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Global climate responses to perturbations in Antarctic Intermediate Water

Global climate responses to perturbations in Antarctic Intermediate Water. Jennifer Graham Prof K. Heywood, Prof. D. Stevens, Dr Z. Wang (BAS). Antarctic Intermediate Water (AAIW). Dark green = extent of Antarctic Intermediate Water. AAIW in HadCM3.

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Global climate responses to perturbations in Antarctic Intermediate Water

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

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

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

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

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

  6. Jennifer Graham, Ocean Modelling 2011 Atmospheric Response Warmer, saltier AAIW Cooler, fresher AAIW • 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.

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

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

  10. Jennifer Graham, Ocean Modelling 2011 Conclusions • Perturbed AAIW, between 10-20oS • ±1oC, compensating salinity decrease, constant density. • Anomalies found at surface: • Equatorial regions. • Higher latitudes (>30oS). • 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. Finally, thanks to The Challenger Society for proving me with funding for this conference.

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

  12. Jennifer Graham, Ocean Modelling 2011

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

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

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

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

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

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

  19. Jennifer Graham, Ocean Modelling 2011

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