Global climate responses to perturbations in Antarctic Intermediate Water Jennifer Graham Prof K....

Global climate responses to perturbations in Antarctic Intermediate Water

Jennifer GrahamProf K. Heywood, Prof. D. Stevens, Dr Z. Wang (BAS)

Antarctic Intermediate Water (AAIW)

Dark green = extent of Antarctic Intermediate Water

Jennifer Graham, Ocean Modelling 2011

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?


Perturbation ExperimentSeparate 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

Surfacing Anomalies• Surfacing in equatorial regions and higher latitudes (>30oS).• Mechanisms:

• A = equatorial or coastal upwelling.• B = shoaling isopycnals and deeper mixed layers.


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.


Warmer, saltier AAIW Cooler, fresher AAIW

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.


Mean SAT anomaly for years 51-100 [oC].

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.

Non-linear responses - II


• Density anomalies lead to changes in ocean circulation.e.g. Meridional overturning circulation (MOC),

Antarctic Circumpolar Current (ACC).

Ensemble-mean anomalies, EXP-Ctl.Red = +1oC (saltier); Blue = -1oC (fresher).

MOC Index

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.

Any questions...?


Equatorial response - ISalinity anomalies found in equatorial regions.

▫Surfacing through equatorial upwelling.



… but no corresponding trend in surface temperature.



Equatorial response - II

Higher latitudes - ISalinity anomalies found in the Southern

Ocean.▫Shoaling isopycnals and regions of deeper

mixed layers.



Higher latitudes - II



Temperature anomalies are also found in the Southern Ocean.▫Responses are not equal and opposite.

Non-linear responses


▫ Density anomalies lead to changes in ocean circulation.▫ e.g. Meridional overturning circulation (MOC),

Antarctic Circumpolar Current (ACC).

Global climate responses to perturbations in Antarctic Intermediate Water Jennifer Graham Prof K....

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