Distinguishing coupled ocean–atmosphere interactions from background noise in the North Pacific

When considering physical mechanisms for decadal-timescale climate variability in the North Pacific, it is useful to describe in detail the expected response of the ocean to the chaotic atmospheric forcing. The expected response to this white-noise forcing includes strongly enhanced power in the decadal frequency band relative to higher frequencies, pronounced changes in basin-wide climate that resemble regime shifts, preferred patterns of spatial variability, and a depth-dependent profile that includes variability with a standard deviation of 0.2–0.4°C over the top 50–100 m. Weak spectral peaks are also possible, given ocean dynamics. Detecting coupled ocean–atmosphere modes of variability in the real climate system is difficult against the spectral and spatial structure of this ‘null-hypothesis’ of how the ocean and atmosphere interact, especially given the impossibility of experimentally decoupling the ocean from the atmosphere. Turning to coupled ocean–atmosphere models to address this question, a method for identifying coupled modes by using models of increasing physical complexity is illustrated. It is found that a coupled ocean–atmosphere mode accounts for enhanced variability with a time scale of 20 years/cycle in the Kuroshio extension region of the model's North Pacific. The observed Pacific Decadal Oscillation (PDO) has many similarities to the expected noise-forced response and few similarities to the model's coupled ocean–atmosphere variability. However, model deficiencies and some analyses of observations by other workers indicate that the possibility that part of the PDO arises from a coupled ocean–atmosphere mode cannot be ruled out.     

Climate and carbon cycle variations in the 20th and 21st centuries in a model of intermediate complexity

The climate model of intermediate complexity developed at the Oboukhov Institute of Atmospheric Physics, Russian Academy of Sciences (IAP RAS CM), has been supplemented by a zero-dimensional carbon cycle model. With the carbon dioxide emissions prescribed for the second half of the 19th century and for the 20th century, the model satisfactorily reproduces characteristics of the carbon cycle over this period. However, with continued anthropogenic CO₂ emissions (SRES scenarios A1B, A2, B1, and B2), the climate-carbon cycle feedback in the model leads to an additional atmospheric CO₂ increase (in comparison with the case where the influence of climate changes on the carbon exchange between the atmosphere and the underlying surface is disregarded). This additional increase is varied in the range 67–90 ppmv depending on the scenario and is mainly due to the dynamics of soil carbon storage. The climate-carbon cycle feedback parameter varies nonmonotonically with time. Positions of its extremes separate characteristic periods of the change in the intensity of anthropogenic emissions and of climate variations. By the end of the 21st century, depending on the emission scenario, the carbon dioxide concentration is expected to increase to 615–875 ppmv and the global temperature will rise by 2.4–3.4 K relative to the preindustrial value. In the 20th–21st centuries, a general growth of the buildup of carbon dioxide in the atmosphere and ocean and its reduction in terrestrial ecosystems can be expected. In general, by the end of the 21st century, the more aggressive emission scenarios are characterized by a smaller climate-carbon cycle feedback parameter, a lower sensitivity of climate to a single increase in the atmospheric concentration of carbon dioxide, a larger fraction of anthropogenic emissions stored in the atmosphere and the ocean, and a smaller fraction of emissions in terrestrial ecosystems.     

Sensitivity analysis of the climate of a chaotic system

This paper addresses some fundamental methodological issues concerning the sensitivity analysis of chaotic geophysical systems. We show, using the Lorenz system as an example, that a naïve approach to variational ("adjoint") sensitivity analysis is of limited utility. Applied to trajectories which are long relative to the predictability time scales of the system, cumulative error growth means that adjoint results diverge exponentially from the "macroscopic climate sensitivity"(that is, the sensitivity of time‐averaged properties of the system to finite‐amplitude perturbations). This problem occurs even for time‐averaged quantities and given infinite computing resources. Alternatively, applied to very short trajectories, the adjoint provides an incorrect estimate of the sensitivity, even if averaged over large numbers of initial conditions, because a finite time scale is required for the model climate to respond fully to certain perturbations. In the Lorenz (1963) system, an intermediate time scale is found on which an ensemble of adjoint gradients can give a reasonably accurate (O(10%)) estimate of the macroscopic climate sensitivity. While this ensemble‐adjoint approach is unlikely to be reliable for more complex systems, it may provide useful guidance in identifying important parameter‐combinations to be explored further through direct finite‐amplitude perturbations.     

Significance of non‐rotational wind stress in the seasonal variation of continental torque

The physical mechanism by which seasonally varying atmospheric wind stress exerted on the sea surface is communicated to the solid earth as oceanic pressure torque (continental torque) and bottom frictional torque is investigated with a linear shallow‐water numerical model of barotropic oceans. The model has a realistic land–ocean distribution and is driven by a seasonally varying climatic wind stress. A novel way to decompose the wind stress into rotational and non‐rotational components is devised. The rotational component drives ocean circulations as classical theories of wind‐driven circulations demonstrate. The non‐rotational component does not produce ocean circulations within the framework of a barotropic shallow‐water model, but balances with the pressure gradient force due to surface displacement in the steady state. Based on this decomposition, it is shown that most of the continental torque which plays a major role in producing the seasonal variation of length of day (LOD) is caused by the non‐rotational component of the wind stress. Both continental torque due to the wind‐driven circulation produced by the rotational component of the wind stress and the bottom frictional torque are of minor importance.     

Modelling snowdrift sublimation and its effect on the moisture budget of the atmospheric boundary layer

A one‐dimensional atmospheric surface layer model including turbulent diffusion and gravitational settling of suspended snow particles is used to simulate vertical profiles of snowdrift sublimation rates and the associated effects on the humidity and temperature profiles in the lowest 10 m. The simulations show that the thermodynamic feedback effects associated with snowdrift sublimation, i.e., strong increases in humidity and cooling, can significantly reduce the snowdrift sublimation rate, in particular in strong winds when large numbers of particles are being suspended. This negative feedback occurs because snowdrift sublimation depends on the undersaturation and temperature. Mechanisms that take away moisture from the surface layer, such as entrainment or horizontal advection of dry air, tend to weaken this feedback and enhance modelled snowdrift sublimation as the air generally remains undersaturated. Near the surface, however, the thermodynamic feedbacks dominate in strong winds, reducing the upward moisture flux from the surface. Then, snowdrift sublimation is the main contributor to the upward moisture flux at 10 m. Interestingly, in strong winds, the simulated total upward moisture flux in snowdrifting conditions is less than that in similar non‐drifting conditions. Hence, the model results indicate that occurrence of snowdrift sublimation may, counterintuitively, eventually lead to a reduction of the surface‐atmosphere moisture transport.     

Validating and assessing the sensitivity of the climate model with an ocean general circulation model developed at the Institute of Atmospheric Physics,Russian Academy of Sciences

A new version of the Institute of Atmospheric Physics, Russian Academy of Sciences (IAP RAS), climate model (CM) has been developed using an ocean general circulation model instead of the statistical-dynamical ocean model applied in the previous version. The spatial resolution of the new ocean model is 3° in latitude and 5° in longitude, with 25 unevenly spaced vertical levels. In the previous version of the oceanic model, as in the atmospheric model, the horizontal resolution was 4.5° in latitude and 6° in longitude, with four vertical levels (the upper quasi-homogeneous layer, seasonal thermocline, abyssal ocean, and bottom friction layer). There is no correction for the heat and momentum fluxes between the atmosphere and ocean in the new version of the IAP RAS CM. Numerical experiments with the IAP RAS CM have been performed under current initial and boundary conditions, as well as with an increasing concentration of atmospheric carbon dioxide. The main simulated atmospheric and oceanic fields agree quite well with observational data. The new version’s equilibrium temperature sensitivity to atmospheric CO₂ doubling was found to be 2.9 K. This value lies in the mid-range of estimates (2–4.5 K) obtained from simulations with state-of-the-art models of different complexities.     

Analysis on the annual variation characteristics of heat exchange on the atmosphere-sea interface over the tropical Pacific

The tropical ocean is the area where the interaction process between atmosphere and sea is most active.To analyze the sensible and latent heat flux (abridged as SHF and LHF hereafter) over the tropical ocean and their spatial and temporal variation as well as the relationship between them and other factors are all essential in understanding the thermo-dynamic interaction mecha nism between atmosphere and sea.These are also useful in the further study on the unusual oceanic and atmospheric circulation and on the climate modelling.By using EOF method,we have discussed the LFH and SFH over the tropical Pacific and the causality factors of the heat flux,the main purpose is to initialize some new approaches with climatic significance.     

Southern Ocean transformation in a coupled model with and without eddy mass fluxes

A coupled air–sea general circulation model is used to simulate the global circulation. Different parameterizations of lateral mixing in the ocean by eddies, horizontal, isopycnal, and isopycnal plus eddy advective flux, are compared from the perspective of water mass transformation in the Southern Ocean. The different mixing physics imply different buoyancy equilibria in the surface mixed layer, different transformations, and therefore a variety of meridional overturning streamfunctions. The coupled‐model approach avoids strong artificial water mass transformation associated with relaxation to prescribed mixed layer conditions. Instead, transformation results from the more physical non‐local, nonlinear interdependence of sea‐surface temperature, air–sea fluxes, and circulation in the model's atmosphere and ocean. The development of a stronger mid‐depth circulation cell and associated upwelling when eddy fluxes are present, is examined. The strength of overturning is diagnosed in density coordinates using the transformation framework.     

基于CCSM3气候模式的同化模拟试验

基于美国NCAR及其他科学家合作发展的共同气候系统模式CCSM3,利用nudging方法开展了把15 m到465 m的次表层海温同化到该模式的研究。1980—2000年的同化试验结果表明,经过同化得到的模拟结果与实际较为一致,较好的再现了中低纬太平洋海洋和大气的平均特征和随时间演变的规律,但仍存在如海表温度偏高、降水偏强等问题。尤其是在大洋的东边界,陆地地形比较陡峭的地区,通常出现较大的偏差。     

Numerical simulation of the world ocean circulation and its climatic variability for 1948–2007 using the INMOM

The results of simulating global ocean circulation and its interannual variability in 1948–2007 using INM RAS ocean general circulation model INMOM (Institute of Numerical Mathematics Ocean Model) are presented. One of the INMOM versions is also used for the Black Sea dynamics simulation. The CORE datasets were used to set realistic atmospheric forcing. Sea ice area decrease by 2007 was reproduced in the Arctic Ocean that is in good agreement with observations. The interdecadal climatic variability was revealed with significant decrease of Atlantic thermohaline circulation (ATHC) and meridional heat transport (MHT) in North Atlantic (NA) since the late 1990’s. MHT presents decrease of heat transport from NA to the atmosphere since the mid-1990’s. Therefore the negative feedback is revealed in the Earth climate system that leads to reducing of climate warming caused primarily by anthropogenic factor for the last decades. Long-term variability (60 years) of ATHC is revealed as well which influences NA thermal state with 10 year delay. The assumption is argued that this mechanism can make a contribution in the ATHC own long-term variability.     

A spectral barotropic model of the wind-driven world ocean

A global spectral barotropic ocean model is introduced to describe the depth-averaged flow. The equations are based on vorticity and divergence (instead of horizontal momentum); continents exert a nearly infinite drag on the fluid. The coding follows that of spectral atmospheric general circulation models using triangular truncation and implicit time integration to provide a first step for seamless coupling to spectral atmospheric global circulation models and an efficient method for filtering of ocean wave dynamics. Five experiments demonstrate the model performance: (i) Bounded by an idealized basin geometry and driven by a zonally uniform wind stress, the ocean circulation shows close similarity with Munk’s analytical solution. (ii) With a real land–sea mask the model is capable of reproducing the spin-up, location and magnitudes of depth-averaged barotropic ocean currents. (iii) The ocean wave-dynamics of equatorial waves, excited by a height perturbation at the equator, shows wave dispersion and reflection at eastern and western coastal boundaries. (iv) The model reproduces propagation times of observed surface gravity waves in the Pacific with real bathymetry. (v) Advection of tracers can be simulated reasonably by the spectral method or a semi-Langrangian transport scheme. This spectral barotropic model may serve as a first step towards an intermediate complexity spectral atmosphere–ocean model for studying atmosphere–ocean interactions in idealized setups and long term climate variability beyond millennia.     

Evaluation of a dynamically downscaled atmospheric reanalyse in the prospect of forcing long term simulations of the ocean circulation in the Gulf of Lions

The paper evaluates atmospheric reanalysis as possible forcing of model simulations of the ocean circulation inter-annual variability in the Gulf of Lions in the Western Mediterranean Sea between 1990 and 2000. The sensitivity of the coastal atmospheric patterns to the model resolution is investigated using the REMO regional climate model (18 km, 1 h), and the recent global atmospheric reanalysis ERA40 (125 km, 6 h). At scales from a few years to a few days, both atmospheric data sets exhibit a very similar weather, and agreement between REMO and ERA40 is especially good on the seasonal cycle and at the daily variability scale. At smaller scales, REMO reproduces more realistic spatio-temporal patterns in the ocean forcing: specific wind systems, particular atmospheric behaviour on the shelf, diurnal cycle, sea-breeze. Ocean twin experiments (1990–1993) clearly underline REMO skills to drive dominant oceanic processes in this microtidal area. Finer wind patterns induce a more realistic circulation and hydrology of the shelf water: unique shelf circulation, upwelling, temperature and salinity exchanges at the shelf break. The hourly sampling of REMO introduces a diurnal forcing which enhances the behaviour of the ocean mixed layer. In addition, the more numerous wind extremes modify the exchanges at the shelf break: favouring the export of dense shelf water, enhancing the mesoscale variability and the interactions of the along slope current with the bathymetry.     

Natural air–sea flux of CO2 in simulations of the NASA-GISS climate model: Sensitivity to the physical ocean model formulation

Results from twin control simulations of the preindustrial CO₂ gas exchange (natural flux of CO₂) between the ocean and the atmosphere are presented here using the NASA-GISS climate model, in which the same atmospheric component (modelE2) is coupled to two different ocean models, the Russell ocean model and HYCOM. Both incarnations of the GISS climate model are also coupled to the same ocean biogeochemistry module (NOBM) which estimates prognostic distributions for biotic and abiotic fields that influence the air–sea flux of CO₂. Model intercomparison is carried out at equilibrium conditions and model differences are contrasted with biases from present day climatologies. Although the models agree on the spatial patterns of the air–sea flux of CO₂, they disagree on the strength of the North Atlantic and Southern Ocean sinks mainly because of kinematic (winds) and chemistry (pCO₂) differences rather than thermodynamic (SST) ones. Biology/chemistry dissimilarities in the models stem from the different parameterizations of advective and diffusive processes, such as overturning, mixing and horizontal tracer advection and to a lesser degree from parameterizations of biogeochemical processes such as gravitational settling and sinking. The global meridional overturning circulation illustrates much of the different behavior of the biological pump in the two models, together with differences in mixed layer depth which are responsible for different SST, DIC and nutrient distributions in the two models and consequently different atmospheric feedbacks (in the wind, net heat and freshwater fluxes into the ocean).     

Multidecadal climate variability in the subtropics/mid‐latitudes of the Southern Hemisphere oceans

Observations of multidecadal variability in sea surface temperature (SST), surface air temperature and winds over the Southern Hemisphere are presented and an ocean general circulation model applied towards investigating links between the SST variability and that of the overlying atmosphere. The results suggest that the dynamical effect of the wind stress anomalies is significant mainly in the neighbourhood of the western boundary currents and their outflows across the mid‐latitudes of each Southern Hemisphere basin (more so in the South Indian and South Atlantic than in the South Pacific Ocean) and in the equatorial upwelling zones. Over most of the subtropics to mid‐latitudes of the Southern Hemisphere oceans, changes in net surface heat flux (particularly in latent heat) appear to be more important for the SST variability than dynamical effects. Implications of these results for modelling and understanding low frequency climate variability in the Southern Hemisphere as well as possible links with mechanisms of decadal/interdecadal variability in the Northern Hemisphere are discussed.     

Atmosphere-ocean general circulation model with the carbon cycle

Results from numerical experiments with an atmosphere-ocean general circulation model coupled to the carbon evolution cycle are analyzed. The model is used to carry out an experiment on the simulation of the climate and carbon cycle change in 1861–2100 under a specified scenario of the carbon dioxide emission from fossil fuel and land use. The spatial distribution of vegetation, soil, and oceanic carbon in the 20th century is generally close to available estimates from observational data. The model adequately reproduces the observed growth of atmospheric CO₂ in the 20th century and the uptake of excess carbon by land ecosystems and by the ocean in the 1980s and 1990s. By 2100, the atmospheric CO₂ concentration is calculated to reach 742 ppmv under emission and land-use scenario A1B. The feedback between climate change and the carbon cycle in the model is positive, with a coefficient close to the mean of all the current models. The ocean and land uptakes of the CO₂ emission by 2100 in the model are 25 and 19%, which are also close to the mean over all models.     

FORECAST OF SEA ICE REGIME

It has been found that the sea-surface temperature and the characteristics of atmospheric circulation in the preceding months are closely related to the temperature and ice regime in winter months. This relationship is strikingly reflected over the strong ocean current regions and over the regions with quasi-permanent atmospheric center actions. It has also been shown that the influence of the ocean on the atmosphere is more pronounced over these regions. This relationship may offer a key for long-term forecasting of the sea-ice regime in winter. In addition, because there is an obvious instability, the stabilities of the correlation coefficients are analysed. In consideration of the fact that the formation of weather process changes with the variations of time scale, predictions for longer and shorter time scale processes are discussed separately.In conclusion, some forecasting results obtained and tests made in recent years are given.     

ENSO事件次表层海温的两个模态及其对大气环流的影响

利用SODA海洋同化资料和NCEP再分析大气资料,分析了热带太平洋次表层海温异常(subsurfaceoceantemperatureanomaly,SOTA)与厄尔尼诺与南方涛动(ElNi?o-SouthernOscillation,ENSO)循环的联系,及SOTA对大气环流的影响。回顾传统ENSO研究,指出存在的问题,提出了ENSO影响大气研究的新思路,得到以下结果:(1)以SOTA为基本资料的研究发现, ENSO事件有两个模态,主要出现在冬季的第一模态对冬季及夏季亚洲-北太平洋-北美地区上空中高纬大气环流有重要影响,主要出现在夏季的第二模态对该地区上空夏季热带和副热带大气系统有重要作用。(2)ENSO事件通过与ENSO相联系的热带太平洋海面温度异常(ENSO-relatedseasurface temperatureanomaly,RSSTA)对大气的异常热通量输送,强迫Walker环流和Hadley环流变化,导致热带和北太平洋及周边地区上空大气环流异常,进而影响相关地区冬季和夏季的气候。(3)海表面温度异常(seasurfacetemperatureanomaly,SSTA)包含RSSTA和大气异常导致的海温变化(sea temperature anomaly caused by atmospheric anomaly, STA)两部分, RSSTA是ENSO事件过程中海洋内部热动力结构调整导致的海面温度变化,在海洋对大气的热输送过程中,它随ENSO事件演变不断更新;STA是大气受RSSTA海洋异常加热后导致的大气环流异常对海面温度的影响,在海洋浅表层STA对RSSTA有重大影响。本文最后讨论了ENSO事件期间热带海洋对大气热输送过程,指出ENSO事件通过海洋内部热动力结构调整产生RSSTA,它直接对大气异常加热,导致大气环流和气候异常,局地海气之间负反馈过程产生STA,反过来抑制RSSTA。结果还指出,人们常用的SSTA变率实际上主要由秋冬季节RSSTA主导,丢失了春夏季ENSO信息,用SSTA研究ENSO事件存在局限性,这也可能是ENSO事件春季预报障碍的原因之一。     

Model estimates for the sensitivity of atmospheric centers of action to global climate changes

The sensitivity of the characteristics of atmospheric centers of action (ACAs) in the Northern Hemisphere to global climate changes is analyzed on the basis of models of different complexity, including the climate model of intermediate complexity of the Institute of Atmospheric Physics, Russian Academy of Sciences and the ECHAM4/OPYC3 and HadCM3 general circulation models of the atmosphere and ocean. The emphasis is on the analysis of trends of the change in ACA characteristics in winter, when the long-term global warming is most considerable. The global climate models are shown to be able to describe not only the intermediate regimes of ACAs but also their dynamics. In particular, ECHAM4/OPYC3 is capable of reproducing the statistically significant connection of the characteristics of the North Pacific centers of action with El Niño/La Niña events, revealed from observational data. With the use of the results of the global climate models, the possible changes in the characteristics of centers of action in the 21st century are estimated for an increased content of greenhouse gases in the atmosphere.     

DMS and its oxidation products in the remote marine atmosphere: implications for climate and atmospheric chemistry

DMS emitted into the atmosphere over the global oceans has a range of effects upon atmospheric composition (mediated through various oxidation products) that may be significant with regard to issues as important as climate regulation, and the trace gas oxidation capacity of the marine atmospheric boundary layer. The roles played by DMS oxidation products within these contexts are diverse and complex, and in many instances are not well understood. Here we summarize what is known, and suspected, about the couplings between the marine atmospheric sulfur cycle, other atmospheric chemical cycles, and the dynamics and microphysics of the marine atmospheric boundary layer. This overview focuses heavily on measurements carried out in clean Southern Ocean air masses in association with the Australian Baseline Air Pollution Station located at Cape Grim (40° 40′ 56″S, 144° 41′ 18″ E), Tasmania. The data confirm that in the remote marine atmosphere, DMS is a central player in a variety of important atmospheric processes, reinforcing the need to understand quantitatively the factors that regulate DMS emissions from the ocean to the atmosphere.     

Downward longwave radiation in general circulation models: a case study at a semi‐arid continental site

The present case study evaluates the downward longwave radiation at the surface (DLR) in several high‐resolution (≈1°) general circulation models (GCMs) using surface observations from a semiarid continental site in New South Wales, Australia (Uardry, 34.39°S, 142.30°E). This site is located on a large grassland plain uniform in both its land use and landcover type, and is therefore particularly well suited for a comparison with GCM grid mean values. Monthly averages of newly constructed clear‐sky and all‐sky DLR climatologies and the resulting cloud‐radiative forcing are compared. It is shown that the GCMs exceed the observed DLR under cloud‐free conditions by 10–20 W m⁻² at this semiarid site on an annual basis, with a strong seasonal dependence. The calculated clear‐sky fluxes are overestimated during the warmer summer season, with large absolute values of DLR, while the biases are reduced in the colder and dryer winter season with smaller fluxes. This gives direct support for recent evidence that the DLR model biases depend systematically on the thermal and humidity structure of the cloudless atmosphere. Fluxes from strongly emitting atmospheres tend to be overestimated, but may be underestimated from atmospheres with smaller emission. This points to common problems inherent in the simulation of the emission from the cloudless atmosphere in current longwave radiation codes.
The comparisons of the all‐sky climatologies at Uardry show that the clear‐sky biases are partly masked in the models with an insufficient cloud‐radiative forcing, thereby counterbalancing the excessive DLR of the cloud‐free atmosphere. On the other hand, when the cloudradiative forcing is improved, the biases in the cloud‐free atmosphere become fully apparent in the all‐sky fluxes.