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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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50 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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52 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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54 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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56 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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58 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: