Snowball oceanograph

yYosef Ashkenazy

Bluastein Institute for Desert Research, Ben-Gurion University

www.bgu.ac.il/~ashkena

Collaborators: Hezi Gildor, Martin Losch, Francis A. Macdonald, Daniel P. Schrag, & Eli Tziperman

What is Snowball Earth?Most extreme climate event in Earth history.Characteristics: • Occurred at least twice between 750-

635 Ma. • Global (or almost global) ice coverage.• More than 1 km thick sea-glacier.• Mean global temperature: -44oC.

(1992)

How do we know about snowball?

Figs from Hoffman & Schrag 2002For more: www.snowballearth.org

Glacial deposits:Dropstone

Glacial deposits at low paleo-latitudeEvidence for Snowball:1) Low latitudes

glacial deposit.2) Open water deposit.3) Carbon isotope

ratio.4) Banded iron

formation. 5) Cap carbonate

rocks. …

Goal

• Improve understanding of the climate system.

• Improve climate models.• Photosynthetic life under the thick ice?

What do we do?

We use and coupled the following models:• Ice-flow model of Tziperman et al.

(2012).• Oceanic MITgcm using shelf-ice package

and bottom geothermal heating. Idealized BC.

• Ice-flow and ocean models exchange information every few hundred years (300 yr).

Motivation Study ocean circulation under global ice-cover.

Models’ coupling

(i) Lat./depth ocean (1D ice): 1o resolution (82oS to 82oN) with 32 levels with 10 m resolution in vicinity of ice. Ocean depth of 2 km plus 1 km ice.

(ii) Eddy resolving (1/8o), equatorial sector (0o—45oE and 10oS—10oN)

(iii)3D ocean (2D ice), 2o resolution globally. 73 levels.

q—melting/freezing rateTf—freezing temperatureh—sea-glacier depthT(z=0)—ice temp. at z=0.

Results: 2D ocean, 1D ice

Summary of the 2D results (i) Strong equatorial currents.

(ii) Enhanced equatorial concentrated meridional overturning circulation (MOC) cell.

(iii) Anti-symmetric and broad zonal vel. (u).(iv) Symmetric & confined meridional vel. (v).(v) u, v change sign with depth. w and MOC

maximal at mid-depth.(vi) No MOC above above the maximum heating. (vii) Difference in temperature of 0.2 oC.(viii) Difference in salinity of 0.5 ppt.We wish to understand why: (i)—(vi). Study a simplified set of

equations

(i) Strong equatorial currents.(ii) Enhanced equatorial concentrated

meridional overturning circulation (MOC) cell.

(iii) Anti-symmetric and broad zonal vel. (u).(iv) Symmetric & confined meridional vel. (v).(v) u, v change sign with depth. w and MOC

maximal at mid-depth.(vi) No MOC above above the maximum heating. (vii) Difference in temperature of 0.2 oC.(viii) Difference in salinity of 0.5 ppt.

ModelAssumptions: (i) 2D (latitude-depth) (∂/∂x =0), (ii) constant ice depth, (iii) steady state (∂/∂t =0), (iv) β-plane.

××× ×× ×

Neglect terms based on “scaling” or numeric.

Equator: Pressure gradient is balanced by viscosity.

Off-equator: “geostrophy”.

z=0 at mid depth.

(i) Strong equatorial currents.(ii) Enhanced equatorial concentrated

meridional overturning circulation (MOC) cell.

(iii) Anti-symmetric and broad zonal vel. (u).(iv) Symmetric & confined meridional vel. (v).(v) u, v change sign with depth. w and MOC

maximal at mid-depth.(vi) No MOC above above the maximum heating.

Equator: Pressure gradient is balanced by viscosity.

Off-equator: “geostrophy”.

z=0 at mid depth.

(i) Strong equatorial currents.(ii) Enhanced equatorial concentrated

meridional overturning circulation (MOC) cell.

(iii) Anti-symmetric and broad zonal vel. (u).(iv) Symmetric & confined meridional vel. (v).(v) u, v change sign with depth. w and MOC

maximal at mid-depth.(vi) No MOC above above the maximum heating.

Equator: Pressure gradient is balanced by viscosity.

Off-equator: “geostrophy”.

z=0 at mid depth.

(i) Strong equatorial currents.(ii) Enhanced equatorial concentrated

meridional overturning circulation (MOC) cell.

(iii) Anti-symmetric and broad zonal vel. (u).(iv) Symmetric & confined meridional vel. (v).(v) u, v change sign with depth. w and MOC

maximal at mid-depth.(vi) No MOC above above the maximum heating.

Equator: Pressure gradient is balanced by viscosity.

Off-equator: “geostrophy”.

z=0 at mid depth.

(i) Strong equatorial currents.(ii) Enhanced equatorial concentrated

meridional overturning circulation (MOC) cell.

(iii) Anti-symmetric and broad zonal vel. (u).(iv) Symmetric & confined meridional vel. (v).(v) u, v change sign with depth. w and MOC

maximal at mid-depth.(vi) No MOC above above the maximum heating.

Equator: Pressure gradient is balanced by viscosity.

Off-equator: “geostrophy”.

z=0 at mid depth.

(i) Strong equatorial currents.(ii) Enhanced equatorial concentrated

meridional overturning circulation (MOC) cell.

(iii) Anti-symmetric and broad zonal vel. (u).(iv) Symmetric & confined meridional vel. (v).(v) u, v change sign with depth. w and MOC

maximal at mid-depth.(vi) No MOC above above the maximum heating.

Most features are explained!

Equatorial sector—high resolution (1/8o simulation) simulationWhy? (i) Parametrization of eddy viscosity coefficient.(ii) Turbulence under complete ice cover?Setup: (i) Equatorial section: 10oS to 10oN & 0oE to 45oE

with 1/8o resolution (360x168 grid); fixed (uniform) ice depth; 20 vertical level (100 m each);

(ii) Two configurations: with and without island.(iii)Maximum geothermal heating at 6oN.(iv)Much lower viscosity coefficient!

Turbulence.

Melting rate• Almost one

order of magnitude larger than atmospheric value.

• The enhance melting is associated with upwelling of warm water.

• Can enhanced melting create hole in the ice?• Can this resolve the question of

photosynthetic life under hard Snowball conditions?

Summary(i) The ocean Snowball condition if far from being stagnant. Rich and enhanced dynamics.

(ii)Mainly equatorial dynamics. Strong zonal jet; strong & confined meridional overturning circulation (MOC) cell as a result of rotation, geothermal heating, and horizontal viscosity.

(iii)Turbulence.

Main oceanic characteristics are robust!