Role of the ocean in controlling atmospheric CO2 concentration in the course of global glaciations

Responses of ocean circulation and ocean carbon cycle in the course of a global glaciation from the present Earth conditions are investigated by using a coupled climate-biogeochemical model. We investigate steady states of the climate system under colder conditions induced by a reduction of solar constant from the present condition. A globally ice-covered solution is obtained under the solar constant of 92.2% of the present value. We found that because almost all of sea water reaches the frozen point, the ocean stratification is maintained not by temperature but by salinity just before the global glaciation (at the solar constant of 92.3%). It is demonstrated that the ocean circulation is driven not by the surface cooling but by the surface freshwater forcing associated with formation and melting of sea ice. As a result, the deep ocean is ventilated exclusively by deep water formation in southern high latitudes where sea ice production takes place much more massively than northern high latitudes. We also found that atmospheric CO₂ concentration decreases through the ocean carbon cycle. This reduction is explained primarily by an increase of solubility of CO₂ due to a decrease of sea surface temperature, whereas the export production weakens by 30% just before the global glaciation. In order to investigate the conditions for the atmospheric CO₂ reduction to cause global glaciations, we also conduct a series of simulations in which the total amount of carbon in the atmosphere?Cocean system is reduced from the present condition. Under the present solar constant, the results show that the global glaciation takes place when the total carbon decreases to be 70% of the present-day value. Just before the glaciation, weathering rate becomes very small (almost 10% of the present value) and the organic carbon burial declines due to weakened biological productivity. Therefore, outgoing carbon flux from the atmosphere?Cocean system significantly decreases. This suggests the atmosphere?Cocean system has strong negative feedback loops against decline of the total carbon content. The results obtained here imply that some processes outside the atmosphere?Cocean feedback loops may be required to cause global glaciations.     

Impact of Surface Exchange Coefficients and Sea Surface Cooling on Tropical Cyclone Simulation

The surface flux exchange associated with the exchange coefficients and upper ocean conditions is essential to the development of tropical cyclones (TCs). Using the Weather Research and Forecasting (WRF) model, the present study has investigated the impact of exchange coefficients and ocean coupling during Super Typhoon Saomai (2006). Firstly, two experiments with different formula of roughness are conducted. The experiment with the Donelan formula for drag coefficient (Cd) and ramped formula for enthalpy coefficient (Ck) can simulate stronger intensity compared to other experiments due to the increased surface wind and enthalpy fluxes. That is because the new formulas allows for a smaller Cd and larger Ck in the high wind regime than the former formulas did. Moreover, two coupled simulations between WRF and a one-dimensional ocean model are conducted to examine the feedback of sea surface cooling to the TC. In the experiments with a horizontal uniform mixed layer depth of 70 m, the sea surface cooling is too weak to change the evolution of TC. While in the experiment with an input mixed layer calculated using the Hybrid Coordinate Ocean Model (HYCOM) data, the significant sea surface cooling induces obvious impact on TC intensity and structure. Under the negative feedback of sea surface cooling, the sensible and latent heat fluxes decreases, especially in the right part of Saomai (2006). The negative feedback with coupled ocean model plays a vital role in simulating the intensity and structure of TC.     

SURFACE OBSERVATIONS IN THE TROPICAL CYCLONE ENVIRONMENT OVER THE SOUTH CHINA SEA

In this paper,the observational data from Marine and Meteorological Observation Platform(MMOP)at Bohe,Maoming and buoys located in Shanwei and Maoming are used to study the characteristics of air-sea temperature and specific humidity difference and the relationship between wind and wave with the tropical cyclones over the South China Sea(SCS).The heat and momentum fluxes from eddy covariance measurement(EC)are compared with these fluxes calculated by the COARE 3.0 algorithm for Typhoon Koppu.The results show that at the developing and weakening stages of Koppu,both these differences between the sea surface and the near-surface atmosphere from the MMOP are negative,and data from the buoys also indicate that the differences are negative between the sea surface and near-surface atmosphere on the right rear portion of tropical cyclones(TCs)Molave and Chanthu.However,the differences are positive on the left front portion of Molave and Chanthu.These positive differences suggest that the heat flux is transferred from the ocean to the atmosphere,thus intensifying and maintaining the two TCs.The negative differences indicate that the ocean removes heat fluxes from the atmosphere,thus weakening the TCs.The wind-wave curves of TCs Molave and Chanthu show that significant wave height increases linearly with 2-min wind speed at 10-m height when the wind speed is less than 25 m/s,but when the wind speed is greater than 25 m/s,the significant wave height increases slightly with the wind speed.By comparing the observed sensible heat,latent heat,and friction velocity from EC with these variables from COARE 3.0 algorithm,a great bias between the observed and calculated sensible heat and latent heat fluxes is revealed,and the observed friction velocity is found to be almost the same as the calculated friction velocity.     

Adjustment and feedbacks in a global coupled ocean-atmosphere model

 We report the analysis of two 20-year simulations performed with the low resolution version of the IPSL coupled ocean-atmosphere model, with no flux correction at the air-sea interface. The simulated climate is characterized by a global sea surface temperature warming of about 4 °C in 20 years, driven by a net heat gain at the top of the atmosphere. Despite this drift, the circulation is quite realistic both in the ocean and the atmosphere. Several distinct periods are analyzed. The first corresponds to an adjustment during which the heat gain weakens both at the top of the atmosphere and at the ocean surface, and the tropical circulation is slightly modified. Then, the surface warming is enhanced by an increase of the greenhouse feedback. We show that the mechanisms involved in the model share common features with sensitivity experiments to greenhouse gases or to SST warming. At the top of the atmosphere, most of the longwave trapping in the atmosphere is driven by the tropical circulation. At the surface, the reduction of longwave cooling is a direct response to increased temperature and moisture content at low levels in the atmospheric model. During the last part of the simulation, a regulation occurs from evaporation at the surface and longwave cooling at TOA. Most of the model drift is attributed to a too large heating by solar radiation in middle and high latitudes. The reduction of the north–south temperature gradient, and the related changes in the meridional equator-to-pole ocean heat transport lead to a warming of equatorial and subtropical regions. This is also well demonstrated by the difference between the two simulations which differ only in the parametrization of sea-ice. When the sea-ice cover is not restored to climatology the model does not maintain sea-ice at high latitudes. The climate warms more rapidly and the water vapor and clouds feedback occurs earlier. Received: 24 May 1996 / Accepted: 29 November 1996     

南海季风区冰相相变潜热对中尺度对流云和降水影响作用的数值模拟研究

利用可分辨云模式及中国南海北部试验区加密探空的平均水平风场、位温场和水汽场模拟分析了1998年5月15日至6月11日中国南海北部地区中尺度对流系统(Mesoscal Convective System,简称MCS)中冰相相变潜热对云和降水、辐射传输以及大尺度环境场的影响作用。研究表明,冰相相变潜热总体上不会引起明显的大气辐射通量的变化,但会引起较明显的下垫面热通量的变化。凝华潜热释放显著地增加了大气稳定度,造成对流和下垫面热通量的减弱,从而导致地面降水减小10.11%。碰冻潜热释放也使得大气稳定度增加,不利于中尺度对流系统对流的发展,区域累积降水量减小2.2%。融化潜热的冷却效应,使得融化层以下的大气降温,从而增加了低层大气的不稳定性,有利于海面热通量的输送,导致MCS降水增加4.1%。因此,冰相相变潜热对降水的影响主要是通过影响大气环境稳定,进而影响洋面感热通量和潜热通量的垂直输送和对流的发展,导致区域降水改变。     

Feedbacks affecting the response of the thermohaline circulation to increasing CO2: a study with a model of intermediate complexity

A three-dimensional ocean model with an idealized geometry and coarse resolution coupled to a two-dimensional (zonally averaged) statistical-dynamical atmospheric model is used to simulate the response of the thermohaline circulation to increasing CO ₂ concentration in the atmosphere. The relative roles of different factors in the slowing down and recovery of the thermohaline circulation were studied by performing simulations with ocean only and partially coupled models. The computational efficiency of the model allows an extensive and thorough study of the causes of changes in the strength of the thermohaline circulation, through a large number of extended runs. The evolution of the atmosphere-to-ocean surface heat fluxes is shown to be the dominant factor in causing the weakening of the circulation in response to an increasing external forcing as well as in controlling the subsequent recovery. The feedback between heat flux and the sea surface temperature is necessary for the ocean circulation to recover. The rate of the recovery, however, is not sensitive to the magnitude of the feedback, and changes in the atmosphere, while contributing to the recovery, play a secondary role. In the case of very strong feedback, substantial changes in the SST structure are shown not to be a necessary condition for the recovery of the circulation. Subsurface changes in the density structure accompany recovery despite nearly fixed SST in one of the uncoupled experiments. Changes in the zonal distribution of heat fluxes serve as a positive feedback for both decrease and recovery of the meridional overturning, and are as important as changes in the zonal-mean values of heat fluxes. The secondary role of the moisture fluxes is explained by a smaller magnitude of their contribution to the surface buoyancy flux. Imposing amplified changes in the moisture fluxes leads to a significant slow down of the circulation, accompanied, however, by changes in the heat flux. The changed heat flux, in its turn, makes a significant contribution to the future slow down. This feedback complicates the evaluation of the relative roles of the different fluxes.     

Reduced Horizontal Sea Surface Temperature Gradients Under Conditions of Clear Skies and Weak Winds

Consideration of the dependence of various components of the sea-surface heat and momentum fluxes on sea surface temperature (SST) leads to an explanation for the observed reduction in the horizontal temperature gradients in the uppermost layer of the ocean (a few to 10 m in depth). Horizontal temperature gradients within the mixed layer can be masked by a near-surface layer of warm water. This camouflage of horizontal temperature gradients has importance for the remote sensing of SST used by the fishing industry, for the estimation of acoustic transmission, and for the forecasting of hurricane development, among many uses of SST data. Diurnal warming conditions in the Straits of Florida are examined by a simulation calculation and by analysis of observations obtained on moorings deployed on the south-east Florida shelf. When there is net heating (i.e., the solar input is stronger than the combined latent, sensible and longwave radiative heat losses) the originally warmer water experiences less heating than the colder water, leading to a weakening of the horizontal SST gradients as seen by surface buoys or satellites. The warmer water also experiences more mixing and therefore less increase in temperature. The strongest effect of the diurnal heating on wind stress occurs when the SST starts out cooler than the air temperature and the atmosphere is stably stratified. Diurnal warming can then rapidly increase the SST above the air temperature because of reduced wind stress and reduced upper-ocean mixing. After that the wind stress increases as convectively driven turbulence contributes to the atmospheric exchange.     

‘Observations and Modelling of Cold-air Advection over Arctic Sea Ice’

Aircraft observations of the atmospheric boundary layer (ABL) over Arctic sea ice were made during non-stationary conditions of cold-air advection with a cloud edge retreating through the study region. The sea-ice concentration, roughness, and ABL stratification varied in space. In the ABL heat budget, 80% of the Eulerian change in time was explained by cold-air advection and 20% by diabatic heating. With the cloud cover and inflow potential temperature profile prescribed as a function of time, the air temperature and near-surface fluxes of heat and momentum were well simulated by the applied two-dimensional mesoscale model. Model sensitivity tests demonstrated that several factors can be active in generating unstable stratification in the ABL over the Arctic sea ice in March. In this case, the upward sensible heat flux resulted from the combined effect of clouds, leads, and cold-air advection. These three factors interacted non-linearly with each other. From the point of view of ABL temperatures, the lead effect was far less important than the cloud effect, which influenced the temperature profiles via cloud-top radiative cooling and radiative heating of the snow surface. The steady-state simulations demonstrated that under overcast skies the evolution towards a deep, well-mixed ABL may take place through the merging of two mixed layers one related to mostly shear-driven surface mixing and the other to buoyancy-driven top-down mixing due to cloud-top radiative cooling.     

Effects of Sea Spray Evaporation and Dissipative Heating on Intensity and Structure of Tropical Cyclone

To examine effects of sea spray evaporation and dissipative heating on structure and intensity of a real tropical cyclone,the sea spray flux parameterization scheme was incorporated into the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model(MM5).Sensitivity tests were performed with varying the spray source function intensities and with and without dissipation heating.The numerical results indicate that sea spray evaporation increases the interfacial sensible heat flux,which is increased by 16% for the moderate spray and 47% for the heavy spray,but has little effect on the interfacial latent heat flux.The net effect of sea spray evaporation is to decrease the total sensible heat flux and to increase the total latent heat flux.The total enthalpy flux is increased by 1% and 12% with moderate and strong spray amounts,respectively.Consistent with these results,the intensity of the tropical cyclone is increased by 5% and 16% in maximum 10-m wind speed,respectively,due to sea spray evaporation.Sea spray evaporation and dissipative heating modify the tropical cyclone structure in important but complex ways.The effect of sea spray on the near-surface temperature and moisture depends on the spray amounts and its location within the tropical cyclone.Within the high-wind region of a tropical cyclone,the lower atmosphere becomes cooler and moister due to the evaporation of sea spray.However,the dissipative heating offsets the cooling due to sea spray evaporation,which makes the lower atmosphere warmer.     

On the climate response of the low-latitude Pacific Ocean to changes in the global freshwater cycle

Under global warming, the predicted intensification of the global freshwater cycle will modify the net freshwater flux at the ocean surface. Since the freshwater flux maintains ocean salinity structures, changes to the density-driven ocean circulation are likely. A modified ocean circulation could further alter the climate, potentially allowing rapid changes, as seen in the past. The relevant feedback mechanisms and timescales are poorly understood in detail, however, especially at low latitudes where the effects of salinity are relatively subtle. In an attempt to resolve some of these outstanding issues, we present an investigation of the climate response of the low-latitude Pacific region to changes in freshwater forcing. Initiated from the present-day thermohaline structure, a control run of a coupled ocean–atmosphere general circulation model is compared with a perturbation run in which the net freshwater flux is prescribed to be zero over the ocean. Such an extreme experiment helps to elucidate the general adjustment mechanisms and their timescales. The atmospheric greenhouse gas concentrations are held constant, and we restrict our attention to the adjustment of the upper 1,000 m of the Pacific Ocean between 40°N and 40°S, over 100 years. In the perturbation run, changes to the surface buoyancy, near-surface vertical mixing and mixed-layer depth are established within 1 year. Subsequently, relative to the control run, the surface of the low-latitude Pacific Ocean in the perturbation run warms by an average of 0.6°C, and the interior cools by up to 1.1°C, after a few decades. This vertical re-arrangement of the ocean heat content is shown to be achieved by a gradual shutdown of the heat flux due to isopycnal (i.e. along surfaces of constant density) mixing, the vertical component of which is downwards at low latitudes. This heat transfer depends crucially upon the existence of density-compensating temperature and salinity gradients on isopycnal surfaces. The timescale of the thermal changes in the perturbation run is therefore set by the timescale for the decay of isopycnal salinity gradients in response to the eliminated freshwater forcing, which we demonstrate to be around 10–20 years. Such isopycnal heat flux changes may play a role in the response of the low-latitude climate to a future accelerated freshwater cycle. Specifically, the mechanism appears to represent a weak negative sea surface temperature feedback, which we speculate might partially shield from view the anthropogenically-forced global warming signal at low latitudes. Furthermore, since the surface freshwater flux is shown to play a role in determining the ocean’s thermal structure, it follows that evaporation and/or precipitation biases in general circulation models are likely to cause sea surface temperature biases.     

Uncertainty in the ocean–atmosphere feedbacks associated with ENSO in the reanalysis products

The evolution of El Ni?o-Southern Oscillation (ENSO) variability can be characterized by various ocean–atmosphere feedbacks, for example, the influence of ENSO related sea surface temperature (SST) variability on the low-level wind and surface heat fluxes in the equatorial tropical Pacific, which in turn affects the evolution of the SST. An analysis of these feedbacks requires physically consistent observational data sets. Availability of various reanalysis data sets produced during the last 15?years provides such an opportunity. A consolidated estimate of ocean surface fluxes based on multiple reanalyses also helps understand biases in ENSO predictions and simulations from climate models. In this paper, the intensity and the spatial structure of ocean–atmosphere feedback terms (precipitation, surface wind stress, and ocean surface heat flux) associated with ENSO are evaluated for six different reanalysis products. The analysis provides an estimate for the feedback terms that could be used for model validation studies. The analysis includes the robustness of the estimate across different reanalyses. Results show that one of the “coupled” reanalysis among the six investigated is closer to the ensemble mean of the results, suggesting that the coupled data assimilation may have the potential to better capture the overall atmosphere–ocean feedback processes associated with ENSO than the uncoupled ones.     

Effects of tropical instability wave (TIW)-induced surface wind feedback in the tropical Pacific Ocean

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 (τ_TIW) part is diagnostically determined using an empirical τ_TIW 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 τ_TIW 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 τ_TIW–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.     

An OGCM Simulation of Seasonal and Interannual Variabilities in the Surface—Layer Pacific of the Equatorial Band

The heat budget is analyzed in the surface-layer (0-50 m) Pacific of the equatorial band (10°S-10°N),using the simulation of an ocean general circulation model from 1945 to 1993. The analysis indicates that downward net surface heat flux from the atmosphere and ocean advective heat fluxes play distinct roles in seasonal and interannual variabilities of surface-layer ocean temperature. The surface heat flux dominantly determines the ocean temperature in the seasonal time-scale. But, it has a negative feedback to the ocean temperature in the interannual time-scale. The interannual variability of ocean temperature is largely associated with the cold advection from off-equatorial divergent flow in the central Pacific and from upwelling in the cold tongue. Both the surface heat flux and ocean advective heat fluxes are important to the ocean temperature during an El Nino event. The ocean advective heat fluxes are further associated with local westward trade wind in the central Pacific. These results are largely consistent with some regional observational analyses.     

赤道不稳定波海气相互作用的数值模拟分析

赤道不稳定波(tropical instability waves)存在于热带东太平洋赤道附近, 通常于每年的春末夏初出现, 以约0.6 m/s速度向西传播, 波周期为20～40天左右, 波长约为1000～2000 km。本文利用一个全球高分辨率海气耦合模式对赤道不稳定波在赤道附近的热量输送进行分析, 表明赤道不稳定波产生指向赤道的热通量, 从而部分抵消了热带东太平洋地区由Ekman辐散和温度平流导致的强冷却效应, 维持热带地区的热量平衡。其对赤道冷舌区的增暖作用可以消除和减弱气候模式中热带东太平洋地区的系统性冷偏差, 能使冷舌的强度和分布得到合理的改善, 对气候模式的改进和发展具有潜在贡献。赤道不稳定波还可以改变赤道海洋上空低层大气层结稳定度, 导致近地层强的风场辐合辐散, 并进一步影响大气混合层的温度、 风场等气象要素。模拟分析结果还表明, 赤道不稳定波对大气强迫产生二次响应, 改变赤道上空逆温层的垂直位移和逆温强度。研究赤道不稳定波对热带海洋气候及其海气相互作用机理的理解具有重要意义。     

Extratropical large-scale air-sea interaction in a coupled and uncoupled ocean-atmosphere model

Spatial patterns of mid-latitude large-scale ocean-atmosphere interaction on monthly to seasonal time scales have been observed to exhibit a similar structure in both the North Pacific and North Atlantic basins. These patterns have been interpreted as a generic oceanic response to surface wind anomalies, whereby the anomalous winds give rise to corresponding anomalous regions of surface heat flux and consequent oceanic cooling. This mechanistic concept is investigated in this study using numerical models of a global atmosphere and a mid-latitude ocean basin (nominally the Atlantic). The models were run in both coupled and uncoupled mode. Model output was used to generate multi-year time series of monthly mean fields. Empirical orthogonal function (EOF) and singular value decomposition (SVD) analyses were then used to obtain the principal patterns of variability in heat flux, air temperature, wind speed, and sea surface temperature (SST), and to determine the relationships among these variables. SVD analysis indicates that the turbulent heat flux from the ocean to the atmosphere is primarily controlled by the surface scalar wind speed, and to a lesser extent by air temperature and SST. The principal patterns of air-sea interaction are closely analogous to those found in observational data. In the atmosphere, the pattern consists of a simultaneous strengthening (or weakening) of the mid-latitude westerlies and the easterly trades. In the ocean there is cooling (warming) under the anomalously strong (weak) westerlies and trade winds, with a weaker warming (cooling) in the region separating the westerly and easterly wind regimes. These patterns occur in both coupled and uncoupled models and the primary influence of the coupling is in localizing the interaction patterns. The oceanic patterns can be explained by the principal patterns of surface heat flux and the attendant warming or cooling of the ocean mixed layer.     

Sensitivity experiments performed with an energy balance atmosphere model coupled to an advection-diffusion ocean model

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

南海海域海-气耦合模式及其数值模拟试验

在ＮＣＡＲ区域气候模式ＲｅｇＧＭ２和普林斯顿海洋模式ＰＯＭ基础上发展适用于区域海－气相互作用研究的区域海－气耦合模式,模式采用同步耦合、海洋模式将海表温度提供给大气模式,大气模式为海洋模式提供太阳短波辐射、感热能量、潜热通量。海洋与大气模式每１５ｍｉｎ交换一次通量。耦合过程没有使用通量校正。使用该模式对中国南海区域１９９５年５－７月大气和海洋进行了模拟试验,将模拟结果与ＣＯＡＤＳ通量强迫的模拟结果     

中国地区夏季平均加热率的时空分布特征

The latitude-altitude distributions of radiative fluxes and heating rates are investigated by utilizing CloudSat satellite data over China during summer. The Tibetan Plateau causes the downward shortwave fluxes of the lower atmosphere over central China to be smaller than the fluxes over southern and northern China by generating more clouds. The existence of a larger quantity of clouds over central China reflects a greater amount of solar radiation back into space. The vertical gradients of upward shortwave radiative fluxes in the atmosphere below 8 km are greater than those above 8 km. The latitudinal-altitude distributions of downward longwave radiative fluxes show a slantwise decreasing trend from low latitudes to high latitudes that gradually weaken in the downward direction. The upward longwave radiative fluxes also weaken in the upward direction but with larger gradients. The maximum heating rates by solar radiation and cooling rates by longwave infrared radiation are located over 28-40°N at 7-8 km mean sea level (MSL), and they are larger than the rates in the northern and southern regions. The heating and cooling rates match well both vertically and geographically.     

Influence of coupling on atmosphere, sea ice and ocean regional models in the Ross Sea sector, Antarctica

Air–sea ice–ocean interactions in the Ross Sea sector form dense waters that feed the global thermohaline circulation. In this paper, we develop the new limited-area ocean–sea ice–atmosphere coupled model TANGO to simulate the Ross Sea sector. TANGO is built up by coupling the atmospheric limited-area model MAR to a regional configuration of the ocean–sea ice model NEMO. A method is then developed to identify the mechanisms by which local coupling affects the simulations. TANGO is shown to simulate realistic sea ice properties and atmospheric surface temperatures. These skills are mostly related to the skills of the stand alone atmospheric and oceanic models used to build TANGO. Nonetheless, air temperatures over ocean and winter sea ice thickness are found to be slightly improved in coupled simulations as compared to standard stand alone ones. Local atmosphere ocean feedbacks over the open ocean are found to significantly influence ocean temperature and salinity. In a stand alone ocean configuration, the dry and cold air produces an ocean cooling through sensible and latent heat loss. In a coupled configuration, the atmosphere is in turn moistened and warmed by the ocean; sensible and latent heat loss is therefore reduced as compared to the stand alone simulations. The atmosphere is found to be less sensitive to local feedbacks than the ocean. Effects of local feedbacks are increased in the coastal area because of the presence of sea ice. It is suggested that slow heat conduction within sea ice could amplify the feedbacks. These local feedbacks result in less sea ice production in polynyas in coupled mode, with a subsequent reduction in deep water formation.     

On the subarctic North Atlantic cooling due to global warming

Various ocean reanalysis data reveal that the subarctic Atlantic sea surface temperature (SST) has been cooling during the twentieth century. A similar cooling pattern is found in the doubling CO₂ experiment obtained from the CMIP3 (coupled model intercomparison project third phase) compared to the pre-industrial experiment. Here, in order to investigate the main driver of this cooling, we perform the heat budget analysis on the subarctic Atlantic upper ocean temperature. The net surface heat flux associated with the increased concentration of greenhouse gases heats the subarctic ocean surface. In the most of models, the longwave radiation, latent heat flux, and sensible heat flux exert a warming effect, and the shortwave radiation exerts a cooling effect. On the other hand, the thermal advection by the meridional current reduces the subarctic upper ocean temperature in all models. This cold advection is attributed to the weakening of the meridional overturning circulation, which is related to the reduction in the ocean surface density. In particular, greater warming of the surface air than of the sea surface results in the reduction of surface evaporation and thereby enhanced freshening of the ocean surface water, while precipitation change was smaller than evaporation change. The thermal advections by both the wind-driven Ekman current and the density-driven geostrophic current contribute to cooling in most of the models, where the heat transport by the geostrophic current tends to be larger than that by the Ekman current.