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Equatorial Atmosphere and Ocean Dynamics

MS320 Wilkin. Equatorial Atmosphere and Ocean Dynamics. Global winds east-west Walker circulation north-south Hadley cells Ekman currents divergence and upwelling convergence/divergence establishes north-south pressure gradients Zonal (east-west) currents geostrophic (off-equator)

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Equatorial Atmosphere and Ocean Dynamics

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  1. MS320 Wilkin Equatorial Atmosphere and Ocean Dynamics • Global winds • east-west Walker circulation • north-south Hadley cells • Ekman currents • divergence and upwelling • convergence/divergence establishes north-south pressure gradients • Zonal (east-west) currents • geostrophic (off-equator) • wind-driven (on-equator) • East-west pressure gradients • sloping thermocline • baroclinic vertical structure • equatorial undercurrent Given the winds, how does the ocean respond… But first: a quick review of atmosphere dynamics (All of global scale meteorology in 5 minutes…)

  2. Inter-tropical Convergence Zone (ITCZ) = location of the doldrums Seasonal shift with monsoontoward summer hemisphere July NH summer … All in a hot and copper sky,The bloody Sun, at noon,Right up above the mast did stand,No bigger than the Moon. Day after day, day after day,We stuck, nor breath nor motion;As idle as a painted shipUpon a painted ocean. … from The Rime of the Ancient Mariner, S. T. Coleridge January SH summer

  3. The large scale pattern of winds is predominantly responsible for the upper ocean wind driven gyres and boundary currents.

  4. rising air | sinking air 500 hPa vertical velocity (Pa/S) in July from ERA-40 reanalysis, 1979-2001 average. Negative (blue) values represent rising air; positive (red) values indicate sinking air. This illustrates the Hadley cell.

  5. O X Light on the right

  6. July • On average: • ITCZ is north of equator • equatorial winds are easterly • eastern boundary upwelling region winds are equatorward January

  7. O O O O X X X X

  8. South Equatorial Eq. Counter North Eq. Current Current Current

  9. Figure 14.3 Average currents at 10m calculated from the Modular Ocean Model driven by observed winds and mean heat fluxes from 1981 to 1994. The model, operated by the NOAA National Centers for Environmental Prediction, assimilates observed surface and subsurface temperatures. From Behringer, Ji, and Leetmaa (1998). http://oceanworld.tamu.edu/resources/ocng_textbook/chapter14/chapter14_01.htm

  10. Winds toward the west Sea level slopes up from east to west West Pacific Warm Pool Thermocline slopes up from west to east

  11. Wind stress (here positive is toward the west) Sea level slopes down from west to east West Pacific Warm Pool Thermocline slopes up from west to east

  12. remember … no Coriolis Pressure force due to sloping sea surface balances wind

  13. to east 110W 155W to west

  14. to east to west

  15. Integral over upper ocean depth range

  16. Thermocline slopes up from west to east

  17. time Months before June 1998 Izumo T., J. Picaut et B. Blanke, 2002: Tropical pathways, equatorial undercurrent variability and the 1998 La Niña. Geophys. Res. Lett., 29 (22), 2080-2083:

  18. Depth Izumo T., J. Picaut et B. Blanke, 2002: Tropical pathways, equatorial undercurrent variability and the 1998 La Niña. Geophys. Res. Lett., 29 (22), 2080-2083:

  19. Air rises at equator so air from off equator converges toward ITCZ • Equator-ward air flow turns west • get Trade winds • SE Trades on equator drive divergent Ekman currents • get equatorial upwelling • Weak winds in doldrums cause convergent Ekman currents • get local maximum in sea level • get “reversed” N-S sea level gradient downward toward north • This is off equator so is balanced by a geostrophic flow toward east • get NECC • Further north winds are stronger and Ekman currents diverge • get NEC • SEC pushes water toward west setting up a sloping sea level • get WPWP • Pressure force due to sea level slope balances winds at surface • Below surface the pressure force dominates and drives water east • get EUC • Water from off equator is drawn in to the EUC • NEXT LECTURE: What happens if the Trade winds weaken, or the WPWP expands and atmospheric convection moves east?

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