Principal Investigator: Helge Drange, Nansen Environmental and
Remote Sensing Center (NERSC) and Dep. of Geophysics, Univ. of Bergen
Project description by: Helge Drange (NERSC), Joe LaCasce (met.no),
Arne Melsom (met.no), Ole Anders Nøst (Norwegian Polar Institute),
Tore Furevik (GfI-UiB), Peter M. Haugan (GfI-UiB),
To examine the forcing, structure and sensitivity of the Atlantic Meridional
Overturning Circulation in response to buoyancy forcing, internal mixing
and wind driving, in idealized and realistic basin configurations
The stability and variability properties of the Atlantic Meridional Overturning
Circulation (AMOC) have been the focus of a series of observational and
numerically based studies. There are several reasons for the general interest
in the AMOC: Time series of temperature proxies from polar ice caps and
marine sediments (Bond et al. 1993; Clark et al. 2002) indicate that the
North Atlantic-European region has experienced changes in the surface
temperature of 5-10 ?C on multi-annual to decadal time scales, the last
warming taking place under the transition from the last glacial maximum
to the Holocene. These transitions, known as Dansgaard-Oeschger (rapid
warming) and Heinrich (rapid cooling) events (Alley 1998), have been linked
to variations in the strength of the AMOC (Clark et al. 2002).
For the present day climate system, the Atlantic Ocean transports about
1.2 PW of heat poleward of 25 ºN, or 20-30% of the total heat flux
carried by the atmosphere-ocean system at this latitude (Hall and Bryden
1982). The northward heat transport in the Atlantic Ocean is therefore
one of the reasons for the anomalously mild climate over large parts of
the North Atlantic and Europe (e.g. Rahmstorf and Ganopolski 1999). Consequently,
changes in the AMOC have the potential to change the climate in this region,
and this is considered as one of the major challenges in climate change
(O'Neill and Oppenheimer 2002): How stable is the AMOC to human induced
greenhouse warming, and is a weakening or even a (near) collapse of the
AMOC likely to occur in the near future? Model results are by no means
conclusive regarding this question (Cubasch et al. 2001).
Direct observations of the strength and variability of the AMOC have not
been conducted, mainly due to the 3-D basin scale extent of the circulation.
In the simplest description of the system, warm and saline surface waters
of tropical-subtropical origin flow poleward. During the northward flow,
heat is lost to the atmosphere, leading to a gradual increase of the density.
At locations in the Greenland-Iceland-Norwegian (GIN) and Labrador Seas
region (Dickson and Brown 1994), the surface water may become sufficiently
dense during late winter, triggering convective mixing to intermediate
or abyssal depths. These dense and cold water masses, together with subsurface
water masses of polar origin flowing southward through the Fram Strait,
constitute the major source waters of the southward deep-flowing branch
of the AMOC.
There is observational-based evidence for a decreasing trend in the southward
flow of dense waters from the Nordic Seas (Hansen et al. 2001). A freshening
of the overflow waters is also observed (Dickson et al. 2002). Since these
water masses are part of the AMOC, this may indicate that the strength
or the structure of the circulation is changing. Furthermore, analyses
of observed hydrography from the Atlantic sub-tropical and sub-polar gyres
indicate that the AMOC-system exhibits strong variations on multi-annual
to decadal time scales (e.g. Curry and McCartney 2001). It is therefore
hard, if at all possible, to assess whether the observed changes of the
properties of the overflow waters reflect natural variability modes, or
are signals of human-induced changes in the climate system.
Using 3-dimensional atmosphere-ocean general circulation models (AOGCMs),
simulations of future climate scenarios exhibit a 30-50% reduction, or
a near stabilization of the AMOC at the end of the 21st century (Cubash
et al., 200). Furthermore, climate simulations going beyond 2100, usually
performed with models of intermediate complexity, show generally a reduced
or even a complete shutdown of the AMOC (Schmittner and Weaver 2001).
The obtained differences in the model-predicted evolution of the AMOC
may have a series of explanations. First, the fate of the AMOC may depend
on the maximum atmospheric CO2 concentration and on the rate of CO2 increase
(e.g. Stocker and Schmittner 1997; Clark et al. 2002) (the various emission
scenarios become more uncertain, and consequently diverge, with time).
Secondly, the simulated stability of the AMOC may depend on the actual
model system (GCM versus model of simplified complexity), the horizontal
and vertical resolution, the parameterisation of unresolved topographic
features and horizontal/vertical mixing processes, the realism of the
sea-ice component, the treatment of continental run-off, the ability for
the model to describe the nature of the major natural climate variability
modes, the use of heat and/or fresh water flux adjustments, the actual
spatial temperature and salinity distributions in key regions (especially
in the sinking regions of the North Atlantic Ocean), the basin scale vertical
density distribution, and so on.
It is clear from the complexity and the still incomplete theoretical understanding
of the system (e.g. Yang 1999; Jayne and Marotzke 2001; Johnson and Marshall
2002), that in-depth studies involving theoretical and observational analyses,
together with (idealised and realistic) model studies, are required to
significantly improve our understanding of the driving mechanisms and
stability properties of the AMOC.
Improved knowledge of the present-day system has the potential to reduce
the uncertainty in climate change predictions in general, and for the
Atlantic-European climate system in particular. In addition, improved
understanding of the major natural variability modes is a prerequisite
for performing - if possible - multi-annual to decadal predictions of
the Atlantic-European climate.
Work addressing the forcing, structure and stability of the modern AMOC
have not received too much attention in Norway up to now, and only a few
papers by Norwegian scientists exist on the topics (e.g. Mauritzen and
Häkkinen 1999, Bentsen et al. 2002, Zhou et al. 2002). We propose
to build such an expertise in NOClim Phase II by funding several national
research institutes, and by a two-way visiting plan. As the available
funding is limited, the work will be bridged with existing funding to
ensure that the activities have a sufficient focus to meet the stated
Methods and tools
To develop Norwegian expertise in the field of idealized numerical model
study of the AMOC, a suitable tool is required. The project also aims
for more realistic simulations of the AMOC, which at the same time calls
for a full primitive equation (PE) model. Given the demonstrated sensitivity
to lateral and vertical diffusion in previous studies, it is believed
that an isopycnic model (one with density as the vertical coordinate)
is a natural choice. Specifying diapycnal fluxes is straightforward in
an isopycnic model, thereby reducing spurious mixing which one usually
finds in standard level (or z-coordinate) models. Norwegian scientists
already have experience with the Miami Isopycnic Coordinate Ocean Model
MICIOM (e.g., Furevik et al. 2002, Bentsen et al. 2002, Zhou et al. 2002).
Therefore, MICOM - and the recent modification of MICOM named HYCOM -
will be used as the major modelling tools in Module A. In addition, and
for direct comparison with earlier studies, comparisons will be made with
a dynamically simpler planetary geostrophic (PG) model. For the idealized
model experiments, various features embedded into the HYCOM system will
be switched off. For instance, the first task will be to adapt the HYCOM
model to an idealised, hemispherical "wedge" geometry. In its
current release the model is configured for the North Atlantic, but this
version should be amenable to the idealized basin (E. Chassignet, pers.
comm. 2002). This is the same configuration for the PG model, which will
facilitate comparisons between the two.
The tasks outlined below represent a suite of experiments of increasing
complexity. They are intimately related, and the results of one will benefit
the others. As in many previous studies, the first tasks will focus on
the AMOC in a flat-bottom ocean. Obviously the ocean is not flat, but
in beginning this way, we will be able to confirm that our experiments
are in line with previous studies. Moreover, we believe there are issues
even in this simple environment which deserve more attention, specifically
the question of model resolution.
Thereafter, we alter the model to make it more realistic. The first major
step will be to add topography. There is substantial observational evidence
for ocean currents being steered by topography, most frequently from subsurface
float and surface drifter data because these indicate the actual motion
of fluid parcels. By examination of the influence by topography and f/H
(the ambient barotropic potential vorticity, or PV) on subsurface floats,
LaCasce (2000) found a strong topographic influence throughout the water
column in the North Atlantic. Similar indications of topographic control
have been given by Fratantoni (2001) from surface drifters in the North
Atlantic, and by Poulain et al. (1996) and Jakobsen et al. (2002) from
surface drifters in the Nordic Seas.
How can topography affect the AMOC? First, topography can alter the vertical
character of the circulation (e.g. Pedlosky, 1987). Second, topography
can alter the stability of baroclinic mean flows, which can in turn affect
variability (Winton, 1997). Third, topography can alter the path of currents
(as suggested by the Lagrangian observations) and thereby affect atmospheric
density fluxes. And fourth, sills and ridges could obviously block baroclinic
flows, in turn altering the location and perhaps magnitude of convection.
After examining the topographic role, we turn to the winds. In the initial
experiments, we will use either steady winds or no winds at all. This
is because we wish to examine in isolation the affects of oceanic instability,
without the added distraction of wind variations. In reality of course
the atmosphere is fully variable, and its changes will be directly reflected
in the ocean. So we next turn to variable forcing.
It is widely debated whether the ocean, atmosphere or both are responsible
for climatic variations. Of the two, the ocean has the much larger heat
capacity and longer adjustment times, so it was long thought that the
ocean forced the long-term changes. In fact, idealized ocean models driven
by stochastic (white noise) wind forcing exhibit decadal variability (e.g.
Frankignoul et al., 1997). On the other hand, it has also been shown that
a model atmosphere with fixed oceanic conditions can exhibit persistent
climatic states (James and James, 1989) and even features resembling observed
modes of variability, like the North Atlantic Oscillation (NAO) (Barnett,
1985). Nature is possibly somewhere in between. The atmosphere exhibits
a red spectrum (Deser and Blackmon, 1993) which is quite different from
that obtained with atmosphere-only models and also different from spectra
found in ocean-only models with stochastic forcing. The question of the
respective roles of atmosphere and ocean in climate change is still debated.
Atmospheric variations encompass both wind and buoyancy forcing. Changes
in buoyancy forcing would directly impact the AMOC by altering the convected
water and therefore the large-scale density gradients. But wind fluctuations
should also affect the AMOC, by altering the upper ocean gyres and also
possibly by changing the ocean vertical diffusivity (Munk and Wunsch,
1998). One could thus imagine idealized experiments in which one of these
forcings was varied and the other held fixed.
But such experiments are obviously artificial. Consider just one aspect,
the effect of mid-latitude sea surface temperature (SST) anomalies on
the atmosphere. While this is still much debated (Frankignoul, 1985; Rahmstorf
and Willebrand, 1995; Kushnir and Held, 1996; Elliott et al. 2001 and
references therein), its impact on a coupled system would be quite different
than that on an ocean-only system. Consider for instance: if a warm SST
anomaly causes low level winds to intensify, those stronger winds could
then alter the surface temperature. If the SST anomaly then intensified,
the winds would grow, etc. Such feedback (seen in the idealized model
of Goodman and Marshall, 1999) would produce unstable growth, perhaps
causing this circulation mode to dominate all others.
Hereafter, we present more detail with regard to the tasks outlined above.
We emphasize though that the goals are two-fold: to improve our own competence
in the area of idealized models of the AMOC, and to enhance the understanding
of fundamental dynamical elements related to it. Both should benefit the
state of climate modelling in Norway, and - during the project period
- put Norwegian scientists at the forefront of the topic.
Each task constitutes a self-contained entity aimed at assessing the importance
for the forcing, variability and stability of the AMOC. The results of
each task represent a significant contribution to the overall objective
of Module A, and are relevant for the overall aim of NOClim Phase II.
Each task will have a small set of milestones for targeting the progress
of work in time as detailed in Table 6 in the formal application form.
2.1 Task descriptions
Task A.1 Establish an idealized model to study the AMOC (months 1-12)
The initial work will be oriented toward reproducing prior results, albeit
with a model (HYCOM) not used before in this context. Comparisons will
be made with a dynamically simpler planetary geostrophic (PG) model (Samelson
and Vallis, 1997).
The first task will therefore be to adapt the HYCOM model to an idealised,
hemispherical "wedge" geometry. The same configuration will
be used for the PG model, facilitating comparisons. Companion experiments
will be performed with the HYCOM and PG models, with boundary conditions
consistent with previous studies. This will give us confidence that we
can reproduce the same circulations. In addition, no one has to our knowledge
performed a side-by-side comparison of these two models. Such a comparison
will be useful to quantify the shortcomings of the PG model, and also
to identify the parameter ranges where the two models agree. Knowing how
well the PG system mimics the full system will be useful for knowing when
to use the PG model in simplified climate experiments, as well as for
developing analytical models.
Milestone A.1.1: Idealized set-up of HYCOM (months 1-3); A.1.2: Perform
companion experiments (months 3-8); A.1.3: Analyse results (months 8-12);
A.1.4: Write technical report/article (months 8-12).
Task A.2 Quantify role of applied model resolution (months 12-18)
Here the question of model resolution will be addressed. Recently, Marotzke
(1997; with a PE model) and Samelson (1998; with a PG model) examined
the effect of localized vertical density mixing. Vertical diffusivity
plays an important (and possibly controlling) role in determining the
strength of the overturning. The observations however show unequivocally
that vertical mixing is highly inhomogeneous in the ocean (Munk and Wunsch,
1998 and refs. therein), and this spatial variation should impact overturning;
the results of Martozke and Samelson confirm this. Confining diffusion
to the lateral boundaries, for example, causes all the upwelling and downwelling
to occur there, yielding a boundary-intensified circulation quite different
from classical perceptions (Stommel and Arons, 1960).
The studies of Marotzke and Samelson were made with coarse resolution
models (grid spacing of several degrees of latitude and longitude). It
is of interest to assess the effect of higher resolution, with correspondingly
narrower boundary layers at the walls. In previous works on resolution
dependency (Bryan, 1991; Fanning and Weaver, 1998), the thermohaline circulation
becomes unstable at higher resolution (if the horizontal diffusivity is
simultaneously decreased). Instability means time variability, and so
is of interest for climatic variations.
Pertinent questions include: does the model become unstable? If it does,
how does this instability manifest itself (for instance through the instability
of a boundary current)? What is the role of eddies, if any, in heat transport?
And is the spectrum of the response smooth, or are there dominant frequencies?
Besides being of numerical interest, these higher resolution simulations
are relevant to the ocean, with its localized mixing and very large Reynolds
Relation to Overall aim: Tasks A.1 and A.2 will address the Overall aim
in Module A by establishing basic experience that is needed for understanding
the AMOC, and examining the role of phenomena on various spatial scales.
Milestone A.2.1: Perform model integrations with increased resolution
(months 12-14); A.2.2: Analysis of results (months 14-18); A.2.3: Write
report/article (months 14-18).
Task A.3 Examine role of topography (months 18-48)
To address this issue, we propose to extend the study in Tasks A.1 and
A.2 by adding topographic perturbations to the idealized geometry. A flat
bottom control run will be made, forced by a (steady) density flux and
an idealized wind stress; then we will methodically alter the topography.
Possible choices include a continental slope (e.g. Winton, 1997), to assess
model stability, and a subcontinent (e.g. Greenland), to see whether the
model develops multiple convection regions (like the Labrador and Nordic
Seas). We would also examine an idealized ridge (like the mid-Atlantic
ridge) and an idealized sill (like the Iceland-Scotland sill). And eventually
we would consider a topographic configuration like that in the North Atlantic
and Nordic seas.
In all cases, we would gauge alterations in the AMOC, in the current paths,
in convection and in stability. In addition, we will examine closely variation
in the bottom velocities (without which there is no topographic influence);
as noted, there remain questions about the abyssal flow even in flat bottom
simulations. For topographic steering to take place there must be geostrophic
velocities near the bottom. Over steep topography these velocities will
follow contours of f/H. Turning of the velocity with depth will only occur
if vertical velocities is induced in the water column, or if the density
is not materially conserved in the flow (see Stommel and Schott, 1977;
Schott and Stommel, 1978). The flow may turn unhindered across a level
of no motion (the turning is then 180 degress, see Hughes and Killworth,
1995), but this means that the flow is still following bathymetric contours.
Because the possibility of turning with depth is limited also the surface
flow is observed to follow topographic contours (Poulain et al. 1996;
LaCasce 2000; Fratantoni 2001; Jakobsen et al. 2002).
One question to be investigated is why the Gulf Stream system in the Atlantic
is topographically steered (That it is topographically steered is quite
obvious from the drifter analysis of Fratantoni, 2001). Among the processes
that may drive the near bottom flow in these regions, there are two possibilities
to be studied:
a) In a classical circulation picture the dense water that sinks in the
polar regions and spreads southwards along the bottom. Are the bottom
geostrophic velocities forced this way able to explaing the topographically
steered Gulf stream system? This will be investigated with the PG model
including a simplified topography. The aim will be to investigate if the
southward spreading dense water may cause the northward flowing warm water
to be topographically steered.
b) The results of Nøst and Isachsen (submitted to JMR 2002) suggests
that the potential vorticity input by winds in the Nordic Seas and Arctic
Ocean (on long time scales) is transmitted to the bottom forcing the near
bottom currents and causing strong topographic steering. Could this happen
in the Gulf stream system in the Atlantic Ocean? One possibility is that
mixing caused by baroclinic instabilities in the Gulf Stream system may
lead to fluxes of potential vorticity that connect the surface wind forcing
with the near bottom layers. The topography itself may also influence
the baroclinic instabilities (see Pedlosky 1962, "Baroclinic instability
in two layer systems") and the effect of this will also be studied.
This may cause bottom geostrophic velocities and topographic steering
of the whole water column. This problem will be addressed by studying
the effect of baroclinic instabilities over a sloping bottom using simplified
process models. This may be models we develop ourselves or a simplified
setup of an existing model. The results of these studies will be attempted
to include in the PG model used in 1a.
If there is topographic steering, a ridge at the sea floor may effectively
block transport across it. However, diapycnal mixing or possibly the beta
effect may lead to a turning of the direction of the current with depth
(see Stommel and Schott, 1977; Schott and Stommel, 1978; Hughes and Killworth,
1995). The processes leading to turning of the velocity vector with depth
should be studied in order to understand where the northward flowing warm
water near the surface is topographically steered, and where it may flow
across f/H contours. This will be done studying an idealized underwater
ridge, with either self developed process models or simplified setup of
an existing model. Answers to the following questions will be sought:
How will the beta effect and the mixing of water masses across the ridge
affect the turning angle of the current with depth? Can different mixing
parameterizations affect this turning angle? On what conditions will the
blocking of the currents be most/least efficient?
The AMOC is probably to a high degree controlled by topographic steering.
By understanding the processes forcing the near bottom currents and the
processes controlling the blocking of baroclinic currents we may be able
to understand and predict possible changes to the system caused by a changing
forcing fields. In the end of the project we should study possible changes
in the forcing fields that could increase or decrease the effect of topography.
The change in the forcing fields should be selected from the results of
the studies described above. These study will be done using a PG model.
The work described above is highly relevant for the work in A4 and A2,
as topography may be very important in localising mixing and diffusion
processes. Mixing and instabilities may be important for bottom velocities
and topographic steering as well as baroclinic currents above topography.
The work described may then increase our understanding on how the AMOC
depends on mixing processes.
Relation to Overall aim: Task A.3 will address the Overall aim in Module
A by shifting our focus from understanding the general dynamics of the
MOC to a setting that resembles the Atlantic Ocean. Milestone A.3.1 (Melsom
and LaCasce): Perform model integrations with topographic perturbations
(months 18-24); A.3.2 (Melsom and LaCasce): Analyse results (months 20-30);
A.3.3 (Melsom and LaCasce): Write report/article (months 24-30). A.3.4
(Nøst): Forcing of near bottom flow (months 12-18). A.3.5 (Nøst):
Blocking of baroclinic currents (months 12-30). A.3.6 (Nøst): Long
term changes in the AMOC (months 14-36).
Task A.4 Assess role of horizontal and vertical mixing parameterisations
(months 1-48, NERSC)
Several GCM simulations show that the natural variability modes of the
AMOC, and also the threshold between on and off modes of the AMOC, are
critically dependent on the actual strength and spatial distribution of
the vertical and horizontal mixing (Bryan 1987; Schmittner and Weaver
2000; Scott and Marotzke 2001). Consequently, Schmittner and Weaver (2000)
argue that it is extremely difficult to assign a probability of future
abrupt change in the AMOC, a question of potential key importance for
the North-Western Europe.
In this task, a theoretical approach will be taken to assess the role
of vertical and horizontal mixing as controlling agents for the AMOC.
Thermodynamic and advection-diffusion considerations of the ocean circulation
will be applied following the numerical experiments presented by e.g.
Scott and Marotzke (2001). Special emphasis will be paid to mixing at
thermocline and abyssal depths, and to boundary mixing versus planetary
geostrophic vorticity balance in the ocean interior. The theoretical work
will be accomplished by analyses of available hydrographic observations,
microstructure and tracer release measurements of mixing in the ocean
(e.g., Ledwell et al. 1993; Ledwell et al. 2000), and results from numerical
ocean models of various complexities and resolutions.
The work will be carried out in close collaboration with Jochem Marotzke
at the Southampton Oceanography Centre. Marotzke will visit Bergen for
2-3 weeks each year in the period 2003-2006. Likewise, Helge Drange (NERSC)
will visit Marotzke and possibly the University of Reading (Rowan Sutton
and co-workers) for 2-4 weeks each year.
Relation to Overall aim: Task A.4 addresses the Overall aim in Module
A by assessing the AMOC dependence and stability to vertical and horizontal
mixing. The obtained results will be of direct use to Task A.2 and A.3.
Milestone A.4.1: Review existing literature (months 1-6); A.4.2: Develop
theoretical framework for mixing at thermocline and abyssal depths (months
1-30); A.4.3: Develop theoretical framework for boundary mixing versus
boundary mixing versus planetary geostrophic vorticity balance (months24-42);
A.4.4: Writing of three papers (months 18, 30, 48).
3. Links to other modules of NOClim II
Module A will develop Norwegian expertise in the theoretical and numerical
understanding of the variability and stability of the AMOC. It is linked
to Modules B and C since both paleo and instrumental observations are
needed for the theoretical and modelling work, and since the expertise
will provide a dynamic understanding to the observations.
4. Links to other co-ordinated projects, free projects under KlimaProg,
and to the Polar Climate research initiative
The activity proposed in this description will be an integrated part in
a broader range of modelling work at the newly awarded Bjerknes Centre
of Excellence in Climate Research. Also, the activity proposed under RegClim
will be useful as the simulations there will be actively used and disseminated
in Task A.4. Likewise, the findings from Module A will be used to improve
the model and analyses tools in RegClim. NOClim Phase II and RegClim Phase
II are therefore complementary activities.
5. Other national and international collaboration
The participating groups in Bergen, Oslo and Tromsø have continuous
contact and collaboration with a series of international groups and projects.
Projects of particular relevance are the EU-funded PRISM, PREDICATE, NOCES
and TRACTOR projects. Particular collaboration exists with the Southampton
Oceanography Centre, the University of Reading, Meteo France, and the
University of Miami.
6. Scientific publications
We plan to prepare and submit at least 8 manuscripts to peer-reviewed
journals over the four-year period, distributed over Tasks A.2-A.4.
7. Expected results of interest for other research
To build top international expertise in the study of the stability and
variability of the AMOC. This expertise is a prerequisite for assessing
the climate development in our region, both on land and in the ocean.
Such an expertise is also needed to ensure balanced information to the
public, and to act as a scientifically based and credible information
source for politicians and the government administration.
8. Need for computer resources
The proposed activity is moderately demanding in terms of computer resources,
and it is assumed that the required resources will be made available through
the Norwegian High Performance Computing Consortium.
Alley, R. B. 1998. Paleoclimatology: Icing the North Atlantic. Nature,
Barnett, T.P. (1985): Variations in near-global sea level pressure. J.
Atmos. Sci., 42, 478-501.
Bentsen, M., H. Drange, T. Furevik and T. Zhou (2002), Variability of
the Atlantic thermohaline circulation in an isopycnic coordinate OGCM,
Clim. Dynam., submitted
Bond, G., W. Broecker, S. Johnsen, J. McManus, L. Labeyrie, J. Jouzel,
and M. G. Bonani, 1993: Correlations between climate records from North
Atlantic sediments and Greenland ice. Nature, 365, 143-147
Bryan, K. (1991): Poleward heat transport in the ocean: a review of a
hierarchy of models of increasing resolution. Tellus, 43, 104-115.
Cessi, P. (2000): Thermal feedback on wind-stress as a contributing cause
of climate variability. J. Climate, 13, 232-244.
Clark, P. U., N. G. Pisias, T. F. Stocker, and A. J. Weaver, 2002: The
role of the thermohaline circulation in abrupt climate change. Nature,
Cubasch, U. et al. (2001): Projections of future climate change. In J.
T. Houghton, Y. Ding, M. Nogua, D. Griggs, P. V. Linden, and K. Maskell,
Eds., Climate Change 2001: The Scientific Basis, pages 525-582. Cambridge
Curry, R. and M. S. McCartney (2001): Ocean gyre circulation changes associated
with the North Atlantic Oscillation. J. Phys. Oceanogr., 31, 3374-3400
Deser, C. and M.L. Blackmon (1993): Surface climate variations over the
North Atlantic Ocean during winter: 1900-1989. J. Climate, 6, 1743-1753.
Dickson, B., I. Yashayaev, J. Meincke, B. Turrell, S. Dye, and J. Holfort,
M. (2002): Rapid freshening of the deep North Atlantic Ocean over the
past four decades. Nature, 416, 832-837
Dickson, R. R. and J. Brown, 1994: The production of North Atlantic Deep
Water: Sources, rates, and pathways. J. Geophys. Res., 99, 12,319-12,341
Fanning, A.F. And A.J. Weaver (1998): Thermohaline variability: the effects
of horizontal resolution and diffusion. J. Climate, 11, 709-715.
Frankignoul, C. (1985): Sea surface temperature anomalies, planetary waves
and air-sea feedbacks in the middle latitude. Rev. of Geophys., 23, 357-390.
Frankignoul, C., P. Muller and E. Zorita (1997): A simple model of the
decadal response of the ocean to stochastic wind forcing. J. Phys. Oceanogr.,
Fratantoni, D. M. (2001): North Atlantic surface circulation during the
1990's observed with satellite-tracked drifters. J. Geophys. Res., 106
Furevik, T., M. Bentsen, H. Drange, I. K. T. Kindem, N. G. Kvamstø
and A. Sorteberg (2002): Description and validation of the Bergen Climate
Model: ARPEGE coupled with MICOM, Clim. Dyn., submitted
Goodman, J. And J. Marshall (1999): A model of decadal middle-latitude
atmosphere-ocean coupled modes. J. Climate, 12, 621-641.
Hall, M. M. and H. L. Bryden, 1982. Direct estimates and mechanisms of
ocean heat transport. Deep-Sea Res., 29, 339-359
Hansen, B., W. R. Turrell, and S. Østerhus (2001): Decreasing overflow
from the Nordic seas into the Atlantic Ocean through the Faroe Bank channel
since 1950. Nature, 411, 927-930
Jakobsen, P.K., M.H. Rikergard, D. Quadfasel and T. Schmidt (2002): The
near surface circulation in the northern North Atlantic as inferred from
Lagrangian drifters: variability from the mesoscale to interannual. J.
Geophys. Res. (submitted).
James, I.N. And P.M. James (1989): Ultra-low frequency variability in
a simple atmospheric model. Nature, 342, 53-55.
Jayne, S. and J. Marotzke (2001): The dynamics of ocean heat transport
variability. Rev. Geophys., 39, 385-411
Johnson, H. L. and D. P. Marshall (2002): A theory for the surface Atlantic
response to thermohaline veriability. J. Phys. Oceanogr., 32, 1121-1132
Kushnir, Y. And I.M. Held (1996): Equilibrium atmospheric response to
North Atlantic SST anomalies. J. Climate, 9, 1208-1220.
LaCasce, J.H. (2000): Floats and f/H. J. Mar. Res., 58, 61-95.
Ledwell, J. R., A. J. Watson and C. S. Law (1993): Evidence for slow mixing
across pycnocline from a open-ocean tracer release experiment. Nature.
Ledwell, J. R., E. T. Montgomery, K. L. Polzin, L. C. St. Laurent, R.
W. Schmitt and J. M. Toole (2000). Evidence for enhanced mixing over rough
topography in the abyssal ocean. Nature. 403. 179-182.
Marotzke, J. (1997): Boundary Mixing and the Dynamics of Three-Dimensional
Thermohaline Circulations. J. Phys. Oceanogr., 27, 1713-1728.
Mauritzen, C. and S. Häkkinen (1999): On the relationship between
dense water formation and the ``Meridional Overturning Cell'' in the North
Atlantic Ocean. Deep-Sea Res., 46, 877-894
Munk, W. and C. Wunsch (1998): Abyssal recipes II: Energetics of tidal
and wind mixing. Deep-Sea Res., 45, 1977-2010.
Munnich, M., M. Latif, S. Venze and E. Maier-Reimer (1998): Decadal oscillations
in a simple coupled model. J. Climate, 11, 3309-3319.
O'Neill, B. C. and M. Oppenheimer (2002): Dangerous climate impacts and
the Kyoto Protocol. Science, 296, 1971-1972
Pedlosky, J. (1987): Geophysical Fluid Dynamics, Springer-Verlag, 710
Poulain, P-M., A. Warn-Varnas and P.P. Niiler (1996): Near-surface circulation
of the Nordic seas as measured by Lagrangian drifters. J. Geophys. Res.,
Rahmstorf, S. and A. Ganopolski (1999): Long-term global warming scenarios
computed with an efficient coupled climate model. Climate Change, 43,
Rahmstorf, S. and J. Willebrand (1995): The Role of Temperature Feedback
in Stabilizing the Thermohaline Circulation. J. Phys. Oceanogr., 25, 787-805.
Samelson, R. M. (1998): Large-scale circulation with locally enhanced
vertical mixing. J. Phys. Oceanogr., 28, 712-726.
Samelson, R. M., and G. K. Vallis (1997): Large-scale circulation with
small diapycnal diffusion: the two-thermocline limit. J. Mar. Res., 55,
Schmittner, A. and A. J. Weaver (2001): Dependence of multiple climate
states on ocean mixong parameters. Geophys. Res. Lett., 28, 1027-1030
Scott, J. R. and J. Marotzke (2001): The location of diapycnal mixing
and the meridional overturning circulation. J. Phys. Oceanogr. submitted
Stommel, H. and A.B. Arons (1960): On the abyssal circulation of the world
ocean---I. Stationary planetary flow patterns on a sphere. Deep-sea Res.,
Sura, P., F. Lunkeit and K. Fraedrich (2000): Decadal variability in a
simplified wind-driven ocean model. J. Phys. Oceanogr., 30, 1917-1930.
Yang, J. (1999): A Linkage between Decadal Climate Variations in the Labrador
Sea and the Tropical Atlantic Ocean. Geophys. Res. Lett., 26, 1023-1026
Zhou, T., T. Furevik, H. Drange and M. Bentsen (2002): Interannual Scale
Adjustment of the North Atlantic Thermohaline Circulation to the Atmospheric
Forcing in a Global Air-Sea Coupled Model, J. Climate, submitted
Winton, M. (1997): The damping effect of bottom topography on internal
decadal-scale oscillations of the thermohaline circulation. J. Phys. Oceanogr.,
Appendix - Description of PhD project module A
The PhD project will be devoted to study the role of horizontal and vertical
mixing on the AMOC. The work will start by performing a literature review
on the problem, including the papers by Bryan (1987), Winton (1997), Fanning
and Weaver (1998), Schmittner and Weaver (2000), Jayne and Marotzke (2001),
Scott and Marotzke (2001), Johnson and Marshall (2002), Nilson and Walin
(2001), and Bentsen et al. (2003). This activity will be performed in
close collaboration with Jochem Marotzke at SOC, Southampton.
The second step will be to apply theoretical considerations to assess
the role of vertical and horizontal mixing as controlling agents for the
AMOC. Thermodynamic and advection-diffusion considerations of the ocean
circulation will be applied following the numerical experiments presented
by e.g. Scott and Marotzke (2001). Special emphasis will be paid to mixing
at the thermocline and at abyssal depths, and to boundary mixing versus
planetary geostrophic vorticity balance in the ocean interior. The theoretical
work will be accomplished by analyses of available hydrographic observations,
microstructure and tracer release measurements of mixing in the ocean
(e.g., Ledwell et al. 1993; Ledwell et al. 2000), and results from numerical
ocean models of various complexities, resolutions and forcings. The latter
analyses will be carried out in close collaboration with the BCM modelling
group at the Bjerknes Centre of Climate Research in Bergen (Furevik et
Tentative work plan:
Month 1-6: Review of existing literature
Month 7-24: Develop theories of the role horizontal and vertical mixing
have on the AMOC; writing of a peer-refereed paper.
Month 18-33: Analyse results from OGCMs of various complexities, resolutions
and forcings, and compare with the theoretical findings; writing of two
Month 34-36: Finalize PhD-work.