Some Impacts Of Marginal-Sea Overflow on an Idealized Meridional Overturning Circulation

1. Some Impacts Of Marginal-Sea Overflow on an Idealized Meridional Overturning Circulation Jiayan Yang Dept. of Physical Oceanography Woods Hole Oceanographic Institution Woods Hole, MA 02543 jyang@whoi.edu The 2nd Density-Current CPT Workshop Nov. 9-10, 2004, Providence, RI

2. Compare the MOC stability and sensitivity from two idealized models, one with and one without marginal-sea coupling to examine: (i) the multiple equilibrium states (Stommel, 1961); (ii) the halocline catastrophe (paleoclimate implication); (iii) the MOC responses to external forcing (how much forcing is needed and how quickly the MOC can be shut down? and can it be restored?) ……………………………………….. What I was originally planning to talk about was: But understanding what has happened in the subpolar North Atlantic Ocean is probably more relevant to this CPT.

3. Salinity change in the central Labrador Sea (from Dickson et al., 2002, Nature)

5. Potential Temperature change in the central Labrador Sea (from Igor Yashayaev et al., BIO)

6. From Igor Yashayaev et al.

7. Hydrographic change in Labrador Sea in the last 4 decades according to Dickson et al (2002): • The steric height was reduced by 8-10 cm in the central Labrador Sea; • Equivalent to continuous heat loss of 8 Wm-2; • Equivalent to mixing 6 m of fresh water; • Arguably the largest full-depth changes ever observed in modern oceanographic record.

8. What is known or have been suggested are: • The change was likely related • to the prolonged positive NAO • phase since 1960s; • The change in the deep layer • can be traced to the upstream • Nordic Basin; • Sea-ice flux from the Arctic • probably played a role for • changes in the Nordic Seas.

9. What is known or have been suggested are: • The change was likely related • to the prolonged positive NAO • phase since 1960s; • The change in the deep layer • can be traced to the upstream • Nordic Basin; • Sea-ice flux from the Arctic • probably played a role for • changes in the Nordic Seas.

10. What is known or have been suggested are: • The change was likely related • to the prolonged positive NAO • phase since 1960s; • The change in the deep layer • can be traced to the upstream • Nordic Basin; Low AO state High AO state Calculated from the daily 25km sea-ice motion vectors derived from SSMI, SMMR, AVHRR and buoys (data from NSIDC, http://www.nsidc.org) (Yang, 2004). • Sea-ice flux from the Arctic • probably played a role for • changes in the Nordic Seas. AO index and sea-ice area flux (using satellite passive microwave data) through Fram Strait (from Kwok and Rothrock, 1999, JGR)

11. How the overflow responds to changes in the Nordic Seas? How changes in both Nordic and Labrador Seas affect the entrainment? How the MOC is affected by and feedback to the overflow process? …………. The challenge or the opportunity for this CPT:

12. The Schematic of the OGCM-MS Coupled Model:

13. The Marginal-Sea Exchange Model: The zero PV rotating hydraulic solution from Whitehead, Leetmaa and Knox (1974). Geophysical Fluid Dynamics, 6, 101-125. Qwind h1 (T1,S1) Qin (Toc,Soc) hu Qout (T2,S2) The outflow transport: Qout=g’hu2/2f

14. Scenario 1: If r1>r2 Deep convection and MS becomes 1 layer Buoyancy Flux Qwind Deep Convection hu Qin Qout (Toc,Soc) (T2,S2)

15. Scenario 2: a 2-layer marginal sea Qwind h1 (T1,S1) (Toc,Soc) hu (T2,S2) Qin Qout Lower layer: Upper layer:

16. Ice-flux from Arctic (2790 km3/year) Precipitation-evaporation runoff Saline water import From N. Atlantic Fresh-water budget in the GIN Seas (from Aagaard and Carmack, 1989, JGR)

17. The entrainment at shelf break is calculated by using the simpler end-point version of Price-Baringer model:

18. Summary of the OGCM/Marginal-sea Overflow Model

19. The Zonally Averaged Meridional Overturning Circulation (MOC)

20. Ice-flux from Arctic (2790 km3/year) Precipitation-evaporation runoff Saline water import From N. Atlantic Fresh-water budget in the GIN Seas (from Aagaard and Carmack, 1989, JGR)

21. Sea-ice transport from the Arctic Ocean 4500 km3/year 3000 km3/year Spin-up Enhanced ice-flux run T=0 T=50 years

25. ~20Wm-2

26. Same model without marginal-sea coupling

28. Current Work and Future Plan: More realistic simulations of the N. Atlantic Ocean; The role of PV flux associated with the overflow and entrainment (lesson learned from the AOMIP consortium);

29. Two examples of AOMIP model results: Among 11 model ouputs posted in AOMIP website (www.planetwater.ca\research\AOMIP), 6 are cyclonic and 5 are anti-cyclonic.

31. Numerical Model Experiments: Russia • Realistic bathymetry (AOMIP); • No wind- and ice-stress; • The Forcing (from Rudels • and Friedrich, 2000): • Inflow: • 0.8 Sv at Bering Strait • 2.0 Sv at Barents Sea • 1.2 Sv at Fram Strait • Outflow: • 4.0 Sv at Fram Strait 0.8Sv Alaska Lomonosov Ridge 2Sv Markarov Basin Alpha-Mendeleyev Ridge Amundsen Basin Nansen Basin Canada Basin 1.2Sv Canada 4Sv