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…)

MS320 Wilkin

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July

January

Inter-tropical Convergence Zone (ITCZ) = location of the doldrums

Seasonal shift with monsoon

toward summer

hemisphere

…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

NH summer

SH summer

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

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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.

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OX

Light on the right

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July

January

On average:

• ITCZ is north of equator

• equatorial winds are easterly

• eastern boundary upwelling region winds are equatorward

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OX OX OX

OX

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South Equatorial Eq. Counter North Eq. Current Current Current

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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 11

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Winds toward the

west

Thermocline slopes up

from west to east

West Pacific

Warm Pool

Sea level slopes up

from east to west

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Wind stress (here positive is toward the

west)

Sea level slopes down from west to

east

Thermocline slopes up

from west to east

West Pacific Warm Pool

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Pressure force dueto sloping sea surfacebalances wind

remember … no

Coriolis

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to east

to wes

t

155W 110W

to east

to wes

t

Integral over upper ocean depth

range

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Thermocline slopes up

from west to east

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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:

time

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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:

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Depth

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• 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?