1. 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)
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…)
Overview of lecture topic
General features of the large scale atmosphere circulation as it affects sea level winds
Ekman current response to mean winds
Equatorial upwelling
Convergence divergence near the equator
Geostrophic flow off-equator
Simpler dynamics on the equator

2. Inter-tropical Convergence Zone (ITCZ) = location of the doldrums
July
Seasonal shift with monsoon toward summer hemisphere
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 ship Upon a painted ocean.
from The Rime of the Ancient Mariner, S. T. Coleridge
This equatorial convergence occurs in a zone referred to the Inter-tropical Convergence Zone (ITCZ)
The ITCZ shifts seasonally toward the summer hemisphere.
This is most pronounced in Indian Ocean. The northern summer is the time of the southwest monsoon, with strong heating over the land causing low air pressure and moist warm winds approach the Indian subcontinent from the southwest.
In the Pacific Ocean this seasonal movement is much less pronounced because there are no heating extremes over surrounding land. Nevertheless, the imbalance in the distribution of the continents globally means that the location of the ITCZ is biased toward the northern hemisphere.
The ITCZ is a region of low wind speeds – a quiet zone between the relentless, steady, easterly flow of the Trade winds. These are the Doldrums.
In Moby Dick, Captain Ahab and crew of the Pequod languished for weeks in the doldrums of the Pacific Ocean before they could continue Ahab’s obsessive quest to find the white whale.
The doldrums do not fall on the equator, so there is a modest easterly Trade wind along the equator in the Pacific. This has important consequences for the oceanic circulation at the equator.
Notice eastern boundary winds are upwelling favorable.
January
SH summer

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

4. Winds for a planet with no continents
Low pressure along the equator because solar heating warms the air, it expands, and rises.
Flow (at sea level) from higher latitudes toward the equator is turned by Coriolis toward the west to form the Trade winds.
Moist warm air ascending at the equator is deflected toward the east (aloft). Short-circuits hemispheric scale convection cells of a stationary-earth model.
The deflected air aloft (cooled and dried) sinks around lat 30, and moves either equatorward (Trades) or poleward (westerlies).
High altitude air at the pole cools and sinks, moving to lower latitudes as the polar easterlies. It gains heat and moisture and meets the poleward surface winds at lat 60. Here the air rises again.
3-cell system in each hemisphere (Hadley and Ferrel cells).

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

7. O X
Light on the right
Equatorward flow turns westward at 0-30 (the Trade winds) and (the polar easterlies), but between these bands lie poleward flow deflected to the east (the westerlies).
Zones of descending air at latitude 30 and 90 are regions of low precipitation and clear skies.
At 0 and 60, moist low density air rises - regions of clouds and rain.
The regions of vertical motion between the major wind belts are places of variable and unreliable winds: the doldrums near 0, and the horse latitudes near 30.

8. ITCZ is north of equator
July
On average:
ITCZ is north of equator
equatorial winds are easterly
eastern boundary upwelling region winds are equatorward
This equatorial convergence occurs in a zone referred to the Intertropical Convergence Zone (ITCZ)
The ITCZ shifts seasonally toward the summer hemisphere.
This is most pronounced in Indian Ocean. The northern summer is the time of the southwest monsoon, with strong heating over the land causing low air pressure and moist warm winds approach the Indian subcontinent from the southwest.
In the Pacific Ocean this seasonal movement is much less pronounced because there are no heating extremes over surrounding land. Nevertheless, the imbalance in the distribution of the continents globally means that the location of the ITCZ is biased toward the northern hemisphere.
The ITCZ is a region of low wind speeds – a quiet zone between the relentless, steady, easterly flow of the Trade winds. These are the Doldrums.
In Moby Dick, Captain Ahab and crew of the Pequod languished for weeks in the doldrums of the Pacific Ocean before they could continue Ahab’s obsessive quest to find the white whale.
The doldrums do not fall on the equator, so there is a modest easterly Trade wind along the equator in the Pacific. This has important consequences for the oceanic circulation at the equator.
Notice eastern boundary winds are upwelling favorable
January

9. O O O O X X X X Off equator currents
Just off the equator, easterly flow generates Ekman currents in the two hemispheres that diverge, causing upwelling and locally decreased sea level.
Low sea level at the equator leads to geostrophic westward currents off the equator, i.e. flowing in the same direction as the directly wind driven surface at the equator.
North of the equator but south of the doldrums:
Northward Ekman transport is greater to the south, so there is a convergence that locally elevates the sea level
The convergence is greatest where the winds are changing most rapidly.
South of the locally high sea level, the geostrophic flow is westward, and this is still considered part of the SEC O X

10. Trade winds drive Ekman currents Equatorial upwelling
South Equatorial Eq. Counter North Eq.
Current Current Current
Trade winds drive Ekman currents
Equatorial upwelling
At the equator – upwelling and westward downwind flow
At the equator, there is no Coriolis force, and the easterly wind blows water toward the west. The accumulation of warm equatorial water in the western Pacific is the source of the West Pacific Warm Pool.
Off equator currents
Just off the equator, easterly flow generates Ekman currents in the two hemispheres that diverge, causing upwelling and locally decreased sea level.
Low sea level at the equator leads to geostrophic westward currents off the equator, i.e. flowing in the same direction as the directly wind driven surface at the equator.

11. Figure 14.3 Average currents at 10m calculated from the Modular Ocean Model driven by observed winds and mean heat fluxes from 1981 to The model, operated by the NOAA National Centers for Environmental Prediction, assimilates observed surface and subsurface temperatures. From Behringer, Ji, and Leetmaa (1998).

12. Ocean primary productivity is kicked off by the new nutrient source, causing a local maximum in phytoplankton biomass close to the equator (0.5 – 1.0o latitude)
Herbivorous zooplankton increase with the available food supply, and subsequently carnivorous zooplankton increase.
Predatory fish tend to congregate near the convergence at the boundary between the SEC and NECC, because prey species in the plankton accumulate there.

13. West Pacific warm pool
Atmospheric convection is greatest over the warmest water, so there a zonal (west-east) pattern ins convection and subsidence.

14. Sea level slopes up from east to west West Pacific Warm Pool
Winds toward the west
Sea level slopes up from east to west
West Pacific Warm Pool
Thermocline slopes up from west to east
At the equator – upwelling and westward downwind flow
At the equator, there is no Coriolis force, and the easterly wind blows water toward the west. The accumulation of warm equatorial water in the western Pacific is the source of the West Pacific Warm Pool.

15. West Pacific warm pool
Atmospheric convection is greatest over the warmest water, so there a zonal (west-east) pattern ins convection and subsidence.

16. Wind stress (here positive is toward the west)
Sea level slopes down from west to east
West Pacific Warm Pool
Right at the equator we forget about the whole Coriolis business.
Water flows downwind just like in ought to.
Steady easterly winds blow water westward, and these waters continually warm the whole way. This leads to an accumulation of very warm water in the western Pacific:
The West Pacific Warm Pool is a huge mass of very warm water much of it greater than 28oC.
The accumulated warm pool depresses the thermocline in the west, and accordingly sea level is elevated in the WPWP compared to the east Pacific.
Thermocline slopes up from west to east

17. Pressure force due to sloping sea surface balances wind
remember … no Coriolis
Pressure force due
to sloping sea surface
balances wind
So there is a pressure force directed from west to east. This is obviously not sufficient to overcome the westward flow of warm water at the surface (otherwise we’d never get the Warm Pool).
But below the upper ocean mixed layer, where the direct influence of the winds is lost, the pressure gradient remains and a west to east undercurrent forms at the bottom of the mixed layer.
This Equatorial Undercurrent (EUC) is a remarkable flow. It is effectively a ribbon of intense flow with speeds approaching 1.5 m s-1 only 200 m deep yet 300 km wide. It is deepest in the west and gradually shoals eastward along with the thermocline itself.

18. to east
110W
155W
to west

19. to east
to west

20. Integral over upper ocean depth range

21. Thermocline slopes up from west to east
A down pressure gradient flow like this can only occur at the equator because otherwise geostrophic balance would require the current to flow perpendicular to the pressure gradient.
However, Coriolis still plays a role in stabilizing the EUC.
Any strong current is prone to instabilities that cause it to meander. We’ve seen this in the meandering path of the Gulf Stream and its tendency to spawn cyclonic and anticyclonic eddies.
If the EUC strays from its zonal path, say into the Southern hemisphere, the Coriolis force starts to come into play deflecting the current left and back toward the equator. Similarly, northward meanders are deflected southward, and the EUC is trapped.

22. The waters of the EUC originate from the southern hemisphere, at the western boundary, from the New Guinea Coastal Current, though there is some exchange with surrounding waters during the course of the EUC transit across the basin.
The NECC is supplied primarily by northern hemisphere waters, also originating in the west from the Mindanao Eddy.

23. 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), :

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

25. 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?