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4
OCEAN HEAT UPTAKE AND
OCEAN CIRCULATION FEEDBACKS
SUMMARY
The rate of global warming and the spatial distribution of warming are
influenced by the uptake of heat by the oceans. Sea surface temperature
(SST) is set by the ocean's heat uptake from the atmosphere; upper ocean
stratification, which depends on temperature, salinity, and winds; and ocean
currents, which are driven by the atmosphere. The SST is in turn a surface
boundary condition for the atmosphere, impacting large-scale atmospheric
wind patterns, most directly in the tropics, and impacting storm tracks and
intensity at mid-latitudes. Ocean surface temperatures also impact ice cover
at high latitudes and thus influence albedo, which influences the atmosphere
and hence the ocean temperatures. Poor knowledge of all these processes
and consequently their parameterizations in climate models contribute to
widely varying climate model projections.
To better represent the exchange of heat and carbon dioxide at the air-
sea interface, physical representations of upper ocean processes need to be
improved in climate models based on experimental studies of the vertical
structure of temperature, salinity, and absorption of solar radiation
representative of different ocean environments, including high northern and
southern latitudes. Improved definition of the time-dependent temperature
and salinity distribution in the global ocean is essential, including the air-sea
fluxes of heat and freshwater. This will require full implementation of a
system with the capabilities of the current and planned ocean-observing
satellites, the Argo global array of profiling floats, the in situ tropical ocean
observation networks, and a strategy for monitoring key regions of the ocean
where deep-water formation important for the thermohaline circulation
occurs, such as in the Labrador, Greenland-Iceland-Norwegian, Weddell,
and Ross seas.
48
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OCEAN HEA T UPTAKE AND OCEAN CIRCULA TION FEEDBACKS
49
The enormous volume and thermal inertia of the ocean moderate the
daily, seasonal, and interannual temperature fluctuations. The ocean also
acts as a large-scale conveyor of heat from low to high latitudes in response
to differential heat and freshwater exchange with the atmosphere. The ocean
is a vast reservoir of carbon dioxide, thereby providing both a potential
source and sink of this radiatively important greenhouse gas.
As the climate is warmed by human activities the processes that regulate
heat uptake in the ocean will have a strong influence on the rate of warming
(Wiebe and Weaver, 1999~. As the winds and the difference between
precipitation and evaporation change in a warming climate, the processes
that determine ocean surface temperature, particularly in the tropics, will
strongly influence the regional responses around the globe to the warming,
and may also influence the rate of warming (Manatee et al., 1990~. The
evolving surface temperature will in turn affect the winds and sea ice. If the
effects of a warmed climate weaken the overturning of the ocean at middle
and high latitudes, the warming will proceed more rapidly, and large
regional shifts in climate may occur.
The discussion in this chapter deals primarily with the role of the ocean
in feeding back on anthropogenic climate change. Ocean heat uptake is also
discussed here with regard to the role it plays in projections of future climate
change. We recognize that strictly speaking the latter subject matter is
outside the scope of this report on feedbacks; however, due to the
scientifically inseparable nature of ocean heat uptake and climate change
feedbacks and the importance of heat uptake for projecting future climate
change, we have included both here.
Coupling and feedback between ocean processes and the atmosphere
involves the ocean's dynamical state, including overturning, mixing, and
stratification in the ocean's surface layer, as well as movement of heat and
freshwater from one region to another, horizontally and vertically, mainly
driven by the winds. Significant portions of even the thermohaline
(conveyor) circulation are wind-forced, through advection and upwelling in
the upper ocean. To the extent that the atmosphere is sensitive to ocean
conditions, the winds are then affected and in turn force the ocean, the
necessary ingredients for feedback. For example, changes in the strength of
the Arctic oscillation may affect the strength of the overturning, which could
in turn feed back on the strength of the Arctic oscillation.
Direct atmospheric sensitivity to the oceans is generally strongest in the
tropics. At higher latitudes sea-ice cover and hence albedo is an important
factor, and thus climate is indirectly sensitive to ocean conditions affecting
sea ice at these latitudes. But the overall ocean's surface temperature is a
function of the ocean's heat capacity, upper ocean salinity stratification, and
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UNDERSTANDING CLIMATE CHANGE FEEDBACKS
large-scale advection patterns. Major dislocations of SST, such as those that
occur during large climate changes, have an effect on the atmosphere and its
dynamics.
The ocean's impact on the atmosphere and hence climate is through
SST, which affects the overlying atmosphere's heat content, winds, storms,
and water vapor content, and through its role in biogeochemical cycles (see
Chapter 8~. The best-known and possibly strongest climate feedback
involving the ocean is the E1 Nino-Southern Oscillation (ENSO), which is
centered in the equatorial Pacific and produces a strong interannual climate
variation that impacts a large portion of the globe. ENSO and its modulation
through additional physical processes (discussed in Chapter 9), as well as
other natural modes of variability are outlined in Chapter 9. Feedbacks
associated with sea ice were described in Chapter 4. Here we single out
feedbacks that involve mixing, local air-sea fluxes, and thermohaline (deep
ocean) circulation processes.
MIXING, OCEAN HEAT UPTAKE, AND CLIMATE FEEDBACKS
To understand the transient nature of ocean heat and carbon uptake and
how they affect atmospheric SST and carbon dioxide (CO2) levels, one must
consider the processes whereby heat and carbon are exchanged at the surface
and whereby these changes are communicated between the surface and the
deeper layers of the ocean. The exchanges of freshwater at the surface
through evaporation, precipitation, and freezing are also critical for setting
the density structure of the ocean at its surface. Mixing processes
communicate the effects of surface freshwater fluxes into the deeper ocean,
where they affect the density-driven circulation.
Exchanges at the Surface of the Ocean
The interface between the atmosphere and ocean is critical for coupling
and feedbacks that involve both systems. The transfers of heat, moisture,
momentum, and carbon across the air-sea interface are crucial in
determining the potential for ocean heat uptake and circulations to feed back
on climate change. Better understanding of the physics of exchanges at the
air-sea interface is needed. This requires observational and process-oriented
research efforts designed to better characterize and reduce uncertainties in
both the observation and parameterization of air-sea fluxes and the physics
of boundary layer transfer. Another overarching issue is that better estimates
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OCEAN HEAT UPTAKE AND OCEAN CIRCULATIONFEEDBACKS
51
of observed fluxes of heat and moisture over the oceans are needed so that
they can be used as metrics to evaluate the performance of climate models.
Oceanic Mixed-Layer Processes
Conditions in the ocean mixed layer directly affect the atmosphere,
while atmospheric forcing is communicated to the ocean through the mixed
layer. The mixed layer is also important for its influence on ice formation.
Surface waves and associated surface turbulence are the main interface for
ocean-atmosphere gas and momentum exchange, and for injection of some
aerosols into the atmosphere. Sea state (wave conditions) depends on wind
speed. As winds increase, sea state increases and air-sea exchange
accelerates. Wind speed and sea state also affect the ocean's evaporation
rate, and hence heat exchange between the ocean and atmosphere. These
turbulent exchanges are usually parameterized with bulk coefficients
multiplying the relevant parameter such as wind speed or humidity, rather
than relying on detailed prediction or observation of the waves themselves.
SSTs are an integral part of this system. They not only affect the atmosphere
through their influence on heat exchange but the horizontal gradients of SST
also affect the strength of winds. An example is the trade wind response to
changes in tropical Pacific SST gradients.
Surface layer mixing is driven by turbulence associated with wind speed
and by convection due to surface cooling. Mixed layer depth and properties
also depend on the density difference between the mixed layer and
underlying water. Both temperature and salinity stratification are important.
Salinity stratification was often ignored in the past in the search for
simplified solutions, but in the 1990s it became widely recognized that
salinity is the dominant factor in near-surface stratification in the tropics and
in subpolar and high-latitude regions (i.e. in regions with excess
precipitation and runoff).
The ocean absorbs incoming solar radiation over a depth that sometimes
exceeds the mixed layer depth. This absorption warms the water column.
Absorption depends on how clear the water is. In the presence of large
sediment loads or large biological productivity, absorption is limited to
shallower depths and SST can be significantly higher (3-4OC) (Denman,
1973; Martin, 1985~. The vertical distribution of absorption affects upper
layer stratification, thereby influencing mixed layer dynamics and SST. If
absorption is shallower and SST is increased, upper layer stratification
increases, thus furthering the SST increase. Higher temperature may also
increase biological productivity, which also produces a positive feedback.
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UNDERSTANDING CLIA~1TE CHANGE FEEDBACKS
Modeling of surface layer processes should be improved through
incorporation of information from focused observational activities.
Sustained observations at a number of locations that represent the variety of
ocean conditions should involve air-sea fluxes of heat and freshwater, upper
ocean temperature and salinity structure, and absorption of solar radiation.
These observations must be conducted in several regions (including high
northern and southern latitudes) representative of different ocean
environments. The seasonal and sub-seasonal variations in these profiles and
in the measurements of the upper ocean from expendable bathythermographs
(XBTs), the Argo array of profiling floats, the Tropical Ocean Global
Atmosphere program's Tropical Atmosphere Ocean project (TOGA/TAO),
the Pilot Research Moored Array in the Tropical Atlantic (PIRATA),
undersea gliders, and a combination of satellite measurements of sea surface
height and ocean color can be used as metrics to test understanding of the
uptake of heat by the ocean and their simulation in climate models.
Some of the important variables for this are large-scale sea surface
height and temperature, upper ocean heat content, and pycnocline depth
patterns in the North Pacific, in the North Atlantic, and in the mid-latitude
Southern Ocean. These can be used to measure and diagnose the circulation
strength and phase of natural decadal time-scale modes, including the Arctic
(North Atlantic) and Antarctic oscillations and the Pacific decadal mode.
Changes in the strength or phase of these natural modes are likely to occur
with anthropogenic forcing.
Interior Ocean Diffusivity Changes
Interior ocean mixing is very important for determining the rate of
uptake of heat and carbon. Observational evidence suggests that diapycnal
diffusivity (i.e., across levels of equal density known as isopycnals) is a
strong function of location. Along isopycnal (i.e., nearly horizontal)
diffusivity is also sometimes modeled as a function of location, depending
on the strength and hence instability of currents. If current strengths decrease
in response to climate warming, mesoscale energy also decreases, providing
less mixing along isopcynals. This could increase SST gradients, which
would provide a feedback. likely negative, increasing winds and thus
increasing current strengths.
Diapycnal diffusivity is due to interior turbulence, mainly caused by
internal waves. Internal waves are forced by tides and by the wind. The
connections between tides, winds, internal waves and diffusivity are being
examined now through intensive in situ experiments. Diapycnal diffusivity
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OCEAN HEA T UPTAKE AND OCEAN CIRCULA TION FEEDBACKS
53
is high in the surface layer where winds have a direct effect. Here mixing is
enhanced with strong winds, with potential feedbacks as described above.
Recent observations also show that diapycnal diffusivity is high near the
ocean's solid boundaries, and is especially high where topography is rough.
It is not yet clear whether long-period variations in, say, tidal amplitudes,
could be amplified enough to produce climatically significant variations in
diapycnal diffusivity, although there are some proponents of this idea.
Ocean Circulation and Parameter~zation of Diffusivity in Simplified
Ocean Models
The ocean models used in climate modeling are sometimes extremely
simplified in order to test parameter ranges and scenarios for phenomena
that do not depend strongly on the ocean. These simplified ocean models
usually have very coarse resolution and hence are very viscous. They cannot
provide insight for feedbacks that actively involve the ocean. The central
issues for these simplified ocean models are their absorption and advection
of heat and accurate representation of SST.
Many modeling studies have shown a strong dependence of the climate
response to radiative forcing on the parameterization of sub-grid-scale ocean
mixing (see Griffies et al. t2000] for a review). It is clear that there is an
urgent need for improved parameterizations of ocean mixing that account for
the observed spatial inhomogeneity of both diapycnal and isopycnal ocean
mixing. These improvements should be developed through theoretical work
coordinated with ongoing observational programs and field studies. To
reduce dependence on sub-grid-scale parameterization, climate-modeling
groups should continue moving toward improving both the resolution and
physics of the ocean in climate models used to make future projections of
climate change.
THERM:OHALINE CIRCULATION FEEDBACKS
The thermohaline circulation is defined as the component of the ocean
circulation driven by fluxes of heat and fresh water through the ocean
surface. In the present climate the North Atlantic and Southern oceans are
the two regions of deepwater formation where warmer surface waters are
converted to colder deepwaters through intense heat loss to the atmosphere.
In the North Atlantic high-latitude cooling together with low-latitude heating
accelerates the thermohaline circulation (Atlantic meridional overturning
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UNDERSTANDING CLIMATE CHANGE FEEDBACKS
circulation [MOC]) with poleward flow at the surface. On the other hand net
high-latitude precipitation, runoff, and ice melt and mid-latitude evaporation
tend to oppose the thermally driven thermohaline circulation. The deepwater
formation sites are localized in the Greenland and Labrador seas, hence the
recommendations in this report for monitoring these sites and their outflows.
The northward flow of warm upper ocean water with southward flow of cold
deepwater provides most or all of the northward ocean heat transport in the
Atlantic (Roemmich and Wunsch, 1985~. This northward heat transport
extends much farther north than in the North Pacific, which has no deep-
water formation and where northward heat transport is associated with
shallow overturn only (Talley, 1999~.
Climate feedbacks associated with the Atlantic MOC involve this
northward transport of warm water, which reduces the equator-to-pole
temperature gradient in the North Atlantic. If the Atlantic MOC strength
were reduced through a reduction in high-latitude cooling and/or increase in
high latitude freshwater, upper ocean and hence atmospheric temperatures in
the northern regions would decrease. A concomitant increase in
temperatures might occur in the South Atlantic (Broecker, 1998; Stocker,
1998).
The possibility of an abrupt change in the Atlantic MOC in response to
increases in greenhouse gas concentrations has been demonstrated in a
number of simulations with models of the coupled ocean-atmosphere system
(NRC, 2002~. The inherent nonlinearity of such an abrupt event, together
with the sensitivity of the behavior on poorly constrained parameterizations
of ocean mixing (Schmittner and Weaver, 2001), makes it extremely
difficult if not impossible to assign a probability for the future occurrence of
abrupt climate change over the next century. See the NRC report on Abrupt
Climate Change (NRC, 2002) for an extensive discussion of this issue.
Much less is known about the meridional overturning in the Southern
Ocean, not because of its lack of importance but rather due to the harsh
environmental conditions and lack of nearby populated continental
landmasses. The Southern Ocean meridional overturn has two major
components: upwelling of Northern Hemisphere deepwaters all around
Antarctica, which feeds subsequent bottom water formation along the
continental shelves under the sea ice. Bottom water formation occurs
primarily in the Weddell and Ross seas, with additional sites along the coast
of Adelie Land. Interactions of the overturning circulation, upwelling,
northward Ekman transport, ice edge and albedo, fresh surface layer, and the
polar winds involve numerous feedbacks (e.g., Gnanadesikan and Hallberg,
2000; Keeling and Stephens, 2001; Rind et al., 2001; Toggweiler and
Samuels, 1993; Thompson and Solomon, 2002~. The Southern Ocean is also
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OCEAN HEAT UPTAKE AND OCEAN CIRCULATION FEEDBACKS
55
a major site of carbon exchange between the atmosphere and ocean (Sabine
et al., in press; Sarmiento and Gruber, 2002), primarily because of the
massive upwelling of deepwaters. The dominant mechanisms equivalent to
the Atlantic MOC's impact on long-time-scale climate remains to be
determined.
Atlantic Meridional Overturning-SST Feedback
Some modeling studies have noted that over the course of the next few
centuries, the Atlantic MOC may move to an off state in response to
increasing greenhouse gases (Cubasch et al., 2001~. Some, on the other
hand, find no such reduction (Gent, 2001; Latif et al., 2000) and others find
very little reduction (Cubasch et al., 2001~. The reduction in Atlantic MOC
strength associated with increasing greenhouse gases leads to a negative
feedback to warming in and around the North Atlantic. That is, through
reducing the transport of heat from low to high latitudes, SSTs are cooler
than they would otherwise be if the Atlantic MOC were left unchanged. As
such, warming is reduced over and downstream of the North Atlantic. It is
important to note that in all models where the Atlantic MOC weakens,
warming still occurs downstream over Europe due to the radiative forcing of
increasing greenhouse gases. In different models the competing effects of
differential heat and freshwater flux forcing between low and high latitudes
fundamentally determine the MOC-SST feedback.
There is some suggestion (Stocker, 1998; Broecker; 1998) that a
reduced cross-equatorial heat transport to the North Atlantic with a reduced
Atlantic MOC would at quasi-equilibrium lead to enhanced SSTs in the
South Atlantic. Thus, a reduction in Atlantic MOC and the stabilization of
South Atlantic surface water would suggest a positive feedback to
anthropogenic warming in and around the South Atlantic.
Many future projections show that once the radiative forcing is held
fixed, reestablishment of the Atlantic MOC occurs at a state similar to that
of the present day. During this reestablishment phase the Atlantic MOC acts
as a positive feedback to warming in and around the North Atlantic and at
equilibrium there is close to zero net feedback. Whether reestablishment of
the MOC occurs depends on the parameterization of ocean mixing (Manatee
and Stouffer, 1999), as well as the emission rate and eventual stabilization
scenario for atmospheric greenhouse gases (Stocker and Schmittner, 1997~.
The fundamental MOC-SST feedback is well understood although
different models yield different projections in the strength of the MOC over
the twenty-first century. This is not because the underlying feedback is
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UNDERSTANDING CLIMATE CHANGE FEEDBACKS
unknown but because the feedback is ultimately linked to the air-sea
exchanges of heat and freshwater. The basic physics of the latter is still a
matter of investigation, and the present observational network needed to
constrain the physics and its parameterization in models is far from ideal.
The recommendation regarding air-sea exchanges of fresh water in the
preceding section is thus critical to better understanding the local feedbacks
that might be associated with changes in the Atlantic meridional overturning.
Hydrological Cycle-Meridional Overturning Circulation Feedback
Freshwater export from the Arctic to the North Atlantic Ocean is
governed by the total precipitation and runoff into the Arctic. Coupled
modeling studies (Cubasch et al., 2001) suggest that a warmer world is one
in which the hydrological cycle, and hence runoff into and precipitation over
the Arctic, will be enhanced. Freshwater export from the Arctic can either be
in the form of sea ice or liquid water and can exit the Arctic into the Atlantic
through either Pram Strait or the Canadian archipelago.
All coupled models project an increase in poleward transport of water
vapor from low to high latitudes in the atmosphere under enhanced
greenhouse conditions. In some this leads to a freshening of the high-latitude
North Atlantic, which reduces convection and hence the strength of the
MOC. In others no change in the overturning occurs as compensating
feedbacks come into play (see discussion of MOC-SST feedback).
Melting of sea ice in the Arctic provides a freshwater source to the
North Atlantic, which acts to weaken the conveyor, thereby initiating the
Atlantic MOC-SST feedback (see discussion of MOC-SST feedback).
Melting of existing sea ice is, of course, a small component of the total
change in freshwater export out of the Arctic under enhanced greenhouse
conditions. Changes in freshwater export out of the Arctic are controlled by
the total atmospheric moisture transport into the Arctic.
The basic understanding of the effects of changes in the hydrological
cycle on the MOC is relatively well known. Uncertainty in this area is
directly linked to uncertainty in hydrological cycle feedbacks discussed in
Chapter 6, as well as uncertainty in the sea-ice feedbacks (see Chapter 4~.
Thermohaline Circulation Wind Feedback
The strength of the wind-driven circulation affects the MOC. In the
northern North Atlantic the wind-driven circulation is part of the advection
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OCEAN LIED T UPTAKE AND OCEAN CIRCULA TION FEEDBA CKS
57
of water northward into the water mass formation areas. Intensification of
the low pressure atmospheric system can have several effects: (1) increasing
the strength of the subpolar circulation and hence increasing the inflow of
lower-latitude waters into convection regions and outflow of lower salinity
arctic and subarctic waters to the south, and (2) increasing the heat loss
through increased wind speed in the whole subpolar region as well as in the
deepest convection regions. The first will reduce surface density throughout
the subpolar region, increase density stratification, and could lead to warmer
SST where the northward flow is stronger, and colder SST in the low-
salinity regions where deep convection is inhibited. The second will increase
the propensity for convection and will reduce surface temperatures.
In the Southern Ocean, wind-driven upwelling in and south of the
Antarctic Circumpolar Current is an essential part of the MOC. The strength
of Southern Hemisphere westerlies can affect the strength of this upwelling
and may be the major control on the Southern Hemisphere MOC.
Atmospheric warming would not easily disrupt the temperature, although it
could affect the net upwelling transport (Toggweiler and Samuels, 1995~. A
weakened southern polar vortex would be associated with an equatorward
shift of the storm track and a reduction in intensity of the cyclonic ocean
circulations and upwelling. This might affect the ice edge (reduction in ice
cover), which would exacerbate the warming and further weaken the polar
vortex. However, because of the deep upwelling the impact of the oceanic
portion of this feedback would not be as pronounced as if the upwelling had
a much shallower source.
Observations of the Meridional Overturning Circulation
Our current understanding of Atlantic meridional overturning is the
product of decades of observations and increased modeling capability. It has
been demonstrated that Atlantic MOC processes can effect climate change,
particularly in response to large climate forcing, such as occurred at the
glacial-interglacial transitions or might occur in response to anthropogenic
forcing. As indicated above, much better projection capability will require
many more years of in situ process studies and modeling, and long-term
monitoring with ongoing predictive modeling. Understanding of the impact
of meridional overturning processes in the Southern Ocean is far less
advanced. This is likely a reflection of insufficient resources to undertake
the necessary studies in this region far away from major oceanographic
centers, rather than actual impact of this region on climate. Future
observations and modeling of this region are likely to reveal far more of
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UNDERSTANDING CLIA~1 TE CHANGE FEEDBACKS
interest to the understanding of long-term climate change than is now
known.
One of several metrics that should be employed to advance this
understanding and evaluate progress is total water-column heat content
along decadally monitored transoceanic cross-sections, especially in the
North Atlantic and Southern oceans because of its utility in monitoring the
integrated effect of climate forcings and feedbacks. Another set of metrics is
the heat and freshwater content at locations with existing long time series
that have already been clearly correlated with large-scale climate change and
with conditions in the ocean that are implicated in climate feedbacks,
including near-coastal regions such as Bermuda. While these specific
locations are not in themselves important, the long time series are a proxy
for larger-scale ocean conditions that are important in climate change
feedback processes. Western boundary current transports and properties,
including the Gulf Stream, Kuroshio, Labrador Current, and Oyashio, can be
used as indicators of the natural modes (mentioned previously) and in the
North Atlantic as the feeder for the deep meridional overturning. Western
boundary currents can be observed through a combination of satellite
altimetry, moored arrays, and repeat hydrography.
A decade of planning has gone into the far-reaching programs and suites
of observations of the international Climate Variability and Predictability
(CLIVAR) and the Global Ocean Observing System (GODS) programs. It
would be an error to recommend large programs beyond these fully planned,
complex, far-reaching programs. Therefore, our recommendation is that the
United States support the ocean process studies being planned by the
CLIVAR program in the Atlantic and Southern Oceans, the GOOS program,
and the Global Ocean Data Assimilation Experiment (GODAE), which will
improve understanding of meridional ocean overturning and its potential
sensitivity to global climate change.
The planning for these programs has been based on the extensive array
of ocean-observing satellites and previous global ocean climate programs,
notably the World Ocean Circulation Experiment (WOCE) and Tropical
Ocean-Global Atmosphere (TOGA). CLIVAR implementation
() will, if fully funded, include numerous process
components studying the many aspects of North Atlantic and Southern
Ocean overturn. These process studies are embedded in the GOOS program,
which is also just beginning implementation (~.
This global network of spaceborne and in situ ocean observations is critical
to our long-term ability to measure, model, and project ocean change and its
impact on climate.
Representative terms from entire chapter:
atlantic moc
