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1.
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?=?106?m3?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   

2.
A global, flux-corrected climate model is employed to predict the surface wind stress and associated wind-driven oceanic circulation for climate states corresponding to a doubling and quadrupling of the atmospheric CO2 concentration in a simple 1% per year CO2 increase scenario. The model indicates that in response to CO2 increase, the position of zero wind stress curl in the mid-latitudes of the Southern Hemisphere shifts poleward. In addition, the wind stress intensifies significantly in the mid-latitudes of the Southern Hemisphere. As a result, the rate of water circulation in the subpolar meridional overturning cell in the Southern Ocean increases by about 6 Sv (1 Sv=106 m3 s−1) for doubled CO2 and by 12 Sv for quadrupled CO2, implying an increase of deep water upwelling south of the circumpolar flow and an increase of Ekman pumping north of it. In addition, the changes in the wind stress and wind stress curl translate into changes in the horizontal mass transport, leading to a poleward expansion of the subtropical gyres in both hemispheres, and to strengthening of the Antarctic Circumpolar Current. Finally, the intensified near-surface winds over the Southern Ocean result in a substantial increase of mechanical energy supply to the ocean general circulation.  相似文献   

3.
For all of the IPCC Special Report on Emission Scenarios (SRESs), sea level is projected to rise globally. However, sea level changes are not expected to be geographically uniform, with many regions departing significantly from the global average. Some of regional distributions of sea level changes can be explained by projected changes of ocean density and dynamics. In this study, with 11 available Coupled Model Intercomparison Project Phase 3 climate models under the SRES A1B, we identify an asymmetric feature (not recognised in previous studies) of projected subtropical gyre circulation changes and associated sea level changes between the North and South Pacific, through analysing projected changes of ocean dynamic height (with reference to 2,000 db), depth integrated steric height, Sverdrup stream function, surface wind stress and its curl. Poleward expansion of the subtropical gyres is projected in the upper ocean for both North and South Pacific. Contrastingly, the subtropical gyre circulation is projected to spin down by about 20 % in the subsurface North Pacific from the main thermocline around 400 m to at least 2,000 m, while the South Pacific subtropical gyre is projected to strengthen by about 25 % and expand poleward in the subsurface to at least 2,000 m. This asymmetrical distribution of the projected subtropical gyre circulation changes is directly related to differences in projected changes of temperature and salinity between the North and South Pacific, forced by surface heat and freshwater fluxes, and surface wind stress changes.  相似文献   

4.
Using an intermediate ocean–atmosphere coupled model (ICM) for the tropical Pacific, we investigated the role of the ocean dynamical thermostat (ODT) in regulating the tropical eastern Pacific sea surface temperature (SST) under global warming conditions. The external, uniformly distributed surface heating results in the cooling of the tropical eastern Pacific “cold tongue,” and the amplitude of the cooling increases as more heat is added but not simply linearly. Furthermore, an upper bound for the influence of the equatorially symmetric surface heating on the cold tongue cooling exists. The additional heating beyond the upper bound does not cool the cold tongue in a systematic manner. The heat budget analysis suggests that the zonal advection is the primary factor that contributes to such nonlinear SST response. The radiative heating due to the greenhouse effect (hereafter, RHG) that is obtained from the multi-model ensemble of the Climate Model Intercomparison Project Phase III (CMIP3) was externally given to ICM. The RHG obtained from the twentieth century simulation intensified the cold tongue cooling and the subtropical warming, which were further intensified by the RHG from the doubled CO2 concentration simulation. However, the cold tongue cooling was significantly reduced and the negative SST response region was shrunken toward the equator by the RHG from the quadrupled CO2 concentration simulation, while the subtropical warming increased further. A systematic RHG forced experiment having the same spatial pattern of RHG from doubled CO2 concentration simulation with different amplitude of forcing revealed that the ocean dynamical response to global warming tended to enhance the cooling in the tropical eastern Pacific by virtue of meridional advection and upwelling; however, these cooling effects could not fully compensate a given RHG warming as the external forcing becomes larger. Moreover, the feedback by the zonal thermal advection actually exerted the warming over the equatorial region. Therefore, as the global warming is intensified, the cooling over the eastern tropical Pacific by ODT and the negative SST response area are reduced.  相似文献   

5.
On the predictability of decadal changes in the North Pacific   总被引：2，自引：0，他引：2  
 The predictability of decadal changes in the North Pacific is investigated with an ocean general circulation model forced by simplified and realistic atmospheric conditions. First, the model is forced by a spatially fixed wind stress anomaly pattern characteristic for decadal North Pacific climate variations. The time evolution of the wind stress anomaly is chosen to be sinusoidal, with a period of 20 years. In this experiment different physical processes are found to be important for the decadal variations: baroclinic Rossby waves dominate the response. They move westward and lead to an adjustment of the subtropical and subpolar gyre circulations in such a way that anomalous temperatures in the central North Pacific develop as a delayed response to the preceding wind stress anomalies. This delayed response provides not only a negative feedback but also bears the potential for long-term predictions of upper ocean temperature changes in the central North Pacific. It is shown by additional experiments that once these Rossby waves have been excited, decadal changes of the upper ocean temperatures in the central North Pacific evolve without any further anomalous atmospheric forcing. In the second part, the model is forced by surface heat flux and wind stress observations for the period 1949–1993. It is shown that the same physical processes which were found to be important in the simplified experiments also govern the evolution of the upper ocean in this more realistic simulation. The 1976/77 cooling can be mainly attributed to anomalously strong horizontal advection due to the delayed response to persistent wind stress curl anomalies in the early 1970s rather than local anomalous atmospheric forcing. This decadal change could have been predicted some years in advance. The subsequent warming in the late 1980s, however, cannot be mainly explained by advection. In this case, local anomalous atmospheric forcing needs to be considered. Received: 6 July 1998 / Accepted: 16 October 1999  相似文献   

6.
Gong  Yuhan  Li  Tim  Chen  Lin 《Climate Dynamics》2020,55(9-10):2689-2702

The impact of the Atlantic Multi-decadal Oscillation (AMO) on the ENSO amplitude was investigated through observational analyses. During the past 90 years the interdecadal variability of ENSO intensity is significantly correlated with the AMO. ENSO variability was strengthened (weakened) during a negative (positive) AMO phase. An ocean mixed layer heat budget analysis reveals that the thermocline feedback is the main process regulating AMO negative phase dependent ENSO growth characteristic. A further examination indicates that a strengthened atmospheric response to unit SST anomaly, an enhanced thermocline response to unit wind stress forcing and a strengthened subsurface temperature response to unit thermocline variation all contribute to the enhanced thermocline feedback during the negative phase of AMO. Such changes are attributed to the increase of background moisture, the weakening of mean subtropical cell (STC) and increase of upper ocean vertical temperature gradient respectively.

  相似文献   

7.
Tropical instability waves (TIWs) arise from oceanic instability in the eastern tropical Pacific and Atlantic Oceans, having a clear atmospheric signature that results in coupled atmosphere–ocean interactions at TIW scales. In this study, the extent to which TIW-induced surface wind feedback influences the ocean is examined using an ocean general circulation model (OGCM). The TIW-induced wind stress (τTIW) part is diagnostically determined using an empirical τTIW model from sea surface temperature (SST) fields simulated in the OGCM. The interactively represented TIW wind tends to reduce TIW activity in the ocean and influence the mean state, with largest impacts during TIW active periods in fall and winter. In December, the interactive τTIW forcing induces a surface cooling (an order of ?0.1 to ?0.3 °C), an increased heat flux into the ocean, a shallower mixed layer and a weakening of the South Equatorial Current in the eastern equatorial Pacific. Additionally, the TIW wind effect yields a pronounced latitudinal asymmetry of sea level field across the equator, and a change to upper thermal structure, characterized by a surface cooling and a warming below in the thermocline, leading to a decreased temperature gradient between the mixed layer and the thermocline. Processes responsible for the τTIW–induced cooling effects are analyzed. Vertical mixing and meridional advection are the two terms in the SST budget that are dominantly affected by the TIW wind feedback: the cooling effect from the vertical mixing on SST is enhanced, with the maximum induced cooling in winter; the warming effect from the meridional advection is reduced in July–October, but enhanced in November–December. Additional experiments are performed to separate the relative roles the affected surface momentum and heat fluxes play in the cooling effect on SST. This ocean-only modeling work indicates that the effect of TIW-induced wind feedback is small but not negligible, and may need to be adequately taken into account in large-scale climate modeling.  相似文献   

8.
The response of the North Atlantic subpolar gyre (SPG) to a persistent positive (or negative) phase of the North Atlantic oscillation (NAO) is investigated using an ocean general circulation model forced with idealized atmospheric reanalysis fields. The integrations are analyzed with reference to a base-line integration for which the model is forced with idealized fields representing a neutral state of the NAO. In the positive NAO case, the results suggest that the well-known cooling and strengthening of the SPG are, after about 10 years, replaced by a warming and subsequent weakening of the SPG. The latter changes are caused by the advection of warm water from the subtropical gyre (STG) region, driven by a spin-up of the Atlantic meridional overturning circulation (AMOC) and the effect of an anomalous wind stress curl in the northeastern North Atlantic, which counteracts the local buoyancy forcing of the SPG. In the negative NAO case, however, the SPG response does not involve a sign reversal, but rather shows a gradual weakening throughout the integration. The asymmetric SPG-response to the sign of persistent NAO-like forcing and the different time scales involved demonstrate strong non-linearity in the North Atlantic Ocean circulation response to atmospheric forcing. The latter finding indicates that analysis based on the arithmetic difference between the two NAO-states, e.g. NAO+ minus NAO?, may hide important aspects of the ocean response to atmospheric forcing.  相似文献   

9.
The anthropogenic changes during boreal winter in the thermal and zonal flow structure over Eastern Atlantic and Western Europe (EAWE) have been investigated using an ensemble of CMIP3 and CMIP5 models. The ensemble mean change in the zonal wind at 500 hPa over this region is characterized by an eastward extension of the belt of zonal winds. Using the thermal wind relation these wind changes are found to be consistent with the changes in the tropospheric temperature profile. An enhanced warming is simulated in the subtropical upper troposphere and a relative surface cooling in the mid-latitudes. The subtropical upper tropospheric warming is related to the downward branch of the mean meridional circulation, whereas the mid-latitude lower tropospheric relative cooling is linked to the ocean processes that govern changes in its surface temperatures. Inter-model differences in the simulated change of the zonal wind over the EAWE by the CMIP3 and CMIP5 models relate well with differences in the upper tropospheric subtropical warming and the mid-latitude lower tropospheric relative cooling. The simulated change of the zonal wind over the EAWE region by the CMIP3 and CMIP5 models correlates well with changes in the meridional SST gradient. We conclude that uncertainties in the projected changes of the zonal flow over Europe are at least partly due to uncertainties in the response of the North Atlantic Ocean to increased levels of greenhouse gases.  相似文献   

10.
This work is an attempt to simulate the Mediterranean Sea general circulation with a Spectral Finite Element Model. This numerical technique associates the geometrical flexibility of the finite elements for the proper coastline definition with the precision offered by spectral methods. The model is reduced gravity and we study the wind-driven ocean response in order to explain the large scale sub-basin gyres and their variability. The study period goes from January 1987 to December 1993 and two forcing data sets are used. The effect of wind variability in space and time is analyzed and the relationship between wind stress curl and ocean response is stressed. Some of the main permanent structures of the general circulation (Gulf of Lions cyclonic gyre, Rhodes gyre, Gulf of Syrte anticylone) are shown to be induced by permanent wind stress curl structures. The magnitude and spatial variability of the wind is important in determining the appearance or disappearance of some gyres (Tyrrhenian anticyclonic gyre, Balearic anticyclonic gyre, Ionian cyclonic gyre). An EOF analysis of the seasonal variability indicates that the weakening and strengthening of the Levantine basin boundary currents is a major component of the seasonal cycle in the basin.The important discovery is that seasonal and interannual variability peak at the same spatial scales in the ocean response and that the interannual variability includes the change in amplitude and phase of the seasonal cycle in the sub-basin scale gyres and boundary currents. The Coriolis term in the vorticity balance seems to be responsible for the weakening of anticyclonic structures and their total disappearance when they are close to a boundary.The process of adjustment to winds produces a train of coastally trapped gravity waves which travel around the eastern and western basins, respectively in approximately 6 months. This corresponds to a phase velocity for the wave of about 1 m/s, comparable to an average velocity of an internal Kelvin wave in the area.  相似文献   

11.
Stochastic wind forcing of ocean gyre circulations is examined using the ideas of generalized linear stability theory applied to the barotropic vorticity equation of a idealized ocean. The barotropic vorticity equation is linearized about a time-evolving basic state flow, and the spatial patterns of stochastic surface wind stress curl that are optimal for increasing the variability of the ocean are computed. The most disruptive pattern of stochastic forcing is found to be insensitive to: measures of variance, the optimization time, the temporal decorrelation time of the stochastic forcing, the time evolution of the basic state flow, the stability of the basic state flow, basin size, gyre symmetry, and the presence of bathymetry. In addition, the most disruptive pattern of wind stress curl is reminiscent of that which would be associated with individual large-scale weather systems in the atmosphere, and changes in the amplitude of the atmospheric teleconnection patterns. The response of a nonlinear model to stochastic forcing described by the optimal patterns is examined, and the dynamics of the response discussed.  相似文献   

12.
Summary The interannual variability of the monthly mean upper layer thickness for the central Arabian Sea (5°N-15° N and 60° E-70° E) from a numerical model of the Indian Ocean during the period 1954–1976 is investigated in relation to Indian monsoon rainfall variability. The variability in the surface structure of the Somali Current in the western Arabian Sea is also briefly discussed. It is found that these fields show a great deal of interannual variability that is correlated with variability in Indian monsoon rainfall. Model upper layer thickness (H) is taken as a surrogate variable for thermocline depth, which is assumed to be correlated with sea surface temperature. In general, during the period 1967 to 1974, which is a period of lower than normal monsoon rainfall, the upper ocean warm water sphere is thicker (deeper thermocline which implies warmer surface water); in contrast, during the period 1954–1966, which is a period of higher than normal monsoon rainfall, the upper warm water sphere is thinner (shallower thermocline which implies cooler surface water). The filtered time series of uppper layer thickness indieates the presence of a quasi-biennial oscillation (QBO) during the wet monsoon period, but this QBO signal is conspicuously absent during the dry monsoon period.Since model H primarily responds to wind stress curl, the interannual variability of the stress curl is investigated by means of an empirical orthogonal function (EOF) analysis. The first three EOF modes represent more than 72% of the curl variance. The spatial patterns for these modes exhibit many elements of central Arabian Sea climatology. Features observed include the annual variation in the intensity of the summer monsoon ridge in the Arabian Sea and the annual zonal oscillation of the ridge during pre- and post-monsoon seasons. The time coefficients for the first EOF amplitude indicate the presence of a QBO during the wet monsoon period only, as seen in the ocean upper layer thickness.The variability in the model upper layer thickness is a passive response to variability in the wind field, or more specifically to variability in the Findlater Jet. When the winds are stronger, they drive stronger currents in the ocean and have stronger curl fields associated with them, driving stronger Ekman pumping. They transport more moisture from the southern hemisphere toward the Indian subcontinent, and they also drive a greater evaporative heat flux beneath the Findlater Jet in the Arabian Sea. It has been suggested that variability in the heat content of the Arabian Sea drives variability in Indian monsoon rainfall. The results of this study suggest that the opposite is true, that the northern Arabian Sea responds passively to variability in the monsoon system.With 10 Figures  相似文献   

13.
The impact of climate warming on the seasonal variability of the Humboldt Current system ocean dynamics is investigated. The IPSL-CM4 large scale ocean circulation resulting from two contrasted climate scenarios, the so-called Preindustrial and quadrupling CO2, are downscaled using an eddy-resolving regional ocean circulation model. The intense surface heating by the atmosphere in the quadrupling CO2 scenario leads to a strong increase of the surface density stratification, a thinner coastal jet, an enhanced Peru–Chile undercurrent, and an intensification of nearshore turbulence. Upwelling rates respond quasi-linearly to the change in wind stress associated with anthropogenic forcing, and show a moderate decrease in summer off Peru and a strong increase off Chile. Results from sensitivity experiments show that a 50% wind stress increase does not compensate for the surface warming resulting from heat flux forcing and that the associated mesoscale turbulence increase is a robust feature.  相似文献   

14.
Abstract

Six‐hourly surface wind analyses over the North Pacific Ocean covering the 10‐year period 1969–78 are used to describe synoptic storm activity in terms of parameters that are directly related to the atmospheric forcing of the ocean. The cube of the atmospheric friction velocity, u3 * and the curl of the surface wind stress, curl τ, are used because of their relationship to turbulent vertical mixing and Ekman pumping in the ocean, respectively. In an attempt to isolate synoptic disturbances from mean fields, the time series of surface wind components at each individual grid point are partitioned into “high‐pass” (periods shorter than 10 days) and “low‐pass” (periods longer than 10 days) components by means of conventional filtering procedures. The two quantities u3 * and curl τ are then calculated from (a) the high‐pass filtered wind components only, (b) a combination of the filtered wind components that include the interaction between the high‐ and low‐pass fields, and (c) the unfiltered wind components. These quantities describe the atmospheric forcing of the ocean that is attributable to (a) synoptic storm activity by itself, (b) synoptic storm activity in the presence of the low‐pass (mean) flow, and (c) the total spectrum of wind forcing, respectively.

Maps of the long‐term (10‐year) monthly mean u3 * calculated from (a) and (b) are coherent across the mid‐latitude North Pacific and appear to coincide with the normal seasonal evolution of synoptic storm activity in that region. In mid‐latitudes, the values of u3 * calculated from (a) and (b) are 27 and 83%, respectively, of the value of u3 * itself. Thus, a major fraction of the production of turbulent energy available for mixing in the upper layers of the ocean comes from synoptic disturbances with a period shorter than 10 days. Maps of the long‐term monthly mean wind stress curl are quite different in that the mean wind stress curl calculated from (a) is essentially negligible. However, the mean curl calculated from (b) closely resembles the pattern of total curl (c), but with a magnitude of only 41% of (c). Thus, synoptic disturbances with a period shorter than 10 days are also responsible for a significant fraction of the Ekman pumping of the ocean.

Future studies with these data will attempt to determine whether a relationship exists between synoptic storm activity, as measured by the parameters developed in this study, and large‐scale sea‐surface temperature anomalies.  相似文献   

15.
W. Cai  H. B. Gordon 《Climate Dynamics》1998,14(7-8):503-516
 The responses of the CSIRO coupled atmosphere-ocean-sea ice model to two greenhouse gas induced warming scenarios are described and compared to a control run with the current CO2 level. In one scenario, denoted IS92a, the atmospheric CO2 increases such that it reaches doubling after 128 years. In the other, the CO2 increases at 1% per year compounding (doubling after 70 y). As the CO2 increases in both scenarios, the top-of-atmosphere outgoing longwave radiation increases giving enhanced cooling of the coupled system, while the outgoing short wave radiation decreases contributing to a warming of the system. The latter overcompensates the former leading to a global mean net radiative heat gain. The distribution of this heat gain produces the well-known interhemispheric asymmetry in warming, despite a decrease in the sea ice around Antarctica in this model. It is found that the volume mean temperature response over the southern ocean is greater than that over the northern hemispheric oceans, and a maximum warming takes place at the subsurface rather at the surface of the ocean in the southern mid-to-high latitude region. The enhanced high-latitude freshening associated with the strengthened hydrological cycle significantly affects the latitudinal distribution of warming and other responses. It enhances the warming immediately equatorward of the deep water formation regions while produces a reduced warming, even a cooling, in these regions. In both runs, there is a decrease in the large-scale oceanic currents which have a significant thermohaline-driven component. The reduction in these currents reduces the poleward transport of salt out of the tropical and subtropical regions of these oceans. This and the enhanced evaporation contribute to considerable increases in surface salinity in the tropical and subtropical regions. In IS92a, the warming rate before doubling is smaller than that in 1% scenario, but the cumulative effects of the two experiments at the time of doubling are similar. Nevertheless, significant contrasts exist. For example, at the time of doubling in IS92a, the warming of the upper ocean is greater because a more developed temperature-albedo feedback occurs. In addition, a longer time is allowed for heat anomalies to spread downward, and so the effective heat penetration depth is greater than that in the 1% scenario. Thus the oceanic response is influenced by the CO2 increase scenario used. Received: 2 September 1997 / Accepted: 21 January 1998  相似文献   

16.
Based on coupled modelling evidence we argue that topographically-induced modifications of the large-scale atmospheric circulation during the last glacial maximum may have led to a reduction of the westerlies, and a slowdown of the Pacific subtropical gyre as well as to an intensification of the Pacific subtropical cell. These oceanic circulation changes generate an eastern North Pacific warming, an associated cooling in the Kuroshio area, as well as a cooling of the tropical oceans, respectively. The tropical cooling pattern resembles a permanent La Niña state which in turn forces atmospheric teleconnection patterns that lead to an enhancement of the subtropical warming by reduced latent and sensible cooling of the ocean. In addition, the radiative cooling due to atmospheric CO2 and water vapor reductions imposes a cooling tendency in the tropics and subtropics, thereby intensifying the permanent La Niña conditions. The remote North Pacific response results in a warming tendency of the eastern North Pacific which may level off the effect of the local radiative cooling. Hence, a delicate balance between oceanic circulation changes, remotely induced atmospheric flux anomalies as well local radiative cooling is established which controls the tropical and North Pacific temperature anomalies during the last glacial maximum. Furthermore, we discuss how the aftermath of a Heinrich event may have affected glacial temperatures in the Pacific Ocean.  相似文献   

17.
Anthropogenic climate change will continue long after anthropogenic CO2 emissions cease. Atmospheric CO2, global warming and ocean circulation will approach equilibrium on the millennial timescale, whereas thermal expansion of the ocean, ice sheet melt and their contributions to sea level rise are unlikely to be complete. Atmospheric CO2 in year 3000 depends non-linearly on the total amount of CO2 emitted and is very likely to exceed the present level of ∼380 ppmv. CO2 is doubled for ∼2500 GtC emitted, quadrupled if all ∼5000 GtC of conventional fossil fuel resources are emitted, and increases by a factor of ∼32 if a further 20,000 GtC of exotic fossil fuel resources are emitted. Global warming in year 3000 will also depend on climate sensitivity to doubling CO2, which is most probably ∼3 ∘C but highly uncertain. Thermal expansion will contribute 0.5–2 m to millennial sea level rise for each doubling of CO2. The Greenland ice sheet could melt completely within the millennium under > 8×CO2, adding a further ∼7 m to sea level. The rate of melt depends on the magnitude of forcing above a regional warming threshold of 1–3 ∘C. The West Antarctic ice sheet could be threatened by 4–10 ∘C local warming, and its potential contribution to millennial sea level rise exceeds current maximum estimates of ∼1 m. The fate of the ocean thermohaline circulation may depend on the rate as well as the magnitude of forcing.  相似文献   

18.
Observations have shown that the Indian Ocean is consistently warming and its warm pool is expanding, particularly in the recent decades. This paper attempts to investigate the reason behind these observations. Under global warming scenario, it is expected that the greenhouse gas induced changes in air–sea fluxes will enhance the warming. Surprisingly, it is found that the net surface heat fluxes over Indian Ocean warm pool (IOWP) region alone cannot explain the consistent warming. The warm pool area anomaly of IOWP is strongly correlated with the sea surface height anomaly, suggesting an important role played by the ocean advection processes in warming and expansion of IOWP. The structure of lead/lag correlations further suggests that Oceanic Rossby waves might be involved in the warming. Using heat budget analysis of several Ocean data assimilation products, it is shown that the net surface heat flux (advection) alone tends to cool (warm) the Ocean. Based on above observations, we propose an ocean-atmosphere coupled positive feedback mechanism for explaining the consistent warming and expansion of IOWP. Warming over IOWP induces an enhancement of convection in central equatorial Indian ocean, which causes anomalous easterlies along the equator. Anomalous easterlies in turn excite frequent Indian ocean Dipole events and cause anti-cyclonic wind stress curl in south-east and north-east equatorial Indian ocean. The anomalous wind stress curl triggers anomalous downwelling oceanic Rossby waves, thereby deepening the thermocline and resulting in advection of warm waters towards western Indian ocean. This acts as a positive feedback and results in more warming and westward expansion of IOWP.  相似文献   

19.
Several numerical experiments are conducted to examine the influence of mesoscale, bottom topography roughness on the inertial circulation of a wind-driven, mid-latitude ocean gyre. The ocean model is based on the quasi-geostrophic formulation, and is eddy-resolving as it features high vertical and horizontal resolutions (six layers and a 10 km grid). An antisymmetrical double-gyre wind stress curl forces the baroclinic modes and generates a strong surface jet. In the case of a flat bottom, inertia and inverse energy cascade force the barotropic mode, and the resulting circulation features strong, barotropic, inertial gyres. The sea-floor roughness inhibits the inertial circulation in the deep layers; the barotropic component of the flow is then forced by eddy-topography interactions, and its energy concentrates at the scales of the topography. As a result, the baroclinicity of the flow is intesified: the barotropic mode is reduced with regard to the baroclinic modes, and the bottom flow (constrained by the mesoscale sea-floor roughness) is decoupled from the surface flow (forced by the gyre-scale wind). Rectified, mesoscale bottom circulation induces an interfacial form stress at the thermocline, which enhances horizontal shear instability and opposes the eastward penetration of the jet. The mean jet is consequently shortened, but the instantaneous jet remains very turbulent, with meanders of large meridional extent. The sea-floor roughness modifies the energy pathways, and the eddies have an even more important role in the establishment of the mean circulation: below the thermocline, rectification processes are dominant, and eddies transfer energy toward permanent mesoscale circulations strongly correlated with topography, whereas above the thermocline mean flow and eddy generation are influenced by the mean bottom circulation through interfacial stress. The topography modifies the vorticity of the barotropic and highest baroclinic modes. Vorticity accumulates at the small topographic scales, and the vorticity content of the highest modes, which is very weak in the flat-bottom case, increases significantly. Few changes occur in surface-intensified modes. In the deep layers of the model, the inverse correlation between relative vorticity and topography at small scales ensures the homogenization of the potential vorticity, which mainly retains the largest scales of the bottom flow and the scale of β.  相似文献   

20.
Physical processes responsible for tropospheric adjustment to increasing carbon dioxide concentration are investigated using abrupt CO2 quadrupling experiments of a general circulation model (GCM) called the model for interdisciplinary research on climate version 5 with several configurations including a coupled atmosphere–ocean GCM, atmospheric GCM, and aqua-planet model. A similar experiment was performed in weather forecast mode to explore timescales of the tropospheric adjustment. We found that the shortwave component of the cloud radiative effect (SWcld) reaches its equilibrium within 2 days of the abrupt CO2 increase. The change in SWcld is positive, associated with reduced clouds in the lower troposphere due to warming and drying by instantaneous radiative forcing. A reduction in surface turbulent heat fluxes and increase of the near-surface stability result in shoaling of the marine boundary layer, which shifts the cloud layer downward. These changes are common to all experiments regardless of model configuration, indicating that the cloud adjustment is primarily independent of air–sea coupling and land–sea thermal contrast. The role of land in cloud adjustment is further examined by a series of idealized aqua-planet experiments, with a rectangular continent of varying width. Land surface warming from quadrupled CO2 induces anomalous upward motion, which increases high cloud and associated negative SWcld over land. The geographic distribution of continents regulates the spatial pattern of the cloud adjustment. A larger continent produces more negative SWcld, which partly compensates for a positive SWcld over the ocean. The land-induced negative adjustment is a factor but not necessary requirement for the tropospheric adjustment.  相似文献   

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