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Snowball oceanography Slide 2 What is Snowball Earth? Most extreme climate event in Earth history. Characteristics: Occurred at least twice between 750-635 Ma. Occurred at least twice between 750-635 Ma. Global (or almost global) ice coverage. Global (or almost global) ice coverage. More than 1 km thick sea-glacier. More than 1 km thick sea-glacier. Mean global temperature: -44 o C. Mean global temperature: -44 o C. (1992) Slide 3 How do we know about snowball? Figs from Hoffman & Schrag 2002 For more: www.snowballearth.orgwww.snowballearth.org Glacial deposits: Dropstone Glacial deposits at low paleo-latitude Evidence for Snowball: 1)Low latitudes glacial deposit. 2)Open water deposit. 3)Carbon isotope ratio. 4)Banded iron formation. 5)Cap carbonate rocks. Slide 4 Goal Improve understanding of the climate system. Improve understanding of the climate system. Improve climate models. Improve climate models. Photosynthetic life under the thick ice? 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). Ice-flow model of Tziperman et al. (2012). Oceanic MITgcm using shelf-ice package and bottom geothermal heating. Idealized BC. 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). Ice-flow and ocean models exchange information every few hundred years (300 yr). Motivation Study ocean circulation under global ice-cover. Slide 5 Models coupling (i)Lat./depth ocean (1D ice): 1 o resolution (82 o S to 82 o N) with 32 levels with 10 m resolution in vicinity of ice. Ocean depth of 2 km plus 1 km ice. (ii)Eddy resolving (1/8 o ), equatorial sector (0 o 45 o E and 10 o S10 o N) (iii)3D ocean (2D ice), 2 o resolution globally. 73 levels. qmelting/freezing rate T f freezing temperature hsea-glacier depth T(z=0)ice temp. at z=0. Slide 6 Results: 2D ocean, 1D ice Slide 7 Slide 8 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 o C. (viii) Difference in salinity of 0.5 ppt. We wish to understand why: (i)(vi). (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 o C. (viii) Difference in salinity of 0.5 ppt. Slide 9 Model Assumptions: (i) 2D (latitude-depth) ( (ii) constant ice depth, (iii) steady state (, (iv) -plane. Assumptions: (i) 2D (latitude-depth) (/x =0), (ii) constant ice depth, (iii) steady state (/t =0), (iv) -plane. Neglect terms based on scaling or numeric. Slide 10 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. Slide 11 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. Slide 12 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. Slide 13 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. Slide 14 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! Slide 15 Equatorial sector high resolution (1/8 o simulation) simulation Why? (i)Parametrization of eddy viscosity coefficient. (ii)Turbulence under complete ice cover? Setup: (i)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); (ii)Two configurations: with and without island. (iii)Maximum geothermal heating at 6 o N. (iv)Much lower viscosity coefficient! Turbulence. Slide 16 Slide 17 Melting rate Almost one order of magnitude larger than atmospheric value. The enhance melting is associated with upwelling of warm water. The enhance melting is associated with upwelling of warm water. Can enhanced melting create hole in the ice? Can enhanced melting create hole in the ice? Can this resolve the question of photosynthetic life under hard Snowball conditions? Can this resolve the question of photosynthetic life under hard Snowball conditions? Slide 18 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! Slide 19 Slide 20