Week 12
11/10/09
Nate Gwyn
Hemispheric See-Saw and Thermalhaline Circulation
Background on Thermalhaline Circulation
Broecker, W. S., 2004. Future Global Scenarios. Science, v. 304, p. 388.
Rahmstorf. S., 2003, The Current Climate. Nature, 421. p. 699.
Rahmstorf. S., 2006. Thermohaline Ocean Circulation. In, Encyclopedia of Quaternary
Sciences, Edited by S. A. Elias. Elsevier, Amsterdam, 10 pp.
Rahmstorf. S., 2007. Ocean Circulation and Climate During the Past 120,000 Years. Nature, v.
419, p. 207-214.
Wunsch, C., 2002. What Is Thermohaline Circulation? Science, v. 298, p. 1179-1180.
New data papers -- relating NA THC to the Pacific and the Tropics
Change, P. et al. 2008. Oceanic Links Between Abrupt in the North Atlantic Ocean and the
African Monsoon. Nature Geoscience, v. 1, p. 445-448.
Herbert, T. D. et al., 2001. Collapse of the California Current During Glacial Maxima Linked to
Climate Change On Land. Science, v. 293. P. 71-76.
Partin et al., 2007, Millennial-Scale Trends in West Pacific Warm Pool Hydrology Since the Last
Glacial Maximum. Nature, v. 449. p. 452-456, 736.
The papers discussed this week can be broken into two main categories. We started off with several
works better describing the Thermohaline Circulation which we have been discussing the last several
weeks. They did a better job of breaking down the THC and how it is a transfer of energy rather than an
actual current. We also talked about the key features of the THC which included; deep water formation,
the spread of deep water, upwelling of deep waters, and the close of the cycle through the near surface
currents (Rahmstorf. S.,et.al. 2006). We also discussed the Coriolis Effect and how it effects the THC as
well as the Ekman Transport through the Ekman Spiral with its affect on upwelling. Temperature was
shown to have more of an effect on the THC rather than the salinity. We also revisited cycles looking at
Dansgaard-Oeshger cycles and Heinrich events as well as Bond Cycles and how they are related and
affect the three circulation models of the glacial Atlantic. We then started a new topic which focuses
more on the warmer tropical currents of the Pacific and Atlantic. We discussed how the global ocean
circulation was related to the North Atlantic Thermohaline Circulation and how causes in THC, affect
more local currents and ocean regions.
In Collapse of the California Current During Glacial Maxima Linked to Climate Change On Land by
Herbert, T. D. et al it discusses Pacific changes affected by the THC. The proxies used in this paper
include sea surface temperature, del O18, sediment cores, alkenones, and vein calcite. Alkenones are
highly resistant organic double bonded carbon compounds produced by phytoplankton that respond to
water temp and therefore give an estimate of past SST’s. Figure 2 shows the relationship between SST
(surface water) and benthic del O18 (deep water). They line up with each other much better in periods
between the ice volume maximums, but during these times of maximum ice the del O18 drop precedes
decline in SST’s. The paper also states that the del O18 can be used to get the precipitation or the
temperature however it does not state what they use it for. The vein calcite from Devils Hole NV is
actually data from another paper that they refer to since it is just inland from the southern Californian
coast which is where the sediment cores were taken and therefore reflects local climate. The times and
extent or the estimates of SST suggest that the Devils Hole calcite is more of a local rather than a global
past temperature and therefore probably doesn’t represent a good idea of when the glaciations occurred.
They also tried to relate the changes in temperature of the various parts of the California Current with
glacial interglacial transitions. They southern part of the current shows this better than the north until it
reaches the Baja of California where there are also effects of the Davidson Counter Current. Then they
also compared pollen, Sequoias and the estimated SST’s. These tended to correlate fairly well seeing as
the upwelling caused by the current produces fog, a necessity for the Sequoias and therefore the
corresponding pollen, so the Sequoias thrive in times of upwelling caused by the warmer temperatures.
All of the data supports the weakening of the California current during times of peak glaciations. Most of
the data gathered supports the Regional temperature theory instead of global change.
In Millennial-Scale Trends in West Pacific Warm Pool Hydrology Since the Last Glacial Maximum by
Partin et al Partin uses oxygen isotope records from stalagmites in Borneo to show changes in the western
Pacific hydrology over the past 27,000 years. These findings show that the Mid Pacific water cycle is
related to climate processes at both polar regions, and this can cause abrupt changes. Modifications of
ocean cycles such as the meridional overturning circulation MOC can affect the northern hemisphere and
thus change other areas like the Inter-Tropical Convergence Zone ITCZ. The ITCZ lies near the study
area (4o north) causing 5m of rain, and little seasonality throughout the year. Del O18 is inversely
proportional to precipitation so when it increases rainfall decreases and vice versa. Fig. 2 shows the del
O18 records for all three of the stalagmites, and all three show the same change in values through time at
around 16 k yrs ago which corresponds to the Younger Dryas. The period directly after this transition
shows decreasing del O18 values and therefore increased precipitation. They then compare the Borneo
stalagmite with other paleoclimate records. Dry conditions in the study area correlate with a southward
shift of the ITCZ which could be caused from weakening of the MOC from a colder North Atlantic. The
data from Borneo suggest that this shift may have occurred during H1. The records compared with the
Borneo Stalagmite all infer that the ITCZ slowly migrated southward over the course of the Holocene.
The study not only demonstrates that the Pacific is effected by polar processes, but also by radiative
forcing. The tropical Pacific may be able to drive the THC causing climate events by altering heat and
salt amounts gradually throughout the oceans. Overall the climate system of the equatorial Pacific is a
mechanism thought to underlie more large scale abrupt climate changes.
In Oceanic Links Between Abrupt in the North Atlantic Ocean and the African Monsoon by Chang, P.
et al it discusses changes in the African monsoon that are affected by the changes in the THC due to
changes in the North Atlantic. The monsoon in Africa is caused by the temperature contrast between the
cool ocean and the warm land. The monsoon is extremely important to the African continent because it is
the primary source of water throughout the year. They developed a module to show how decreasing the
THC which in turn decreases rainfall in Africa. They say that if the THC is weakened then the Brazil
current is reversed, which is unlikely unless the world spins the other way, granted it may change its
position. This is what they say leads to warmer Sea Surface Temperatures SST’s however the changing
of any number of currents could add to increased SST’s near the equator, if the warmer surface water
currents are disrupted and the warm water builds up. The warmer waters off the coast of Africa have little
contrast with the temp of the land and thus you don’t get the monsoonal climate and that is a problem for
Africa. Past evidence of this is seen during the Younger Dryas due to the widespread cooling of the N
Atlantic and thus the shutdown of the THC. In addition to the decreased rain from the disappearance of
the monsoonal climate, the ITCZ is shifted south as a result of Colder N Atlantic waters. There is
evidence that the Earth’s orbital precession cycle is a dominate factor in the African monsoon variation
however these data indicate that the cycles are decadal to centuries. We also talked about how the
disruption of the currents be they THC or wind driven would have an effect on the upwelling off the coast
and that there would be moisture in the area as a response to that as well. So basically there are two main
factors that relate to the monsoon cycles of the African continent which are the interaction of the THC
and the wind driven currents in the tropical Atlantic.
Overall all the papers this week were tied together by one common theme and that is Thermohaline
Circulation affects everything. The first papers went into more detail and kind of pulled all the papers
from the last couple of weeks together as well as the three new ones we read today. Another
commonality between the new papers was the ITCZ or the Inter-Tropical Convergence Zone and how it
can be affected by the THC and or the time of year. Again a lot of what we went over with the Coriolis
Effect and how it regularly affects ocean currents in the different hemispheres, also tied several of the
new papers together, since we were talking about the more equatorial tropical reigns as opposed to the N
Atlantic. Energy transport rather than an actual current was another topic that was of importance this
week since we were dealing with much warmer waters. Also several of the papers were brought together
by the discussion on upwelling and how it affects the near coast climate on a regional scale. So for the
most part, this week really helped bring a lot of what we have been talking about most of the semester
together.