Yosef Ashkenazy Bluastein Institute for Desert Research BenGurion University wwwbguacil ashkena Collaborators Hezi Gildor Martin Losch Francis A Macdonald Daniel P ID: 275999
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Slide1
Snowball oceanography
Yosef
AshkenazyBluastein Institute for Desert Research, Ben-Gurion Universitywww.bgu.ac.il/~ashkena
Collaborators
:
Hezi
Gildor
, Martin
Losch
, Francis A. Macdonald, Daniel P.
Schrag
, &
Eli
Tziperman
Slide2
What is Snowball Earth?
M
ost 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)Slide3
How do we know about snowball?Figs from Hoffman &
Schrag 2002For more: www.snowballearth.org
Glacial deposits:DropstoneGlacial deposits at low paleo-latitudeEvidence for Snowball:Low latitudes glacial deposit.Open water deposit.Carbon isotope ratio.Banded iron formation. Cap carbonate rocks. … Slide4
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.Slide5
Models’ coupling
Lat./depth ocean (1D ice): 1
o resolution (82oS to 82oN) with 32 levels with 10 m resolution in vicinity of ice. Ocean depth of 2 km plus 1 km ice.Eddy resolving (1/8o), equatorial sector (0o—45oE and 10oS—10oN)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.Slide6
Results
:
2D ocean, 1D iceSlide7Slide8
Summary of the 2D results
Strong equatorial currents.
Enhanced equatorial concentrated meridional overturning circulation (MOC) cell. Anti-symmetric and broad zonal vel. (u). Symmetric & confined meridional vel. (v). u, v change sign with depth. w and MOC maximal at mid-depth. No MOC above above the maximum heating. Difference in temperature of 0.2 oC. Difference in salinity of 0.5 ppt.We wish to understand why: (i)—(vi). Study a simplified set of equations
Strong equatorial currents.
Enhanced
equatorial
concentrated
meridional
overturning circulation (MOC) cell.
Anti-symmetric and broad zonal vel. (u).
S
ymmetric & confined
meridional
vel. (v).
u, v change sign with depth.
w
and MOC maximal at mid-depth.
No
MOC above above the maximum heating
.
Difference in temperature of 0.2
o
C.
Difference
in
salinity
of
0.5 ppt.Slide9
Model
Assumptions: (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.Slide10
Equator
: Pressure gradient is balanced by viscosity.
Off-equator
: “
geostrophy
”.
z
=0 at mid depth.
Strong equatorial currents.
Enhanced
equatorial
concentrated
meridional
overturning circulation (MOC) cell.
Anti-symmetric and broad zonal vel. (u).
S
ymmetric & confined
meridional
vel. (v).
u, v change sign with depth.
w
and MOC maximal at mid-depth.
No
MOC above above the maximum heating
. Slide11
Equator
: Pressure gradient is balanced by viscosity.
Off-equator
: “
geostrophy
”.
z
=0 at mid depth.
Strong equatorial currents.
Enhanced
equatorial
concentrated
meridional
overturning circulation (MOC) cell.
Anti-symmetric and broad zonal vel. (u).
S
ymmetric & confined
meridional
vel. (v).
u, v change sign with depth.
w
and MOC maximal at mid-depth.
No
MOC above above the maximum heating
. Slide12
Equator
: Pressure gradient is balanced by viscosity.
Off-equator
: “
geostrophy
”.
z
=0 at mid depth.
Strong equatorial currents.
Enhanced
equatorial
concentrated
meridional
overturning circulation (MOC) cell.
Anti-symmetric and broad zonal vel. (u).
S
ymmetric & confined
meridional
vel. (v).
u, v change sign with depth.
w
and MOC maximal at mid-depth.
No
MOC above above the maximum heating
. Slide13
Equator
: Pressure gradient is balanced by viscosity.
Off-equator
: “
geostrophy
”.
z
=0 at mid depth.
Strong equatorial currents.
Enhanced
equatorial
concentrated
meridional
overturning circulation (MOC) cell.
Anti-symmetric and broad zonal vel. (u).
S
ymmetric & confined
meridional
vel. (v).
u, v change sign with depth.
w
and MOC maximal at mid-depth.
No
MOC above above the maximum heating
. Slide14
Equator
: Pressure gradient is balanced by viscosity.
Off-equator
: “
geostrophy
”.
z
=0 at mid depth.
Strong equatorial currents.
Enhanced
equatorial
concentrated
meridional
overturning circulation (MOC) cell.
Anti-symmetric and broad zonal vel. (u).
S
ymmetric & confined
meridional
vel. (v).
u, v change sign with depth.
w
and MOC maximal at mid-depth.
No
MOC above above the maximum heating
.
Most features are explained!Slide15
Equatorial sector—high resolution (1/8o simulation) simulation
Why?
Parametrization of eddy viscosity coefficient.Turbulence under complete ice cover?
Setup
:
Equatorial section: 10
o
S to 10
o
N & 0
o
E to 45
o
E with
1/8
o
resolution (360x168 grid)
; fixed (uniform) ice depth; 20 vertical level (100 m each);
Two configurations: with and without island.
Maximum geothermal heating at 6
o
N.
Much lower viscosity coefficient!
Turbulence
.Slide16Slide17
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?Slide18
Summary
The ocean Snowball condition if far from being stagnant. Rich and enhanced dynamics.
Mainly equatorial dynamics. Strong zonal jet; strong & confined meridional overturning circulation (MOC) cell as a result of rotation, geothermal heating, and horizontal viscosity.Turbulence. Main oceanic characteristics are robust! Slide19Slide20