snowball oceanography
DESCRIPTION
Snowball oceanography. Collaborators : Hezi Gildor , Martin Losch , Francis A. Macdonald, Daniel P. Schrag , & Eli Tziperman. Yosef Ashkenazy Bluastein Institute for Desert Research, Ben-Gurion University www.bgu.ac.il /~ ashkena. M ost extreme climate event in Earth history. - PowerPoint PPT PresentationTRANSCRIPT
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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
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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)
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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. …
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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.
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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.
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Results: 2D ocean, 1D ice
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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.
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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.
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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.
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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.
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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.
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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.
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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!
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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.
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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?
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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!
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