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1.
We analyze the sensitivity of the oceanic thermohaline circulation (THC) regarding perturbations in fresh water flux for a range of coupled oceanic general circulation — atmospheric energy balance models. The energy balance model (EBM) predicts surface air temperature and fresh water flux and contains the feedbacks due to meridional transports of sensible and latent heat. In the coupled system we examine a negative perturbation in run-off into the southern ocean and analyze the role of changed atmospheric heat transports and fresh water flux. With mixed boundary conditions (fixed air temperature and fixed surface fresh water fluxes) the response is characterized by a completely different oceanic heat transport than in the reference case. On the other hand, the surface heat flux remains roughly constant when the air temperature can adjust in a model where no anomalous atmospheric transports are allowed. This gives an artificially stable system with nearly unchanged oceanic heat transport. However, if meridional heat transports in the atmosphere are included, the sensitivity of the system lies between the two extreme cases. We find that changes in fresh water flux are unimportant for the THC in the coupled system. 相似文献
2.
The orbital configuration at the end of the last interglacial, 115,000 years BP (115 ky BP), was such that the Northern Hemisphere seasonal contrast was decreased when compared to the last interglacial maximum, 126 ky BP. Climatic reconstructions argue for increased latitudinal surface temperature and salinity gradients in the North Atlantic at 115 ky BP compared to 126 ky BP. According to proxy measurements the high-latitude ocean freshening may be explained by enhanced northward atmospheric moisture advection which would have then led to decreased deep convection activity in the northern seas. To evaluate such re-adjustments of the atmospheric circulation to the insolation forcing changes, we have explored the changes in atmospheric energy balance and transport with two AGCM experiments, one for each climate. We show that the northward increase in static heat transport at 115 ky BP to 126 ky BP constitutes a first order response to the changing insolation. It tends to equalise the heat balance of the atmosphere. Despite sea surface temperatures fixed (SSTs) to present-day this feature is strongly amplified by the air–sea heat flux exchanges. By comparing with OAGCM experiments for the same periods, we find that the simulated surface ocean heat flux responses to insolation forcing are similar whether the ocean is allowed to vary or not. The latent heat transport does not undergo the same changes as the dry static one. On an annual basis, it decreases over the high northern latitudes. This is the result of summer modification of moisture sources and transient activity. The latter appears to affect latent heat transport much more than the dry static one. The winter response, however, differs from the summer response which dominates the annual mean. There is an enhanced northward atmospheric moisture advection during winter at 115 ky BP, which is responsible for the freshening of high-latitude ocean during this season. This result seems to confirm the hypothesis inferred from marine data. 相似文献
3.
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 相似文献
4.
《大气与海洋》2013,51(2):81-92
Abstract Evidence based on numerical simulations is presented for a strong correlation between the North Atlantic Oscillation (NAO) and the North Atlantic overturning circulation. Using an ensemble of numerical experiments with a coupled ocean‐atmosphere model including both natural and anthropogenic forcings, it is shown that the weakening of the thermohaline circulation (THC) could be delayed in response to a sustained upward trend in the NAO, which was observed over the last three decades of the twentieth century, 1970–99. Overall warming and enhanced horizontal transports of heat from the tropics to the subpolar North Atlantic overwhelm the NAO‐induced cooling of the upper ocean layers due to enhanced fluxes of latent and sensible heat, so that the net effect of warmed surface ocean temperatures acts to increase the vertical stability of the ocean column. However, the strong westerly winds cause increased evaporation from the ocean surface, which leads to a reduced fresh water flux over the western part of the North Atlantic. Horizontal poleward transport of salinity anomalies from the tropical Atlantic is the major contributor to the increasing salinities in the sinking regions of the North Atlantic. The effect of positive salinity anomalies on surface ocean density overrides the opposing effect of enhanced warming of the ocean surface, which causes an increase in surface density in the Labrador Sea and in the ocean area south of Greenland. The increased density of the upper ocean layer leads to deeper convection in the Labrador Sea and in the western North Atlantic. With a lag of four years, the meridional overturning circulation of the North Atlantic shows strengthening as it adjusts to positive density anomalies and enhanced vertical mixing. During the positive NAO trend, the salinity‐driven density instability in the upper ocean, due to both increased northward ocean transports of salinity and decreased atmospheric freshwater fluxes, results in a strengthening overturning circulation in the North Atlantic when the surface atmospheric temperature increases by 0.3°C and the ocean surface temperature warms by 0.5° to 1°C. 相似文献
5.
Various ocean reanalysis data reveal that the subarctic Atlantic sea surface temperature (SST) has been cooling during the twentieth century. A similar cooling pattern is found in the doubling CO2 experiment obtained from the CMIP3 (coupled model intercomparison project third phase) compared to the pre-industrial experiment. Here, in order to investigate the main driver of this cooling, we perform the heat budget analysis on the subarctic Atlantic upper ocean temperature. The net surface heat flux associated with the increased concentration of greenhouse gases heats the subarctic ocean surface. In the most of models, the longwave radiation, latent heat flux, and sensible heat flux exert a warming effect, and the shortwave radiation exerts a cooling effect. On the other hand, the thermal advection by the meridional current reduces the subarctic upper ocean temperature in all models. This cold advection is attributed to the weakening of the meridional overturning circulation, which is related to the reduction in the ocean surface density. In particular, greater warming of the surface air than of the sea surface results in the reduction of surface evaporation and thereby enhanced freshening of the ocean surface water, while precipitation change was smaller than evaporation change. The thermal advections by both the wind-driven Ekman current and the density-driven geostrophic current contribute to cooling in most of the models, where the heat transport by the geostrophic current tends to be larger than that by the Ekman current. 相似文献
6.
Observations indicated that for the El Niño/Southern Oscillation (ENSO) there have been eastward displacements of the zonal wind stress (WS) anomalies and surface heat flux (short wave heat flux and latent heat flux) anomalies during El Niño episodes in the 1981–1995 regime relative to the earlier regime of 1961–1975 (without corresponding displacements during La Niña episodes). Our numerical experiments with the Zebiak–Cane coupled model generally reproduced such displacements when the model climatological fields were replaced by the observed climatologies [of sea surface temperature (SST), surface WS and surface wind atmospheric divergence] and simulated climatologies (of oceanic surface layer currents and associated upwelling) for the 1981–1995 regime. Sensitivity tests indicated that the background atmospheric state played a much more important role than the background ocean state in producing the displacements, which enhanced the asymmetry between El Niño and La Niña in the later regime. The later regime climatology state resulted in the eastward shifts in the ENSO system (WS and SST) only during El Niño, through the eastward shift of the atmosphere convergence heating rate in the coupled model. The ENSO period and ENSO predictability were also enhanced in the coupled model under the later regime climatology. That the change in the mean state of the tropical Pacific atmosphere and ocean after the mid 1970s could have produced the observed changes in ENSO properties is consistent with our findings. 相似文献
7.
Influence of coupling on atmosphere, sea ice and ocean regional models in the Ross Sea sector, Antarctica 总被引：1，自引：0，他引：1
Nicolas C. Jourdain Pierre Mathiot Hubert Gallée Bernard Barnier 《Climate Dynamics》2011,36(7-8):1523-1543
Air–sea ice–ocean interactions in the Ross Sea sector form dense waters that feed the global thermohaline circulation. In this paper, we develop the new limited-area ocean–sea ice–atmosphere coupled model TANGO to simulate the Ross Sea sector. TANGO is built up by coupling the atmospheric limited-area model MAR to a regional configuration of the ocean–sea ice model NEMO. A method is then developed to identify the mechanisms by which local coupling affects the simulations. TANGO is shown to simulate realistic sea ice properties and atmospheric surface temperatures. These skills are mostly related to the skills of the stand alone atmospheric and oceanic models used to build TANGO. Nonetheless, air temperatures over ocean and winter sea ice thickness are found to be slightly improved in coupled simulations as compared to standard stand alone ones. Local atmosphere ocean feedbacks over the open ocean are found to significantly influence ocean temperature and salinity. In a stand alone ocean configuration, the dry and cold air produces an ocean cooling through sensible and latent heat loss. In a coupled configuration, the atmosphere is in turn moistened and warmed by the ocean; sensible and latent heat loss is therefore reduced as compared to the stand alone simulations. The atmosphere is found to be less sensitive to local feedbacks than the ocean. Effects of local feedbacks are increased in the coastal area because of the presence of sea ice. It is suggested that slow heat conduction within sea ice could amplify the feedbacks. These local feedbacks result in less sea ice production in polynyas in coupled mode, with a subsequent reduction in deep water formation. 相似文献
8.
S. L. Weber 《Climate Dynamics》1998,14(3):201-212
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 κ0.5
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 相似文献
9.
10.
On the incompatibility of ocean and atmosphere models and the need for flux adjustments 总被引：1，自引：0，他引：1
The surface heat and freshwater fluxes from equilibrium ocean (OGCM) and atmospheric (AGCM) general circulation model climates are examined in order to determine the minimum flux adjustment required to prevent climate drift upon coupling. This is accomplished by integrating an OGCM with specified surface fluxes. It is shown that a dramatic climate drift of the coupled system is inevitable unless ocean meridional heat and freshwater (salt) transports are used as constraints for tuning the AGCM present-day climatology. It is further shown that the magnitude of the mismatch between OGCM and AGCM fluxes is not as important for climate drift as the difference in OGCM and implied AGCM meridional heat and freshwater (salt) transports. Hence a minimum flux adjustment is proposed, which is zonally-uniform in each basin and of small magnitude compared to present flux adjustments. This minimum flux adjustment acts only to correct the AGCM implied oceanic meridional transports of heat and freshwater (salt). A slight extension is also proposed to overcome the drift in the surface waters when the minimum flux adjustment is used. Finally, it is suggested that the flux adjustments which arise from current methods used to determine them are all very similar, leading to adjustment fields which are significantly larger than both AGCM and climatological fields over large regions. 相似文献
11.
1. Introduction Air-sea interaction plays an important role in theglobal seasonal to inter-annual climate variability,most notably, the El Ni?no and Southern Oscillation(ENSO) phenomenon (Webster and Lukas, 1992). Be-cause of its widespread impacts on … 相似文献
12.
This study investigates the mechanisms by which the ocean diurnal cycle can affect the ocean mean state in the North Atlantic region. We perform two ocean-atmosphere regionally coupled simulations (20°N–80°N, 80°W–40°E) using the CNRMOM1D ocean model coupled to the ARPEGE4 atmospheric model: one with a 1 h coupling frequency (C1h) and another with a 24 h coupling frequency (C24h). The comparison between both experiments shows that accounting for the ocean diurnal cycle tends to warm up the surface ocean at high latitudes and cool it down in the subtropics during the boreal summer season (June–August). In the subtropics, the leading cause for the formation of the negative surface temperature anomalies is the fact that the nocturnal entrainment heat flux overcompensates the diurnal absorption of solar heat flux. Both in the subtropics and in the high latitudes, the surface temperature anomalies are involved in a positive feedback loop: the cold (warm) surface anomalies favour a decrease (increase) in evaporation, a decrease (increase) in tropospheric humidity, a decrease (increase) in downwelling longwave radiative flux which in turn favours the surface cooling (warming). Furthermore, the decrease in meridional sea surface temperature gradient affects the large-scale atmospheric circulation by a decrease in the zonal mean flow. 相似文献
13.
COOL AND FRESHWATER SKIN OF THE OCEAN DURING RAINFALL 总被引：2，自引：0，他引：2
Peter Schlössel Alexander V. Soloviev William J. Emery 《Boundary-Layer Meteorology》1997,82(3):439-474
Rainfall over the sea modifies the molecular boundary layers of the upper ocean through a variety of different effects. These cover the freshwater flux stabilizing the near-surface layer, additional heat flux established due to rain versus surface temperature differences, modification of physical parameters by temperature and salinity changes, enhancement of the surface roughness, damping of short gravity waves, surface mixing by rain, and transfer of additional momentum from air to sea. They are separately described and included in our surface renewal model to investigate the rain's influence on the cool skin of the ocean and the creation of a haline molecular diffusion layer. Simulations with the upgraded model show that the most important effect on the conductive layer is that of reduced renewal periods followed by additional surface cooling due to rain on the order of 0.1 K. At rain rates below 50 mm h-1 rainfall is not able to completely destroy the mean temperature difference across the cool skin. A freshwater skin is created that exhibits a salinity difference exceeding 4 under strong rainfall. Comparisons with field data of the cool skin taken during the Coupled Ocean Atmosphere Response Experiment confirm the upgraded renewal model. Surface salinity measurements taken during the same field campaign are consistent with the calculated salinity differences across the freshwater skin. The enhancement of surface roughness by natural rain is less pronounced than described in earlier laboratory studies of rain with large drop sizes only. 相似文献
14.
To understand the influence of the Bering Strait on the World Ocean’s circulation, a model sensitivity analysis is conducted.
The numerical experiments are carried out with a global, coupled ice–ocean model. The water transport through the Bering Strait
is parametrized according to the geostrophic control theory. The model is driven by surface fluxes derived from bulk formulae
assuming a prescribed atmospheric seasonal cycle. In addition, a weak restoring to observed surface salinities is applied
to compensate for the global imbalance of the imposed surface freshwater fluxes. The freshwater flux from the North Pacific
to the North Atlantic associated with the Bering Strait throughflow seems to be an important element in the freshwater budget
of the Greenland and Norwegian seas and of the Atlantic. This flux induces a freshening of the North Atlantic surface waters,
which reduces the convective activity and leads to a noticeable (6%) weakening of the thermohaline conveyor belt. It is argued
that the contrasting results obtained by Reason and Power are due to the type of surface boundary conditions they used.
Received: 27 October 1995/Accepted: 20 November 1996 相似文献
15.
A computational scheme is developed for estimating turbulent surface stress, sensible heat flux and humidity flux from mean velocity, temperature and humidity at a single height in the atmospheric surface layer; conditions at this reference level are presumed known from observations or from a numerical atmospheric circulation model. The method is based on coupling a Monin-Obukhov similarity profile to a force-restore formulation for the evolution of surface soil temperature to yield the local values of shear stress, heat flux and surface temperature. A self-contained formulation is presented including parameterizations for solar and infrared radiant flux at the surface.In addition to reference-level mean flow properties, the parameters needed to implement the scheme are thermal heat capacity of the soil, surface aerodynamic roughness, latitude, solar declination, surface albedo, surface emissivity and atmospheric transmissivity.Sample calculations are presented for (a), constant atmospheric forcing at the reference level, and (b) variable atmospheric forcing corresponding to Kahle's (1977) measurements of windspeed, air temperature and radiometer soil surface temperature under dry vegetatively sparse conditions in the Mohave Desert in California. The latter case simulated the observed diurnal variations resonably well for the parameters used.Consultant, Atmospheric Sciences Division, Department of Energy and Environment, Brookhaven National Laboratory, Upton, N.Y., pc11973, U.S.A. 相似文献
16.
A total of 52 years of data (1949–2000) from the NCEP/NCAR reanalysis are used to investigate mechanisms involved in forcing and damping of sea surface temperature (SST) variability in the South Atlantic Ocean. Organized patterns of coupled ocean–atmosphere variability are identified using EOF and SVD analyses. The leading mode of coupled variability consists of an SST pattern with a strong northeast–southwest gradient and an SLP monopole centered at 15°W, 45°S. The anomalous winds associated with this monopole generate the SST pattern through anomalous latent heat flux and mixed layer deepening. Other heat flux components and anomalous Ekman transport play only a secondary role. Once established, the SST pattern is attenuated through latent heat flux. The higher SST modes are also induced by anomalous winds and destroyed by latent heat flux. It thus appears that the coupled variability in the South Atlantic Ocean consists of atmospheric circulation anomalies that induce SST anomalies through anomalous latent heat fluxes and wind-induced mixed layer deepening. These SST anomalies are destroyed by latent heat flux with no detectable systematic feedback onto the atmospheric circulation. Atmospheric variability in the South Atlantic is found to be largely independent of that elsewhere, although there is a weak relation with ENSO (El Niño-Southern Oscillation). 相似文献
17.
A three-dimensional, coupled atmosphere-ocean general circulation model is developed and effects of the seasonal variation
on forming a steady-state coupled atmosphere-ocean system are studied. Case studies are carried out for a basin of idealized
geometry by changing the period of the seasonal variation in the solar forcing. The coupled system shows significant differences
depending on the existence of the seasonal variation, where the heat transport by the oceanic circulation plays a central
role. For the regular seasonal variation, a stronger oceanic meridional circulation, which accompanies larger poleward heat
transport, is realized compared with that for the annual-mean solar forcing. The stronger meridional circulation is associated
with a larger wintertime meridional gradient of the sea surface temperature. The meridional gradient of the annual-mean sea
surface temperature is smaller for the regular seasonal variation, which results in a smaller atmospheric poleward heat transport.
In consequence, the total of the atmospheric and the oceanic poleward heat transport is almost identical for the two cases.
Cases with the seasonal variation of the doubled period and with the time-filtered flux exchange between the atmosphere and
the ocean are also studied, showing that the system is not sensitive to those factors. 相似文献
18.
P. D. Williams E. Guilyardi R. T. Sutton J. M. Gregory G. Madec 《Climate Dynamics》2006,27(6):593-611
Under global warming, the predicted intensification of the global freshwater cycle will modify the net freshwater flux at the ocean surface. Since the freshwater flux maintains ocean salinity structures, changes to the density-driven ocean circulation are likely. A modified ocean circulation could further alter the climate, potentially allowing rapid changes, as seen in the past. The relevant feedback mechanisms and timescales are poorly understood in detail, however, especially at low latitudes where the effects of salinity are relatively subtle. In an attempt to resolve some of these outstanding issues, we present an investigation of the climate response of the low-latitude Pacific region to changes in freshwater forcing. Initiated from the present-day thermohaline structure, a control run of a coupled ocean–atmosphere general circulation model is compared with a perturbation run in which the net freshwater flux is prescribed to be zero over the ocean. Such an extreme experiment helps to elucidate the general adjustment mechanisms and their timescales. The atmospheric greenhouse gas concentrations are held constant, and we restrict our attention to the adjustment of the upper 1,000 m of the Pacific Ocean between 40°N and 40°S, over 100 years. In the perturbation run, changes to the surface buoyancy, near-surface vertical mixing and mixed-layer depth are established within 1 year. Subsequently, relative to the control run, the surface of the low-latitude Pacific Ocean in the perturbation run warms by an average of 0.6°C, and the interior cools by up to 1.1°C, after a few decades. This vertical re-arrangement of the ocean heat content is shown to be achieved by a gradual shutdown of the heat flux due to isopycnal (i.e. along surfaces of constant density) mixing, the vertical component of which is downwards at low latitudes. This heat transfer depends crucially upon the existence of density-compensating temperature and salinity gradients on isopycnal surfaces. The timescale of the thermal changes in the perturbation run is therefore set by the timescale for the decay of isopycnal salinity gradients in response to the eliminated freshwater forcing, which we demonstrate to be around 10–20 years. Such isopycnal heat flux changes may play a role in the response of the low-latitude climate to a future accelerated freshwater cycle. Specifically, the mechanism appears to represent a weak negative sea surface temperature feedback, which we speculate might partially shield from view the anthropogenically-forced global warming signal at low latitudes. Furthermore, since the surface freshwater flux is shown to play a role in determining the ocean’s thermal structure, it follows that evaporation and/or precipitation biases in general circulation models are likely to cause sea surface temperature biases. 相似文献
19.
On the Significance of the Webb Correction to Fluxes 总被引：2，自引：0，他引：2
For establishing correct mass or energy balances at the Earth's surface, detailed and correct measurements of air constituent fluxes are needed. Flux measurements obtained from the eddy covariance technique have to pass several corrections of different relevance in order to give correct flux data. One of these corrections, the Webb correction, is analysed herein from latent heat flux and CO2 flux data recorded during two field experiments. The significance of this correction for the latent heat flux data varies with the air humidity and the Bowen ratio. The correction changes the latent heat flux values only a little, but significantly (by 2 to 3%). For other air constituents (like CO2), the Webb correction is much more important (20 to 30% of the flux). 相似文献
20.
Stabilization of thermohaline circulation by wind-driven and vertical diffusive salt transport 总被引：1，自引：0，他引：1
The stability of the thermohaline circulation is investigated using an ocean general circulation model coupled to a simple
atmospheric model. The atmospheric model is so developed that it represents the wind stress and the freshwater flux more realistically
than existing energy balance models. The coupled model can reproduce the realistic deep ocean circulation without any flux
adjustment. Effects of the wind stress and the vertical diffusion on the thermohaline circulation are studied by conducting
various experiments with the coupled model. The Ekman upwelling between 60∘N and 90∘N brings up salt to the sea surface, while the compensation flow of the Ekman transport and the wind-driven gyre circulation
between 30∘N and 60∘N carry salt horizontally to the high latitudes. By carrying out experiments where the wind stress is completely or partly
removed, it is demonstrated that either of the vertical or the horizontal salt transport prevents the halocline formation
at high latitudes and maintains the thermohaline circulation. For an experiment in which the vertical diffusivity is enhanced
at high latitudes, it is shown that the vertical diffusion at high latitudes also prevents the halocline formation and stabilizes
the thermohaline circulation. It is also shown that the value of the vertical diffusivity at high latitude affects the existence
of the multiple equilibria of the thermohaline circulation.
Received: 26 April 2000 / Accepted: 10 January 2001 相似文献