U.S. Dept. of Commerce / NOAA
/ OAR / GFDL
*Disclaimer
5. OCEANIC CIRCULATION
GOALS
To predict the response of the World Ocean to changing atmospheric
conditions through the use and development of detailed three-dimensional
ocean circulation models.
To understand the critical role of the oceans in climate variability,
regional climate change, and climate change impacts.
To identify practical applications of oceanic models to human marine
activities by the development of a coastal ocean model.
To understand the biogeochemistry of the ocean through the use of
coupled carbon-cycle/ocean models.
5.1 WORLD OCEAN STUDIES
5.1.1 Modeling Eddies
in the Southern Ocean
A. Gnanadesikan K.S.
Smith
S. Griffies J.R.
Toggweiler
R. Hallberg G.K.
Vallis
B. Samuels
ACTIVITIES FY00
The Southern Ocean
in general, and the Antarctic Circumpolar Current (ACC) in particular,
are emerging as centrally important players in both the general circulation
of the ocean and the earth's climate system. For this reason, the Ocean
Group at GFDL has embarked on an ambitious, multi-year effort to numerically
simulate the Southern Ocean, with sufficient horizontal and vertical resolution
to resolve the ubiquitous energy containing mesoscale eddies.
The Southern
Ocean is unique in containing latitudes that are not blocked by land. Consequently,
the leading-order dynamic balances (e.g., Sverdrup balance) do not
obviously dictate the general structure of the ACC, unlike most of the
other ocean currents. The northward Ekman transport in the unblocked latitudes
must be balanced by the combination of the upwelling of waters from great
depth and meridional eddy mass fluxes. The meridional density gradients
associated with the ACC are intimately tied to the mean stratification
of the entire ocean to the north of the ACC, and it is becoming increasingly
evident that the dynamics of the Southern Ocean are crucial in determining
the stratification and overturning circulation of the entire ocean (1313,
1595, 1700, 1728).
Complete understanding
of the interplay between the intense mesoscale eddies and the mean ocean
circulation, and so of the general oceanic circulation itself, demands
the use of numerical models with sufficiently high resolution to explicitly
resolve these eddies, typically as small as 10 km in the ACC. These processes
are being studied through simulations of the Southern Hemisphere oceans
using GFDL's z-coordinate model (MOM) and GFDL's isopycnal-coordinate model
(HIM). Fig. 5.1 shows a comparison of instantaneous velocities in the ACC
near the tip of Africa. The top two panels show results from HIM and MOM
at 1/2° resolution (25 km at 60°S). The bottom panel shows results
from HIM at 1/4° resolution (13.9 km resolution at 60°S). The ACC
flows toward the east between 40° and 45°S. The strong flow along
the coast of Africa is the Agulhas current flowing toward the southwest.
The Agulhas current in the real world is known to join the ACC south of
Africa by retroflecting back to the east. It is also known to break up
into eddies which propagate westward into the Atlantic. Fig. 5.1 shows
that both HIM and MOM capture the retroflection at 1/2° resolution,
but only the 1/4° model in the bottom panel is able to simulate the
production of Agulhas eddies.
Wind stress perturbations
are being applied to both models over a range of resolutions to study the
sensitivity of the dynamic balances between the mean circulation, eddy
fluxes, and the diabatic circulation in the Southern Ocean. The complete
suite of experiments at 1/2° resolution is nearing completion.
The Southern
Ocean provides an ideal test-bed for exploring the effects of eddy-flux
parameterization schemes and the still more basic scientific issue of geostrophic
turbulence in the ocean. Parameterizing (and therefore understanding) oceanic
mesoscale eddies is necessary for properly representing the ocean in long-term
coupled ocean-atmosphere climate simulations. The re-entrant channel geometry
of the ACC makes it possible to pose the problem in a relatively clean
form and, taken together with the inclusion of sub-tropical gyres and intense
western boundary currents in the proposed simulations, allows the problem
to be studied with unprecedented detail.
PLANS FY01
Sensitivity studies
with both MOM and HIM will be analyzed at 1/2° (isotropic) resolution.
Remaining calculations, including wind stress perturbations, at 1/4°
resolution will be carried out. A similar series of simulations at a resolution
of 1/6° or 1/8° will be initiated. More idealized geometries will
also be used to understand the dynamics.
5.1.2 Southern Ocean
Winds and the Circumpolar Current: Theoretical Studies
A. Gnanadesikan R.W.
Hallberg
ACTIVITIES FY00
A number of theories
have been advanced to predict how the Antarctic Circumpolar Current should
respond to changes in the wind stress over the Southern Ocean. According
to one theory, the strength of the Circumpolar Current should respond to
the mean wind stress within the open latitudes of Drake Passage. According
to another, the meridional gradient of
the wind stress should be the most important factor. Still another theory
predicts that the strength of the ACC is saturated, i.e., additional
energy input from the wind simply makes for more energetic eddies. Until
recently, none of these theories considered the impact of heating and cooling
north and south of the ACC.
A recent study
(1728) suggests that any theory which purports to explain the response
of the ACC to the wind must consider the buoyancy forcing north and south
of the ACC as it pertains to the north-south pressure gradient across the
current. Fig. 5.2 shows schematically
that upwelling in the south forces a conversion of dense deep water
into lighter thermocline waters. This leads to an increase in the depth
of the light-water pool in low latitudes and a larger north-south pressure
gradient in the upper part of the water column. This pressure gradient
increases the overturning in the Northern Hemisphere leading to warmer
and lighter deep waters in the north. Insofar as the density of deep water
in the Southern Ocean remains basically unchanged, the result is a larger
north-south pressure gradient at all depths across the ACC and an increase
in the transport of the Circumpolar Current.
The picture is
complicated by the presence of mesoscale eddies in the ACC. Eddies allow
for a southward mass flux of relatively light water which replaces some
of the dense water being upwelled from the deep ocean by the winds. Eddies
thereby reduce the mass conversion and northern overturning depicted in
Fig. 5.2. An idealized theoretical study to examine the effects of eddies
has been carried out using a two-layer isopycnal primitive-equation model
(nc). The model setup allows for strong topographic effects and the resolution
of baroclinic eddies. A key parameter is the ratio between the Ekman flux,
which lifts the interface between the two layers, and the diapycnal mass
flux, which converts water from one density type to the other. When this
ratio is small, i.e., the circumpolar current is relatively weakly
forced by the wind, the conversion of dense upwelled water into lighter
surface water dominates and the strength of the circumpolar current increases
with a stronger wind stress. An increase in the wind stress is partly compensated
by increases in the pycnocline depth and overturning at the northern wall,
partly compensated by an increase in eddy fluxes. However, when the Ekman
flux dominates at the point of instability, i.e., the conversion
of dense upwelled water into lighter surface water can't keep up with the
Ekman flux, increases in wind stress are compensated almost entirely by
increases in the eddy flux.
Figure 5.3 illustrates
the importance of this non-dimensional ratio in governing the dynamics
of a circumpolar current. It shows two realizations of an idealized ACC,
both with the same ratio of Ekman flux to diapycnal flux. The idealized
ACC in the top panel is forced with a
strong wind stress and a high rate of thermal damping in the sponge
regions to the north and south. The ACC in the bottom panel is forced with
a weaker wind stress and less thermal damping. The ACC transport is almost
identical in the two runs, but the structure of the ACC and associated
eddy fluxes are very different.
PLANS FY01
As illustrated in
Fig. 5.3, the eddy mass flux should be more energetic downstream of large
topographic features, such as the Scotia Arc or Campbell Plateau. Homogeneous
parameterizations of eddy fluxes, like the widely used Gent-McWilliams
parameterization, are unlikely to be able to reproduce this asymmetry.
It is unclear whether this will have important effects on their ability
to predict the large-scale structure and dependence of the Circumpolar
Current. Simulations will be carried out in which various parameterizations
are included in a coarse-resolution version of the eddy resolving model.
Results will be compared with the fully eddy-resolving calculations.
5.1.3 The ACC and
the Ocean's Thermohaline Circulation
H. Bjornsson R.
Hotinski
S. Carson B.
Samuels
A. Gnanadesikan J.R.
Toggweiler
ACTIVITIES FY00
Much of the
meridional heat transport into the earth's temperate and sub polar zones
is carried by the ocean's thermohaline circulation. It has long been supposed
that this circulation is maintained by diabatic heating in the interior
brought about by the downward mixing of heat in low and middle latitudes.
Given the central role of diabatic heating in the theory, the thermohaline
circulation has been thought to carry tropical heat poleward toward both
poles. The archetypal thermohaline circulation in today's ocean, seen in
the Atlantic Ocean, does not readily conform to expectations. Both the
warm upper branch and the cold lower branch of the Atlantic thermohaline
circulation extend across the equator into the circumpolar region in the
south such that much of the heat carried northward into the North Atlantic
comes from outside the Atlantic basin. The cross-equatorial overturning
in the Atlantic weakens the poleward ocean heat transport in the Southern
Hemisphere, while it strengthens that of the Northern Hemisphere. The old
theory requires special factors, e.g., extra salt in the Atlantic,
to account for the interhemispheric nature of the Atlantic overturning.
Earlier studies
(1313, 1562) showed that the Atlantic's thermohaline circulation could
very well exist without diabatic heating if it operates in conjunction
with a wind-driven ACC in the far south. From this perspective, the ocean's
dominant thermohaline circulation is interhemispheric by definition, and
it owes its existence to Drake Passage, the gap between South America and
Antarctica which allows the ACC to exist. An idealized coupled model has
been created in which the climatic effects of an open Drake Passage can
be explicitly simulated (1714). The model describes a mostly water-covered
earth in which a full ocean GCM (MOM) is coupled to an energy balance model
of the atmosphere. Land consists of a thin barrier which extends north
and south between two Antarctica-like polar islands. The opening of Drake
Passage is simulated by removing a section of the barrier near the south
polar island. Air temperatures and oceanic SSTs are prognostic variables
of the coupled system; no restoring boundary conditions are used.
Results from
the "water planet" model show that all the major features of the Atlantic
thermohaline circulation and the ocean's heat transport system suddenly
appear with the opening of Drake Passage. The high latitudes of the Southern
Hemisphere cool down by some 3°C while the high latitudes of the Northern
Hemisphere warm up by about the same amount, in accord with the observed
temperature differences between the North and South Atlantic. A relatively
warm, salty deep water mass forms next to the north polar island and flows
southward across the equator at mid-depths much like North Atlantic Deep
Water (NADW) in the real world. The deep water forming next to the north
polar island is inherently salty in relation to deep water formed elsewhere.
Virtually all
the extra heat given up to the atmosphere in the high latitudes of the
Northern Hemisphere comes from the high latitudes of the Southern Hemisphere,
where cold water upwelled by Ekman divergence south of the model's ACC
is warmed as it moves northward under the influence of the Southern Hemisphere
westerlies. According to the old theory, the thermohaline circulation should
cool the tropics as it transports tropical heat poleward. The tropics in
the water planet model actually warm slightly and contribute no extra heat
to the Northern Hemisphere when Drake Passage is opened. The pattern of
warming and cooling generated by an open Drake Passage is proportional
to the wind stress applied in the latitude band of the model ACC. Stronger
winds thicken the thermocline north of the model's ACC in accord with the
simple predictive scheme in (1595). Stronger winds enhance the amount of
warm water that comes into contact with the atmosphere near the north polar
island and thereby enhance the flux of oceanic heat to the atmosphere.
PLANS FY01
This approach to
the ocean's heat transport suggests that the earth's climate should be
particularly sensitive to tectonic changes that open and close critical
ocean gateways. It is hypothesized, for example, that the equable climates
of the Cretaceous and early Cenozoic were maintained, in part, by the enhanced
ocean heat transport caused by a circumglobal current, analogous to the
ACC, that circled the earth in the latitude band of the northern tropics.
Preliminary work with the water planet model shows that the opening of
a Drake Passage-like gap in the northern tropics cools the entire tropical
ocean by some 2°C and boosts the northward oceanic heat transport out
of the tropics up to 4x1015 W.
5.1.4 Ocean Eddy
Energies, Scales, and Vertical Structure
S. Griffies S.
Smith
T. Huck* G.
K. Vallis
*Universite de Bretagne Occidentale, Brest, France
ACTIVITIES FY00
Turbulent motions
comprise a significant fraction of the oceanic energy budget and transport,
yet remain at the edge of what computer models can resolve. Parameterization
of such motions is thus a continuing effort, but one for which only intermediate
solutions currently exist, all of which have deficiencies. A particular
approach pioneered by Held and Larichev (1362) is based on the fundamental
aspects of fully developed geostrophic turbulence generated by baroclinic
instability, and is thus most likely applicable in regions of highly unstable
flow - a fair characterization of much of the world's ocean. A new project
extends the Held and Larichev formalism to incorporate the effects of arbitrary
stratification and shear. The modified theory specifies a vertical structure
for the eddy diffusivity which automatically ensures conservation of the
appropriate material properties. In particular, the theory specifies a
form for the northward eddy potential vorticity flux, v'q', shown in Fig.
5.4, which always
integrates to 0, thereby ensuring that the parameterization conserves
momentum. The modified theory has been tested in a multi-layer homogeneous
quasi-geostrophic turbulence model with realistic stratification and mean
shear with encouraging results.
PLANS FY01
The proposed parameterization
presumes that an eddy producing flow is highly unstable. While this is
likely the case, no specific analysis of ocean climatology has addressed
this question specifically. Moreover, alternate theories exist which are
likely more appropriate for the weakly unstable limit. In order to determine
the degree to which regions of the global ocean are stable, weakly unstable
or strongly unstable to both baroclinic and barotropic instability, a linearized
balanced model appropriate for large scale and frontal regions will be
used to assess the regional instability of oceanic currents. By comparing
these results to measurements of the four-dimensional spectrum of mesoscale
variability throughout the ocean, an assessment will be made regarding
the degree to which various regions are controlled by linear or non-linear
processes, leading to the determination of appropriate eddy closure schemes
for each region. Plans are underway to test the modified theory in an isopycnal
primitive equation model (HIM) and to extend the theory to incorporate
the effects of non-zonal flow, surface buoyancy flux and topographic roughness.
5.1.5 Geostrophic
Turbulence
G. Boccalletti I.
Marinov
C. Cartwright S.
Smith
A. Fournier* C.-Y.
Tam
N. Griannik G.K.
Vallis
I. Held
*Dept. of Geosciences, Princeton University
ACTIVITIES FY00
An interesting research
project emerged from a graduate class taught in the fall of 1999. In lieu
of a final exam, the students worked on various computational/theoretical
problems in geostrophic and two-dimensional turbulence. These projects
were of sufficient interest and originality that they have been extended
and focussed. The common theme of the work is the inverse cascade in forced-dissipative
turbulence. The inverse cascade is of particular oceanographic significance,
because it is one means whereby mesocale eddies interact with the large-scale
circulation, potentially providing natural variability at the gyre scale.
Here, we will highlight just a few of the results that have emerged so
far in this idealized, homogeneous framework.
Ocean general
circulation models typically use a linear Rayleigh damping on the bottom
layer vorticity to simulate Ekman drag. Because baroclinic modes are primarily
surface trapped, bottom drag acts primarily to damp the barotropic mode.
The degree and mechanism by which drag is involved in setting the observed
eddy scales is an open question. Two-dimensional turbulence is an approximation
of the ocean's barotropic mode, thus a simplified two-dimensional model
can be used to selectively examine the dynamics of linear drag on non-linear
transfers. A theory which predicts the steady-state energy spectrum of
this system has been devised and tested against a simulation of two-dimensional
turbulence in a doubly periodic domain forced at high wave number (Fig.
5.5). Given the highly non-linear character of these flows, it is remarkable
that the theory predicts not only the mean scale of
the eddies, but their magnitude and energy distribution as well. Ongoing
research will address the combined effects of more realistic quadratic
drag, differential rotation and finite radius of deformation on eddy energies
and scales, as well as on tracer variance distributions, which approximate
baroclinic mode energy at large scales.
5.2 MODEL DEVELOPMENT
5.2.1 Modular Ocean
Model
V. Balaji R.
Pacanowski
S. Griffies A.
Rosati
ACTIVITIES FY00
Version 3 of GFDL's
Modular Ocean Model (MOM 3) was developed and optimized for use on vector
computers such as the CRAY T90. It can also run on distributed memory systems
like the CRAY T3E. Model performance suffers in two respects, however,
when MOM3 is run on distributed memory systems. First, the amount of memory
required per processor is not reduced as more processors are used to attack
a problem. This means that large models may not be able to fit in available
memory even though the computational domain is divided among many processors.
The second deficiency is an inadequate increase in speed when the work
load is spread across a large number of processors. Because of the way
that arrays are laid out in memory, it is impossible to address these deficiencies
within MOM 3.
A new version
of MOM (MOM 4) has been created to address these deficiencies.
MOM 4 has been written in Fortran 90 syntax and is structured so that it
can use generalized curvilinear coordinates. Early tests indicate that
MOM 4 memory requirements and speed scale significantly better than in
MOM 3 on the CRAY T3E. Better scaling of memory has been achieved by eliminating
MOM 3's memory window and allocating all arrays to the size of the local
domain on each processor which is determined at run-time.
Better scaling
of model speed across multiple processors has been realized due to the
two-dimensional domain decomposition used in MOM 4. Fig. 5.6 shows a comparison
of model
speedup versus number of processors in MOM 3 and MOM 4. The test configuration
used 64 rows of latitude, 120 longitudes, and 15 vertical levels. Both
versions of the model were run on 2, 4, 8, 16, 32, and 64 processors of
the GFDL CRAY T3E and execution times were normalized by the execution
time on 1 processor. The circles indicate the speedup over one processor
using MOM 4 and the triangles represent the speedup over one processor
using MOM 3. The speedup in MOM 3 peaks at 32 processors and then declines
while the speedup in MOM 4 on 64 processors is still increasing. In absolute
terms, the execution time of MOM 4 on 64 processors is a little less than
one half of the execution time of MOM 3 on 64 processors.
Coding complexity
has also been reduced by removal of the memory window and by the use of
averaging and derivative operators. Further improvements in speed are expected
to follow once operators have been optimized for better use of cache and
registers. MOM 4 is compliant with the Flexible Modeling System (FMS) and
will be added to the FMS repository once the driver for interfacing MOM
4 to the atmosphere and ice models has been written. After MOM 4 has been
added to the FMS repository, MOM 4 will be available for addition of physics
modules.
PLANS FY01
Future plans include
further optimization of speed once the new GFDL computer system is in place.
Parameterizations such as Redi-diffusion and Gent McWilliams skew flux
will have to be rewritten. An adjoint of MOM 4 will be built using the
Giering TAF compiler. MOM 4 will be coupled to ice and atmosphere models
using the FMS exchange grid.
5.2.2 Isopycnal Coordinate
Model Development
ACTIVITIES FY00
Isopycnal coordinate
ocean models offer several potentially important advantages over traditional
level models. But disadvantages (such as the difficulty of representing
the nonlinear equation of state, the need to use a nonlinear and nonlocal
treatment of vertical mixing especially in dense flows over sills, and
the difficulty of representing mixed layer dynamics in the detraining phase)
have in the past been limitations for this class of model. Many of these
difficulties have now been addressed.
GFDL's isopycnal
coordinate ocean model (HIM1.0) was officially released to the community
this past year. New technical features include run-time specification of
many parameters, flexible registration style temporal averaging and determination
of the output fields, full support of NetCDF-based input and output, and
run-time determination of parallel processor count. HIM may be run in parallel
using a one- or two-dimensional domain decomposition, and this past year
HIM was ported to 8 different parallel computers without having to change
a single line of code. Also, the calling interface of HIM1.0 is now essentially
compatible with that of MOM 4. HIM development is now managed with modern
version control software.
The effects of
the nonlinear equation of state of seawater are particularly tricky to
incorporate into a model which uses a density-like vertical coordinate.
In the past year, all of the effects of the nonlinear equation of state,
including thermobaricity and cabbeling, have been incorporated into HIM.
Potential density referenced to an interior pressure (typically 2000 dbar)
is used as the vertical coordinate. But various other terms involving density
gradients use different, locally appropriate measures. Gradients of in
situ density (adjusted by a function of pressure only) are used to
calculate pressure gradient accelerations. Locally referenced potential
density is used to estimate the shear-Richardson-number-dependent diapycnal
mixing (1707). Cabbeling is included by advecting potential temperature
and salinity, and adjusting the vertical mixing to restore the coordinate
density of each layer towards its target value. Mixed layer dynamics use
potential densities referenced to the surface to evaluate the mixed layer
turbulent kinetic energy balances (although convective adjustment is also
done if the coordinate variable is unstably stratified). In short, by separating
the role of density as the coordinate variable from its other dynamic roles,
all of the effects of the nonlinear equation of state of seawater can be
described within an isopycnal coordinate ocean model.
PLANS FY01
Improved treatments
of the mixed layer dynamics will be explored. These may include depictions
of Ekman-driven mixed layer stratification, baroclinic instability within
the mixed layer, penetrative short-wave radiation, and rotational constraints
on convective efficiency. The use of HIM as the ocean component of a coupled
model will be tested. Hybrid pressure-density vertical components and alternative
horizontal grids will also be examined.
5.3 COASTAL OCEAN MODELING AND PREDICTION
5.3.1 East Coast
and North Atlantic Modeling and Forecasting
T. Ezer G.L.
Mellor
H.-C. Lee
ACTIVITIES FY00
A 6-year simulation
of the Atlantic Ocean, west of 55°W, has been carried out using the
sigma coordinate Princeton Ocean Model with a curvilinear grid. A comparison
between experiments with and without surface fluxes shows that the effect
of the surface wind stress and heat fluxes on the Gulf Stream path and
separation is closely related to the intensification of deep circulations
in the northern region. The separation of the Gulf Stream and the downslope
movement of the Deep Western Boundary Current (DWBC) are reproduced in
the model results. The model DWBC crosses under the Gulf Stream southeast
of Cape Hatteras and then feeds the deep cyclonic recirculation east of
the Bahamas. The model successfully reproduces the cross-sectional vertical
structures of the Gulf Stream, such as the asymmetry of the velocity profile
and the eddy activity of the Gulf Stream. Entrainment of the upper layer
slope current into the Gulf Stream occurs near the cross-over point; the
converging cross-stream flow is nearly barotropic.
Using the same
model, the Loop Current and deep circulation in the Gulf of Mexico has
been described. A deep cyclonic circulation, bounded by the deep basin
in the eastern Gulf, is shown to be spun up by the Loop Current. The Loop
Current also induces deep anticyclonic columnar eddies in the eastern Gulf
which decouple from the upper layer Loop Current. The westward translation
speed of a Loop Current Ring is about 2.5-6 cm/s. Lower layer eddies have
a higher speed and lead the rings into the central Gulf. The time-average
surface circulation of the Gulf of Mexico basin is anticyclonic, mainly
due to the transport of anticyclonic vorticity by Loop Current Rings in
the surface layer. An average lower layer cyclonic circulation occurs along
the continental slope of the basin.
The Data
Assimilation and Model Evaluation Experiments in the North Atlantic Basin
(DAMEE-NAB), supported by the Office of Naval Research (ONR), has been
completed with the publication of a special issue that includes an intercomparison
between six different ocean models (1716). Sensitivity studies evaluate
the effect of open boundary conditions, horizontal diffusivity, and model
resolution on model variability and Gulf Stream dynamics (1715).
PLANS FY01
Future research
will focus on the dynamics of the Loop Current and the deep circulation
in the Gulf of Mexico. A coastal forecasting system based on a higher resolution
version of the extended east coast model will be tested.
5.3.2 Turbulent Boundary
Layer Modeling
ACTIVITIES FY00
An improved surface
boundary layer formulation has been developed to improve the Mellor-Yamada
boundary layer model (ky). Unlike the situation in three-dimensional simulations
or in the real ocean, the kinetic energy in one-dimensional surface layer
models can build up and artificially enhance local mixing. Adding a sink
term to the momentum equations counteracts this behavior. The sink term
is a surrogate for energy divergence available to three-dimensional models,
but not to one-dimensional models. The new sink term tends to exacerbate
problems with overly warm summertime surface temperatures. A Richardson
number dependent dissipation term yields a favorable improvement in the
comparison between model calculations and observations.
Further testing
of the modified Mellor-Yamada turbulence scheme with a three-dimensional
North Atlantic Ocean model (1717) shows improvement in the simulations
of the seasonal mixed-layer depth. The mixed layer in the three-dimensional
model is shown to be strongly influenced by errors in the surface heat
flux, the frequency of the forcing wind stress and most importantly, by
penetration of short wave radiation, making the evaluation of the new turbulence
scheme in three-dimensional models much more difficult than it is in one-dimensional
models.
PLANS FY01
Future work will
incorporate the effects of surface waves in both the surface boundary layer
and bottom boundary layers in shallow water.
5.3.3 Princeton Ocean
Model Development and Testing
ACTIVITIES FY00
The Princeton Ocean
Model (POM) users group has continued to grow. It now includes more than
700 users from 54 countries. User support and code development continues
(for more detail consult the POM web page - www.aos.princeton.edu/WWWPUBLIC/htdocs.pom).
Some of the new features that have been tested with POM during the last
year include: a) a modified Mellor-Yamada turbulence scheme (1717, ky)
that improves the simulation of vertical mixing and surface layers; b)
a vertical generalized coordinate system that can accommodate z-level,
sigma-level or other vertical discretizations (ip); c) a multidimensional
positive definite advection transport algorithm; and d) a sixth-order accuracy
combined compact difference scheme that significantly reduces pressure
gradient errors. A parallel version of POM is currently being tested.
PLANS FY01
A new ONR-supported
initiative will foster a collaboration between the POM developers and other
ocean model developers from Rutgers University in order to develop and
improve terrain-following ocean models and community support.
5.3.4 Climate Variability
Studies with POM
ACTIVITIES FY00
Sigma coordinate
ocean models, originally developed for regional coastal studies and prediction,
are now being used for long-term climate simulations. Attention is given
to surface and bottom boundary layers which may play an important role
in surface heat exchange and in deep water formation processes. Observed
decadal variations in the North Atlantic Ocean during the period 1950-1989
have been successfully simulated with an Atlantic Ocean version of POM
(1663) and compared favorably with a simple wind-driven Rossby wave model
(ny). The simulations demonstrate the important role played by westward
propagating planetary waves in affecting the subtropical gyre and Gulf
Stream variations. An interesting finding was that, on decadal time scales,
the ocean model responds in a linear fashion to the combined effect of
surface temperature and wind stress anomalies. Simulations of the response
of the ocean to heat and fresh water flux anomalies in high latitudes,
a result of possible future climate change, show an adjustment process
that takes a few decades (nz). Spatial climatic changes in circulation
patterns and in coastal sea level will be studied in detail.
PLANS FY01
Research will focus
on the interaction between climatic changes in the open ocean and variability
in the coastal ocean. The effect of bottom boundary layers and the way
they are formulated in ocean models on deep water formation will be further
studied.
5.3.5 Coastal Models
of the West Coast and the Gulf of Mexico
H.-C. Lee M.
Wei
L.-Y. Oey
ACTIVITIES FY00
Research on the
coastal observing systems off the west coast of the U.S. have started up
recently as part of the National Ocean Partnership Program (NOPP). This
work shows that details of the wind field, particularly the local wind
curl, can be an important forcing to coastal circulation (1611). Studies
based on coastal observations and models that continue to deploy sparse
arrays of wind stations and coarse-resolution wind products (e.g.,
NCEP or ECMWF) may therefore be incomplete. Hindcasts of the coastal variability
in the Santa Barbara Channel, carried out in collaboration with the Scripps
Institute of Oceanography, show that the high-resolution wind fields give
the best results.
Observations
have suggested that episodic subsurface current events in the Gulf of Mexico
are caused by topographic Rossby waves (TRW) forced by Loop Current pulsation
(north/south extrusion and retraction) and eddy shedding in the eastern
Gulf. However, the existence of TRWs in coastal forecast models has never
been rigorously established. A ten-year simulation has been analyzed to
isolate the TRWs. Over 70% of the simulated subsurface energy was observed
to reside in the 20 to 100 day periods in narrow bands over the continental
slope and rise. Bottom intensification has been shown to exist in these
high-energy bands. While the modeled TRWs are unambiguously forced by variability
induced by the Loop Current and Loop Current Eddies, precise mechanism(s)
through which energy is transmitted to lower layers is not yet understood.
PLANS FY01
High-resolution
simulations of coastal variability along the west coast and Gulf of Mexico
will continue.
5.4 GLOBAL BIOGEOCHEMISTRY AND THE
CARBON CYCLE
5.4.1 Terrestrial
Carbon Cycling
S.-M. Fan S.
Pacala*
G.C. Hurtt* J.L.
Sarmiento
P.C. Milly E.
Shevliakova**
P. Moorcroft
*Institute for the Study of Earth, Oceans, and Space, Univ. of New
Hampshire
**Dept. of Ecology and Evolutionary Biology, Princeton University
ACTIVITIES FY00
Through a detailed
analysis of forest inventory data, the relative contribution of land use,
carbon dioxide fertilization, and nitrogen fertilization to carbon sequestration
in U.S. forests has been quantified. Results show that land-use is the
primary factor governing the rate of carbon sequestration in U.S. forests.
This conclusion is further supported by a second analysis that compares
U.S. carbon budgets obtained through inventories of forests and other components
of the terrestrial carbon cycle with U.S. carbon budgets obtained through
inverse modeling of CO2 flask sample measurements.
The findings of this study support the conclusion that changes in land-use
account for most of the carbon being sequestered within the U.S., and reconciles
a longstanding perceived inconsistency in the estimates obtained by these
different methods.
The first implementation
of a new terrestrial biosphere model, the Ecosystem Demography model (ED
model), has been documented. A second implementation of the off-line model
that examines the impact of human land-use on the terrestrial carbon cycle
of North America is currently being completed. The results of this study
show how historical changes in patterns of land-use across the continent
during the past three centuries account for the spatio-temporal distribution
of carbon stocks and fluxes during this period and how these effects are
likely to continue into the next century.
The ED model
has also been successfully coupled to a mesoscale model of the atmosphere
(MM5 V3.3), and significant progress has been made towards integrating
ED into GFDL's new FMS. Energy and moisture exchange parameterizations
in MM5 have been modified to account for the sub-grid heterogeneity in
vegetation distribution predicted by ED. The sub-grid scale heterogeneity
includes ecosystem age and species type, and vertical profiles of plant
canopy characteristics, in particular, the Leaf Area Index (LAI) and stomatal
closure. The coupled ED-MM5 model is being used to explore the role of
vegetation structure on diurnal patterns of fluxes between land surface
and the lower atmosphere. Preliminary results obtained using the original
OSU-ETA soil hydrology model show that the diurnal mode is able to capture
the diurnal pattern of moisture and heat fluxes between atmosphere and
biosphere in the tropical region.
A canopy interface
that couples the atmosphere with vegetation and soil processes modeled
by ED has also been developed. In this parameterization, sensible heat
and moisture fluxes from leaves and soil surface are calculated using a
multiple resistance parameterization and turbulent mixing inside the canopy
is assumed to be dominated by gusts with a size characteristic of the height
of canopy. Mass and energy are transported directly from leaf surfaces
to the top of the canopy, while diffusion between leaf layers is assumed
to be small and thus neglected. Effective canopy air temperature and moisture
content near the top of the canopy are calculated as linear combinations
of temperature at the bottom layer of the atmosphere, ground temperature
and leaf temperature through the canopy.
PLANS FY01
The ED model will
be integrated with forest inventory and land-use history data to complete
a global, off-line implementation. The MM5-ED model will be used to: 1)
analyze the impact of anthropogenic land use practices on the regional
distribution and dynamics of CO2 sources
and sinks; and 2) understand the role of plant rooting depth on the biophysical
feedback to local climate. This research will explore a range of perturbations
to the land atmosphere system that represent scenarios of land-use and
vegetation succession in different regions. Once incorporated into FMS,
ED will form the terrestrial component of a global, land-atmosphere-ocean
model that we will use to study the importance of biospheric feedbacks
for global climate. A particular focus will be the identification of regions
where ecosystem dynamics have a significant impact on the regional climate.
5.4.2 Inverse Modeling
of Carbon Isotopic Ratios of CO2 in the
Atmosphere
S.-M. Fan J.L.
Sarmiento
E.M. Gloor*
*Max-Planck Institute for Biogeochemistry, Jena, Germany
ACTIVITIES FY00
Over the past two
years, the steady-state distribution of carbon isotopic ratios in atmospheric
CO2 has been modeled using the GFDL Global
Chemical Tracer Model (GCTM), which uses a year-long record of wind fields
generated previously by a general circulation model. Prescribed air-sea
isotopic fluxes without seasonal variability are used to force the model
over the oceans. The annual isotopic fluxes due to terrestrial net primary
production and respiration were estimated for three land regions (Eurasia,
North America, and the tropics and Southern Hemisphere) by inversions of
atmospheric observations of the isotopic ratios. The inversion results
suggest for the 1993-1995 period the presence of a terrestrial carbon sink
in the midlatitude Northern Hemisphere and a terrestrial carbon source
in the tropics. The terrestrial carbon source and sink were estimated previously
for the 1980s and 1990s from inversions of atmospheric CO2
mixing ratios.
PLANS FY01
The goal of this
work is to estimate, using inverse models, the isotopic carbon fluxes between
air and ocean, as well as between air and land. The isotopic fluxes are
caused by net oceanic CO2 uptake and by
the existence of an air-sea disequilibrium due to a lagged isotopic Suess
Effect in the ocean. On-going global ocean general circulation and biogeochemistry
model simulations by the Carbon Modelling Consortium (CMC) will be used
to predict the net CO2 uptake and the disequilibrium
isotopic carbon fluxes from the pre-industrial times to the present. The
inversion results will be combined with ocean models to improve the estimate
of the oceanic uptake of CO2 and the estimate
of the terrestrial carbon fluxes for different geographical regions. A
time-dependent inverse model of atmospheric carbon dioxide and its isotopic
ratios may be developed towards this goal.
5.4.3 Mixing Parameterizations,
Large-Scale Ocean Circulation, and Global
Biogeochemical
Cycles
A. Gnanadesikan J.L.
Sarmiento
N. Gruber* R.
Slater
I. Marinov P.S.
Swathi
*Dept. of Atmospheric Sciences, UCLA
ACTIVITIES FY00
It has been shown
that the large-scale pycnocline depth can be explained by multiple combinations
of wind stress, Southern Ocean eddy fluxes, and low-latitude diffusion
(1595). However, different combinations of vertical and lateral mixing
result in very different pathways for vertical exchange. A suite of model
runs was performed in which the vertical and lateral diffusion were changed
in a way that produced a similar pycnocline depth. In one case (with low
vertical and lateral mixing) relatively little upwelling of deep water
occurred through the low-latitude pycnocline. In a second case (with high
vertical and lateral mixing) about 18 Sv of upwelling occurred through
the low-latitude pycnocline. The high and low mixing cases are quite similar
in their thermal structure, but the high mixing case has slightly higher
uptake of CFCs and anthropogenic CO2, and
a new production which is twice that of the low mixing case (pc). This
is because the high mixing model has more upwelling at low latitudes and
more convection at high latitudes. Additional model runs were made with
high vertical mixing only in the Southern Ocean. In these runs, the amount
of convection in high latitudes increased, but the low-latitude upwelling
was essentially unchanged.
PLANS FY01
Analysis of the
suite of model runs will continue. Particular attention will be paid to
the effect of nutrient fertilization on anthropogenic CO2
uptake and low-latitude production. Development of the ocean general circulation
model will continue with a focus on improving the deep circulation.
5.4.4 Air-Sea Fluxes
of O2 and CO2
Determined by Inverse Modeling
of
Ocean Bulk Measurements
E.M. Gloor* J.L.
Sarmiento
N. Gruber**
*Max-Planck Institute for Biogeochemistry
**Dept. of Atmospheric Sciences, UCLA
ACTIVITIES FY00
A method has been
developed to estimate the air-sea fluxes of gases from their observed distributions
in the interior of the ocean. An ocean circulation model is used to establish
a relationship between a unit air-sea flux in a given region of the surface
ocean and the concentration at any point in the interior of the ocean.
The surface ocean is divided into 15 regions, each of which contributes
to the interior concentration at any location in the ocean. The tracers
are linear (i.e., additive), so the model predicted concentration
in any given region is equal to the sum of the 15 components. In a final
step, the Singular Value Decomposition is used to estimate the actual magnitude
of the air-sea fluxes in each of the 15 regions by requiring that the predicted
interior concentrations fit the observations.
The method has
been applied to estimating the air-sea flux of heat, water, O2,
and CO2. Before the method could be applied
to O2 and CO2,
the observed concentrations had to be corrected for the changes in water
column chemistry that are due to biological cycling. This was done using
the observed phosphate, nitrate, and alkalinity distributions, and assuming
that the stoichiometric oxygen and carbon to phosphate ratios in organic
matter are constant. The observed CO2 distribution
must also be separated into a pre-industrial component, which is assumed
to be at steady state, and an anthropogenic component. This separation
enables estimation of both the pre-industrial air-sea flux distribution,
and anthropogenic carbon uptake.
PLANS FY01
Estimates of CO2
uptake will be made as more estimates of anthropogenic CO2
become available from the World Ocean Circulation Experiment.
5.4.5 An Ecosystem
Model for Biogeochemical Studies
R. Armstrong* N.
Gruber**
C. Deutsch J.L.
Sarmiento
A. Gnanadesikan R.
Slater
*Marine Sciences Research Center, SUNY-Stony Brook
**Dept. of Atmospheric Sciences, UCLA
ACTIVITIES FY00
A simplified ecosystem
model has been developed for use in biogeochemical studies. The ecosystem
model is simple in the sense that it has few state variables, yet advanced
in the sense that it captures fundamental features concisely. The production
model consists of two state variables representing small and large producer
organisms. These are parameterized in a way that captures the difference
between small and large phytoplankton, as well as the production and temperature
dependent behavior of the f-ratio. The model is simple enough that four
of the parameters needed for its specification can be estimated from the
f-ratio data, and the other two can be estimated from fits to Levitus nutrient
data.
PLANS FY01
The ecosystem model
simulations will be compared with local patterns of seasonal variation
in nutrients and chlorophyll at long term study sites (the Bermuda Atlantic
Time Series and Ocean Weather stations), as well as satellite chlorophyll
data and primary production estimated from these data. These comparisons
give a measure of the ability of the model to reproduce seasonal and interannual
patterns of production, as well as identifying regions where additional
biogeochemical processes such as iron limitation will need to be added.
Future development of the model will include splitting of production among
taxonomic categories (diatoms, calcium depositors, and other algae) to
match observed latitudinal patterns in silicate, phosphate, and alkalinity.
5.4.6 Oceanic Nitrogen
Cycle
C. Deutsch J.L.
Sarmiento
N. Gruber*
*Dept. of Atmospheric Sciences, UCLA
ACTIVITIES FY00
An analysis of nitrogen
cycling in the Pacific has recently been completed based on nutrient data
from the World Ocean Circulation Experiment (WOCE). The analysis uses the
tracer N* (nitrate - 16phosphate)
to identify regions where nitrogen fixation and denitrification occur (1497).
The analysis shows the basin to contain sources and sinks of fixed nitrogen
with magnitudes that are significant on the global scale (about 50 TgN/yr
of water column denitrification - a sink, and about 60 TgN/yr of newly
fixed nitrogen).
PLANS FY01
Work is currently
under way to prepare gridded fields from the high quality nutrient dataset
provided by the WOCE program, with the aim of diagnosing the spatial patterns
and rates of nitrogen fixation and denitrification by damping an OGCM toward
those observed nutrient fields. This will allow exploration of a number
of hypotheses regarding what controls the rates and locations of nitrogen
inputs and outputs to the ocean. It will also enable diagnostic models
of ocean carbon uptake to account for carbon transports that are associated
with source and sink pathways in the marine nitrogen cycle.
5.4.7 Global Patterns
of Marine Silicate, Nitrate, and Alkalinity Cycling
A. Gnanadesikan J.L.
Sarmiento
N. Gruber* P.S.
Swathi
R.M. Key
*Dept. of Atmospheric Sciences/UCLA
ACTIVITIES FY00
The large-scale
field of nutrients has been used to identify regions where particular functional
groups of plankton (in particular, silicifying organisms such as diatoms)
are important. Typical diatoms have a silicate to nitrate molar uptake
ratio of approximately 1:1. This observation is used to define a tracer
Si* = (Si) - (N) of excess silicate concentration relative to the needs
of such diatoms (Fig. 5.7). The dominant feature of deep ocean Si* is an
increase along the pathway of the deep thermohaline circulation from a
low of 0 to 10 mol
kg-1 in the North Atlantic to highs in
excess of 100 mol
kg-1 in the deep North Indian and Pacific
Oceans. This increase results from the well-known preferential dissolution
of silicate in the deep ocean.
The high Si* waters of the abyss reach up to the surface in the Southern
Ocean south of the Antarctic Polar Front. There the silicate is preferentially
stripped out by diatoms with a Si:N ratio of approximately 5:1, consistent
with this being an iron limited region. This generates a broad 20°-wide
band of negative surface Si* around the entire Southern Ocean that subsequently
penetrates to the north at the depth of the Subantarctic Mode Water (SAMW).
The negative Si* supplied by the SAMW is the dominant feature of the upper
kilometer of all ocean basins except the North Pacific, which is the only
other place in the world where high Si* waters of the deep ocean are able
to reach the surface. Consequently, most areas of the ocean outside the
North Pacific and Southern Ocean have a deficit of silicate relative to
nitrate. Within the North Atlantic, the low Si* water is swept downward
with North Atlantic Deep Water. A more detailed examination of Si* shows
how other processes such as shallow remineralization of nitrate relative
to silicate modify the above overview. Of particular interest is the fact
that there is no clear indication of a high Si:N drawdown in the iron limited
North Pacific.
PLANS FY01
The global analysis
will be extended to look at the effects of calcifying organisms. The goal
is to identify regions where coccolithophorids are dominant producers using
the water mass distribution. Results will be compared with satellite estimates
for calcification under way at other institutions. A key product will be
an estimate of the fraction of new production associated with these functional
groups. Simulations which evaluate the effects of changes in the relative
fraction of carbonate and silicate producing organisms on the carbon cycle
will then be carried out.
5.4.8 Analysis of
the Ocean's Carbon Pumps
S. Carson J.L.
Sarmiento
A. Gnanadesikan J.R.
Toggweiler
R. Murnane*
*Bermuda Biological Station for Research
ACTIVITIES FY00
It is almost axiomatic
that temperature anomalies imposed on the ocean near the poles will have
a greater effect on the partitioning of CO2
between the ocean and atmosphere than temperature anomalies imposed in
low latitudes. This is true because most of the ocean's volume comes into
contact with the atmosphere through cold polar outcrops. Two influential
papers published during the last two years1,2
have shown that this polar temperature sensitivity is not nearly as strong
in GCMs as it is in box models. Broecker and Archer suggest that this is
true because vertical mixing in GCMs allows the solubility effect of warm
surface temperatures to be felt well down in the interior of the ocean.
Broecker and Archer imply that the mixing effect in GCMs carries over to
the other "carbon pumps" in the ocean, namely those involving the production
and remineralization of organic particles and CaCO3.
A new method
has been devised for disaggregating the carbon pumps in ocean models to
explore these issues. The results show that the critical distinction between
box models and GCMs has less to do with mixing than with the relative areas
over which atmosphere-deep ocean communication is allowed to occur. GCMs
have less polar sensitivity than box models because the high-latitude gas
exchange that allows CO2 to move between
the deep ocean and atmosphere is restricted by limited areas of convection
and deep water formation. The polar boxes in box models tend to be fairly
large, typically occupying 5-10% of the overall ocean area. With such a
large area, the effect of gas exchange in box models is less limiting.
It is found, furthermore, that the effect of limited gas exchange is different
for the thermal solubility effect than it is for the CO2
pumped down into the deep ocean by organic particles. This is because the
critical region where limited gas exchange influences CO2
solubility is in the North Atlantic while the critical region for the organic
carbon pump is in the Southern Ocean.
PLANS FY01
The new method for
disaggregating carbon pumps described above has been applied so far only
to one GCM-based biogeochemistry simulation (1617). In the coming year,
the new method will be used to examine other GCM simulations and data from
the real ocean.
5.4.9 Response of
Ocean Biology to Future Climate Change
J.L. Sarmiento R.
Stouffer
R. Slater
ACTIVITIES FY00
In collaboration
with modeling groups at CSIRO (Australia), the Hadley Center (United Kingdom),
Max Planck Institute (Germany), the Institute Pierre et Simon Laplace (France),
and NCAR, six different coupled climate model simulations of future climate
change are being examined to determine the range of behavior of those aspects
of global warming simulations that are relevant to the ocean biological
response. The overall response inferred from examining the physical response
of the ocean to global warming is decreased biological production in low
latitude upwelling regions and the poleward half of the subtropical gyres,
and increased production in the polar regions.
Wind-driven upwelling
is the dominant mechanism of nutrient supply along the highly productive
western margins of the continents and in the equatorial regions. Models
predict widely varying results, but the general tendency is towards a reduction
of upwelling in these regions, from which it is inferred that biological
production would decrease. The dominant mechanism for nutrient supply in
the subtropical gyres poleward of the subtropical convergence zone is wintertime
convection. These regions tend to become more stratified with future climate
change, which reduces the depth of wintertime mixing. The expectation,
supported by model predictions, is that this would result in reduced biological
production. The polar regions generally have a high supply of nutrients
due to upwelling and convection, but can suffer from low productivity due
to low light supply in deep mixed layers. Increased stratification, which
occurs in most models, though with a complex pattern, would thus tend to
increase biological production. Exceptions to this would be where low levels
of micronutrient supply by dust limit the production, such as is thought
to be the case in the Southern Ocean and North Pacific, or where the decreased
mixing reduced the nutrient supply to less than the potential biological
uptake.
The mechanism
of nutrient supply to regions between the equatorial upwelling bands and
subtropical convergence is poorly understood and poorly simulated in most
models. It is difficult to determine how these regions will respond to
future climate change. The changes described will also very likely lead
to changes in ocean ecology, as the major phytoplankton groups such as
diatoms, coccolithophorids, flagellates, Phaeocystis, and nitrogen fixers
are sensitive to water column stratification, as well as nutrient content.
PLANS FY01
The analyses made
to date are local analyses, and do not consider impacts of changes in one
part of the ocean on other parts of the ocean. In the coming year, special
attention will be paid to this question. A particular area of interest
is the effect of increased high latitude production on nutrient supply
to the low latitudes.
1. Broecker,
W. S., J. Lynch-Stieglitz, D. Archer, M. Hofman, E. Maier-Reimer, O. Marchal,
T. Stocker, and N. Gruber, How strong is the Harvardton-Bear constraint?,
Global Biogeochem. Cycles, 13, 817-820, 1999.
2. Archer, D.,
G. Eshel, A. Winguth, W. Broecker, R. Pierrehumbert, M. Tobis, and R. Jacob,
Atmospheric CO2 sensitivity to the biological
pump in the ocean, Global Biogeochem. Cycles, in press.
*Portions of this document contain
material that has not yet been formally published and may not be quoted
or referenced without explicit permission of the author(s).