Climate change simulated by full and mixed-layer ocean versions of CSIRO Mk3.5 and Mk3.0: Large-scale sensitivity

The CSIRO Mk3.5 coupled atmosphere-ocean model includes upgrades to atmospheric and oceanic processes that remove a cold bias of the earlier Mk3.0. The global mean warming over the 21st century from Mk3.5 is 3.1 K under the CMIP3 A1B scenario, some 25% larger than that from Mk3.0. Two mixed-layer ocean versions of Mk3.5 were constructed, and these are also more sensitive than Mk3.0. To elucidate these differences, a simple feedback analysis is extended to Mk3.5, using changes for doubled CO₂ in each model version. The net feedback for the low-mid latitude region is the main driver of the sensitivity contrast. The clear-sky component is consistently larger in Mk3.5, as is the increase in specific humidity, even after standardizing by the global warming. Cloud forcing provides a small positive feedback, which is stronger in cases that had larger declines in low-layer cloud. The net positive feedback for the higher-latitude region is larger in the coupled Mk3.5 than Mk3.0, which had more stable Arctic sea ice. However, some contrasts differed among the versions. As for Mk3.0, the surface warming in the coupled Mk3.5 is suppressed over that from the MLO case. Over the ocean, the pattern of suppression is similar to the change in energy flux into the surface in the coupled model. There is also a gradient of equatorial warming in the Asia-Pacific region that relates to the change in net convergence of heat transport by ocean currents. The effect of this pattern on regional rainfall is a focus of Part 2 of the study.     

The transport of CO2-induced warming into the ocean: an analysis of simulations by the OSU coupled atmosphere-ocean general circulation model

The OSU global coupled atmosphere-ocean general circulation model has been used to investigate a 2xCO₂-induced climate change. A previous analysis of the simulated 2xCO₂–1xCO₂ temperature differences showed that the CO₂-induced warming penetrated into the ocean and thereby caused a delay in the equilibration of the climate system with an estimatede-folding time of 50–75 years. The objective of the present study is to determine by what pathways and through which physical processes the simulated ocean general circulation produces the penetration of the CO₂-induced warming into the ocean.A global-mean oceanic heat budget analysis shows that the ocean gains heat at a rate of 3 W/m² due to the CO₂ doubling, and that this heat penetrates downward into the ocean predominantly through the reduction in the convective overturning. A zonal-mean oceanic heat budget analysis shows that the surface warming increases from the tropics toward the midlatitudes of both hemispheres and gradually penetrated into the deeper ocean, with a greater penetration in the subtropics and midlatitudes than in the equatorial region. The zonal-mean heat budget analysis also shows that the CO₂-induced warming of the ocean occurs predominantly through the down-ward transport of heat, with the meridional heat flux being only of secondary importance. In the tropics the penetration of the CO₂-induced heating is minimized by the upwelling of cold water. In the subtropics the heating is transported down-ward more readily by the downwelling existing there. In the high latitudes the suppressed convection plays the dominant role in the downward penetration of the CO₂-induced heating. The latter result should be considered as tentative, however, as the ocean component of the coupled model employed a prescribed surface salinity field and did not include the mechanism of brine rejection when sea water freezes into sea ice.     

Deep-ocean heat uptake and equilibrium climate response

We integrate the coupled climate model ECHAM5/MPIOM to equilibrium under atmospheric CO₂ quadrupling. The equilibrium global-mean surface-temperature change is 10.8 K. The surface equilibrates within about 1,200 years, the deep ocean within 5,000 years. The impact of the deep ocean on the equilibrium surface-temperature response is illustrated by the difference between ECHAM5/MPIOM and ECHAM5 coupled with slab ocean model (ECHAM5/SOM). The equilibrium global-mean surface temperature response is 11.1 K in ECHAM5/SOM and is thus 0.3 K higher than in ECHAM5/MPIOM. ECHAM5/MPIOM shows less warming over the northern-hemisphere mid and high latitudes, but larger warming over the tropical ocean and especially over the southern-hemisphere high latitudes. ECHAM5/MPIOM shows similar polar amplification in both the Arctic and the Antarctic, in contrast to ECHAM5/SOM, which shows stronger polar amplification in the northern hemisphere. The southern polar warming in ECHAM5/MPIOM is greatly delayed by Antarctic deep-ocean warming due to convective and isopycnal mixing. The equilibrium ocean temperature warming under CO₂ quadrupling is around 8.0 K and is near-uniform with depth. The global-mean steric sea-level rise is 5.8 m in equilibrium; of this, 2.3 m are due to the deep-ocean warming after the surface temperature has almost equilibrated. This result suggests that the surface temperature change is a poor predictor for steric sea-level change in the long term. The effective climate response method described in Gregory et al. (2004) is evaluated with our simulation, which shows that their method to estimate the equilibrium climate response is accurate to within 10 %.     

A mixed-flux equilibrium asynchronous coupling scheme for accelerating convergence in ocean-atmosphere models

A coupled atmosphere/mixed-layer ocean energy balance model of intermediate complexity is used to examine the feasibility of a technique to accelerate the model's convergence rate while using equilibrium asynchronous coupling (EAC). EAC consists of an iterative sequence of integrations of an atmospheric model driven to equilibrium with fixed SSTs, the output of which is used to drive an ocean model to equilibrium. In an attempt to accelerate a radiatively perturbed climate to its final equilibrium state, we superimpose on the EAC scheme a mixed-flux condition, in which a portion of the turbulent air-sea heat flux from the most recent atmospheric leg is used in the present oceanic integration. In sensitivity tests using enhanced insolation, this mixed-flux approach strongly hastens the model's convergence rate, whose acceleration is regionally dependent: the tropics show the strongest sensitivity to the mixed-flux condition, while polar regions are least affected. This geographic variation stems from the presence of polar sea-ice, which promotes a temperature inversion and thereby causes a seasonal change in the sign of the total turbulent air-sea heat flux. Potential applications and limitations of this accelerated EAC method are also discussed.     

Transient responses of the CSIRO climate model to two different rates of CO2 increase

 The responses of the CSIRO coupled atmosphere-ocean-sea ice model to two greenhouse gas induced warming scenarios are described and compared to a control run with the current CO₂ level. In one scenario, denoted IS92a, the atmospheric CO₂ increases such that it reaches doubling after 128 years. In the other, the CO₂ increases at 1% per year compounding (doubling after 70 y). As the CO₂ increases in both scenarios, the top-of-atmosphere outgoing longwave radiation increases giving enhanced cooling of the coupled system, while the outgoing short wave radiation decreases contributing to a warming of the system. The latter overcompensates the former leading to a global mean net radiative heat gain. The distribution of this heat gain produces the well-known interhemispheric asymmetry in warming, despite a decrease in the sea ice around Antarctica in this model. It is found that the volume mean temperature response over the southern ocean is greater than that over the northern hemispheric oceans, and a maximum warming takes place at the subsurface rather at the surface of the ocean in the southern mid-to-high latitude region. The enhanced high-latitude freshening associated with the strengthened hydrological cycle significantly affects the latitudinal distribution of warming and other responses. It enhances the warming immediately equatorward of the deep water formation regions while produces a reduced warming, even a cooling, in these regions. In both runs, there is a decrease in the large-scale oceanic currents which have a significant thermohaline-driven component. The reduction in these currents reduces the poleward transport of salt out of the tropical and subtropical regions of these oceans. This and the enhanced evaporation contribute to considerable increases in surface salinity in the tropical and subtropical regions. In IS92a, the warming rate before doubling is smaller than that in 1% scenario, but the cumulative effects of the two experiments at the time of doubling are similar. Nevertheless, significant contrasts exist. For example, at the time of doubling in IS92a, the warming of the upper ocean is greater because a more developed temperature-albedo feedback occurs. In addition, a longer time is allowed for heat anomalies to spread downward, and so the effective heat penetration depth is greater than that in the 1% scenario. Thus the oceanic response is influenced by the CO₂ increase scenario used. Received: 2 September 1997 / Accepted: 21 January 1998     

On the sensitivity of global warming experiments to the parametrisation of sub-grid scale ocean mixing

An ocean general circulation model coupled to an energy-moisture balance atmosphere model is used to investigate the sensitivity of global warming experiments to the parametrisation of sub-grid scale ocean mixing. The climate sensitivity of the coupled model using three different parametrisations of sub-grid scale mixing is 3°C for a doubling of CO₂ (6°C for a quadrupling of CO₂). This suggests that the ocean has only a weak feedback on global mean surface air temperature although significant regional differences, notably at high latitudes, exist with different sub-grid scale parametrisations. In the experiment using the Gent and McWilliams parametrisation for mixing associated with mesoscale eddies, an enhancement of the surface response in the Southern Ocean is found. This enhancement is largely due to the existence of more realistic sea-ice in the climatological control integration and the subsequent enhanced ice-albedo feedback upon warming. In accordance with earlier analyses, the Gent and McWilliams scheme decreases the global efficiency of ocean heat uptake. During the transient phase of all experiments, the North Atlantic overturning initially weakened but ultimately recovered, surpassing its former strength. This suggests that in the region around the North Atlantic the ocean acts as a negative feedback on local warming during the transient phase but a positive feedback at equilibrium. During the transient phase of the experiments with a more sophisticated and realistic parametrisation of sub-grid scale mixing, warmed Atlantic water was found to penetrate at depth into the Arctic, consistent with recent observations in the region. Received: 14 October 1998 / Accepted: 27 April 1999     

海洋对人为CO2吸收的三维模式研究

文中用包含海洋化学过程和一个简单生物过程的三维碳循环模式模拟了海洋对大气CO₂的吸收,并分析了碳吸收的纬度分布。模拟工业革命以来海洋对大气CO₂的吸收表明:海洋碳吸收再加上大气CO₂的增加只占由化石燃料燃烧、森林砍伐和土地利用的变化而释放到大气中的CO₂的2/3。1980～1989年期间海洋年平均吸收2.05GtC。海洋人为CO₂的吸收有明显的纬度特征。模式计算的海洋CO₂的吸收在总量与纬度分布上与观测结果比较相符。     

Future ocean uptake of CO2: interaction between ocean circulation and biology

We discuss the potential variations of the biological pump that can be expected from a change in the oceanic circulation in the ongoing global warming. The biogeochemical model is based on the assumption of a perfect stoichiometric composition (Redfield ratios) of organic material. Upwelling nutrients are transformed into organic particles that sink to the deep ocean according to observed profiles. The physical circulation model is driven by the warming pattern as derived from scenario computations of a fully coupled ocean-atmosphere model. The amplitude of the warming is determined from the varying concentration of atmospheric CO₂. The model predicts a pronounced weakening of the thermohaline overturning. This is connected with a reduction of the transient uptake capacity of the ocean. It yields also a more effective removal of organic material from the surface which partly compensates the physical effects of solubility. Both effects are rather marginal for the evolution of atmospheric pCO₂. Running climate models and carbon cycle models separately seems to be justified. Received: 9 August 1995 / Accepted: 22 April 1996     

Towards quantifying uncertainty in transient climate change

Ensembles of coupled atmosphere–ocean global circulation model simulations are required to make probabilistic predictions of future climate change. “Perturbed physics” ensembles provide a new approach in which modelling uncertainties are sampled systematically by perturbing uncertain parameters. The aim is to provide a basis for probabilistic predictions in which the impact of prior assumptions and observational constraints can be clearly distinguished. Here we report on the first perturbed physics coupled atmosphere–ocean model ensemble in which poorly constrained atmosphere, land and sea-ice component parameters are varied in the third version of the Hadley Centre model (the variation of ocean parameters will be the subject of future study). Flux adjustments are employed, both to reduce regional sea surface temperature (SST) and salinity biases and also to admit the use of combinations of model parameter values which give non-zero values for the global radiation balance. This improves the extent to which the ensemble provides a credible basis for the quantification of uncertainties in climate change, especially at a regional level. However, this particular implementation of flux-adjustments leads to a weakening of the Atlantic overturning circulation, resulting in the development of biases in SST and sea ice in the North Atlantic and Arctic Oceans. Nevertheless, model versions are produced which are of similar quality to the unperturbed and un-flux-adjusted version. The ensemble is used to simulate pre-industrial conditions and a simple scenario of a 1% per year compounded increase in CO₂. The range of transient climate response (the 20 year averaged global warming at the time of CO₂ doubling) is 1.5–2.6°C, similar to that found in multi-model studies. Measures of global and large scale climate change from the coupled models show simple relationships with associated measures computed from atmosphere-mixed-layer-ocean climate change experiments, suggesting that recent advances in computing the probability density function of climate change under equilibrium conditions using the perturbed physics approach may be extended to the transient case.     

Response of Southern Ocean circulation to global warming may enhance basal ice shelf melting around Antarctica

We investigate the large-scale oceanic features determining the future ice shelf–ocean interaction by analyzing global warming experiments in a coarse resolution climate model with a comprehensive ocean component. Heat and freshwater fluxes from basal ice shelf melting (ISM) are parameterized following Beckmann and Goosse [Ocean Model 5(2):157–170, 2003]. Melting sensitivities to the oceanic temperature outside of the ice shelf cavities are varied from linear to quadratic (Holland et al. in J Clim 21, 2008). In 1% per year CO₂-increase experiments the total freshwater flux from ISM triples to 0.09 Sv in the linear case and more than quadruples to 0.15 Sv in the quadratic case after 140 years at which 4 × 280 ppm = 1,120 ppm was reached. Due to the long response time of subsurface temperature anomalies, ISM thereafter increases drastically, if CO₂ concentrations are kept constant at 1,120 ppm. Varying strength of the Antarctic circumpolar current (ACC) is crucial for ISM increase, because southward advection of heat dominates the warming along the Antarctic coast. On centennial timescales the ACC accelerates due to deep ocean warming north of the current, caused by mixing of heat along isopycnals in the Southern Ocean (SO) outcropping regions. In contrast to previous studies we find an initial weakening of the ACC during the first 150 years of warming. This purely baroclinic effect is due to a freshening in the SO which is consistent with present observations. Comparison with simulations with diagnosed ISM but without its influence on the ocean circulation reveal a number of ISM-related feedbacks, of which a negative ISM-feedback, due to the ISM-related local oceanic cooling, is the dominant one.     

Climate change simulated by full and mixed-layer ocean versions of CSIRO Mk3.5 and Mk3.0: The Asia-Pacific region

The patterns of climate change in the Asia-Pacific region simulated by versions of the CSIRO Mk3.5 and Mk3.0 climate models are examined and compared with those from 23 CMIP3 models. Using fields standardized by global warming, it is seen that both CSIRO coupled models simulate larger surface warming in the tropical western Pacific Ocean, and smaller warming in the eastern Indian Ocean, than the CMIP3 average, and also model versions with a mixed-layer ocean. Corresponding differences in the changes in the pressure, winds, rainfall and other quantities were simulated. Introducing the coupled Mk3.5’s sea surface temperature field for the present climate, which has a warm bias, as the base climate for the MLO version had only a minor effect on the MLO version’s pattern of climate change. A Pacific-Indian Dipole index quantifying the amplitude of the warming pattern explains much of the variation in rainfall change simulated by the CMIP3 models over Australia and the Indonesian and Melanesian regions. It relates more strongly to Australian average rainfall than several other indices representing southern hemispheric circulation changes. The decline in Australian rainfall produced by the full ocean coupling is largest in summer, but occurs in each season, and extends across the continent. Further assessment of the importance of the dipole change pattern in new simulations is warranted. Analyses aimed at reducing the uncertainty in its potential amplitude could help narrow the range of projections for change in the Australasian region.     

Effect of ocean thermal diffusivity on global warming induced by increasing atmospheric CO2

A global mean ocean model including atmospheric heating, heat capacity of the mixed layer ocean, and vertical thermal diffusivity in the lower ocean, proposed by Cess and Goldenberg (1981), is used in this paper to study the sen-sitivity of global warming to the vertical diffusivity. The results suggest that the behaviour of upper ocean tempera-ture is mainly determined by the magnitude of upper layer diffusivity and an ocean with a larger diffusivity leads to a less increase of sea surface temperature and a longer time delay for the global warming induced by increasing CO2 than that with smaller one. The global warming relative to four scenarios of CO2 emission assumed by Intergovernmental Panel of Climate Change (IPCC) is also estimated by using the model with two kinds of thermal diffusivities. The result shows that for various combinations of the CO2 emission scenarios and the diffusivities, the oceanic time delay to the global warming varies from 15 years to 70 years.     

Parameter sensitivity of a coupled atmosphere-ocean model

 The sensitivity of a coupled model to the oceanic vertical diffusion coefficient κ _v is examined. This is compared to the sensitivity of an ocean-only model forced by mixed boundary conditions (BC). The atmospheric component of the coupled model is a moist energy balance model. The ocean component is a 12-level geostrophic model, defined on a midlatitude β-plane. Atmosphere and ocean are coupled through the fluxes of heat and moisture at their interface. The coupled model contains a number of feedback processes which are not represented in the ocean-only model. This results in a temperature and salinity response to κ _v which is stronger in the coupled model than in the ocean-only model. On the other hand, there is a weaker response in oceanic processes such as meridional heat transport, deep-water formation at high latitudes, etc. Ocean-only sensitivity experiments were also performed with modified BCs, which parametrise the feedback processes included in the coupled model. These are the modified thermal BC of Rahmstorf and Willebrand and a modified freshwater BC proposed in the present study. Large-scale features of the response in oceanic surface fields are well represented with modified BCs. However, the sensitivity of the deep ocean temperature is only partly captured due to local differences in the surface response. The scaling behavior of the zonal overturning stream function was found to depend on the surface BCs. In contrast to this, the meridional overturning stream function basically scales with κ^0.5 _v in all sensitivity experiments. Differences in the heat transport response among the experiments are thus primarily related to differences in the temperature response. Received: 28 February 1997/Accepted: 12 September 1997     

The variation of ENSO characteristics associated with atmospheric parameter perturbations in a coupled model

We analyse the differences in the properties of the El Niño Southern Oscillation (ENSO) in a set of 17 coupled integrations with the flux-adjusted, 19-level HadCM3 model with perturbed atmospheric parameters. Within this ensemble, the standard deviation of the NINO3.4 deseasonalised SSTs ranges from 0.6 to 1.3 K. The systematic changes in the properties of the ENSO with increasing amplitude confirm that ENSO in HadCM3 is prevalently a surface (or SST) mode. The tropical-Pacific SST variability in the ensemble of coupled integrations correlates positively with the SST variability in the corresponding ensemble of atmosphere models coupled with a static mixed-layer ocean (“slab” models) perturbed with the same changes in atmospheric parameters. Comparison with the respective coupled ENSO-neutral climatologies and with the slab-model climatologies indicates low-cloud cover to be an important controlling factor of the strength of the ENSO within the ensemble. Our analysis suggests that, in the HadCM3 model, increased SST variability localised in the south-east tropical Pacific, not originating from ENSO and associated with increased amounts of tropical stratocumulus cloud, causes increased ENSO variability via an atmospheric bridge mechanism. The relationship with cloud cover also results in a negative correlation between the ENSO activity and the model’s climate sensitivity to doubling CO₂.     

CO2 climate sensitivity and snow-sea-ice albedo parameterization in an atmospheric GCM coupled to a mixed-layer ocean model

The snow-sea-ice albedo parameterization in an atmospheric general circulation model (GCM), coupled to a simple mixed-layer ocean and run with an annual cycle of solar forcing, is altered from a version of the same model described by Washington and Meehl (1984). The model with the revised formulation is run to equilibrium for 1 × CO₂ and 2 × CO₂ experiments. The 1 ×CO₂ (control) simulation produces a global mean climate about 1° warmer than the original version, and sea-ice extent is reduced. The model with the altered parameterization displays heightened sensitivity in the global means, but the geographical patterns of climate change due to increased carbon dioxide (CO₂) are qualitatively similar. The magnitude of the climate change is affected, not only in areas directly influenced by snow and ice changes but also in other regions of the globe, including the tropics where sea-surface temperature, evaporation, and precipitation over the oceans are greater. With the less-sensitive formulation, the global mean surface air temperature increase is 3.5 °C, and the increase of global mean precipitation is 7.12%. The revised formulation produces a globally averaged surface air temperature increase of 4.04 °C and a precipitation increase of 7.25%, as well as greater warming of the upper tropical troposphere. Sensitivity of surface hydrology is qualitatively similar between the two cases with the larger-magnitude changes in the revised snow and ice-albedo scheme experiment. Variability of surface air temperature in the model is comparable to observations in most areas except at high latitudes during winter. In those regions, temporal variation of the sea-ice margin and fluctuations of snow cover dependent on the snow-ice-albedo formulation contribute to larger-than-observed temperature variability. This study highlights an uncertainty associated with results from current climate GCMs that use highly parameterized snow-sea-ice albedo schemes with simple mixed-layer ocean models.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The effect of atmospheric CO2 and ice sheet topography on LGM climate

The role of reduced atmospheric CO₂ concentration and ice sheet topography plus its associated land albedo on the LGM climate is investigated using a coupled atmosphere-ocean-sea ice climate system model. The surface cooling induced by the reduced CO₂ concentration is larger than that by the ice sheet topography plus other factors by about 30% for the surface air temperature and by about 100% for the sea surface temperature. A large inter-hemispheric asymmetry in surface cooling with a larger cooling in the Northern Hemisphere is found for both cases. This asymmetric inter-hemispheric temperature response is consistent in the ice sheet topography case with earlier studies using an atmospheric model coupled with a mixed-layer ocean representation, but contrasts with these results in the reduced CO₂ case. The incorporation of ocean dynamics presumably leads to a larger snow and sea ice feedback as a result of the reduction in northward ocean heat transport, mainly as a consequence of the decrease in the North Atlantic overturning circulation by the substantial freshening of the North Atlantic convection regions. A reversed case is found in the Southern Ocean. Overall, the reduction in atmospheric CO₂ concentration accounts for about 60% of the total LGM climate change.     

Effects of tropical instability wave (TIW)-induced surface wind feedback in the tropical Pacific Ocean

Tropical instability waves (TIWs) arise from oceanic instability in the eastern tropical Pacific and Atlantic Oceans, having a clear atmospheric signature that results in coupled atmosphere–ocean interactions at TIW scales. In this study, the extent to which TIW-induced surface wind feedback influences the ocean is examined using an ocean general circulation model (OGCM). The TIW-induced wind stress (τ_TIW) part is diagnostically determined using an empirical τ_TIW model from sea surface temperature (SST) fields simulated in the OGCM. The interactively represented TIW wind tends to reduce TIW activity in the ocean and influence the mean state, with largest impacts during TIW active periods in fall and winter. In December, the interactive τ_TIW forcing induces a surface cooling (an order of ?0.1 to ?0.3 °C), an increased heat flux into the ocean, a shallower mixed layer and a weakening of the South Equatorial Current in the eastern equatorial Pacific. Additionally, the TIW wind effect yields a pronounced latitudinal asymmetry of sea level field across the equator, and a change to upper thermal structure, characterized by a surface cooling and a warming below in the thermocline, leading to a decreased temperature gradient between the mixed layer and the thermocline. Processes responsible for the τ_TIW–induced cooling effects are analyzed. Vertical mixing and meridional advection are the two terms in the SST budget that are dominantly affected by the TIW wind feedback: the cooling effect from the vertical mixing on SST is enhanced, with the maximum induced cooling in winter; the warming effect from the meridional advection is reduced in July–October, but enhanced in November–December. Additional experiments are performed to separate the relative roles the affected surface momentum and heat fluxes play in the cooling effect on SST. This ocean-only modeling work indicates that the effect of TIW-induced wind feedback is small but not negligible, and may need to be adequately taken into account in large-scale climate modeling.     

Relative roles of dynamic and thermodynamic processes in causing positive and negative global mean SST trends during the past 100 years

Global mean surface temperature (GMST) during 1910–2012 experienced four alternated rapid warming and warming hiatus phases. Such a temporal variation is primarily determined by global mean sea surface temperature (SST) component. The relative roles of ocean dynamic and thermodynamic processes in causing such global mean SST variations are investigated, using two methods. The first method is ocean mixed layer heat budget analysis. The budget diagnosis result shows that the thermodynamic processes dominate in the rapid warming phases, while the ocean dynamics dominate during the hiatus phases. The second method relies on the diagnosis of a simple equilibrium state model. This model captures well the horizontal distribution of SST difference between two warmer and cooler equilibrium states during either the rapid warming or hiatus phases. It is found that the SST difference during the rapid warming phases is primarily controlled by the increase of downward longwave radiation as both column integrated water vapor and CO₂ increase during the phases. During the hiatus phases, the water vapor induced greenhouse effect offsets the CO₂ effect, and the SST cooling tendency is primarily determined by the ocean dynamics over the Southern Ocean and tropical Pacific. The SST pattern associated with the Interdecadal Pacific Oscillation (IPO) might be responsible for the remote and local ocean dynamic responses through induced wind change.     

The role of the individual air-sea flux components in CO2-induced changes of the ocean's circulation and climate

 In this study we investigate the role of heat, freshwater and momentum fluxes in changing the oceanic climate and thermohaline circulation as a consequence of increasing atmospheric CO₂ concentration. Two baseline integrations with a fully coupled ocean atmosphere general circulation model with either fixed or increasing atmospheric CO₂ concentrations have been performed. In a set of sensitivity experiments either freshwater (precipitation, evaporation and runoff from the continents) and/or momentum fluxes were no longer simulated, but prescribed according to one of the fully coupled baseline experiments. This approach gives a direct estimate of the contribution from the individual flux components. The direct effect of surface warming and the associated feedbacks in ocean circulation are the dominant processes in weakening the Atlantic thermohaline circulation in our model. The relative contribution of momentum and freshwater fluxes to the total response turned out to be less than 25%, each. Changes in atmospheric water vapour transport lead to enhanced freshwater input into middle and high latitudes, which weakens the overturning. A stronger export of freshwater from the Atlantic drainage basin to the Indian and Pacific ocean, on the other hand, intensifies the Atlantic overturning circulation. In total the modified freshwater fluxes slightly weaken the Atlantic thermohaline circulation. The contribution of the modified momentum fluxes has a similar magnitude, but enhances the formation of North Atlantic deep water. Salinity anomalies in the Atlantic as a consequence of greenhouse warming stem in almost equal parts from changes in net freshwater fluxes and from changes in ocean circulation caused by the surface warming due to atmospheric heat fluxes. Important effects of the momentum fluxes are a poleward shift of the front between Northern Hemisphere subtropical and subpolar gyres and a southward shift in the position of the Antarctic circumpolar current, with a clear signal in sea level. Received: 3 May 1999 / Accepted: 11 December 1999