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Aspects of Atmosphere-Ocean Circulation

Aspects of Atmosphere-Ocean Circulation. Brian Hoskins Director, Grantham Institute for Climate Change, Imperial College London Professor of Meteorology, University of Reading. Some Basic numbers. mass of10m of ocean = mass of atmosphere. 0 < ρ A < 1.3. 1020 < ρ O < 1040.

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Aspects of Atmosphere-Ocean Circulation

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  1. Aspects of Atmosphere-Ocean Circulation Brian Hoskins Director, Grantham Institute for Climate Change, Imperial College London Professor of Meteorology, University of Reading

  2. Some Basic numbers mass of10m of ocean = mass of atmosphere 0 < ρA < 1.3 1020 < ρO < 1040 thermal inertia of 2.5 m of ocean = that of atmosphere Atmosphere: well mixed gases + water vapour +… Ocean: water + salts + …..

  3. Evaporation-Precipitation & Salinity of Ocean E-P Salinity

  4. Density variations & radiative transfer atmosphere ocean α 30 times and γ 2.104 larger for atmosphere Salinity (S) variations (~ 33-36%o) as important as T variations for ocean Water vapour (0-50g/kg) impact on ρ often ignored for atmosphere Radiative transfer atmosphere: ½ solar through to surface of the Earth long wave loss to space from mid-troposphere ocean: solar absorbed in top 10m or so (55% in top 1m) long wave loss from top few mm

  5. Vertical structure Atmosphere 50km Ocean surface T 5km surface θ

  6. Energy Atmosphere Ocean = 0 except near the surface Poleward energy transport by Hadley Cell = M (cpΔT + ΔΦ + L Δq) ~ ( -4.8 + 7.5 - 1.8) 1015 W = 0.9 PW

  7. Topographic confinement Δp =pE- pW Ocean confined in basins ∫ρvgdx = Δp/f However Antarctic Circumpolar Current

  8. Winds, Currents & Mass Transport Westerly wind & an easterly current! vA ~ 10 ms-1vO ~ 1 cm s-1 → ρAvA ~ ρOvO Mass flux by jets/currents ~ ρ V H W V H e.g. Gulf Stream mass flux ~ 103 . 1 . 1.103 . 1.105 ~ 100.109 kg s-1 ~ 100 Sv 1 Sv (= 106 m3 s-1 ) = 1.109 kg s-1 W Westerly Jet mass flux ~ 0.6 . 20 . 4.103 . 2.106 ~ 96.109 kg s-1 ~ 100 Sv From “Diagnostics of the Global Atmospheric Circulation”, UGAMP technical report #7

  9. Westward relative movement of Rossby Waves: contrasting arguments Atmosphere positive vorticity independent of y Ocean Low pressure/free surface independent of y + large f large f y + - + - - - + + - small f

  10. Quasi-geostrophic potential vorticity: = LR2/L2 where LR = NH/f N = 10-2s-1, f =10-4s-1; H = 10km , LR = 1000km; H = 100m, LR = 10km Ertel PV = ρ-1ζ. grad θ Atmosphere ~ ρ(z)-1 (f+ξ) δθ/δz Ocean ~ ρ0-1 f/H H

  11. Frictional Stress at the atmosphere-ocean interface z PGF CF Geostrophic motion in the atmosphere L CF PGF L Balance of forces with surface drag L Drag

  12. z hA surface τ hO =

  13. Wind-driven Ocean Circulation Sverdrup relation βv = f wz

  14. Overturning circulations & northward energy transport N S Poleward energy transport = McpΔT Northward energy transport (Trenberth & Caron 2001) total 4PW A O

  15. Evaporation-Precipitation & Salinity of Ocean E-P Salinity

  16. Storm-tracks Northern Hemisphere (Dec-Feb) time-mean Eady growth parameter in the lower troposphere. Shading indicates high values. Blacked areas are regions of high orography. Hoskins and Valdes, 1990 NH (DJF) Standard deviation of 2-6 day band pass filtered relative vorticity on 850 hPa. Hoskins and Hodges, 2002

  17. El Niňo Southern Oscillation SOI: Tahiti – Darwin SLP Niňo3.4 SST

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