2. Global Ocean Circulation (2)
Focus on conservation principles: Balancing heat, salt, etc. into and out of boxes
Wind-driven gyre-scale circulation of the surface ocean and upper thermocline
Global heat and freshwater water transport, conservation properties
Water mass properties, polar convection and mixing, tracers, global thermohaline circulation
© Donglai Gong

4. Why should the thermocline slope down toward the west
Why should the thermocline slope down toward the west? It does this in the subtropical gyres of all the oceans.
Westward propagation of Rossby waves

5. Sea surface topography variations
Phenomenon
Typical Surface Expression
Period of Variability
Comments
Western boundary currents (Gulf Stream, Kuroshio)
130 cm/100 km
Days to years
Variability in position, and 25% variability in transport
Large gyres
50 cm/ 3000 km
One to many years
25% variability expected
Eastern boundary currents
30 cm/100 km
100% variability expected, possible direction reversals
Mesoscale eddies
25 cm/100 km
100 days
100% variability
Rings
100 cm/100 km
Weeks to years
100% variability, growth and decay
Equatorial currents
30 cm/5000 km
Months to years
Tides
100cm/5000 km
Hours to years
Aliased to low frequency

6. Box A: Between the Trades and the westerlies the Ekman transport is converging
Box B is the same: the reversal in direction of the Ekman transport in the Southern Hemisphere means this is still convergence

7. + =

8. http://ferret. pmel. noaa. gov/nvods/UI

9. East-west slope of thermocline comes about because of balance between Ekman pumping and westward traveling Rossby waves.
With a map of north-south velocity to go with this temperature map, we can calculate total north-south heat transport.
Everywhere between the maximum of the westerlies, and the maximum of the Trades, there is convergence of Ekman transport

10. With a map of north-south velocity to go with this temperature map, we can calculate total north-south heat transport.
wind

11. zone of Ekman convergence
max. of westerlies
westerlies
easterlies
zone of Ekman convergence
max. easterlies
East-west slope of thermocline comes about because of balance between Ekman pumping and westward traveling Rossby waves.
With a map of north-south velocity to go with this temperature map, we can calculate total north-south heat transport.

13. Bowl of subtropical gyre shows westward and eastward flow into and out of the page forming the gyre circulation.

14.
Mean SST shows cooler regions in the east, why?
Warm Pool.
Example slices through World Ocean Atlas
Xy:
Surface O2 (follows cold water)
Salinity at 1000m (Med Sea outflow)
Yz: (Atlantic)
Salt
Silicate
Yz: (Pacific)
Salt – fresher in far north – tongue of subducting water

15. With a map of north-south velocity to go with this temperature map, we can calculate total north-south heat transport.
wind

16. Review of air-sea heat exchange processes

17. Positive implies ocean warming Warming everywhere – of course
Shortwave
Everywhere warming the ocean (obviously)
- Where is greatest shortwave, and why?

18. Net longwave is less in WPWP than in east yet water is warmer?
Positive implies ocean warming Negative implies ocean cooling
Net longwave
Values are negative => net longwave loses heat from the ocean
Sum of downward longwave emitted by clouds and water vapor
Why is net upward longwave less in WPWP than East Pacific?

19. Positive implies ocean cooling
Western boundary currents have greatest air-sea temperature difference
Sensible
Positive implies cooling – the atmosphere cools the ocean on average everywhere
Local hot spots in Kuroshio and Gulf Stream
have greatest air-sea temperature difference

20. Positive implies ocean cooling Distribution largely follows SST
Latent
Positive: ocean is cooling.
Distribution largely follows SST
Elevated values
in WBC
southeast Pacific (warm and relatively clear skies – presumably less water vapor)
COADS monthly climatology

21. Shaded is net cooling Shaded is net cooling White is net warming
Western boundary currents are all cooling
Equator is warming – especially in the east

22. Western boundary currents are all cooling
Shaded is net cooling
Western boundary currents are all cooling
Equator is warming – especially in the east
Shaded is net cooling
White is net warming
Western boundary currents are all cooling
Equator is warming – especially in the east

23. Shaded is net cooling Shaded is net cooling White is net warming
Western boundary currents are all cooling
Equator is warming – especially in the east

24. Shaded is net cooling Shaded is net cooling White is net warming
Western boundary currents are all cooling
Equator is warming – especially in the east

25. Oceanic heat transport.
Box balance of fluxes integrated across the entire basin from coast to coast.
These values agree reasonably with direct estimates from ocean observations.
Notice again that the Atlantic is different – the heat transport is northward at al latitudes

26. RT = Radiation total at the top of the atmosphere from Earth Radiation Budget Experiment OT = ocean transport from net air-sea heat flux AT = atmospheric transport (calculated from RT – OT)
Radiation Total (RT) at the top of the atmosphere from the Earth Radiation Budget Experiment (ERBE) satellites.
Ocean Transport from air-sea flux divergence. AT from the residual. Similar pattern with atmosphere doing 2 to 3 times as much transport of heat.

27. East-west slope of thermocline comes about because of balance between Ekman pumping and westward traveling Rossby waves.
With a map of north-south velocity to go with this temperature map, we can calculate total north-south heat transport.

31. Bars show direct estimates from oceanographic cross-sections
Oceanic heat transport.
Box balance of fluxes integrated across the entire basin from coast to coast.
These values agree reasonably with direct estimates from ocean observations.
Notice again that the Atlantic is different – the heat transport is northward at al latitudes

32. Atlantic has northward heat transport everywhere
Heat transport from ocean observations constrained by conservation principles
Atlantic has northward heat transport everywhere

33. Ocean heat transport implied from the divergence of the air-sea heat flux.
Notice again that the Atlantic is different.
These calculations are made assuming conservation of salt.

34. Atlantic heat transport is so different because there is a vertical over-turning circulation that shows up clearly in patterns of salinity
Why the Atlantic is so different
Overturning circulation
why doesn’t it overturn at the equator?

35. Where evaporation is strong we tend to have high salinity
Low salinity in north but no large negative E-P. What other freshwater sources are there?
South
North
Now consider freshwater balances.
Global zonal average – no surprises here.
Very low salinity in the northern more latitudes
but not especially low E-P
what freshwater sources are not included in E-P?
- river runoff
- floating pack ice being exported from the Arctic

36. Precipitation minus evaporation (m year-1)
Shaded implies net precipitation
Earth’s water budget is dominated by P-E over the ocean (80% occurs there)
Precipitation minus evaporation (m year-1)
Max P-E in East Pacific is not exactly over the equator – why?
Earth’s water budget is dominated by E-P over the ocean, about 80% occurs there.
White = net evaporation
Shaded = net precipitation
Max in east pacific is not exactly over the equator, why?

37. Surface Atlantic is saltier than the Pacific
Many large rivers drain into the Atlantic and the Arctic Sea, so why is the Atlantic saltier than the Pacific?
Low salinity follows the regions of greatest negative E-P.
The mass of salt in the ocean stays constant, with inputs from erosion via rivers balancing deposition to sediments.
The atmosphere and ocean can exchange freshwater, but not salt.
So conservation of salt principles are coupled to freshwater transport.
Atlantic is saltier than the Pacific
Surface Atlantic is saltier than the Pacific

38. Broecker (1997) showed that 0
Broecker (1997) showed that 0.32 Sv of the water evaporated from the Atlantic does not fall as rain on land. It is carried by winds into the Pacific.
The quantity is small, equivalent to little more than the flow in the Amazon River.
But were this flux not compensated by an exchange of more salty Atlantic waters for less salty Pacific waters, the salinity of the Atlantic would rise about 1 gram per liter per millennium.
Many large rivers drain into the Atlantic and the Arctic Sea, so why is the Atlantic saltier than the Pacific?
Broecker (1997) showed that 0.32 Sv of the water evaporated from the Atlantic does not fall as rain on land.
It is carried by winds into the Pacific.
The quantity is small, equivalent to a little more than the flow in the Amazon River.
Were this flux not compensated by an exchange of more salty Atlantic waters for less salty Pacific waters, the salinity of the entire Atlantic would rise about 1 gram per liter per millennium.”

39. © Donglai Gong

40. (a) (b)
Make the plot of the 26N section more obvious with the boundary current
Extedn the n-s secion in the quiz document
(c) (d)