The  transient  response  of  the  Southern  Ocean  to  ozone  depletion  

in  the  GFDL  ESM2Mc  model  

William  SeviourJohns  Hopkins  University

Darryn Waugh,  Anand Gnanadesikan,  Jordan  Thomas

FESD  Virtual  Meeting,   20  November  2015

24  Ensemble  members  run  for  48  years  each

12  use  daily  ozone  values12  use  monthly  means  of  daily  values

12  on  ‘warm’  start  dates12  on  ‘cold’  start  dates

How  does  the  climate  system  respond  to  instantaneously  introducing  ozone  depletion?

How  does  the  climate  system  respond  to  instantaneously  introducing  ozone  depletion?

24  Ensemble  members  run  for  48  years  each

12  use  daily  ozone  values12  use  monthly  means  of  daily  values

12  on  ‘warm’  start  dates12  on  ‘cold’  start  dates

Ensemble  mean  SST  response

Initial  cooling  ~0.1  K,  for  about  30  years  

Daily  vs.  Monthly  mean  ozone

Atmospheric  Response

~50%  increase  in  peak  windstress anomaly  going  from  monthly  -­‐>  daily  ozone

Consistent  with  Neely  et  al.  2014

Position  of  peak  windstress in  control  simulation

MOC  Response

colors:  Temp.  stratification  in  control  (c.i. 2K)black:  MOC  anomaly  (c.i. 0.5  Sv)

Anomalous   overturning   circulation  ~50%  stronger  going   from  monthly   -­‐>  daily  ozone  

MOC  Response

Same  contour  levels  

MITgcm (Ferreira  et  al.,  2014)

Weaker  stratification  below  100  m  in  GFDL  ESM2Mc.

Stronger  cooling  over  first  100  m.

Transient  temperature  response

!"#$%&

!' ≈ −𝑤′ !",!-

Agrees  with  Ferreira  et  al.,  2014

𝑇/012 reaches  0.1K  (magnitude  of  surface  cooling)  at  about  30  years

Transient  temperature  response

Temperature  trend  similar  despite  stronger  upwelling.  Is  upwelling  really  driving  temperature  trend?

Heat  flux  budget:  Work  in  Progress

advection

eddies

column

𝑤2 is  positive,  but   !"!-

2is  negative  so  vertical  advection  contibution is  small  

Daily  vs.  monthly  ozone  SST  (50-­‐70S)

Difference  in  SST  response  is  small  relative  to  differences  due  to  initial  conditions.

Warm  start  vs.  Cold  start

Role  of  the  initial  state

Path  of  natural  (i.e.  unforced)  variability  determined  from  the  autocorrelation  of  control  simulation.

Shaded  region  =  95%  uncertainty  of  natural  variability

Role  of  the  initial  state

Natural  variability  – simulated  SST

Similar  response  relative  to  natural  variability.

Less  warming  in  cold  start,  but  probably  not  statistically  significant.

Role  of  the  initial  state

Ferreira  et  al.,  2014

~Similar  magnitude

Regional  responses

Ross  Sea

Weddell  Sea

Natural  variability  larger  in  Weddell  Sea,  forced  response  larger  in  Ross  Sea.

Timescales  different  (~10  year  cooling  in  Ross  Sea,  30  years  in  Weddell).

Role  of  the  initial  state

Natural  variability  dominates  in  Weddell  Sea,  not  in  Ross  Sea

Conclusions

• Two-­‐timescale  response  is  reproduced  by  GFDL  ESM2Mc,  with  ~30  year  initial  cooling.

• Daily  ozone  produces  stronger  windstress anomaly,  stronger  ocean  circulation  response,  but  little  temperature  difference.

• Response  relative  to  natural  variability  does  not  depend  strongly  on  the  initial  state.

• Natural  variability  is  large  relative  to  response  to  ozone  over  much  of  Southern  Ocean,  but  not  the  Ross  Sea.

• Timescales  of  response  are  regionally  dependent.

Daily  vs  Monthly  ozone

Sea  ice  

Sea  ice