1. Deep Ocean Circulation
Present, Past, and Future
Andreas Schmittner
College of Earth, Ocean, and Atmospheric Sciences
Oregon State University
Portland State University Nov. 6, 2017
2. The deep ocean is dark because light is absorbed by water near the surface. This is why all photosynthetic organisms have to live there, in what we call the euphoric
zone (~100 m). Below that is the twighlight zone and below about 500 m there is almost no light.
3. World Ocean Atlas 2013
Heating the surface from above leads to a stable layering in most of the ocean because warmer water is lighter and more buoyant than cold water. This is in contrast to
the atmosphere, where light penetrates through the atmosphere and heats it from the bottom (of the atmosphere, which is the surface of the ocean/land), which leads to
convection (rising air) almost everywhere. The stable stratification of the ocean is illustrated here showing a section of observed temperature across the Atlantic ocean
from the Antarctic (left) to the Arctic (right). The vertical axis represents depth and the white shading is the topography on the seafloor (bathymetry). It is only at high
latitudes where the surface ocean water gets cold and dense enough to sink to great depths.
4. dense
cold & salty
light
warm & fresh
The density of seawater depends not only on temperature (vertical axis) but also on salinity (horizontal axis). Oceanographers like to use potential density, which is the
density a water parcel had if it was brought to the surface without changing its energy content. For convenience we subtract 1000 from the absolute numbers. Warm and
fresh water is light, whereas cold and salty water is heavy.
5. World Ocean Atlas 2013
Salinity observations show a distinct layering in the Atlantic. A tongue of relatively fresh water apparently spreads north from the Southern Ocean at about 1 km depth.
Below that is a layer of relatively salty water with a northern origin between about 1.5 and 4 km depth. At the bottom a relatively fresh layer emanating from the Antarctic
spreads northward.
6. World Ocean Atlas 2013
Antarctic
Bottom Water
Antarctic
Intermediate Water
North Atlantic
Deep Water
Meridional Overturning Circulation (aka thermohaline circ. / great conveyor belt)
This is the same as the figure before but now the flow direction and water mass names are indicated. In the Atlantic there are two overturning cells. The upper cell
consists of sinking in the north and southward flow at depth. Some North Atlantic Deep Water upwells in the Southern Ocean but some flows into the Indian and Pacific
oceans (see Pacific figure below). The lower cell consists of sinking near Antarctica and northward flow along the bottom, upwelling and return flow at slightly shallower
depths.
7. World Ocean Atlas 2013
Northward flow of warm, near surface water and southward flow of cold, deep water imply a net northward heat transport from the southern to the northern hemisphere
by the upper overturning cell.
8. World Ocean Atlas 2013
Flip back and forth between this and the next figure to illustrate the differences in salinity between the Atlantic and the Pacific Ocean.
9. Pacific (150°W)
The Pacific is much fresher than the Atlantic, particularly in the north and near the surface.
10. Pacific
Deep Water
Circumpolar
Deep Water
North Pacific
Intermediate Water
Pacific (150°W)
Because of this layer of fresh and buoyant water sinking and vertical in the North Pacific is very shallow as indicated by the North Pacific Intermediate Water arrow. In
contrast to the Atlantic no large and vigorous upper overturning cell exists. The deep flow is rather dominated by inflow from the Southern Ocean near the bottom,
upwelling, and return flow at depths.
11. Global Deep Water Circulation
a.k.a.Thermohaline Circulation: driven by temperature and salinity (density) differences
or Meridional (North-South) Overturning Circulation (MOC)
This is another schematic of the global deep overturning circulation. Due to the ribbon-like appearance it has been called the great conveyor belt, but the
real circulation is much more complicated that that and includes lots of eddies and other turbulent features. Deep Water Formation refers to the sinking of
surface waters to depths. Comparing the Atlantic and Pacific prompts the following question.
12. Why is there no deep
water formation in North
Pacific?
We already know part of the answer: because the Pacific is fresher than the Atlantic. But why is the Pacific fresher and the Atlantic saltier? Somebody suggested rivers.
What are some of the biggest rivers? The Amazon. Where does it flow to? Into the Atlantic. The Mississippi, Kongo, and Rhine rivers also flow into the Atlantic. The three
large Siberian rivers Ob, Lena and Yenisei flow into the Arctic, which is more connected to the Atlantic than to the Pacific. There are also large rivers flowing into the
Pacific such as the Yellow river and the Columbia. But overall more rivers flow into the Atlantic than into the Pacific. Thus the differences in river runoff should make the
Atlantic fresher than the Pacific.
13. Sea Surface Salinity Observations
Rockies
Andes
Water vapor
transport from
Pacific to
Atlantic
blocked by
mountains
Water vapor
transport from
Atlantic
to Pacific
across Central
America
Atlantic = salty
Pacific = fresh (g salt per kg water)
E.Africa
Previous scientists have suggested that the differences in evaporation minus precipitation (E-P) and effects of topography on atmospheric water vapor
transport play a role. Water vapor is transported mainly by near surface winds because temperature and water vapor quickly decrease with height. Moisture
transport by westerly winds at mid latitudes from the Pacific into the Atlantic is blocked by the Rocky mountains and the Andes, whereas gaps in the
mountains over Central America allow trade winds to transport water vapor from the Atlantic to the Pacific. We have explored this issue quantitatively by
using a climate model.
14. Climate Models
Equations based on
conservation of
• mass
• momentum
(Navier-Stokes)
• energy
• water, salt
• carbon
• ...
Interactive Components:
• Atmosphere
• Ocean
• Sea Ice
• Land Surface (vegetation,
snow, soil moisture, runoff)
Prescribed (fixed):
• Land Ice Sheets
The ocean and atmosphere is divided into boxes. In each box conservation equations are solved and variables such as velocities, temperature, humidity, precipitation,
and salinity are calculated. Properties and mass is exchanged between boxes through advective (transport by the fluid) and diffusive (generally representing unresolved,
small scale processes) fluxes through the boundaries.
15. Flat World
Real World
Schmittner et al. (2011) J. Climate
Model streamfunction
2 Sv isolines
(1 Sv = 106 m3 s-1)
(Amazon = 0.2 Sv)
Climate Model Simulations
In order to test the idea that topography causes the differences between the Pacific and Atlantic we have created a model without topography (Flat World) and compare
that with the model that has realistic topography (Real World). In the Flat World model the circulation pattern is switched such that deep water now forms in the Pacific
and no longer in the Atlantic. This confirms the hypothesis.
16. Conclusions Present
• Deep ocean circulation transports heat from southern to
northern hemisphere
• North Atlantic is saltier than North Pacific because of
topographic effects of mountains on atmospheric water
vapor transport
• This causes surface waters to sink in the North Atlantic,
setting up the global deep water circulation
18. Method:
Produce different AMOC model states
Compare with sediment reconstructions
Reduced southward moisture flux in SH
causes saltier and stronger AABW and
weaker NADW and AMOC
We don’t claim that this is the correct
mechanism for changing the circulation.
Test method first with modern data.
Muglia et al. (in review) Sci. Adv.Funded by NSF
19. Can we reconstruct the modern Atlantic Meridional Overturning Circulation (AMOC)
using carbon isotopes (14C and 13C) only at sparse locations of LGM sediment cores?
Muglia et al. (in review) Sci. Adv.
Yes, we can!
25. Conclusions Past
• During the Last Glacial Maximum the Atlantic overturning
circulation was much weaker (by about 50%) and
shallower than today
• Increased iron fertilization from dust enhanced
phytoplankton productivity and deep ocean carbon
storage
• Together, these two effects explain much of the increased
carbon storage in the ice age oceanduring the Last
Glacial Maximum
27. Bakker et al. (2016) Geophys. Res. Let.Funded by NOAA
Greenland
Ice Sheet
Melting
only
Probabilistic
Projections with
Simple Box Model
Comprehensive Models
28.
29. Conclusions Future
• Climate models project a decrease of the Atlantic
overturning circulation due to warming, intensification of
the atmospheric hydrological cycle, and (to a lesser
degree) melting of the Greenland ice sheet.
• Higher carbon emissions will lead to a stronger decline.
• A collapse of the circulation can be avoided by reducing
carbon emissions.
31. Frajka-Williams (2015) Geophys. Res. Let.
In Situ
Observations
Linear Trend
-0.2 Sv/yr
Reconstruction
Based on Satellite Data
-0.13 Sv/yr
1 Sv = 106 m3 s-1
For Comparison:
Model Trend post 2006
-0.04 Sv/yr
Model Natural Decadal
Variability
+-0.08 Sv/yr
Observations
32. Final Thought
• Currently observed decrease is likely mostly due to
natural variability but anthropogenic could also already
play a role
