Effects of global warming on fast and drift ice introduction and provocation!

Peter M. Haugan 14.11.2000

Preliminaries – Sea ice in the climate system

Level 1: Basic effects of sea ice on climate

• Changing the global radiation balance and equilibrium surface temperature • Shutting off the oceanic heat source to the atmosphere

Level 2: How variations in ice cover may affect global and regional climate

• Ice albedo feedback • Ice freezing (brine rejection and deep water ventilation), ice motion, and ice melting (stabilization and reduction of overturning circulation) • Effects of surface temperature and roughness on atmospheric circulation • Effects of ice cover and wind driven ice drift on ocean circulation (does sea ice reduce sensitivity of THC ?)

Preliminaries (cont’d)

Level 3: Processes affecting ice cover characteristics

• Run-off, precipitation and melting/freezing effects on upper ocean stability • Precipitation effects on ice surface • Surface heat balance effect on ice growth • Penetration of short wave radiation and heat supply via the surface layer • Turbulent heat transport in the water column • Wind and current driven lead/polynya and ridge formation • Wind and current driven ice drift

Level 4: Selected topics for today

• snow ice, polynyas and ice freezing, mesoscale atmospheric forcing and oceanic response, link to ventilation • other specific topics to be discussed?

Anticipated changes associated with global warming • Increased surface air temperature • Increased atmospheric water vapour content • Increased precipitation and runoff • More energetic wind field

Effects on sea ice ?

Likely effects on landfast ice (modified after Wadhams, 1998) • Reduced freezing-degree-days gives thinner ice and longer ice-free season • More open water, higher humidity and more precipitation increases snow thickness, further decreasing ice growth • (Note that increased sea-air heat transfer occurs due to thinner ice)

Potential complications:

• Snow ice • Snow protection against surface melt in summer (“floating glaciers”) • Earlier freezing due to freshened surface water?

Likely effects on drift ice (modified after Wadhams, 1998) • Mean ice thickness is largely

insensitive

to air temperature, humidity and surface heat balance • Main sensitivity of ice growth is to wind driven deformation creating leads • (Note that increased air temperature decreases air-sea heat transfer since most occurs in leads)

Potential complications:

• Different dominating mechanisms in different areas. The above may hold in the central Arctic and north of Greenland, but what about Barents and Antarctic with larger ocean heat supply?

• Ice edge regions

The provocation… • Fast ice response to global warming is well understood and can be estimated by simple downscaling from regional atmospheric scenarios.

• Meaningful estimates of drift ice response requires coupled atmosphere-ice-ocean scenarios with unrealistic detail and incorporation of ice processes which are not amenable to grid models.

Prove me wrong!

(Plus do not spend all energy on global warming – Also care about fundamental processes and effects of sea ice on atmosphere and ocean)

Summary of Visbeck, Fischer & Schott JGR 1995 • Wind driven ice drift from Nordbukta • Average ice export of 5-8 mmd -1 was required in otherwise 1-D ocean model to achieve deep convection and match observations • Suggest haline convection phase initially associated with ice export • Later ”thermal” convection phase • The haline convection in agreement with earlier work by Rudels • The ”thermal” convection likely affected by thermobaricity (Aagaard & Carmack, 1989)

Summary of Thorkildsen & Haugan DSR 1999 • Steady state plume model including nonlinear equation of state, dynamic pressure and both components of earth rotation.

• New scaling of plume radius.

• To achieve deep convection: Small elevation in density O (10 -4 kgm -3 ) is sufficient; ambient must be only weakly stratified; thermobaricity is the most important driving force.

• Inserting a warm and saline layer 600-1400m with same dr / d z gives increased penetration for plumes which otherwise would have stopped in the layer, and decreased penetration for plumes which would have traveled through.

Convection in the Deep Ocean

Attribute Importanc e for deep water renewal Weddell Sea

Potentially large

Importanc e for ice cover

Potentially large

Greenland Sea

Believed to be large, but recently questioned

Labrador Sea

Large

Gulf of Lions

Small Relatively small Small None

Mechanis ms

Ice export followed by thermal convection

Role of thermobar icity Controls on depths and rates

Trigger catastrophic overturning ?

Reduces S cr , makes plume penetration more effective Ice divergence, cold air outbreaks, doming of isopycnals in gyre Small ?

Thermal Negligible Wind speed and air temperature (surface cooling), preconditioni ng

Summary of Løyning & Weber JGR 1997 • Infinitesimal perturbations can trigger thermobaric instability. This is shown by classical linear stability analysis applied to Boussinesq type equations with expansion in d/H a where d is layer thickness, as well as from nonlinear 2D equations.

• Thermobaric convection, i.e. when thermobaricity is the sole cause of convection, a s D T < bD S < ( a s + a 1 d) D T, gives asymmetric cells with stronger circulation in the lower part. This is shown from linear approximation and from numerical solution of nonlinear equations.

• (Weber & Løyning 2000 Techn. Rept. Univ. Oslo: ”Thermobaric effect on symmetric instability” show that thermobaricity destabilises stratified, geostrophic flows. Small-amplitude rolls have centers of circulation which are shifted towards the lower part of the fluid layer).