1. Global climate responses to perturbations in Antarctic Intermediate Water Jennifer Graham Prof K. Heywood, Prof. D. Stevens, Dr Z. Wang (BAS)

2. Antarctic Intermediate Water (AAIW) Dark green = extent of Antarctic Intermediate Water Jennifer Graham, Ocean Modelling 2011

3. AAIW in HadCM3 Zonal-mean salinity from 500-year control Characteristic salinity minimum found at intermediate depths in each ocean basin. Key Questions:: Can changes in AAIW affect the climate system? If so, how? Jennifer Graham, Ocean Modelling 2011

4. Perturbation Experiment Separate experiments for Atlantic, Pacific and Indian oceans. Temperature perturbation of ±1 o C, made between 10-20 o S. Density-compensating change in salinity. Simulations run for 100 years, using 9 member ensembles. Jennifer Graham, Ocean Modelling 2011 Example: perturbing Atlantic AAIW

5. Surfacing Anomalies Surfacing in equatorial regions and higher latitudes (>30 o S). Mechanisms: A = equatorial or coastal upwelling. B = shoaling isopycnals and deeper mixed layers. Jennifer Graham, Ocean Modelling 2011

6. Atmospheric Response Surface air temperature (SAT) anomalies found over regions of sea surface temperature (SST) anomalies. Response is not equal and opposite for warming or cooling perturbations. Jennifer Graham, Ocean Modelling 2011 Warmer, saltier AAIWCooler, fresher AAIW

7. Mean SAT anomaly for each experiment Response for Atl- is most significant. Cooling in the North Atlantic Current (NAC) and subpolar gyre (SPG). Reduced SAT over cooler surface ocean. Jennifer Graham, Ocean Modelling 2011 Mean SAT anomaly for years 51-100 [ o C].

8. Non-linear responses - I Jennifer Graham, Ocean Modelling 2011 Non-linear response to initially density- compensating perturbations. When anomalies surface, anomalous ocean-atmosphere heat fluxes cause changes in density. Density anomalies cause fresher AAIW to spend greater length of time at surface than the saltier AAIW. Graham et al., 2011, Clim Dyn.

9. Non-linear responses - II Jennifer Graham, Ocean Modelling 2011 Density anomalies lead to changes in ocean circulation. e.g. Meridional overturning circulation (MOC), Antarctic Circumpolar Current (ACC). Ensemble-mean anomalies, EXP-Ctl. Red = +1 o C (saltier); Blue = -1 o C (fresher). MOC Index

10. Conclusions Perturbed AAIW, between 10-20 o S ±1 o C, compensating salinity decrease, constant density. Anomalies found at surface: Equatorial regions. Higher latitudes (>30 o S). More significant response for cooler, fresher perturbations i.e. non-linear response. North Atlantic particularly sensitive as decreased MOC strength reduces northward ocean heat transport. Jennifer Graham, Ocean Modelling 2011 Finally, thanks to The Challenger Society for proving me with funding for this conference.

11. Any questions...? Jennifer Graham, Ocean Modelling 2011

13. Equatorial response - I Salinity anomalies found in equatorial regions. Surfacing through equatorial upwelling. Ensemble-mean anomalies, EXP-Ctl. Red = +1 o C (saltier); Blue = -1 o C (fresher). Jennifer Graham, Ocean Modelling 2011

14. … but no corresponding trend in surface temperature. Ensemble-mean anomalies, EXP-Ctl. Red = +1 o C (saltier); Blue = -1 o C (fresher). Jennifer Graham, Ocean Modelling 2011 Equatorial response - II

15. Higher latitudes - I Salinity anomalies found in the Southern Ocean. Shoaling isopycnals and regions of deeper mixed layers. Ensemble-mean anomalies, EXP-Ctl. Red = +1 o C (saltier); Blue = -1 o C (fresher). Jennifer Graham, Ocean Modelling 2011

16. Higher latitudes - II Ensemble-mean anomalies, EXP-Ctl. Red = +1 o C (saltier); Blue = -1 o C (fresher). Jennifer Graham, Ocean Modelling 2011 Temperature anomalies are also found in the Southern Ocean. Responses are not equal and opposite.

17. Ensemble-mean anomalies, EXP-Ctl. Red = +1 o C (saltier); Blue = -1 o C (fresher). Jennifer Graham, Ocean Modelling 2011

18. Non-linear responses Jennifer Graham, Ocean Modelling 2011 Density anomalies lead to changes in ocean circulation. e.g. Meridional overturning circulation (MOC), Antarctic Circumpolar Current (ACC).

19. Jennifer Graham, Ocean Modelling 2011