ALBERT

Description: Scattered groups of these ancient fish may all stem from a single remote population. Coelacanths were discovered in the Comoros archipelago to the northwest of Madagascar in 1952. Since then, these rare, ancient fish have been found to the south off Mozambique, Madagascar and South Africa, and to the north off Kenya and Tanzania — but it was unclear whether these are separate populations or even subspecies. Here we show that the genetic variation between individuals from these different locations is unexpectedly low. Combined with earlier results from submersible and oceanographic observations1, 2, our findings indicate that a separate African metapopulation is unlikely to have existed and that locations distant from the Comoros were probably inhabited relatively recently by either dead-end drifters or founders that originated in the Comoros.

Description: There has been concern about recent temperature trends and the future effects of CO2 concentrations in the atmosphere1,2; but instrumental records only cover a few decades to a few centuries and it is essential that proxy data sources, such as pollen spectra from peats and lake sediments, be carefully interpreted as climate records. Several workers have shown statistically significant associations between the modern pollen rain and climatic parameters, an approach that by-passes the recognition of pollen/vegetation units. Statistically defined equations that associate abiotic and biotic elements are called transfer functions. We report here on the application of transfer function equations to nine middle and late Holocene peat and lake sediment sequences from northern Canada (Fig. 1).

Description: An empirical model for the temperature of subsurface water entrained into the ocean mixed layer (Te) is presented and evaluated to improve sea surface temperature anomaly (SSTA) simulations in an intermediate ocean model (IOM) of the tropical Pacific. An inverse modeling approach is adopted to estimate Te from an SSTA equation using observed SST and simulated upper-ocean currents. A relationship between Te and sea surface height (SSH) anomalies is then obtained by utilizing a singular value decomposition (SVD) of their covariance. This empirical scheme is able to better parameterize Te anomalies than other local schemes and quite realistically depicts interannual variability of Te, including a nonlocal phase lag relation of Te variations relative to SSH anomalies over the central equatorial Pacific. An improved Te parameterization naturally leads to better depiction of the subsurface effect on SST variability by the mean upwelling of subsurface temperature anomalies. As a result, SSTA simulations are significantly improved in the equatorial Pacific; a comparison with other schemes indicates that systematic errors of the simulated SSTAs are significantly small—apparently due to the optimized empirical Teparameterization. Cross validation and comparisons with other model simulations are made to illustrate the robustness and effectiveness of the scheme. In particular it is demonstrated that the empirical Te model constructed from one historical period can be successfully used to improve SSTA simulations in another.

Description: Climate models used to produce global warming scenarios exhibit widely diverging responses of the thermohaline circulation (THC). To investigate the mechanisms responsible for this variability, a regional Atlantic Ocean model driven with forcing diagnosed from two coupled greenhouse gas simulations has been employed. One of the coupled models (MPI) shows an almost constant THC, the other (GFDL) shows a declining THC in the twenty-first century. The THC evolution in the regional model corresponds rather closely to that of the respective coupled simulation, that is, it remains constant when driven with the forcing from the MPI model, and declines when driven with the GFDL forcing. These findings indicate that a detailed representation of ocean processes in the region covered by the Atlantic model may not be critical for the simulation of the overall THC changes in a global warming scenario, and specifically that the coupled model’s rather coarse representation of water mass formation processes in the subpolar North Atlantic is unlikely to be the primary cause for the large differences in the THC evolution. Sensitivity experiments have confirmed that a main parameter governing the THC response to global warming is the density of the intermediate waters in the Greenland–Iceland–Norwegian Seas, which in turn influences the density of the North Atlantic Deep Water, whereas changes in the air–sea heat and freshwater fluxes over the subpolar North Atlantic are only of moderate importance, and mainly influence the interannual–decadal variability of THC. Finally, as a consequence of changing surface fluxes, the Labrador Sea convection ceases by about 2030 under both forcings (i.e., even in a situation where the overall THC is stable) indicating that the eventual breakdown of the convection is likely but need not coincide with substantial THC changes.

Description: This study explores the influence of phytoplankton on the tropical Pacific heat budget. A hybrid coupled model for the tropical Pacific that is based on a primitive equation reduced-gravity multilayer ocean model, a dynamic ocean mixed layer, an atmospheric mixed layer, and a statistical atmosphere is used. The statistical atmosphere relates deviations of the sea surface temperature from its mean to wind stress anomalies and allows for the rectification of the annual cycle and the El Niño–Southern Oscillation (ENSO) phenomenon through the positive Bjerknes feedback. Furthermore, a nine-component ecosystem model is coupled to the physical variables of the ocean. The simulated chlorophyll concentrations can feed back onto the ocean heat budget by their optical properties, which modify solar light absorption in the surface layers. It is shown that both the surface layer concentration as well as the vertical profile of chlorophyll have a significant effect on the simulated mean state, the tropical annual cycle, and ENSO. This study supports a previously suggested hypothesis (Timmermann and Jin) that predicts an influence of phytoplankton concentration of the tropical Pacific climate mean state and its variability. The bioclimate feedback diagnosed here works as follows: Maxima in the subsurface chlorophyll concentrations lead to an enhanced subsurface warming due to the absorption of photosynthetically available shortwave radiation. This warming triggers a deepening of the mixed layer in the eastern equatorial Pacific and eventually a reduction of the surface ocean currents (Murtugudde et al.). The weakened south-equatorial current generates an eastern Pacific surface warming, which is strongly enhanced by the Bjerknes feedback. Because of the deepening of the mixed layer, the strength of the simulated annual cycle is also diminished. This in turn leads to an increase in ENSO variability.

Description: The authors present the first quantitative comparison between new velocity datasets and high-resolution models in the North Atlantic subpolar gyre [1/10° Parallel Ocean Program model (POPNA10), Miami Isopycnic Coordinate Ocean Model (MICOM), ° Atlantic model (ATL6), and Family of Linked Atlantic Ocean Model Experiments (FLAME)]. At the surface, the model velocities agree generally well with World Ocean Circulation Experiment (WOCE) drifter data. Two noticeable exceptions are the weakness of the East Greenland coastal current in models and the presence in the surface layers of a strong southwestward East Reykjanes Ridge Current. At depths, the most prominent feature of the circulation is the boundary current following the continental slope. In this narrow flow, it is found that gridded float datasets cannot be used for a quantitative comparison with models. The models have very different patterns of deep convection, and it is suggested that this could be related to the differences in their barotropic transport at Cape Farewell. Models show a large drift in watermass properties with a salinization of the Labrador Sea Water. The authors believe that the main cause is related to horizontal transports of salt because models with different forcing and vertical mixing share the same salinization problem. A remarkable feature of the model solutions is the large westward transport over Reykjanes Ridge [10 Sv (Sv ≡ 106 m3 s−1) or more]

Description: In the context of gradual Cenozoic cooling, the timing of the onset of significant Northern Hemisphere glaciation 2.7 million years ago is consistent with Milankovitch's orbital theory, which posited that ice sheets grow when polar summertime insolation and temperature are low. However, the role of moisture supply in the initiation of large Northern Hemisphere ice sheets has remained unclear. The subarctic Pacific Ocean represents a significant source of water vapour to boreal North America, but it has been largely overlooked in efforts to explain Northern Hemisphere glaciation. Here we present alkenone unsaturation ratios and diatom oxygen isotope ratios from a sediment core in the western subarctic Pacific Ocean, indicating that 2.7 million years ago late-summer sea surface temperatures in this ocean region rose in response to an increase in stratification. At the same time, winter sea surface temperatures cooled, winter floating ice became more abundant and global climate descended into glacial conditions. We suggest that the observed summer warming extended into the autumn, providing water vapour to northern North America, where it precipitated and accumulated as snow, and thus allowed the initiation of Northern Hemisphere glaciation.

Description: The dependence of the air–sea interactions over the North Atlantic on the ocean dynamics is explored by analyzing multicentury integrations with two different coupled ocean–atmosphere models. One is a coupled general circulation model (CGCM), in which both the atmospheric and the oceanic components are represented by general circulation models (GCMs). The second coupled model employs the same atmospheric GCM, but the oceanic GCM is replaced by a fixed-depth mixed layer model, so that variations of the ocean dynamics are excluded. The coupled model including active ocean dynamics simulates strong multidecadal variability in the sea surface temperature (SST) of the North Atlantic, with a monopolar spatial structure. In contrast, the coupled model that employs an oceanic mixed layer model and thus does not carry active ocean dynamics simulates a tripolar SST anomaly pattern at decadal time scales. The tripolar SST anomaly pattern is characterized by strong horizontal gradients and is by definition the result of the action of surface heat flux anomalies on the oceanic mixed layer. The differences in the spatial structures of the dominant decadal SST anomaly patterns yield rather different atmospheric responses. While the response to the monopolar SST anomaly pattern is shallow and thermal, the response to the tripolar SST anomaly pattern involves changes in the transient eddy statistics. The latter can be explained by the strong horizontal SST gradients that affect the surface baroclinicity, which in turn affects the growth rate of the transient eddies. The differences in the atmospheric response characteristics yield completely different response patterns. In the coupled run with active ocean dynamics, the sea level pressure (SLP) anomalies exhibit a rather homogeneous pattern that resembles somewhat the East Atlantic Pattern (EAP), while a dipolar (North Atlantic Oscillation) NAO-like SLP anomaly pattern is simulated in the coupled run without active ocean dynamics.

Description: The interaction of clouds with solar and terrestrial radiation is one of the most important topics of climate research. In recent years it has been recognized that only a full three-dimensional (3D) treatment of this interaction can provide answers to many climate and remote sensing problems, leading to the worldwide development of numerous 3D radiative transfer (RT) codes. The international Intercomparison of 3D Radiation Codes (I3RC), described in this paper, sprung from the natural need to compare the performance of these 3D RT codes used in a variety of current scientific work in the atmospheric sciences. I3RC supports intercomparison and development of both exact and approximate 3D methods in its effort to 1) understand and document the errors/limits of 3D algorithms and their sources; 2) provide “baseline” cases for future code development for 3D radiation; 3) promote sharing and production of 3D radiative tools; 4) derive guidelines for 3D radiative tool selection; and 5) improve atmospheric science education in 3D RT. Results from the two completed phases of I3RC have been presented in two workshops and are expected to guide improvements in both remote sensing and radiative energy budget calculations in cloudy atmospheres.

Description: Changes of the North Atlantic thermohaline circulation (THC) excite wave patterns that readjust the thermocline globally. This paper examines the impact of a freshwater-induced THC shutdown on the depth of the Pacific thermocline and its subsequent modification of the El Niño–Southern Oscillation (ENSO) variability using an intermediate-complexity global coupled atmosphere–ocean–sea ice model and an intermediate ENSO model, respectively. It is shown by performing a numerical eigenanalysis and transient simulations that a THC shutdown in the North Atlantic goes along with reduced ENSO variability because of a deepening of the zonal mean tropical Pacific thermocline. A transient simulation also exhibits abrupt changes of ENSO behavior, depending on the rate of THC change. The global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales. Simulated multidecadal global sea surface temperature (SST) patterns show a large degree of similarity with previous climate reconstructions, suggesting that the observed pan-oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections.

Description: The interannual heat content variability in the tropical south Indian Ocean (SIO) and its relationship with El Niño–Southern Oscillation (ENSO) is studied. The baroclinic ocean response to stochastic wind stress predicted by a simple analytical model is compared with two integrations of the ECHO-G coupled general circulation model. In one integration, ocean–atmosphere interactions are suppressed in the tropical Pacific Ocean, so that this integration does not simulate ENSO. In the other integration, interactions are allowed everywhere and ENSO is simulated. The results show that basinwide variability in the SIO heat content can be produced by two mechanisms: 1) oscillatory forcing by ENSO-related wind stress and 2) temporally stochastic and spatially coherent wind stress forcing. Previous studies have shown that transmission of energy from the tropical Pacific to the southern Indian Ocean occurs through coastal Kelvin waves along the western coast of Australia. The results in this paper confirm the occurrence of such transmission. In the ECHO-G simulations, this transmission occurs both at the annual time scale and at interannual time scales. Generation of offshore Rossby waves by these coastal Kelvin waves at interannual time scales—and, in particular, at the ENSO time scale—was found.

Description: A coupled global atmosphere–ocean model of intermediate complexity is used to study the influence of glacial boundary conditions on the atmospheric circulation during the Last Glacial Maximum in a systematical manner. A web of atmospheric interactions is disentangled, which involves changes in the meridional temperature gradient and an associated modulation of the atmospheric baroclinicity. This in turn drives anomalous transient eddy momentum fluxes that feed back onto the zonal mean circulation. Moreover, the modified transient activity (weakened in the North Pacific and strengthened in the North Atlantic) leads to a meridional reorganization of the atmospheric heat transport, thereby feeding back onto the meridional temperature structure. Furthermore, positive barotropic conversion and baroclinic production rates over the Laurentide ice sheets and the far eastern North Pacific have the tendency to decelerate the westerlies, thereby feeding back to the stationary wave changes triggered by orographic forcing.

Description: Two state-of-the-art profiling floats were equipped with novel optode-based oceanographic oxygen sensors. Both floats were simultaneously deployed in the central Labrador Sea gyre on 7 September 2003. They drift at a depth of 800 db and perform weekly profiles of temperature, salinity, and oxygen in the upper 2000 m of the water column. The initial results from the first 6 months of operation are presented. Data are compared with a small hydrographic oxygen survey of the deployment site. They are further examined for measurement quality, including precision, accuracy, and drift aspects. The first 28 profiles obtained are of high quality and show no detectable sensor drift. A method of long-term drift control is described and a few suggestions for the operation protocol are provided.

Description: The dominant pattern of atmospheric variability in the North Atlantic sector is the North Atlantic Oscillation (NAO). Since the 1970s the NAO has been well characterized by a trend toward its positive phase. Recent atmospheric general circulation model studies have linked this trend to a progressive warming of the Indian Ocean. Unfortunately, a clear mechanism responsible for the change of the NAO could not be given. This study provides further details of the NAO response to Indian Ocean sea surface temperature (SST) anomalies. This is done by conducting experiments with a coupled ocean–atmosphere general circulation model (OAGCM). The authors develop a hypothesis of how the Indian Ocean impacts the NAO.

Description: The Coupled Model Intercomparison Project (CMIP) involves study and intercomparison of multimodel simulations of present and future climate. The simulations of the future use idealized forcing in which CO, increase is compounded 1% yr(-1) until it doubles (near year 70) with global coupled models that contain, typically, components representing atmosphere, ocean, sea ice, and land surface. Results from CMIP diagnostic sub-projects were presented at the Second CMIP Workshop held at the Max Planck Institute for Meteorology in Hamburg, Germany, in September 2003. Significant progress in diagnosing and understanding results from global coupled models has been made since the time of the First CMIP Workshop in Melbourne, Australia, in 1998. For example, the issue of flux adjustment is slowly fading as more and more models obtain stable multicentury surface climates without them. El Nino variability, usually about half the observed amplitude in the previous generation of coupled models, is now more accurately simulated in the present generation of global coupled models, though there are still biases in simulating the patterns of maximum variability. Typical resolutions of atmospheric component models contained in coupled models are now usually around 2.5degrees latitude-longitude, with the ocean components often having about twice the atmospheric model resolution, with even higher resolution in the equatorial Tropics. Some new-generation coupled models have atmospheric resolutions of around 1.5degrees latitude - longitude. Modeling groups now routinely run the CMIP control and 1% CO2 simulations in addition to twentieth- and twenty-first-century climate simulations with a variety of forcings e.g., volcanoes, solar variability, anthropogenic sulfate aerosols, ozone, and greenhouse gases, with the anthropogenic forcings for future climate as well. However, persistent systematic errors noted in previous generations of global coupled models are still present in the current generation (e.g., overextensive equatorial Pacific cold tongue, double ITCZ). This points to the next challenge for the global coupled climate modeling community. Planning and commencement of the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4) has prompted rapid coupled model development, which is leading to an expanded CMIP-like activity to collect and analyze results for the control, 1% CO2, and twentieth-, twenty-first, and twenty-second-century simulations performed for the AR4. The international climate community is encouraged to become involved in this analysis effort.

Description: The solar radiative properties of cirrus clouds depend on ice particle shape, size, and orientation, as well as on the spatial cloud structure. Radiation schemes in atmospheric circulation models rely on estimates of cloud optical thickness only. In the present work, a Monte Carlo radiative transfer code is applied to various cirrus cloud scenarios to obtain the radiative response of uncertainties in the above-mentioned microphysical and spatial cloud properties (except orientation). First, plane-parallel homogeneous (0D) clouds with different crystal shapes (hexagonal columns, irregular polycrystals) and 114 different size distributions have been considered. The resulting variabilities in the solar radiative fluxes are in the order of a few percent for the reflected and about 1% for the diffusely transmitted fluxes. Largest variabilities in the order of 10% to 30% are found for the solar broadband absorptance. However, these variabilities are smaller than the flux differences caused by the choice of ice particle geometries. The influence of cloud inhomogeneities on the radiative fluxes has been examined with the help of time series of Raman lidar extinction coefficient profiles as input for the radiative transfer calculations. Significant differences between results for inhomogeneous and plane-parallel clouds were found. These differences are in the same order of magnitude as those arising from using extremely different crystal shapes for the radiative transfer calculations. From this sensitivity study, the ranking of cirrus cloud properties according to their importance in solar broadband radiative transfer is optical thickness, ice crystal shape, ice particle size, and spatial structure.

Description: The Labrador Sea is one of the few regions of the World Ocean where deep convection takes place. Several moorings across the Labrador continental slope just north of Hamilton Bank show that convection does take place within the Labrador Current. Mixing above the lower Labrador slope is facilitated by the onshore along-isopycnal intrusions of low-potential-vorticity eddies that weaken the stratification, combined with baroclinic instability that sustains slanted mixing while restratifying the water column through horizontal fluxes. Above the shelf break, the Irminger seawater core is displaced onshore while the stratification weakens with the increase in isopycnal slope. The change in stratification is partially due to the onshore shift of the “classical” Labrador Current, baroclinic instability, and possibly slantwise convection.

Description: Repeated shipboard observation sections across the boundary flow off northeastern Brazil as well as acoustic Doppler current profiler (ADCP) and current-meter records from a moored boundary array deployed during 2000–04 near 11°S are analyzed here for both the northward warm water flow by the North Brazil Undercurrent (NBUC) above approximately 1100 m and the southward flow of North Atlantic Deep Water (NADW) underneath. At 5°S, the mean from nine sections yields an NBUC transport of 26.5 ± 3.7 Sv (Sv ≡ 106 m3 s−1) along the boundary; at 11°S the mean NBUC transport from five sections is 25.4 ± 7.4 Sv, confirming that the NBUC is already well developed at 11°S. At both latitudes a persistent offshore southward recirculation between 200- and 1100-m depth reduces the net northward warm water flow through the 5°S section (west of 31.5°W) to 22.1 ± 5.3 Sv and through the 11°S section to 21.7 ± 4.1 Sv (west of 32.0°W). The 4-yr-long NBUC transport time series from 11°S yields a seasonal cycle of 2.5 Sv amplitude with its northward maximum in July. Interannual NBUC transport variations are small, varying only by ±1.2 Sv during the four years, with no detectable trend. The southward flow of NADW within the deep western boundary current at 5°S is 25.5 ± 8.3 Sv with an offshore northward recirculation, yielding a nine-section mean of 20.3 ± 10.1 Sv west of 31.5°W. For Antarctic Bottom Water, a net northward flow of 4.4 ± 3.0 Sv is determined at 5°S. For the 11°S section, the moored array data show a pronounced energy maximum at 60–70-day period in the NADW depth range, which was identified in related work as deep eddies translating southward along the boundary. Based on a kinematic eddy model fit to the first half of the moored time series, the mean NADW transfer by the deep eddies at 11°S was estimated to be about 17 Sv. Given the large interannual variability of the deep near-boundary transport time series, which ranged from 14 to 24 Sv, the 11°S mean was considered to be not distinguishable from the mean at 5°S

Description: Analysis of aeolo-marine dust deposits in the subtropical eastern Atlantic enables the strength of the major wind patterns during the late Quaternary to be evaluated and gives an insight into the climate of North Africa.

Description: Sexual mimicry among animals is widespread, but does it impart a fertilization advantage in the widely accepted ‘sneak–guard’ model of sperm competition? Here we describe field results in which a dramatic facultative switch in sexual phenotype by sneaker-male cuttlefish leads to immediate fertilization success, even in the presence of the consort male. These results are surprising, given the high rate at which females reject copulation attempts by males, the strong mate-guarding behaviour of consort males, and the high level of sperm competition in this complex mating system

Description: Analyses of a 500-yr control integration with the non-flux-adjusted coupled atmosphere–sea ice–ocean model ECHAM5/Max-Planck-Institute Ocean Model (MPI-OM) show pronounced multidecadal fluctuations of the Atlantic overturning circulation and the associated meridional heat transport. The period of the oscillations is about 70–80 yr. The low-frequency variability of the meridional overturning circulation (MOC) contributes substantially to sea surface temperature and sea ice fluctuations in the North Atlantic. The strength of the overturning circulation is related to the convective activity in the deep-water formation regions, most notably the Labrador Sea, and the time-varying control on the freshwater export from the Arctic to the convection sites modulates the overturning circulation. The variability is sustained by an interplay between the storage and release of freshwater from the central Arctic and circulation changes in the Nordic Seas that are caused by variations in the Atlantic heat and salt transport. The relatively high resolution in the deep-water formation region and the Arctic Ocean suggests that a better representation of convective and frontal processes not only leads to an improvement in the mean state but also introduces new mechanisms determining multidecadal variability in large-scale ocean circulation.

Description: The tropics have been suggested as the drivers of global ocean and atmosphere circulation and biogeochemical cycling during the extreme warmth of the Cretaceous period1, 2; but the links between orbital forcing, freshwater runoff and the biogeochemistry of continental margins in extreme greenhouse conditions are not fully understood. Here we present Cretaceous records of geochemical tracers for freshwater runoff obtained from a sediment core off the Ivory Coast that indicate that alternating periods of arid and humid African climate were driven by orbital precession. Our simulations of the precession-driven patterns of river discharge with a global climate model suggest that ocean anoxia and black shale sedimentation were directly caused by high river discharge, and occurred specifically when the northern equinox coincided with perihelion (the minimum distance between the Sun and the Earth). We conclude that, in a warm climate, the oceans off tropical continental margins respond rapidly and sensitively to even modest changes in river discharge.

Description: The ocean area south of the Antarctic Circumpolar Current (ACC) frontal system is a region of major watermass modification. Influx of North Atlantic Deep Water (NADW), small-scale mixing, eddy transport and diffusion, as well as the fluxes of momentum and buoyancy at the sea surface combine in a complex array of processes to generate the unique stratification of the Southern Ocean with its southward uprising isopycnals and northward flux of Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water. Comprehensive analytical models of this scenario are rare. The authors develop and apply a model based on zonally and temporally averaged theory to explain the conversion of NADW into AAIW with all of the aforementioned processes contained in an extremely simplified way. Eddies appear via a transformed Eulerian mean (TEM) approach with a conventional downgradient parameterization of the meridional density flux. The structure of the eddy coefficient is estimated from hydrographic and wind stress data by a simple inverse approach. Mixing is limited to a near-surface layer and is treated in a most simple entrainment form. The model determines the zonal mean density stratification in the Southern Ocean and the baroclinic transport of the ACC from the applied wind stress and the surface density flux and unravels the role and importance of the different processes responsible for shaping the stratification (Ekman and eddy-induced advection and pumping, mixing, surface buoyancy flux, and eddy-induced diffusion). All of these processes must be present to yield an agreement between the simulated stratification and the observed one, but details of their parameterization might not be too critical. The ACC transport is shown to have a contribution forced by the local wind stress as well as another contribution relating to the nonlocal forcing by wind stress and density flux over the entire Antarctic zone.

Description: We have analyzed the decadal-scale variability in the Tropical Pacific by means of observations and numerical model simulations. The two leading modes of the sea surface temperature (SST) variability in the central western Pacific are a decadal mode with a period of about 10 years and the ENSO mode with a dominant period of about four years. The SST anomaly pattern of the decadal mode is ENSO-like. The decadal mode, however, explains most variance in the western equatorial Pacific and off the equator. A simulation with an ocean general circulation model (OGCM) forced by reanalysis data is used to explore the origin of the decadal mode. It is found that the variability of the shallow subtropical-tropical overturning cells (STCs) is an important factor in driving the decadal mode. This is supported by results from a multi-century integration with a coupled ocean-atmosphere general circulation model (CGCM) that realistically simulates Tropical Pacific decadal variability. Finally, the sensitivity of the STCs to greenhouse warming is discussed by analyzing the results of a scenario integration with the same CGCM.