Sea Ice Extent

Sea Ice
Trends

Arctic sea ice = declining

Antarctic sea ice = increased over past 20 years

Antarctic Δ contradicts global warming ? Why?

Proposed
Cause Ant Δ

Why Antarctic sea ice increased despite global warming ?

Change in winds due to ozone depletion

How Winds
Δ Sea Ice?
Turner 2009

Change in winds due to ozone depletion

  1. Cooling due to ozone depletion

since ozone absorbs UV ∴ heating stratosphere

  1. Low pressure trough generated
    as cooling modifies air pressure
  1. Westerly winds strengthened over S. Ocean
    by pressure anomaly (cooling)
  1. Equatorward Ekman transport strengthened
    driving Ekman flux away from Antarctica
  1. Cold surface waters extend further North ∴ freezing & sea ice extends further N

Some disagree Sigmond & Fyfe 2013

Coupled climate models

Modelled effects of
ozone depletion:

  1. Does increase wind stress

2. Decreased sea ice extent due to strengthened winds

  1. Contradicts observations...

Ozone hole

most ozone depletion = above Antarctica

Short & Long Term Response
Ferreira et al 2015

Modelling response to ozone depletion

Two-fold Effect Ozone Depletion

1. Moves equatorward strong core of Westerlies

i.e. latitude @ which Westerlies = most intense

2. Strengthens Westerlies

  1. induces geostrophic circulation in upper atmos around S Pole
  1. low pressure above antarctica

Westerlies
Effect Nutshell

  1. Westerlies move further N in S Ocean

3. Sea ice extent moves N
(Colder waters brough further N ∴ sea surface freezes)

  1. Strengthens equatorward Ekman flux

Fast (years)

Sea ice advances North

Slow
(decades)

Warm water upwelling at depth

Short term ~ year

Sea ice retreats

Ekman pumping equatorwards etc

If Ekman flux imposed for long enough

Authors found 2 phases of response

idealised 'aquaplanet' - no continents

Concurs with observed expansion

Sea ice response timescales

Varies as f(how model represents ocean upwelling)

i.e. how aggressively upwelling simulated

upwelling =
difficult to model

Theory is viable

Different models show same qualitiative behaviour

Timings differ
btwn models

Ferreira - retreat after 15 years

Sigmond - retreat after 5 years

alters ocean circulation

subject to feedbacks