Climate and Global Dynamics Division
Oceanography Section
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The mission of the Oceanography
Section (OS) is understanding the large-scale ocean circulation and the
dynamics of climate through studies of the important processes in the ocean
and sea-ice, in air-sea-ice interactions, and in coupled systems.
Global Ocean Modeling
The OS continues to maintain and upgrade the ocean component of the Community
Climate System Model (CCSM).
OS members Julianna Chow, Gokhan Danabasoglu, Matthew Hecht, Brian Kauffman,
and Nancy Norton spend most of their time upgrading and running the ocean and
coupler components. This work is done in close cooperation with the ocean modeling
group at the Los Alamos National Laboratory (LANL), which includes Richard Smith,
John Dukowicz, Philip Jones, Matt Maltrud, and Wooyoung Choi. The transition
to using the Parallel Ocean Program (POP) code is nearly completed, but the
new model needs to be fully tested and the final parameter values determined
by long integrations of the model. In addition, Kauffman is working closely
with Tom Bettge (Climate Change Research (CCR)
Section), Tony Craig (CCR), and Mariana Vertenstein (Climate Modeling Section,
CMS) on an updated version of the
CCSM coupler that will execute more efficiently on IBM and SGI computers. This
will be the coupler component for the new version of the CCSM-2.
A major element of the potential impact of global warming on society is coastal
inundation due to sea level rise. While tide gauge records provide one of the
longest duration measures of the ocean climate, their distribution over the
globe is extremely sparse and non-uniform. In consequence, there are large uncertainties
in our current estimates of sea level change over the last century. Frank Bryan
(OS), along with Craig Tierney and John Wahr (both of University of Colorado,
Boulder, CU) uses results from the Climate System Model (CSM) Climate of the
20th and 21st Century simulations to explore the issue of the adequacy of the
global tide gauge network to monitor global mean sea level rise. In particular,
they addressed the question of aliasing of non-steric regional changes in sea
level (e.g., due to changes in wind stress forcing of the gyre circulations)
into estimates of the global mean rate of sea level rise. They showed that such
changes could exceed the uncertainty level of published sea level rise estimates,
but strong deep ocean drift in the model simulations made the results equivocal.
Peter Gent (OS) has studied the North Atlantic Ocean thermohaline circulation
in three experiments using the CSM-1.3. The experiments are a control integration
for 1870 conditions and particular emission scenarios for the 20th and 21st
centuries. He found that the strength of the thermohaline circulation does not
change significantly over the 21st century. This also occurs in all the 21st
century scenario runs using the CSM. This result contrasts with other recent
studies done at other climate centers that have projected a significant reduction
over the 21st century. The reason for the difference is that the Northwest Atlantic
becomes warmer and more saline in the CSM. These changes combine to make little
change to the surface ocean density in this region, and hence to the rate of
deep water formation. A paper, in press at Geophysical Research Letters
discusses caveats about the CSM and other climate models.
Scott Doney, William Large, James McWilliams, Danabasoglu, and Stephen Yeager
(all of OS) have completed a series of simulations of the global NCAR CSM Ocean
Model (NCOM) using realistic atmospheric forcing from 1958-1997. Initial analysis
indicates that a substantial fraction of the observed interannual-to-decadal
oceanic variability can be reconstructed from the model solutions in this fashion.
Work in progress includes: (a) evaluating the magnitude and phasing of the simulated
ocean variability and (b) exploring the dynamical processes that govern the
oceanic response to anomalous wind and buoyancy forcing. In particular, the
formation of surface water mass anomalies and their propagation into the ocean
interior is being examined.
In the OS ocean models, the key thermohaline process of deep oceanic convection
is subgridscale and accomplished through the K-Profile Parameterization (KPP)
of vertical mixing. Is the standard scheme adequate, or should there be specific
modifications for deep convection, or is a fundamentally different parameterization
called for? These questions were addressed by comparing a one-dimensional (1-D)
KPP model to the results of a three-dimensional, non-hydrostatic, large eddy
simulation (LES). Parameters were chosen to be representative of the Labrador
Sea, one of the few known sites of oceanic deep convection. The key result is
that the 1-D KPP does reproduce faithfully the resolved LES vertical buoyancy
flux from the surface to below 1000 m. The important technical requirements
are that the forcing (a large cooling over a disk in the LES) be compatible
and that the 1-D model take full account of the large-scale flow that develops
within the LES domain. The most crucial physics is non-local buoyancy flux by
which an upward flux of buoyancy is maintained counter to the stable local buoyancy
gradient throughout most of the mixing layer. The gradient is unstable only
very near the surface, and local downgradient diffusion dominates. This physics
was found to be well parameterized by the non-local buoyancy flux term in the
KPP model. A turbulent kinetic energy mixing scheme without such a term was
found to produce a very different buoyancy profile. To supply the upward surface
buoyancy flux of the forcing, a very strong and much too deep unstable layer
was formed. The LES results also provided evidence for a non-local vertical
momentum transfer and would be valuable "truth" for the development
of a parameterization of this physics within KPP. However, such a development
is not a very high priority, because the momentum driven turbulence in deep
ocean convection is secondary to the buoyancy.
McWilliams (OS) and Danabasoglu (OS) have analyzed a global ocean general circulation
model for the tropical meridional overturning circulation due to mesoscale eddy,
Lagrangian mass transport. The analysis was inspired by recent measurements
in the tropical North Pacific Ocean by Roemmich and Gilson (both of Scripps
Institution of Oceanography). Good qualitative agreement is found between the
measurements and the model in a way that allows an inference of the best-fit
value of the effective eddy transport coefficient for the region. The model
representation for the eddy-induced circulation is the Gent and McWilliams parameterization.
From the model, we conclude that the eddy-induced circulation is similar in
all tropical basins. It has a strength of about 10% of the Eulerian (Ekman)
circulation, and its contribution to the meridional heat flux is a similar fraction.
Its pattern is one of double cells in the vertical and antisymmetry of the meridional
overturning streamfunction about the equator. Near the equator, there is downwelling
above the undercurrent and upwelling below, with the return circulations mostly
closed within the upper 250 m and 5 degrees north and south. Off the equator
there are overturning cells in the opposite sense in each basin that reach deeper
into the main pycnocline. As with the Eulerian meridional overturning circulation,
the seasonal cycle in the eddy-induced circulation has a magnitude comparable
to the time-mean circulation, although for an entirely different dynamical reason
associated with seasonal changes in the buoyancy field that are largely diabatic.
There is also a substantial El Niño-Southern Oscillation (ENSO) anomaly in this
circulation, centered on the undercurrent. The rather good agreement between
the measurements and the model solution gives support to the theory underlying
the parameterization of eddy-induced circulation.
Ilana Wainer (visitor, University of Sao Paulo, Brazil), Gent (OS), and Gustavo
Goni (Atlantic Oceanographic and Meteorology Laboratory, Miami) have compared
the mean and seasonal variability of the circulation in the Southwest Atlantic
from the 300-year control integration of the CSM-1.0 with observations. The
CSM transport values in the region of 38S are consistent with hydrographically
derived values. The transport of the CSM Brazil Current is higher during austral
summer and smaller during austral winter. Conversely, the Malvinas Current transport
is weaker during austral summer and stronger during austral winter. This is
also consistent with observations. The CSM seasonal cycle in transports of the
Brazil and Malvinas Currents and their meridional displacement is closely linked
to seasonal variations in the local wind stress curl.
However, the displacement is much smaller in the model than in observations.
The CSM results show that the latitudinal displacement of the 24C and 17C isotherms
at the South American coast between austral summer and winter is 20 degrees
and 12 degrees, respectively. This is very similar to the displacement seen
in observations. This study shows that a coarse resolution climate model, such
as the CSM, can successfully reproduce major characteristics of the Brazil-Malvinas
confluence seasonality, although the mesoscale features involving recirculation
and meander dynamics are not resolved. A manuscript on this work is in press
at the Journal of Geophysical Research.
North Atlantic Basin Studies
Bryan (OS) and Hecht (OS) continue the analysis of a series of integrations
at 0.4, 0.2, and 0.1 degrees horizontal resolution that were carried out in
collaboration with Richard Smith (visitor, LANL) and Matthew Maltrud (LANL).
Additional experiments at all three resolutions were carried out, exploring
issues of sensitivity of the solutions to dissipation and mixed-layer parameterizations.
An analysis of the dynamics of eddy-mean-flow interaction in the simulated Gulf
Stream was initiated, including a direct comparison against corresponding analyses
using dense observations obtained during the Synoptic Ocean Prediction (SYNOP)
program. The geographical distributions and magnitudes of mean flow and eddy
energy appear to be realistic. However, discrepancies are apparent in the spatial
distribution of eddy-mean flow energy conversions. In particular, the relationships
of eddy generation with respect to a time mean trough axis near 68 degrees west
longitude does not agree well with observations. This may be indicative of remaining
problems in simulating instability processes in the Gulf Stream, even at horizontal
resolutions below 10 km.
Bryan (OS) and Doney (OS) continue a collaborative project with Dennis McGillicuddy
(Woods Hole Oceanographic Institution, WHOI), Smith (LANL), and Maltrud (LANL)
to study mesoscale biological-physical interactions in the framework of eddy-resolving
POP North Atlantic Basin simulations. The scientific rationale for this work
springs from a number of recent observational and modeling studies showing eddy
induced enhancement of biological productivity in nutrient-poor regions of the
subtropical gyre. By displacing isopycnal surfaces upward into the euphotic
(well-lit) zone where phytoplankton can take up the nutrients, mesoscale eddies
can generate an elevated, effective vertical nutrient flux. We are pursuing
two experimental tracks: (a) simplified nutrient uptake simulation at the highest
available spatial resolution, and (b) full ecosystem simulations at both eddy-permitting
and non-eddy-resolving resolutions. A preliminary high-resolution (1/10 degree)
nutrient uptake simulation was designed and completed in FY99 using computational
resources at LANL. Analysis in FY00 showed several problems with this simulation
and a revised simulation was begun towards the end of FY00. An NCAR postdoctoral
fellow, Ivan Lima (OS), has implemented a more complete nitrogen-based ecosystem
model into a North Atlantic version of POP, and the model-data evaluation is
underway.
Ocean Biogeochemical Modeling
Marine biogeochemical processes as they relate to the global carbon cycle and
climate system are under study by Doney (OS). Three paths are currently being
pursued: (a) ecosystem modeling and remote sensing, (b) global ocean tracer
and biogeochemical modeling, and (c) observational data analysis. A global,
mixed-layer marine ecosystem model has been developed, based on extensions of
the simple, nitrogen-based ecosystem model for the Sargasso Sea. The mixed-layer
model is being used by Doney (OS) and Keith Moore, a joint Advanced Studies
Program (ASP)/CGD postdoctoral fellow, as a test bed for evaluating and improving
biological parameterizations for such things as iron limitation, zooplankton
grazing, nitrogen fixation, and calcification. Satellite ocean color images,
a proxy for surface phytoplankton distributions, play an important role in evaluating
model ecosystem solutions. In collaboration with David Glover (WHOI) and Montserrat
Fuentes (North Carolina State University), a statistical data analysis is underway
of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite ocean color
images to determine the magnitude and spatial scales of mesoscale variability
in the biology.
On a second front, a series of ocean tracer and carbon cycle calculations have
been completed in the global CSM ocean model. The tracer work provides valuable
information on the ventilation rates of the ocean model on timescales from several
years out to centuries. In work with Hecht (OS), simulations of deep water chlorofluorocarbons
demonstrated that the formation of Antarctic Bottom Water in the base NCAR ocean
climate model was too weak. Significant improvements were found by modifying
the surface salinity/freshwater forcing in the Ross and Weddell Seas. The carbon
simulations are used to study the physical and biological factors governing
the ocean inorganic carbon system, as well as the links with the atmospheric
and terrestrial biosphere. Doney (OS) and Keith Lindsay (OS) are participating
in the international Ocean Carbon Model Intercomparison Project (OCMIP) in collaboration
with Ray Najjar (Pennsylvania State University) and James Orr (Commissariat
à l'Energie Atomique, France). OCMIP has a standard set of tracer simulations,
and a number of new tracer simulations have been generated in the CSM ocean
component over the last two years: natural equilibrium radiocarbon and abiotic
carbon; equilibrium biotic carbon; anthropogenic perturbation radiocarbon and
anthropogenic carbon; and chlorofluorocarbons. Analysis of the solutions, in
the context of OCMIP, is currently being carried out. We are also porting the
biogeochemical model codes into the POP framework.
Observational and data analysis studies make up the third activity in marine
biogeochemistry and are part of long-term collaborations with scientists at
other institutions. In collaboration with Inez Fung (University of California,
Berkeley), a major project was completed in FY99 on the supply and phytoplankton
demand for the limiting element iron to the ocean surface euphotic zone. The
atmospheric deposition rates of dust are now being used to drive the marine
ecosystem model. Work was initiated with Natalie Mahowald (visitor, University
of California, Santa Barbara, UCSB) to explore the changes in dust deposition
over paleo and historical time periods. Joan Kleypas (OS) collaborates extensively
with Bob Buddemeier (University of Kansas), Jean-Pierre Gattuso (Observatoire
Oceanologique, France), and Bradly Opdyke (Australian National University) on
the role of coral reefs and other shallow carbonate systems in the global carbon
cycle.
Sea-Ice Modeling
The OS also continues to maintain and upgrade the sea-ice component of the CCSM.
This work is now a collaborative effort between NCAR [Bruce Briegleb (OS), Tony
Craig (CCR), Marika Holland (OS), and Julie Schramm (OS)], the University of
Washington (Cecilia Bitz and Dick Moritz), and LANL (Elizabeth Hunke and Bill
Lipscomb). One particular improvement is that a subgridscale ice thickness distribution
developed by Bitz (visitor, University of Washington, UW), in collaboration
with Holland (OS), Andrew Weaver, and Michael Eby (both of University of Victoria,
Canada), is now included in the CCSM sea-ice component. This allows for
simulation of the high spatial variability that is present in observations of
sea-ice cover. A paper documenting this parameterization and its influence on
the Arctic climate system is in press at the Journal of Geophysical Research.
Briegleb (OS) has developed an active-ice-only system for testing the sea-ice
component. The system includes the sea-ice model coupled to a slab ocean model
driven by atmospheric forcing. It is run through the CCSM coupled system. Numerous
tests, including the sensitivity to various model parameterizations and forcing,
have been made by groups at NCAR (Briegleb, Holland, and Schramm), the University
of Washington (Bitz), and Los Alamos National Laboratory (Hunke and Lipscomb).
Variability simulations from 1958-1998 have been performed with this model system
by Holland (OS). The strength of feedback mechanisms and the influence of various
sea-ice model parameterizations on these feedbacks is being evaluated in this
context. It is found that ocean mixed-layer feedbacks, particularly those associated
with the albedo feedback mechanism, have a strong influence on the sea-ice variability,
accounting for up to 60% of the summertime sea-ice thickness and concentration
variance in the central Arctic. Additionally, resolving the ice thickness distribution
modifies the impact of the feedback mechanism due to its influence on the sea-ice
strength and open water formation. A manuscript describing this study is currently
in preparation for submission to the Journal of Climate.
Air-Sea-Ice Interactions
Holland (OS), in collaboration with Bitz (UW) and Weaver (University of Victoria),
has examined the influence of sea-ice model parameterizations, particularly
ice dynamics and the ice thickness distribution, on climate change in an intermediate
complexity climate model. The results of this study are discussed in a manuscript
submitted to the Journal of Geophysical Research. The study found that
the mean ice state, which is strongly affected by the sea-ice parameterizations,
influences both the atmosphere and ocean response to climate change. The ice
parameterizations also affect feedbacks in the system that modify the climate
response to increasing atmospheric CO2. The ocean thermohaline
circulation is strongly influenced by the ice model parameterizations and has
a smaller initial response to global warming when ice dynamics and an ice thickness
distribution are included in the model.
The influence of simulated Arctic sea-ice variability on ice-ocean interactions
and the ocean thermohaline circulation has been examined by Holland (OS) in
collaboration with Bitz (UW), Weaver, and Eby (both of University of Victoria).
A manuscript describing this study is currently in press at the Journal
of Climate. Under stochastic wind forcing of the ice cover, the thermohaline
circulation responds with variability that is approximately 10% of the mean.
This variability occurs predominantly on interdecadal timescales that are concentrated
at approximately 20 years. It is forced by fluctuations in the export of ice
from the Arctic into the northern North Atlantic and the subsequent variations
in sea-ice melting that occur in this region. The ice melt acts as a stochastic
forcing of the surface ocean and appears to excite a damped ocean-only mode
of variability. The ice-ocean thermal coupling damps the thermohaline variability,
causing a 25% reduction in the standard deviation.
A further study was performed by Holland (OS), Aaron Brasket (CU) and Weaver
(University of Victoria) to determine how increasing atmospheric CO2
modifies the ice-ocean interactions and variability. This is discussed in a
manuscript published in Geophysical Research Letters. Under a global
warming scenario, variations in the volume of ice exported from the Arctic are
reduced, leading to a smaller thermohaline circulation variability. The thermohaline
circulation variability is also sensitive to the location of ice melt, which
is shifted poleward under the 2xCO2 forcing. This study points out
that ice-ocean exchange in the northern North Atlantic has the potential to
reduce climate variability under warmer conditions.