What controls the location of the sea ice edge in the N. Atlantic? Why do we care?

1. What controls the location of the sea ice edge in the N. Atlantic?Why do we care? • Motivation • Impact of atmosphere on sea ice and ocean • Impact of ocean heat transport (OHT) on sea ice • Feedbacks between the ocean and the atmosphere, mediated by the sea ice • Atmosphere -> Sea Ice -> Atmosphere • Atmosphere -> Ocean -> Sea Ice -> Atmosphere • Will Greenhouse Warming initiate a thermohaline catastrophe, and subsequently a large rapid cooling of Europe and beyond? David S. Battisti University of Washington

2. 1. Motivation: Who cares about the location of the sea ice edge? • Climate models forced by increasing Greenhouse Gas Concentrations project a slowdown in the ocean thermohaline circulation (THC) -- the main deliverer of heat by the ocean to the N. Atlantic*. • Even if the OTC shuts down, these models project the large climate changes will likely be confined to over the N. Atlantic Ocean. Temperature Change 20-30 years after a shutdown of the Ocean Thermohaline Circulation (Villenga et al. 2003)

3. 1. Motivation: Who cares about the location of the sea ice edge? Temperature Change: “Heat Transport” minus “No Heat Transport” (Seager et al. 2002) • If reductions in the ocean heat transport (OHT) by the OTC cause the sea ice edge to migrate significantly southward, a large rapid cooling of the N. Hemisphere would occur. (see the movie).

4. In the Greenland-Iceland-Norwegian (GIN) Seas, the heat supplied by the ocean circulation is comparable to the energy absorbed from sunlight. Winter Surface Heat Flux Q Ocean Heat Flux Convergence D Release of Heat in Winter due to summer Storage Q-D Also, only a small change in the ocean heat flux convergence is required to melt a substantial fraction of the sea ice in the Arctic.

5. 2. What influences location of the sea ice edge (part 1)? Advection by the wind The year-to-year variability in the sea ice edge is mainly driven by atmospheric variability: • The leading pattern of atmospheric variability in N. Hemisphere winter is the Arctic Oscillation, the AO (aka the North Atlantic Oscillation, the NAO)

6. 2. What influences location of the sea ice edge? Advection by the wind • The winter averaged NAO index is highly correlated with the leading pattern of SST and sea ice variability: Sea Ice anomalies are due to wind stress changes (advection) Less Ice r=-0.55 More Ice r= 0.4 SST anomalies due to wind induced changes in turbulent heat fluxes and mixing/convection (Visbeck et al. 2003 & refs. therein) ( e.g., Chapman & Walsh 1983; Bitz 1996; Deser et al. 2000)

7. 2. Impact of the atmosphere on the ocean heat transport (interannual to decadal time scales) • Changes in ocean thermohaline circulation (THC) on decadal and shorter time scales are predominately due to forcing by the atmosphere: • Changes in temperature that are forced by atmospheric induced flux anomalies and then advected/convected/mixed by mean ocean currents; • Changes in ocean currents due to changes in wind stress and buoyancy flux gradients. (e.g., Mauritzen and Hakkinen 1999; Holland et al. 2001; Visbeck et al. 2003 & refs therein).

8. 2. Impact of atmosphere on ocean heat transport (interannual time scales) • Changes in ocean currents due to the NAO: • Ekman changes enhance the temperature anomalies due to the surface heat flux. • Gyre changes due to wind stress curl changes also enhances the SST anomalies driven by the NAO related turbulent heat fluxes: • E.g., Positive NAO creates turbulent cooling in the subpolar gyre, anomalous Ekman currents that cool the Southern subpolar gyre; and enhanced gyre circulation that further cools the Labrador Sea.

9. 3. What influences location of the sea ice edge (part 2)? Variability in ocean heat transport • On Centennial and longer time scales, changes in the ocean currents may be due to internal ocean variability/instability. • Example: Dansgaard/Oeschger events? • D/O events & Greenland ‘temperature’: • Rapid onset ( 10 K in < 30 years!) • Long-lived (~ 200 - 600 years) • Abrupt warming in Greenland is consistent with a melt-back of sea ice in wintertime: • Temperature (N fractionation) • delta 18O • Accumulation Li et al (2004). See poster 28 (Li et al do not comment on the cause of sea ice retreat)

10. 4. Feedbacks: Atmospheric response to changes in the location of the sea ice edge • On interannual time scales, the atmosphere forces changes in the sea ice edge (e.g., through NAO forcing) • The response of the atmosphere to those changes in sea ice is a moderate negative feedback (Magnusdottir et al. 2004) • 40 year (1954-94) trends in Z500 and (2x) Sea Ice Concentrations • The same relationship holds on interannual time scales.

11. 4. Feedbacks: Atmospheric response to changes in the location of the sea ice edge • The response of the atmosphere to the changes in sea ice is a moderate (50%) negative feedback (Magnusdottir et al. 2004) • Most of the response is due to the GIN sea ice changes Sea Ice Response Atmospheric Feedback Forcing

12. 4. Feedbacks: (NAO induced) Ocean Circulation Changes impacting the sea ice edge • On the interannual time scales, the dynamical response of the ocean to NAO forcing is to enhance the SST anomalies driven by the NAO induced surface heat anomalies. • On longer time scales (e.g, decadal), the subpolar gyre circulation will be affected by changes in the steric height in the Labrador Sea. Lab Sea P.E. Lab Sea 1500db Temp 1950 2000 • (Curry and McCartney 2001)

13. 4. Feedbacks: (NAO induced) Ocean Circulation Changes impacting the sea ice edge • In turn, NAO induced changes in the steric height in the Labrador Sea will lead to changes in the strength of the subpolar gyre: Lab Sea P.E. Lab Sea Temp SubpolarGyre Transport NAO Index 2000 1950 • (Curry and McCartney 2001)

14. 4. Feedbacks: (NAO induced) Ocean Circulation Changes impacting the sea ice edge • The baroclinic adjustment time scale of the subpolar gyre is O(decade). Thus, there there should be a lagged response in the ocean circulation change to NAO-induced changes in ocean heat content (Visbeck et al. 2003): Sustained NAO+ forcing 0.5 yr (surf. Heat fluxes) 0.5 yr (wind) Cooling in Lab Sea/Increased Convection 1-2 yr (Sverd) Decrease sea ice in GIN Seas (wind) ? 5-10 yrs baro-clinic adj. Increased Subpolar Gyre 0.5 yrs increased ocean heat and salt transport

15. 5. Will Greenhouse Warming initiate a thermohaline catastrophe, and thus cause a large rapid cooling of Europe and beyond? • Increasing CO2 will increase the Greenhouse Effect and cause warming of the planet. • If Global warming is responsible for the positive trend in the NAO due to atmospheric processes (e.g., Shindell et al.; Hoerling et al., etc) … • the feedback loops acting on sea ice in the N. Atlantic appear to be positive because the subpolar gyre appears to be responsive to thermal anomalies in the Labrador Sea. Hence, increased heat and salt import into the GINs seas should be expected, along with the continual retreat of sea ice; hence, no abrupt European cooling. 0 PW 40N 90N Change in Ocean Heat Transport at the time of doubling of CO2. (from CIMP runs; Holland and Bitz 2004)

16. 5. Will Greenhouse Warming initiate a thermohaline catastrophe, and thus cause a large rapid cooling of Europe and beyond? • A fly in the ointment: the GIN sea and the subpolar gyre are have undergone a freshening over the past 30 years. • The equivalent of an extra 0.2m of fresh water per year in the GIN Seas for 30 years. • Where is this freshwater coming from, and could the freshening rate overtake the positive ocean feedbacks to shutdown the thermohaline circulation, ice over the North Atlantic and freeze Europe?

17. 5. Will Greenhouse Warming initiate a thermohaline catastrophe? • Where did this freshwater come from? Plausible explanations: Increased runoff; increased precip - evap (displaced storm track), increased advection of sea ice (melt), and increased freshwater import from the Arctic Ocean via currents. (Vinje 2001, Peterson et al. 2002, Curry et al. 2003 & refs therein)

18. 5. Will Greenhouse Warming initiate a thermohaline catastrophe? Surface Temperature Change • A rapid shutdown of the thermohaline circulation requires enormous volumes of freshwater in a short time. • For example, Vellinga et al (2003) place 16 Sv of freshwater on the N. Atlantic in one year -- over 200 times the observed freshening rate and more than 100 times the annual average precipitation over the N. Atlantic. • Even with such unrealistic* dollops of freshwater, the sea ice returns to its normal state by 20 years and temperature changes over the land are modest. Ice Line; T=0, T > 20 yrs 0 < T < 20 yrs (Villenga et al. 2003)

19. SummaryWill Greenhouse Warming initiate a thermohaline catastrophe, and a subsequent abrupt large cooling in parts of the NH? • Greenhouse Warming is thought to be responsible for the upward trend in the NAO/AO which has, in turn, been shown to be responsible for the reduction of sea ice in the GIN Seas (IPCC 2001 and refs therein). • The resultant changes in ocean gyre circulation appear to be a positive feedback on the ice -- more heat imported into the GIN seas. Hence, the feedbacks associated with subpolar gyre changes would enhance warming in the North Atlantic. Answer: Not likely. • The fly in the ointment: the (GH-NAO induced?) trend in the net “P-E plus runoff plus sea ice melt” exceeds the increase in salt imported by the (GH/NAO-induced) spin-up of the subpolar gyre, for a net increase in buoyancy in the GIN seas.

20. Summary (cont): Would 200 more years of freshening at the observed rate cause a slowdown the THC and its heat transport into the N. Atlantic? • If the increase in buoyancy due to the net increase in “P-E plus runoff plus sea ice melt” exceeds the decrease in buoyancy due to the GH/NAO spin-up of the subpolar gyre, a gradual regional cooling (as in G&R). Answer: Not likely. • The cooling would be superimposed on the large-scale warming due to (by then) large increases in GH gases. Could (a) GH warming cause half of Greenland to slide catastrophically into the north Atlantic, thereby (b) initiating a THC shutdown and an abrupt large-scale cooling? • If (a) is true, then (b) should be true -- at least for a few decades. (but c.f. Heinrich events and other large freshwater surges that don’t appear to cause abrupt global cooling).