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1.
The long-term variability of an aquaplanet climate is analyzed with a coupled atmosphere–ocean–sea ice general circulation model. The main result of the 20,000 years simulation is a very dominant low-frequency oscillation with a period of approximately 700 years. All compartments of the aquaplanet (atmosphere, ocean, and sea ice) are involved as the climate alternates between warmer and colder states. Comprehensive time series analyses, as well as a comparison between mean states of cold and warm phases, give a detailed picture of the life cycle of the low-frequency oscillation. The warm phases are characterized by ice-free polar waters and a weaker meridional overturning circulation. During cold phases, the poles are completely covered by sea ice (down to 65° N/S) and the overturning cells in the ocean are stronger. The climate state changes throughout atmosphere and ocean; however, surface areas in high latitudes are especially affected due to the changing sea ice cover. The meridional energy transport in atmosphere and ocean alternates with the climate regime, since the ocean is more efficient in transporting heat poleward when the poles are ice-free.  相似文献   

2.
The stability of the thermohaline circulation of modern and glacial climates is compared with the help of a two dimensional ocean—atmosphere—sea ice coupled model. It turns out to be more unstable as less freshwater forcing is required to induce a polar halocline catastrophy in glacial climates. The large insulation of the ocean by the extensive sea ice cover changes the temperature boundary condition and the deepwater formation regions moves much further South. The nature of the instability is of oceanic origin, identical to that found in ocean models under mixed boundary conditions. With similar strengths of the oceanic circulation and rates of deep water formation for warm and cold climates, the loss of stability of the cold climate is due to the weak thermal stratification caused by the cooling of surface waters, the deep water temperatures being regulated by the temperature of freezing. Weaker stratification with similar overturning leads to a weakening of the meridional oceanic heat transport which is the major negative feedback stabilizing the oceanic circulation. Within the unstable regime periodic millennial oscillations occur spontaneously. The climate oscillates between a strong convective thermally driven oceanic state and a weak one driven by large salinity gradients. Both states are unstable. The atmosphere of low thermal inertia is carried along by the oceanic overturning while the variation of sea ice is out of phase with the oceanic heat content. During the abrupt warming events that punctuate the course of a millennial oscillation, sea ice variations are shown respectively to damp (amplify) the amplitude of the oceanic (atmospheric) response. This sensitivity of the oceanic circulation to a reduced concentration of greenhouse gases and to freshwater forcing adds support to the hypothesis that the millennial oscillations of the last glacial period, the so called Dansgaard—Oeschger events, may be internal instabilities of the climate system.  相似文献   

3.
During the last glacial, major abrupt climate events known as Heinrich events left distinct fingerprints of ice rafted detritus, and are thus associated with iceberg armadas; the release of many icebergs into the North Atlantic Ocean. We simulated the impact of a large armada of icebergs on glacial climate in a coupled atmosphere–ocean model. In our model, dynamic-thermodynamic icebergs influence the climate through two direct effects. First, melting of the icebergs causes freshening of the upper ocean, and second, the latent heat used in the phase-transition of ice to water results in cooling of the iceberg surroundings. This cooling effect of icebergs is generally neglected in models. We investigated the role of the latent heat by performing a sensitivity experiment in which the cooling effect is switched off. At the peak of the simulated Heinrich event, icebergs lacking the latent heat flux are much less efficient in shutting down the meridional overturning circulation than icebergs that include both the freshening and the cooling effects. The cause of this intriguing result must be sought in the involvement of a secondary mechanism: facilitation of sea-ice formation, which can disturb deep water production at key convection sites, with consequences for the thermohaline circulation. We performed additional sensitivity experiments, designed to explore the effect of the more plausible distribution of the dynamic icebergs’ melting fluxes compared to a classic hosing approach with homogeneous spreading of the melt fluxes over a section in the mid-latitude North Atlantic (NA) Ocean. The early response of the climate system is much stronger in the iceberg experiments than in the hosing experiments, which must be a distribution-effect: the dynamically distributed icebergs quickly affect western NADW formation, which synergizes with direct sea-ice facilitation, causing an earlier sea-ice expansion and climatic response. Furthermore, compared to dynamic-thermodynamic icebergs, a homogeneous hosing overestimates the fresh water flux in the Eastern Ruddiman belt, causing a fresh anomaly in the Eastern North Atlantic, leading to a delayed recovery of the circulation after the event.  相似文献   

4.
A number of transient climate runs simulating the last 120?kyr have been carried out using FAMOUS, a fast atmosphere–ocean general circulation model (AOGCM). This is the first time such experiments have been done with a full AOGCM, providing a three-dimensional simulation of both atmosphere and ocean over this period. Our simulation thus includes internally generated temporal variability over periods from days to millennia, and physical, detailed representations of important processes such as clouds and precipitation. Although the model is fast, computational restrictions mean that the rate of change of the forcings has been increased by a factor of 10, making each experiment 12?kyr long. Atmospheric greenhouse gases (GHGs), northern hemisphere ice sheets and variations in solar radiation arising from changes in the Earth’s orbit are treated as forcing factors, and are applied either separately or combined in different experiments. The long-term temperature changes on Antarctica match well with reconstructions derived from ice-core data, as does variability on timescales longer than 10 kyr. Last Glacial Maximum (LGM) cooling on Greenland is reasonably well simulated, although our simulations, which lack ice-sheet meltwater forcing, do not reproduce the abrupt, millennial scale climate shifts seen in northern hemisphere climate proxies or their slower southern hemisphere counterparts. The spatial pattern of sea surface cooling at the LGM matches proxy reconstructions reasonably well. There is significant anti-correlated variability in the strengths of the Atlantic meridional overturning circulation (AMOC) and the Antarctic Circumpolar Current (ACC) on timescales greater than 10?kyr in our experiments. We find that GHG forcing weakens the AMOC and strengthens the ACC, whilst the presence of northern hemisphere ice-sheets strengthens the AMOC and weakens the ACC. The structure of the AMOC at the LGM is found to be sensitive to the details of the ice-sheet reconstruction used. The precessional component of the orbital forcing induces ~20?kyr oscillations in the AMOC and ACC, whose amplitude is mediated by changes in the eccentricity of the Earth’s orbit. These forcing influences combine, to first order, in a linear fashion to produce the mean climate and ocean variability seen in the run with all forcings.  相似文献   

5.
Freshwater (FW) leaves the Arctic Ocean through sea-ice export and the outflow of low-salinity upper ocean water. Whereas the variability of the sea-ice export is known to be mainly caused by changes in the local wind and the thickness of the exported sea ice, the mechanisms that regulate the variability of the liquid FW export are still under investigation. To better understand these mechanisms, we present an analysis of the variability of the liquid FW export from the Arctic Ocean for the period 1950–2007, using a simulation from an energy and mass conserving global ocean–sea ice model, coupled to an Energy Moisture Balance Model of the atmosphere, and forced with daily winds from the NCEP reanalysis. Our results show that the simulated liquid FW exports through the Canadian Arctic Archipelago (CAA) and the Fram Strait lag changes in the large-scale atmospheric circulation over the Arctic by 1 and 6 years, respectively. The variability of the liquid FW exports is caused by changes in the cyclonicity of the atmospheric forcing, which cause a FW redistribution in the Arctic through changes in Ekman transport in the Beaufort Gyre. This in turn causes changes in the sea surface height (SSH) and salinity upstream of the CAA and Fram Strait, which affect the velocity and salinity of the outflow. The SSH changes induced by the large-scale atmospheric circulation are found to explain a large part of the variance of the liquid FW export, while the local wind plays a much smaller role. We also show that during periods of increased liquid FW export from the Arctic, the strength of the simulated Atlantic meridional overturning circulation is reduced and the ocean heat transport into the Arctic is increased. These results are particularly relevant in the context of global warming, as climate simulations predict an increase in the liquid FW export from the Arctic during the twenty-first century.  相似文献   

6.
The interplay between the North Atlantic Oscillation (NAO) and the large scale ocean circulation is inspected in a twentieth century simulation conducted with a state-of-the-art coupled general circulation model. Significant lead–lag covariance between oceanic and tropospheric variables suggests that the system supports a damped oscillatory mode involving an active ocean–atmosphere coupling, with a typical NAO-like space structure and a 5 years timescale, qualitatively consistent with a mid-latitude delayed oscillator paradigm. The two essential processes governing the oscillation are (1) a negative feedback between ocean gyre circulation and the high latitude SST meridional gradient and (2) a positive feedback between SST and the NAO. The atmospheric NAO pattern appears to have a weaker projection on the ocean meridional overturning, compared to the gyre circulation, which leads to a secondary role for the thermohaline circulation in driving the meridional heat transport, and thus the oscillatory mode.  相似文献   

7.
A simple coupled ocean, atmosphere and sea-ice model is presented. The idealised model consists of a zonally averaged land and ocean strip of constant angular width extending from pole to pole. The meridional energy transport in the ocean is modelled by contributions from the large scale thermohaline overturning cells and from horizontal diffusive fluxes. The atmospheric meridional energy transports are parametrised as diffusive fluxes in addition to advective transports in the Hadley domain. This parametrisation resolves the equatorward moisture transport as well as the poleward transport of potential energy in the upper branch of the Hadley circulation. The model reproduces the annual averaged meridional energy transports in the climate system with a small number of free model parameters. The basic feedbacks between the three climatic components are studied by investigating the model's sensitivity towards reductions in the solar insolation. It is found that the meridional energy transport in the ocean does not amplify the ice albedo feedback. This has important implications for modelling the climate sensitivity in atmosphere-only models, as these would exaggerate the sensitivity to changes in the solar insolation if their parametrisations of the meridional energy transport are constrained by surface temperatures. The role of the dependence of the atmospheric transports on the meridional temperature gradient is shown to have a significant influence on the sensitivity on the coupled model, and the inclusion of seasonal cycles greatly increase the models sensitivity. The Hadley circulation does significantly alter the strength of the ice-albedo feedback in the coupled model. The idealised configuration of the model makes it a useful tool for studying the feedbacks in the ocean-atmosphere-sea ice system in the context of the "Snowball Earth" hypothesis.  相似文献   

8.
Three different reconstructed wind-stress fields which take into account variations of the North Atlantic Oscillation, one general circulation model wind-stress field, and three radiative forcings (volcanic activity, insolation changes and greenhouse gas changes) are used with the UVic Earth System Climate Model to simulate the surface air temperature, the sea-ice cover, and the Atlantic meridional overturning circulation (AMOC) since 1500, a period which includes the Little Ice Age (LIA). The simulated Northern Hemisphere surface air temperature, used for model validation, agrees well with several temperature reconstructions. The simulated sea-ice cover in each hemisphere responds quite differently to the forcings. In the Northern Hemisphere, the simulated sea-ice area and volume during the LIA are larger than the present-day area and volume. The wind-driven changes in sea-ice area are about twice as large as those due to thermodynamic (i.e., radiative) forcing. For the sea-ice volume, changes due to wind forcing and thermodynamics are of similar magnitude. Before 1850, the simulations suggest that volcanic activity was mainly responsible for the thermodynamically produced area and volume changes, while after 1900 the slow greenhouse gas increase was the main driver of the sea-ice changes. Changes in insolation have a small effect on the sea ice throughout the integration period. The export of the thicker sea ice during the LIA has no significant effect on the maximum strength of the AMOC. A more important process in altering the maximum strength of the AMOC and the sea-ice thickness is the wind-driven northward ocean heat transport. In the Southern Hemisphere, there are no visible long-term trends in the simulated sea-ice area or volume since 1500. The wind-driven changes are roughly four times larger than those due to radiative forcing. Prior to 1800, all the radiative forcings could have contributed to the thermodynamically driven changes in area and volume. In the 1800s the volcanic forcing was dominant, and during the first part of the 1900s both the insolation changes and the greenhouse gas forcing are responsible for thermodynamically produced changes. Finally, in the latter part of the 1900s the greenhouse gas forcing is the dominant factor in determining the sea-ice changes in the Southern Hemisphere.
Jan SedláčekEmail:
  相似文献   

9.
The recent decline in Arctic sea-ice cover (SIC) shows seasonal and regional characteristics. The retreat of summer sea ice has occurred mainly in the Pacific sector of the Arctic. In this study, using the moving t-test, we found an abrupt change event in the long-term sea-ice area in the Pacific sector in summer 1989. This event was linked to the phase shift of the Arctic Oscillation (AO) or the Northern Annular Mode (NAM). Corresponding with the AO/NAM phase shift from negative to positive, the area of the northern hemisphere stratospheric polar vortex decreased abruptly in winter 1988/89. Comparisons of two periods before (1979–1988) and after (1989–1993) the abrupt decrease in sea ice show that an anomalous winter sea level pressure (SLP) was induced by changes in the polar vortex leading to an anomalous cyclonic ice drift in the Pacific sector. The changes in SLP and wind field persisted into the following spring, resulting in a decrease in SIC and warming of the surface air temperature (SAT). The influence of the spring SLP and SAT on ice persisted into the following summer. Meanwhile, the increased summer net surface heat flux over the ocean and sea ice as a result of the decreased spring ice cover further contributed to the summer sea-ice melt.  相似文献   

10.
It has recently been suggested that the structure and strength of the meridional overturning circulation in the global ocean is governed by the input of mechanical energy to the system by winds and tides. However, it is not clear how this suggestion relates to the existence of multiple equilibria of the meridional overturning circulation, which depends on thermohaline feedbacks and is more consistent with a buoyancy-driven view of the circulation. Both theories have been illustrated by box models in the past (Stommel in Tellus 13:224–230, 1961; Gnanadesikan in Science 283:2077–2079, 1999). Here we incorporate these two theories into a single box model in an attempt to reconcile the roles of mechanical and buoyancy forcing in driving the meridional overturning circulation. The box model has two equilibrium solutions, one with sinking at high northern latitudes as in the present-day Atlantic, and one without. The circulation is mechanically driven, but the northern sinking can be thought of as a release valve which acts as a sink of potential energy when the surface water at high northern latitudes is dense enough to convect. While the source of energy comes from mechanical forcing, the presence or otherwise of multiple equilibria is therefore determined by thermohaline feedbacks. In some areas of parameter space an oscillation between the model’s two circulation regimes occurs, reminiscent of a bipolar seesaw.  相似文献   

11.
We use the coupled atmosphere–ocean general circulation model ECHAM5/MPI-OM to investigate the transition from the present-day climate to a modern Snowball Earth, defined as the Earth in modern geography with complete sea-ice cover. Starting from the present-day climate and applying an abrupt decrease of total solar irradiance (TSI) we find that the critical TSI marking the Snowball Earth bifurcation point is between 91 and 94% of the present-day TSI. The Snowball Earth bifurcation point as well as the transition times are well reproduced by a zero-dimensional energy balance model of the mean ocean potential temperature. During the transition, the asymmetric distribution of continents between the Northern and Southern Hemisphere causes heat transports toward the more water-covered Southern Hemisphere. This is accompanied by an intensification of the southern Hadley cell and the wind-driven subtropical ocean cells by a factor of 4. If we set back TSI to 100% shortly before the transition to a modern Snowball Earth is completed, a narrow band of open equatorial water is sufficient for rapid melting. This implies that for 100% TSI the point of unstoppable glaciation separating partial from complete sea-ice cover is much closer to complete sea-ice cover than in classical energy balance models. Stable states can have no greater than 56.6% sea-ice cover implying that ECHAM5/MPI-OM does not exhibit stable states with near-complete sea-ice cover but open equatorial waters.  相似文献   

12.
Summary In this paper a simple climate model is presented which is used to perform some sensitivity experiments. The atmospheric part is represented by a vertically and zonally averaged layer in which the surface air temperature, radiative fluxes at the surface and at the top of the atmosphere, the turbulent fluxes between atmosphere and surface and the snow cover are calculated. This atmospheric layer is coupled to a two-dimensional advection-diffusion ocean model in which the zonal overturning pattern is prescribed. The ocean model evaluates the temperature distribution, the amount of sea-ice and the meridional and vertical heat fluxes. The present-day climate simulated by the model compares reasonably well with observations of the seasonal and latitudinal distribution of temperature, radiation, surface alebdo, sea-ice and snow cover and meridional energy fluxes. Then, the sensitivity of the model-simulated present-day climate to perturbations in the incident solar radiation at the top of the atmosphere is investigated. The temperature response displays large latitudinal and seasonal variations, which is in qualitative agreement with results obtained with other climate models. It is found that the seasonal variation of sea-ice cover (and hence, the effective oceanic heat capacity) is one of the most important elements determining seasonal variations in climate sensitivity. Differences in sensitivity between the seasonal and annual mean version of the model are discussed. Finally, the equilibrium response to perturbations in some selected model variables is presented; these variables include meridional diffusion coefficients, drag coefficient, sea-ice thickness, atmospheric CO2-concentration and cloud optical thickness.With 13 Figures  相似文献   

13.
The Community Climate System Model version 3, (CCSM3) is used to investigate the effect of the high latitude North Atlantic subsurface ocean temperature response in idealized freshwater hosing experiments on the strength of the Atlantic meridional overturning circulation (AMOC). The hosing experiments covered a range of input magnitudes at two locations in a glacial background state. Subsurface subpolar ocean warms when freshwater is added to the high latitude North Atlantic (NATL cases) and weakly cools when freshwater is added to the Gulf of Mexico (GOM cases). All cases show subsurface ocean warming in the Southern Hemisphere (SH). The sensitivity of the AMOC response to the location and magnitude of hosing is related to the induced subsurface temperature response, which affects the magnitude of the large-scale meridional pressure gradient at depth through the effect on upper ocean density. The high latitude subsurface warming induced in the NATL cases lowers the upper ocean density in the deepwater formation region enhancing a density reduction by local freshening. In the GOM cases the effect of SH warming partially offsets the effect of the high latitude freshening on the meridional density gradient. Following the end of hosing, a brief convective event occurs in the largest NATL cases which flushes some of the heat stored in the subsurface layers. This fuels a rapid rise in AMOC that lasts less than a couple of decades before subsequent freshening from increases in precipitation and sea ice melt reverses the initial increase in the meridional density gradient. Thereafter AMOC recovery slows to the rate found in comparable GOM cases. The result for these glacial transient hosing experiments is that the pace of the longer recovery is not sensitive to location of the imposed freshwater forcing.  相似文献   

14.
A suite of decadal predictions has been conducted with the NASA Global Modeling and Assimilation Office’s (GMAO’s) GEOS-5 Atmosphere–Ocean general circulation model. The hind casts are initialized every December 1st from 1959 to 2010, following the CMIP5 experimental protocol for decadal predictions. The initial conditions are from a multi-variate ensemble optimal interpolation ocean and sea-ice reanalysis, and from GMAO’s atmospheric reanalysis, the modern-era retrospective analysis for research and applications. The mean forecast skill of a three-member-ensemble is compared to that of an experiment without initialization but also forced with observed greenhouse gases. The results show that initialization increases the forecast skill of North Atlantic sea surface temperature compared to the uninitialized runs, with the increase in skill maintained for almost a decade over the subtropical and mid-latitude Atlantic. On the other hand, the initialization reduces the skill in predicting the warming trend over some regions outside the Atlantic. The annual-mean atlantic meridional overturning circulation index, which is defined here as the maximum of the zonally-integrated overturning stream function at mid-latitude, is predictable up to a 4-year lead time, consistent with the predictable signal in upper ocean heat content over the North Atlantic. While the 6- to 9-year forecast skill measured by mean squared skill score shows 50 % improvement in the upper ocean heat content over the subtropical and mid-latitude Atlantic, prediction skill is relatively low in the subpolar gyre. This low skill is due in part to features in the spatial pattern of the dominant simulated decadal mode in upper ocean heat content over this region that differ from observations. An analysis of the large-scale temperature budget shows that this is the result of a model bias, implying that realistic simulation of the climatological fields is crucial for skillful decadal forecasts.  相似文献   

15.
A change in a sea-ice parameter in a global coupled climate model results in a reduction in amplitude (of about 60%) and a shortening of the predominant period of decadal low frequency variability in the time series of globally averaged surface air temperature. These changes are global in extent and also are reflected in time series of area-averaged SSTs in the equatorial eastern Pacific Ocean, the principal components of the first EOFs of global surface air temperature and sea level pressure, Asian monsoon precipitations and other quantities. Coupled ocean-atmosphere-sea ice processes acting on a global scale are modified to produce these changes. Global climate sensitivity is reduced when ice albedo feedback is weakened due to the change in sea ice that makes it more difficult to melt. The changes in the amplitude and time scale of the low frequency variability in the model are traced to changes in the base state of the climate simulations as affected by modifications associated with the changes in sea ice. Making sea ice more difficult to melt results in increased sea-ice area, colder high latitudes, increased meridional surface temperature gradients, and, to a first order, stronger surface winds in most regions which strengthen near-surface currents, particularly in the Northern Hemisphere, and decreases the advection time scale in the upper ocean gyres. Additionally, in the North Atlantic there is enhanced meridional overturning due to increased density mainly in the Greenland Sea region. This also contributes to an intensified North Atlantic gyre. The changes in base state due to the sea ice change result in a more predominant decadal time scale of near 14 years and significantly reduced contributions from lower frequencies in the range of 15–40 year periods. Received: 11 December 1998 / Accepted: 4 October 1999  相似文献   

16.
An ocean–atmosphere–sea ice model is developed to explore the time-dependent response of climate to Milankovitch forcing for the time interval 5–3 Myr BP. The ocean component is a zonally averaged model of the circulation in five basins (Arctic, Atlantic, Indian, Pacific, and Southern Oceans). The atmospheric component is a one-dimensional (latitudinal) energy balance model, and the sea-ice component is a thermodynamic model. Two numerical experiments are conducted. The first experiment does not include sea ice and the Arctic Ocean; the second experiment does. Results from the two experiments are used to investigate (1) the response of annual mean surface air and ocean temperatures to Milankovitch forcing, and (2) the role of sea ice in this response. In both experiments, the response of air temperature is dominated by obliquity cycles at most latitudes. On the other hand, the response of ocean temperature varies with latitude and depth. Deep water formed between 45°N and 65°N in the Atlantic Ocean mainly responds to precession. In contrast, deep water formed south of 60°S responds to obliquity when sea ice is not included. Sea ice acts as a time-integrator of summer insolation changes such that annual mean sea-ice conditions mainly respond to obliquity. Thus, in the presence of sea ice, air temperature changes over the sea ice are amplified, and temperature changes in deep water of southern origin are suppressed since water below sea ice is kept near the freezing point.  相似文献   

17.
Greenland ice cores, as well as many other paleo-archives from the northern hemisphere, recorded a series of 25 warm interstadial events, the so-called Dansgaard-Oeschger (D-O) events, during the last glacial period. We use the three-dimensional coupled global ocean–atmosphere–sea ice model ECBILT-CLIO and force it with freshwater input into the North Atlantic to simulate abrupt glacial climate events, which we use as analogues for D-O events. We focus our analysis on the Northern Hemisphere. The simulated events show large differences in the regional and seasonal distribution of the temperature and precipitation changes. While the temperature changes in high northern latitudes and in the North Atlantic region are dominated by winter changes, the largest temperature increases in most other land regions are seen in spring. Smallest changes over land are found during the summer months. Our model simulations also demonstrate that the temperature and precipitation change patterns for different intensifications of the Atlantic meridional overturning circulation are not linear. The extent of the transitions varies, and local non-linearities influence the amplitude of the annual mean response as well as the response in different seasons. Implications for the interpretation of paleo-records are discussed.  相似文献   

18.
The significance of the Atlantic meridional overturning circulation (MOC) for regional and hemispheric climate change requires a complete understanding using fully coupled climate models. Here we present a persistent, decadal oscillation in a coupled atmosphere–ocean general circulation model. While the present study is limited by the lack of comparisons with paleo-proxy records, the purpose is to reveal a new theoretically interesting solution found in the fully-coupled climate model. The model exhibits two multi-century-long stable states with one dominated by decadal MOC oscillations. The oscillations involve an interaction between anomalous advective transport of salt and surface density in the North Atlantic subpolar gyre. Their time scale is fundamentally determined by the advection. In addition, there is a link between the MOC oscillations and North Atlantic Oscillation (NAO)-like sea level pressure anomalies. The analysis suggests an interaction between the NAO and an anomalous subpolar gyre circulation in which sea ice near and south of the Labrador Sea plays an important role in generating a large local thermal anomaly and a meridional temperature gradient. The latter induces a positive feedback via synoptic eddy activity in the atmosphere. In addition, the oscillation only appears when the Nordic Sea is completely covered by sea ice in winter, and deep convection is active only near the Irminger Sea. Such conditions are provided by a substantially colder North Atlantic climate than today.  相似文献   

19.
A box model of the inter-hemispheric Atlantic meridional overturning circulation is developed, including a variable pycnocline depth for the tropical and subtropical regions. The circulation is forced by winds over a periodic channel in the south and by freshwater forcing at the surface. The model is aimed at investigating the ocean feedbacks related to perturbations in freshwater forcing from the atmosphere, and to changes in freshwater transport in the ocean. These feedbacks are closely connected with the stability properties of the meridional overturning circulation, in particular in response to freshwater perturbations. A separate box is used for representing the region north of the Antarctic circumpolar current in the Atlantic sector. The density difference between this region and the north of the basin is then used for scaling the downwelling in the north. These choices are essential for reproducing the sensitivity of the meridional overturning circulation observed in general circulation models, and therefore suggest that the southernmost part of the Atlantic Ocean north of the Drake Passage is of fundamental importance for the stability of the meridional overturning circulation. With this configuration, the magnitude of the freshwater transport by the southern subtropical gyre strongly affects the response of the meridional overturning circulation to external forcing. The role of the freshwater transport by the overturning circulation (M ov ) as a stability indicator is discussed. It is investigated under which conditions its sign at the latitude of the southern tip of Africa can provide information on the existence of a second, permanently shut down, state of the overturning circulation in the box model. M ov will be an adequate indicator of the existence of multiple equilibria only if salt-advection feedback dominates over other processes in determining the response of the circulation to freshwater anomalies. M ov is a perfect indicator if feedbacks other than salt-advection are negligible.  相似文献   

20.
A coupled atmosphere-ocean-sea ice model is applied to investigate to what degree the area-thickness distribution of new ice formed in open water affects the ice and ocean properties. Two sensitivity experiments are performed which modify the horizontal-to-vertical aspect ratio of open-water ice growth. The resulting changes in the Arctic sea-ice concentration strongly affect the surface albedo, the ocean heat release to the atmosphere, and the sea-ice production. The changes are further amplified through a positive feedback mechanism among the Arctic sea ice, the Atlantic Meridional Overturning Circulation (AMOC), and the surface air temperature in the Arctic, as the Fram Strait sea ice import influences the freshwater budget in the North Atlantic Ocean. Anomalies in sea-ice transport lead to changes in sea surface properties of the North Atlantic and the strength of AMOC. For the Southern Ocean, the most pronounced change is a warming along the Antarctic Circumpolar Current (ACC), owing to the interhemispheric bipolar seasaw linked to AMOC weakening. Another insight of this study lies on the improvement of our climate model. The ocean component FESOM is a newly developed ocean-sea ice model with an unstructured mesh and multi-resolution. We find that the subpolar sea-ice boundary in the Northern Hemisphere can be improved by tuning the process of open-water ice growth, which strongly influences the sea ice concentration in the marginal ice zone, the North Atlantic circulation, salinity and Arctic sea ice volume. Since the distribution of new ice on open water relies on many uncertain parameters and the knowledge of the detailed processes is currently too crude, it is a challenge to implement the processes realistically into models. Based on our sensitivity experiments, we conclude a pronounced uncertainty related to open-water sea ice growth which could significantly affect the climate system sensitivity.  相似文献   

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