves strong vertical motion at several locations. In an ocean located on a rotating earth, the dynamics of this overturning is not obvious.

7. The Forcing

8. Cooling Heating Heating at the equator cooling at the poles The cold water near the poles are denser than the warm water at the equator.

9. Dense and light water Light Dense

10. The trade winds is an important forcing for the Gulf Stream system

11. Transport of Atlantic Water at the Svinøy section correlated to wind stress curl in the Atlantic. (from Orvik and Skagseth, 2004)

12. Effect of rotation To understand the path of the Gulf Stream, we need to consider the effect of rotation.

13. Rotation about a vertical axis (perpendicular to the surface of Earth)

14. A simple experiment Non-rotating Rotating

15. Overturning can not occur on large-scale (>10 km)

16. Topographic steering: Angular momentum conservation of a water column. A change in depth causes a change in circulation which forces the circulation to follow constant depth.

17. Western intensification and the variation of rotation with latitude. The western intensification of the ocean currents is due to the variation of rotation with latitude (Stommel, 1958).

18. Another effect of Earth rotation is that sinking motion occurs on scales in the order of 10 km. The net effect of all the mesoscale (~10km) eddies is to set up the large scale overturning circulation.

19. Will the Gulf Stream stop in our time? I will try to answer this through a summary of what I have just said.

20. The path of the Gulf Stream Western intensification: Controlled by the variation of Earth rotation with latitude. Crossing of the mid ocean ridge: Seems to cross at the only possible location due to topographic steering. Further northward flow: Topographic steering and north-south density difference Conclusion: The path of the Gulf Stream is mainly controlled by factors that does not change in time (rotation, topography). The path is therefore a robust feature which is not likely to change in near future.

21. Cooling Heating The strength of the Gulf Stream. The Gulf Stream strength is given by the north-south density difference in the ocean and the strength of the trade winds. Melting ice at high latitudes may reduce density difference. However, the ocean will always be denser at high latitudes. Conclusion: The Gulf Stream may vary in strength. However, it is not likely to stop.

22. Transport of Atlantic Water along the Norwegian Coast (Orvik and Skagseth, 2004).

23. The structure of the thermohaline circulation. The structure of the thermohaline circulation depends on the mesoscale (~10km) eddies. These eddies are poorly understood. The resolution of climate models is in the order of 100 km. In order to predict changes to the Gulf Stream system we need better models, and better understanding of the dynamics of the ocean currents.