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1.
We have developed a new method to accelerate tracer simulations to steady-state in a 3-D global ocean model, run off-line.
Using this technique, our simulations for natural 14C ran 17 times faster when compared to those made with the standard non-accelerated approach. For maximum acceleration we
wish to initialize the model with tracer fields that are as close as possible to the final equilibrium solution. Our initial
tracer fields were derived by judiciously constructing a much faster, lower-resolution (degraded), off-line model from advective
and turbulent fields predicted from the parent on-line model, an ocean general circulation model (OGCM). No on-line version
of the degraded model exists; it is based entirely on results from the parent OGCM. Degradation was made horizontally over
sets of four adjacent grid-cell squares for each vertical layer of the parent model. However, final resolution did not suffer
because as a second step, after allowing the degraded model to reach equilibrium, we used its tracer output to re-initialize
the parent model (at the original resolution). After re-initialization, the parent model must then be integrated only to a
few hundred years before reaching equilibrium. To validate our degradation-integration technique (DEGINT), we compared 14C results from runs with and without this approach. Differences are less than 10‰ throughout 98.5% of the ocean volume. Predicted
natural 14C appears reasonable over most of the ocean. In the Atlantic, modeled Δ 14C indicates that as observed, the North Atlantic Deep Water (NADW) fills the deep North Atlantic, and Antartic Intermediate
Water (AAIW) infiltrates northward; conversely, simulated Antarctic Bottom Water (AABW) does not penetrate northward beyond
the equator as it should. In the Pacific, in surface eastern equatorial waters, the model produces a north–south assymetry
similar to that observed; other global ocean models do not, because their resolution is inadequate to resolve equatorial dynamics
properly, particularly the intense equatorial undercurrent. The model’s oldest water in the deep Pacific (at −239‰) is close
to that observed (−248‰), but is too deep. Surface waters in the Southern Ocean are too rich in natural 14C due to inadequacies in the OGCM’s thermohaline forcing.
Received: 18 March 1997 / Accepted: 27 July 1997 相似文献
2.
We use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the Southern Ocean and North Atlantic and the Atlantic meridional overturning circulation. In several experiments, we artificially modify the meridional density gradients by applying different magnitudes of the Gent–McWilliams isopycnal eddy diffusion coefficients in the Southern Ocean and in the North Atlantic and investigate the response of the simulated Atlantic meridional overturning to such changes. The simulations are carried out close to the limit of no diapycnal mixing, with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities. Our results reveal that changes in eddy diffusivities in the North Atlantic affect the maximum of the Atlantic meridional overturning, but not the outflow of North Atlantic Deep Water into the Southern Ocean. In contrast, changes in eddy diffusivities in the Southern Ocean affect both the South Atlantic outflow of North Atlantic Deep Water and the maximum of the Atlantic meridional overturning. Results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the Atlantic meridional overturning. Pressure gradients and overturning are found to be linearly related. We show that, in our simulations, zonally averaged deep pressure gradients are very weak between 20°S and about 30°N and that between 30°N and 60°N the zonally averaged pressure grows approximately linearly with latitude. This pressure difference balances a westward geostrophic flow at 30–40°N that feeds the southbound deep Atlantic western boundary current. We extend our analysis to a large variety of experiments in which surface freshwater forcing, vertical mixing and winds are modified. In all experiments, the pycnocline depth, assumed to be the relevant vertical scale for the northward volume transport in the Atlantic, is found to be approximately constant, at least within the coarse vertical resolution of the model. The model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of Atlantic meridional flow, and seems conceptually closer to Stommel’s box model. In all our simulations, the Atlantic overturning seems to be mainly driven by Southern Ocean westerlies. However, the actual strength of the Atlantic meridional overturning is not determined solely by the Southern Ocean wind stress but as well by the density/pressure gradients created between the deep water formation regions in the North Atlantic and the inflow/outflow region in the South Atlantic. 相似文献
3.
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
4.
The Atlantic meridional overturning circulation (AMOC) in a 600?years pre-industrial run of the newly developed EC-EARTH model features marked interdecadal variability with a dominant time-scale of 50–60?years. An oscillation of approximately 2 Sverdrup (1?Sv?=?10 6?m 3?s ?1) is identified, which manifests itself as a monopole causing the overturning to simultaneously strengthen (/weaken) and deepen (/shallow) as a whole. Eight years before the AMOC peaks, density in the Labrador-Irminger Sea region reaches a maximum, triggering deep water formation. This density change is caused by a counterclockwise advection of temperature and salinity anomalies at lower latitudes, which we relate to the north-south excursions of the subpolar-subtropical gyre boundary and variations in strength and position of the subpolar gyre and the North Atlantic Current. The AMOC fluctuations are not directly forced by the atmosphere, but occur in a delayed response of the ocean to forcing by the North Atlantic Oscillation, which initiates “intergyre”-gyre fluctuations. Associated with the AMOC is a 60-year sea surface temperature variability in the Atlantic, with a pattern and timescale showing similarities with the real-world Atlantic Multidecadal Variability. This good agreement with observations lends a certain degree of credibility that the mechanism that is described in this article could be seen as representative of the real climate system. 相似文献
5.
The South Atlantic response to a collapse of the North Atlantic meridional overturning circulation (AMOC) is investigated in the ECHAM5/MPI-OM climate model. A reduced Agulhas leakage (about 3.1?Sv; 1?Sv?=?10 6?m 3?s ?1) is found to be associated with a weaker Southern Hemisphere (SH) supergyre and Indonesian throughflow. These changes are due to reduced wind stress curl over the SH supergyre, associated with a weaker Hadley circulation and a weaker SH subtropical jet. The northward cross-equatorial transport of thermocline and intermediate waters is much more strongly reduced than Agulhas leakage in relation with an AMOC collapse. A cross-equatorial gyre develops due to an anomalous wind stress curl over the tropics that results from the anomalous sea surface temperature gradient associated with reduced ocean heat transport. This cross-equatorial gyre completely blocks the transport of thermocline waters from the South to the North Atlantic. The waters originating from Agulhas leakage flow somewhat deeper and most of it recirculates in the South Atlantic subtropical gyre, leading to a gyre intensification. This intensification is consistent with the anomalous surface cooling over the South Atlantic. Most changes in South Atlantic circulation due to global warming, featuring a reduced AMOC, are qualitatively similar to the response to an AMOC collapse, but smaller in amplitude. However, the increased northward cross-equatorial transport of intermediate water relative to thermocline water is a strong fingerprint of an AMOC collapse. 相似文献
6.
To stimulate a discussion on the role of tropical atmospheric circulation versus thermohaline circulation changes for tropical Atlantic sea-surface temperature (SST) variations, we present a record of the SST contrast (SST) between the tropical northwest and southeast Atlantic from the Last Glacial Maximum to the Late Holocene. The SST was calculated from two alkenone-derived SST records; one from the Caribbean Sea and the other from the Angola Basin. Changes in the cross-equatorial SST were then compared with an abundance record of Florisphaera profunda from the equatorial Atlantic, which is indicative of SE trade-wind induced variations in thermocline depth in the equatorial divergence zone. This comparison implies that the Last Glacial Maximum, the Younger Dryas, and the Mid to Late Holocene were periods of strong SE trade winds, which led to an intense upwelling-related cooling in the southeast Atlantic and concurrently enhanced advection of warm tropical South Atlantic waters into the western tropical Atlantic. Accordingly, a coupled ocean-atmospheric process has probably created a dipole-like SST distribution pattern in the tropical Atlantic during these three distinct climatic periods. In contrast, Heinrich Event 1, the Bølling-Allerød, and the Early Holocene were intervals of weakened SE trade winds, causing a warming in the southeast Atlantic. However, synchronous warming in both regions during Heinrich Event 1 can be partially attributed to a weakening of thermohaline overturning which caused a reduced northward heat transport from the low-latitude to the high-latitude North Atlantic. 相似文献
7.
The McGill Paleoclimate Model-2 (MPM-2) is employed to study climate–thermohaline circulation (THC) interactions in a pre
-industrial climate, with a special focus on the feedbacks on the THC from other climate system components. The MPM-2, a new
version of the MPM, has an extended model domain from 90S to 90N, active winds and no oceanic heat and freshwater flux adjustments.
In the MPM-2, there are mainly two stable modes for the Atlantic meridional overturning circulation (MOC) under the ‘present-day’
forcing (present-day solar forcing and the pre-industrial atmospheric CO 2 level of 280 ppm). The ‘on’ mode has an active North Atlantic deep water formation, while the ‘off’ mode has no such deep
water formation. By comparing the ‘off’ mode climate state with its ‘on’ mode analogue, we find that there exist many large
differences between the two climate states, which originate from large changes in the oceanic meridional heat transports.
By suppressing or isolating each process associated with a continental ice sheet over North America, sea ice, the atmospheric
hydrological cycle and vegetation, feedbacks from these components on the Atlantic MOC are investigated. Sensitivity studies
investigating the role of varying continental ice growth and sea ice meridional transport in the resumption of the Atlantic
MOC are also carried out. The results show that a fast ice sheet growth and an enhanced southward sea ice transport significantly
favor the resumption of the Atlantic MOC in the MPM-2. In contrast to this, the feedback from the atmospheric hydrological
cycle is a weak positive one. The vegetation-albedo feedback could enhance continental ice sheet growth and thus could also
favor the resumption of the Atlantic MOC. However, before the shut-down of the Atlantic MOC, feedbacks from these components
on the Atlantic MOC are very weak. 相似文献
8.
Seawater property changes in the North Atlantic Ocean affect the Atlantic meridional overturning circulation (AMOC), which transports warm water northward from the upper ocean and contributes to the temperate climate of Europe, as well as influences climate globally. Previous observational studies have focused on salinity and freshwater variability in the sinking region of the North Atlantic, since it is believed that a freshening North Atlantic basin can slow down or halt the flow of the AMOC. Here we use available data to show the importance of how density patterns over the upper ocean of the North Atlantic affect the strength of the AMOC. For the long-term trend, the upper ocean of the subpolar North Atlantic is becoming cooler and fresher, whereas the subtropical North Atlantic is becoming warmer and saltier. On a multidecadal timescale, the upper ocean of the North Atlantic has generally been warmer and saltier since 1995. The heat and salt content in the subpolar North Atlantic lags that in the subtropical North Atlantic by about 8–9 years, suggesting a lower latitude origin for the temperature and salinity anomalies. Because of the opposite effects of temperature and salinity on density for both long-term trend and multidecadal timescales, these variations do not result in a density reduction in the subpolar North Atlantic for slowing down the AMOC. Indeed, the variations in the meridional density gradient between the subpolar and subtropical North Atlantic Ocean suggest that the AMOC has become stronger over the past five decades. These observed results are supported by and consistent with some oceanic reanalysis products. 相似文献
9.
参照Griffies et al.(2009)提出的海洋—海冰耦合模式参考试验(Coordinated Ocean-ice Reference Experiments,COREs),设计了一个800年积分的数值试验,对一个质量严格守恒的压力坐标海洋环流模式(Pressure Coordinate Ocean Model,PCOM1.0)的基本模拟性能进行了评估,并与观测资料和再分析资料进行了对比。结果表明,PCOM1.0模拟的温盐场和基本流场与COREs模式的模拟水平基本接近。其中,模拟的大西洋经向翻转流在45°N附近达到18 Sv(1 Sv=106 m3 s-1),与观测估计值接近;对海表面温度的模拟误差主要集中在北太平洋黑潮区和北大西洋湾流区等中高纬度急流区;模拟的热带太平洋温跃层过于深厚;模拟的经德雷克海峡的体积输送达130 Sv,比大部分COREs模式及再分析资料都更接近于观测估计值。 相似文献
10.
Summary ?This paper presents an objective analysis of the structure of daily rainfall variability over the South American/South Atlantic
region (15°–60° W and 0°–40° S) during individual austral summer months of November to March. From EOF analysis of satellite
derived daily rainfall we find that the leading mode of variability is represented by a highly coherent meridional dipole
structure, organised into 2 extensive bands, oriented northwest to southeast across the continent and Atlantic Ocean. We argue
that this dipole structure represents variability in the meridional position of the South Atlantic Convergence Zone (SACZ).
During early and later summer, in the positive (negative) phase of the dipole, enhanced (suppressed) rainfall over eastern
tropical Brazil links with that over the subtropical and extra-tropical Atlantic and is associated with suppressed (enhanced)
rainfall over the sub-tropical plains and adjacent Atlantic Ocean. This structure is indicative of interaction between the
tropical, subtropical and temperate zones. Composite fields from NCEP reanalysis products (associated with the major positive
and negative events) show that in early and late summer the position of the SACZ is associated with variability in: (a) the
midlatitude wave structure, (b) the position of the continental low, and (c) the zonal position of the South Atlantic Subtropical
High. Harmonic analysis of the 200 hPa geopotential anomaly structure in the midlatitudes indicates that reversals in the
rainfall dipole structure are associated primarily with variability in zonal wave 4. There is evidence of a wave train extending
throughout the midlatitudes from the western Pacific into the SACZ region. During positive (negative) events the largest anomalous
moisture advection occurs within westerlies (easterlies) primarily from Amazonia (the South Atlantic). In both phases a convergent
poleward flow results along the leading edge of the low-level trough extending from the tropics into temperate latitudes.
High summer events differ from those in early and late summer in that the rainfall dipole is primarily associated with variability
in the phase of zonal wave 3, and that tropical-temperate link is not clearly evident in positive events.
Received May 31, 2001; revised October 17, 2001; accepted June 13, 2002 相似文献
11.
We investigate the formation process and pathways of deep water masses in a coupled ice–ocean model of the Arctic and North
Atlantic Oceans. The intent is to determine the relative roles of these water masses from the different source regions (Arctic
Ocean, Nordic Seas, and Subpolar Atlantic) in the meridional overturning circulation. The model exhibits significant decadal
variability in the deep western boundary current and the overturning circulation. We use detailed diagnostics to understand
the process of water mass formation in the model and the resulting effects on the North Atlantic overturning circulation.
Particular emphasis is given to the multiple sources of North Atlantic Deep Water, the dominant deep water masses of the world
ocean. The correct balance of Labrador Sea, Greenland Sea and Norwegian Sea sources is difficult to achieve in climate models,
owing to small-scale sinking and convection processes. The global overturning circulation is described as a function of potential
temperature and salinity, which more clearly signifies dynamical processes and clarifies resolution problems inherent to the
high latitude oceans. We find that fluxes of deep water masses through various passages in the model are higher than observed
estimates. Despite the excessive volume flux, the Nordic Seas overflow waters are diluted by strong mixing and enter the Labrador
Sea at a lighter density. Through strong subpolar convection, these waters along with other North Atlantic water masses are
converted into the densest waters [similar density to Antarctic Bottom Water (AABW)] in the North Atlantic. We describe the
diminished role of salinity in the Labrador Sea, where a shortage of buoyant surface water (or excess of high salinity water)
leads to overly strong convection. The result is that the Atlantic overturning circulation in the model is very sensitive
to the surface heat flux in the Labrador Sea and hence is correlated with the North Atlantic Oscillation. As strong subpolar
convection is found in other models, we discuss broader implications. 相似文献
12.
Summary Tropical ocean thermocline variability is studied using gridded data assimilated by an ocean model in the period 1950–2000.
The dominant patterns and variability are identified using EOF analysis applied to E–W depth slices of sea temperatures averaged
over the tropics. After removing the annual cycle, an east–west ‘see-saw’ with an interannual to decadal rhythm is the leading
mode in each of the tropical basins. In the case of the leading mode in the Pacific, the thermocline oscillation forms a dipole
structure, but in the (east) Atlantic and (southwest) Indian Ocean there is a single center of action. The interaction of
the ocean thermocline and atmospheric Walker circulations is studied through cross-modulus analysis of wavelet-filtered EOF
time scores.
Our study demonstrates how tropical ocean thermocline variability contributes to zonal circulation anomalies in the atmosphere.
The equatorial Pacific thermocline oscillation explains 62 and 53% of the variability of the Pacific and Atlantic zonal overturning
circulations, the latter driving convective polarity between North Africa and South America. The Pacific sea-saw leads the
Atlantic zonal circulation by a few months. 相似文献
13.
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
14.
The stability of the Atlantic thermohaline circulation against meltwater input is investigated in a coupled ocean-atmosphere
general circulation model. The meltwater input to the Labrador Sea is increased linearly for 250 years to a maximum input
of 0.625 Sv and then reduced again to 0 (both instantaneously and linearly decreasing over 250 years). The resulting freshening
forces a shutdown of the formation of North Atlantic deepwater and a subsequent reversal of the thermohaline circulation of
the Atlantic, filling the deep Atlantic with Antarctic bottom water. The change in the overturning pattern causes a drastic
reduction of the Atlantic northward heat transport, resulting in a strong cooling with maximum amplitude over the northern
North Atlantic and a southward shift of the sea-ice margin in the Atlantic. Due to the increased meridional temperature gradient,
the intertropical convergence zone over the Atlantic is displaced southward and the westerlies in the Northern Hemisphere
gain strength. We identify four main feedbacks affecting the stability of the thermohaline circulation: the change in the
overturning circulation of the Atlantic leads to longer residence times of the surface water in high-northern latitudes, which
allows them to accumulate more precipitation and runoff from the continents. As a consequence the stratification in the North
Atlantic becomes more stable. This effect is further amplified by an enhanced northward atmospheric water vapour transport,
which increases the freshwater input into the North Atlantic. The reduced northward oceanic heat transport leads to colder
sea-surface temperatures and an intensification of the atmospheric cyclonic circulation over the Norwegian Sea. The associated
Ekman transports cause increased upwelling and increased freshwater export with the East Greenland Current. Both the cooling
and the wind-driven circulation changes largely compensate for the effects of the first two feedbacks. The wind-stress feedback
destabilizes modes without deep water formation in the North Atlantic, but has been neglected in almost all studies so far.
After the meltwater input stops, the North Atlantic deepwater formation resumed in all experiments and the meridional overturning
returned within 200 years to a conveyor belt pattern. This happened although the formation of North Atlantic deep water was
suppressed in one experiment for more than 300 years and the Atlantic overturning had settled into a circulation pattern with
Antarctic bottom water as the only source of deep water. It is a clear indication that cooling and wind-stress feedback are
more effective, at least in our model, than advection feedback and increased atmospheric water vapour transport. We conclude
that the conveyor belt-type thermohaline circulation seems to be much more stable than hitherto assumed from experiments with
simpler models.
Received 31 January 1996/Accepted 22 August 1996 相似文献
15.
Summary ?Thirty years (1958–1987) of daily rainfall data for Kenya and north eastern Tanzania are analysed with the aim to characterize
the interannual variability of the onset and cessation of the East African “long rains” (boreal spring). The leading principal
component (PC1) depicts consistent rainfall variations over much of the region. Cumulative PC1 scores for each year serve
to identify onset and cessation dates. The robustness of the dates derived from this method is demonstrated through the use
of an independent sample of stations. Their spatial representativity is assessed by daily rainfall composites. Average onset
occurs on March 25 th, and cessation on May 21 st. The interannual variability of the onset (standard deviation of 14.5 days) is larger than that of the withdrawal (10.3 days),
but the onset is also spatially much more consistent. Mean dates and dates in selected anomalous years agree well with previous
studies.
The relationship between onset time-series and large-scale atmospheric and oceanic fields is analysed. On a monthly time-scale,
interannual variations in “long rains” onset are associated with sea-surface temperature (SST) and sea-level pressure (SLP)
patterns that have a different sign for the Atlantic and Indian Oceans. A warm South Atlantic and a cool Indian Ocean are
associated with low and high SLP anomalies, respectively. These patterns are conducive to enhanced equatorial easterlies and
surface divergence over East Africa. This maintains the meridional branch (north–south orientated) of the Intertropical Convergence
Zone (ITCZ) further west, and the net result is a delayed onset of the “long rains”. Some of the South Atlantic features are
already present during January–February, suggesting some potential for monitoring interannual variations in the wet season
onset, based on SST and SLP patterns. Additional signals are found over Europe and the Mediterranean Sea in terms of the interaction
between the Northern Hemisphere extratropics and equatorial eastern Africa. A surge in the mid-tropospheric northerlies at
this time induces instability that may lead to an early onset event.
Received July 3, 2002; revised November 28, 2002; accepted December 7, 2002
Published online March 17, 2003 相似文献
16.
We use a reduced complexity climate model with a three-dimensional ocean component and realistic topography to investigate
the effect of stratification-dependent mixing on the sensitivity of the North Atlantic subpolar gyre (SPG), and the Atlantic
meridional overturning circulation (AMOC), to idealized CO 2 increase and peaking scenarios. The vertical diffusivity of the ocean interior is parameterized as κ ∼ N
−α, where N is the local buoyancy frequency. For all parameter values 0 ≤ α ≤ 3, we find the SPG, and subsequently the AMOC, to weaken
in response to increasing CO 2 concentrations. The weakening is significantly stronger for α ≥ α cr ≈ 1.5. Depending on the value of α, two separate model states develop. These states remain different after the CO 2 concentration is stabilized, and in some cases even after the CO 2 concentration has been decreased again to the pre-industrial level. This behaviour is explained by a positive feedback between
stratification and mixing anomalies in the Nordic Seas, causing a persistent weakening of the SPG. 相似文献
17.
Experiments with the coupled climate model CLIM BER-3α, which contains an oceanic general circulation model, show deep upwelling in the Southern Ocean to be proportional to
the surface wind stress in the latitudinal band of Drake Passage. At the same time, the distribution of the Southern Ocean
upwelling onto the oceanic basins is controlled by buoyancy distribution; the inflow into each basin being proportional to
the respective meridional density difference. We observe approximately the same constant of proportionality for all basins,
and demonstrate that it can be directly related to the flow geometry. For increased wind stress in the Southern Ocean, the
overturning increases both in the Atlantic and the Indo-Pacific basin. For strongly reduced wind stress, the circulation enters
a regime where Atlantic overturning is maintained through Pacific upwelling, in order to satisfy the transports set by the
density differences. Previous results on surface buoyancy and wind stress forcing, obtained with different models, are reproduced
within one model in order to distill a consistent picture. We propose that both Southern Ocean upwelling and meridional density
differences set up a system of conditions that determine the global meridional overturning circulation. 相似文献
18.
Analyzed is the interannual variability of the meridional mass transport ψ S in the North Atlantic based on the Sverdrup relation. The continuous (1980–2005) monthly wind stress dataset with the spatial resolution of 1 × 1° was used as the initial data. Sverdrup transport analysis performed for different latitudinal transects within the North Atlantic subtropical gyre demonstrated that the maximum long-term Sverdrup transport (?25.2 Sv) can be found at 33°N. Studied is a mechanism of the interaction between the meridional Sverdrup transport and the water flow in the Florida Strait. The significant correlation coefficient (0.5) is revealed for the Florida Strait water discharge and the mass transport at 27°N. Analyzed is the relationship between ψ S and the North Atlantic Oscillation index and the statistically significant correlation coefficient (0.45) is obtained for the Sverdrup transport at 49°N. 相似文献
19.
The response of a two-dimensional thermohaline ocean circulation model to a random freshwater flux superimposed on the usual mixed boundary conditions for temperature and salinity is considered. It is shown that for a wide range of vertical and horizontal diffusivities and a box geometry that approximates the Atlantic Ocean, 200–300 yr period oscillations exist in the basic-state, interhemispheric meridional overturning circulation with deep convection in the north. These fluctuations can also be described in terms of propagating salinity anomalies which travel in the direction of the thermohaline flow. For large horizontal ( K
h
= 15 × 103 m 2/s) and small vertical ( K
v
= 0.5 × 10 –4 m 2/s) diffusivities, the random forcing also excites deca-millennial oscillations in the basic structure of the thermohaline circulation. In this case, the meridional circulation pattern slowly oscillates between three different stages: a large positive cell, with deep convection in the North Atlantic and upwelling in the south; a symmetric two-cell circulation, with deep convection in both polar regions and upwelling near the equator; and a large negative cell, with deep convection in the South Atlantic and upwelling in the north. Each state can persist for 0 (10 kyr). 相似文献
20.
General circulation models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are examined with respect to their ability to simulate the mean state and variability of the tropical Atlantic and its linkage to the tropical Pacific. While, on average, mean state biases have improved little, relative to the previous intercomparison (CMIP5), there are now a few models with very small biases. In particular the equatorial Atlantic warm SST and westerly wind biases are mostly eliminated in these models. Furthermore, interannual variability in the equatorial and subtropical Atlantic is quite realistic in a number of CMIP6 models, which suggests that they should be useful tools for understanding and predicting variability patterns. The evolution of equatorial Atlantic biases follows the same pattern as in previous model generations, with westerly wind biases during boreal spring preceding warm sea-surface temperature (SST) biases in the east during boreal summer. A substantial portion of the westerly wind bias exists already in atmosphere-only simulations forced with observed SST, suggesting an atmospheric origin. While variability is relatively realistic in many models, SSTs seem less responsive to wind forcing than observed, both on the equator and in the subtropics, possibly due to an excessively deep mixed layer originating in the oceanic component. Thus models with realistic SST amplitude tend to have excessive wind amplitude. The models with the smallest mean state biases all have relatively high resolution but there are also a few low-resolution models that perform similarly well, indicating that resolution is not the only way toward reducing tropical Atlantic biases. The results also show a relatively weak link between mean state biases and the quality of the simulated variability. The linkage to the tropical Pacific shows a wide range of behaviors across models, indicating the need for further model improvement. 相似文献
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