THE EFFECTS OF MECHANICAL FORCING ON THE MEAN MERIDIONAL CIRCULATION AND TRANSFER PROPERTIES OF THE ATMOSPHERE

Experiments using a quasi-geostrophic model and the ECMWF T21 spectral model with and without orography are performed to investigate the effects of mechanical forcing on the mean meridional circulation. Results show that mechanical forcing intensifies the horizontal poleward heat flux and redistributes the eddy angular momentum in the vertical, and that this changes significantly the intensity and location of the mean meridional circulation centres.It is shown how the mean meridional circulation is set up in such a way to satisfy both the dynamical and thermodynamical transport requirements of the model atmosphere. Whenever external forcing changes the eddy fluxes, the Coriolis torques from the upper horizontal branches of the mean meridional circulations change to balance the extra divergence of eddy momentum flux, and additional adiabatic heating is produced by the vertical branches of the toroids to balance the extra divergence of eddy heat flux. The mean meridional circulation is, therefore, confirm     

Arctic Climate Changes Based on Historical Simulations(1900-2013) with the CAMS-CSM

The Chinese Academy of Meteorological Sciences Climate System Model(CAMS-CSM) is a newly developed global climate model that will participate in the Coupled Model Intercomparison Project phase 6. Based on historical simulations(1900-2013), we evaluate the model performance in simulating the observed characteristics of the Arctic climate system, which includes air temperature, precipitation, the Arctic Oscillation(AO), ocean temperature/salinity,the Atlantic meridional overturning circulation(AMOC), snow cover, and sea ice. The model-data comparisons indicate that the CAMS-CSM reproduces spatial patterns of climatological mean air temperature over the Arctic(60°-90°N) and a rapid warming trend from 1979 to 2013. However, the warming trend is overestimated south of the Arctic Circle, implying a subdued Arctic amplification. The distribution of climatological precipitation in the Arctic is broadly captured in the model, whereas it shows limited skills in depicting the overall increasing trend. The AO can be reproduced by the CAMS-CSM in terms of reasonable patterns and variability. Regarding the ocean simulation, the model underestimates the AMOC and zonally averaged ocean temperatures and salinity above a depth of 500 m, and it fails to reproduce the observed increasing trend in the upper ocean heat content in the Arctic. The largescale distribution of the snow cover extent(SCE) in the Northern Hemisphere and the overall decreasing trend in the spring SCE are captured by the CAMS-CSM, while the biased magnitudes exist. Due to the underestimation of the AMOC and the poor quantification of air–sea interaction, the CAMS-CSM overestimates regional sea ice and underestimates the observed decreasing trend in Arctic sea–ice area in September. Overall, the CAMS-CSM reproduces a climatological distribution of the Arctic climate system and general trends from 1979 to 2013 compared with the observations, but it shows limited skills in modeling local trends and interannual variability.     

A Note on the Role of Meridional Wind Stress Anomalies and Heat Flux in ENSO Simulations

Four comparative experiments and some supplementary experiments were conducted to examine the role of meridional wind stress anomalies and heat flux variability in ENSO simulations by using a high-resolution Ocean General Circulation Model （OGCM）. The results indicate that changes in the direction and magnitude of meridional wind stress anomalies have little influence on ENSO simulations until meridional wind stress anomalies are unrealistically enlarged by a factor of 5.0. However, evidence of an impact on ENSO simulations due to heat flux variability was found. The simulated Nino-3 index without the effect of heat flux anomalies tended to be around 1.0° lower than the observed, as well as the control run, during the peak months of ENSO events.     

Evaluation of Arctic Sea-ice Cover and Thickness Simulated by MITgcm

A regional Arctic Ocean configuration of the Massachusetts Institute of Technology General Circulation Model(MITgcm)is applied to simulate the Arctic sea ice from 1991 to 2012.The simulations are evaluated by comparing them with observations from different sources.The results show that MITgcm can reproduce the interannual and seasonal variability of the sea-ice extent,but underestimates the trend in sea-ice extent,especially in September.The ice concentration and thickness distributions are comparable to those from the observations,with most deviations within the observational uncertainties and less than 0.5 m,respectively.The simulated sea-ice extents are better correlated with observations in September,with a correlation coefficient of 0.95,than in March,with a correlation coefficient of 0.83.However,the distributions of sea-ice concentration are better simulated in March,with higher pattern correlation coefficients(0.98)than in September.When the model underestimates the atmospheric influence on the sea-ice evolution in March,deviations in the sea-ice concentration arise at the ice edges and are higher than those in September.In contrast,when the model underestimates the oceanic boundaries’influence on the September sea-ice evolution,disagreements in the distribution of the sea-ice concentration and its trend are found over most marginal seas in the Arctic Ocean.The uncertainties of the model,whereby it fails to incorporate the atmospheric information in March and oceanic information in September,contribute to varying model errors with the seasons.     

ENSO phase-locking in an ocean-atmosphere coupled model FGCM-1.0

The mean climatology and the basic characteristics of the ENSO cycle simulated by a coupled model FGCM-1.0 are investigated in this study. Although with some common model biases as in other directly coupled models, FGCM-1.0 is capable of producing the interannual variability of the tropical Pacific, such as the ENSO phenomenon. The mechanism of the ENSO events in the coupled model can be explained by “delayed oscillator” and “recharge-discharge” hypotheses. Compared to the observations, the simulated ENSO events show larger amplitude with two distinctive types of phase-locking： one with its peak phase-locked to boreal winter and the other to boreal summer. These two types of events have a similar frequency of occurrence, but since the second type of event is seldom observed, it may be related to the biases of the coupled model. Analysis show that the heat content anomalies originate from the central south Pacific in the type of events peaking in boreal summer, which can be attributed to a different background climatology from the normal events. The mechanisms of their evolutions are also discussed.     

How Are El Nio and La Nia Events Improved in an Eddy-Resolving Ocean General Circulation Model?

The present study compares the performance of two versions of the LASG/IAP(State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics) Climate System Ocean Model(LICOM) in reproducing the interannual variability associated with El Nio and La Nia events in the tropical Pacific. Both versions are forced with the identical boundary conditions from observed or reanalysis data, in which one version has a finer spatial resolution of(1/10)° in the horizontal domain and 55 vertical layers, and the other version has a coarse resolution of 1° in the horizontal domain and 30 vertical layers. ENSO simulations form the two versions are compared with observations and, in particular, the improvements with regard to ENSO by the finer resolution ocean model are emphasized. As a result of the finer spatial resolution, both the vertical temperature gradient and vertical velocity are better represented in the equatorial Pacific than they are by the coarse resolution model; and thus, the corresponding vertical advections of temperature are more reasonable. Besides the mean climatology, simulated ENSO events and relevant feedbacks are much improved in the finer resolution model. A heat budget analysis suggests that both thermocline feedback and Ekman feedback are mainly responsible for the rapid increase in temperature anomalies during the developing and mature phases of ENSO events.     

An Assessment of the Quality of Surface Sensible Heat Flux Derived from Reanalysis Data through Comparison with Station Observations in Northwest China

The present study compares seasonal and interdecadal variations in surface sensible heat flux over Northwest China between station observations and ERA-40 and NCEP-NCAR reanalysis data for the period 1960-2000.While the seasonal variation in sensible heat flux is found to be consistent between station observations and the two reanalysis datasets,both land-air temperatures difference and surface wind speed show remarkable systematic differences.The sensible heat flux displays obvious interdecadal variability that is season-dependent.In the ERA-40 data,the sensible heat flux in spring,fall,and winter shows interdecadal variations that are similar to observations.In the NCEP-NCAR reanalysis data,sensible heat flux variations are inconsistent with and sometimes even opposite to observations.While surface wind speeds from the NCEP-NCAR reanalysis data show interdecadal changes consistent with station observations,variations in land-air temperature difference differ greatly from the observed dataset.In terms of land-air temperature difference and surface wind speed,almost no consistency with observations can be identified in the ERA-40 data,apart from the land-air temperature difference in fall and winter.These inconsistencies pose a major obstacle to the application in climate studies of surface sensible heat flux derived from reanalysis data.     

Extended-Range Forecasts of the Principal 20–30-Day Oscillation of the Circulation over East Asia During the Summer of 2002

Daily 850-hPa meridional wind fields in East Asia from March to September 2002 were used to establish a model of the principal oscillation pattern (POP). This model was then used to conduct independent extended-range forecasts of the principal temporal and spatial variations in the low-frequency meridional wind field on a time scale of 20-30 days. These variations affect the occurrence of heavy precipitation events in the lower reaches of the Yangtze River valley (LYRV). The results of 135 forecast experiments during the summer half year show that the predicted and observed anomalies are strongly correlated at a lead time of 20 days (mean correlation greater than 0.50). This strong correlation indicates that the model is capable of accurately forecasting the low-frequency variations in meridional wind that corresponded to the 3 heavy precipitation events in the LYRV during the summer of 2002. Further forecast experiments based on data from multiple years with significant 20-30-day oscillations show that these prediction modes are effective tools for forecasting the space-time evolution of the low-frequency circulation. These findings offer potential for improving the accuracy of forecasts of heavy precipitation over the LYRV at lead times of 3-4 weeks.     

Simulation of Tropospheric Ozone with MOZART-2： An Evaluation Study over East Asia

Climate changes induced by human activities have attracted a great amount of attention. With this, a coupling system of an atmospheric chemistry model and a climate model is greatly needed in China for better understanding the interaction between atmospheric chemical components and the climate. As the first step to realize this coupling goal, the three-dimensional global atmospheric chemistry transport model MOZART-2 (the global Model of Ozone and Related Chemical Tracers, version 2) coupled with CAM2 (the Community Atmosphere Model, version 2) is set up and the model results are compared against observations obtained in East Asia in order to evaluate the model performance. Comparison of simulated ozone mixing ratios with ground level observations at Minamitorishima and Ryori and with ozonesonde data at Naha and Tateno in Japan shows that the observed ozone concentrations can be reproduced reasonably well at Minamitorishima but they tend to be slightly overestimated in winter and autumn while underestimated a little in summer at Ryori. The model also captures the general features of surface CO seasonal variations quite well, while it underestimates CO levels at both Minamitorishima and Ryori. The underestimation is primarily associated with the emission inventory adopted in this study. Compared with the ozonesonde data, the simulated vertical gradient and magnitude of ozone can be reasonably well simulated with a little overestimation in winter, especially in the upper troposphere. The model also generally captures the seasonal, latitudinal and altitudinal variations in ozone concentration. Analysis indicates that the underestimation of tropopause height in February contributes to the overestimation of winter ozone in the upper and middle troposphere at Tateno.     

What Determines the Amplitude of ENSO Events?

In this paper, the dynamic effect of oceanic upwelling on the intensity of El Nio-Southern Oscillation (ENSO) is studied using a simple coupled model (Zebiak-Cane Model). The term balance analysis in the temperature variability equation shows that the anomalous upwelling of the mean vertical temperature gradient and the mean advection of the anomalous meridional temperature gradient are the two of most important factors that determine the intensity of ENSO events, in which the "vertical oceanic heat flux" in the eastern equatorial Pacific (EEP) is the primary influencing factor. The lag correlation between "vertical heat flux (VHF)" and ENSO intensity shows that the highest correlation occurs when the former leads the latter by one to two weeks. The VHF is positively correlated with the background thermocline strength in the EEP, and an increase of both could result in strong ENSO variability. Comparison of the forced and coupled experiments suggests that the coupled process can affect both the intensity and frequency of ENSO.     

The impact of boreal autumn SST anomalies over the South Pacific on boreal winter precipitation over East Asia

The possible mechanism behind the variability in the dipole pattern of boreal winter precipitation over East Asia is analyzed in this study. The results show that the SST anomalies(SSTAs) over the South Pacific Ocean(SPO) in boreal autumn are closely related to the variability in the dipole pattern of boreal winter precipitation over East Asia. The physical link between the boreal autumn SPO SSTAs and the boreal winter East Asian precipitation dipole pattern is shown to mainly be the seasonal persistence of the SPO SSTAs themselves. The seasonal persistence of the SPO SSTAs can memorize and transport the signal of the boreal autumn SSTAs to the following winter, and then stimulates a meridional teleconnection pattern from the SH to the NH, resulting in a meridional dipole pattern of atmospheric circulation over East Asia in boreal winter. As a major influencing factor, this dipole pattern of the atmospheric circulation can finally lead to the anomalous precipitation dipole pattern over East Asia in boreal winter. These observed physical processes are further confirmed in this study through numerical simulation. The evidence from this study, showing the impact of the SPO SSTAs in boreal autumn,not only deepens our understanding of the variability in East Asian boreal winter precipitation, but also provides a potentially useful predictor for precipitation in the region.     

Preliminary evaluations of FGOALS-g2 for decadal predictions

The Flexible Global Ocean-Atmosphere-Land System model, Grid-point Version 2 (FGOALS-g2) for decadal predictions, is evaluated preliminarily, based on sets of ensemble 10-year hindcasts that it has produced. The results show that the hindcasts were more accurate in decadal variability of SST and surface air temperature (SAT), particularly in that of Nin o3.4 SST and China regional SAT, than the second sample of the historical runs for 20th-century climate (the control) by the same model. Both the control and the hindcasts represented the global warming well using the same external forcings, but the control overestimated the warming. The hindcasts produced the warming closer to the observations. Performance of FGOALS-g2 in hindcasts benefits from more realistic initial conditions provided by the initialization run and a smaller model bias resulting from the use of a dynamic bias correction scheme newly developed in this study. The initialization consists of a 61-year nudging-based assimilation cycle, which follows on the control run on 01 January 1945 with the incorporation of observation data of upper-ocean temperature and salinity at each integration step in the ocean component model, the LASG IAP Climate System Ocean Model, Version 2 (LICOM2). The dynamic bias correction is implemented at each step of LICOM2 during the hindcasts to reduce the systematic biases existing in upper-ocean temperature and salinity by incorporating multi-year monthly mean increments produced in the assimilation cycle. The effectiveness of the assimilation cycle and the role of the correction scheme were assessed prior to the hindcasts.     

Relationships between Interannual and Intraseasonal Variations of the Asian-Western Pacific Summer Monsoon Hindcasted by BCC_CSM1.1（m）

Using hindcasts of the Beijing Climate Center Climate System Model, the relationships between interannual variability （IAV） and intraseasonal variability （ISV） of the Asian-western Pacific summer monsoon are diagnosed. Predictions show reasonable skill with respect to some basic characteristics of the ISV and IAV of the western North Pacific summer monsoon （WNPSM） and the Indian summer monsoon （ISM）. However, the links between the seasonally averaged ISV （SAISV） and seasonal mean of ISM are overestimated by the model. This deficiency may be partially attributable to the overestimated frequency of long breaks and underestimated frequency of long active spells of ISV in normal ISM years, although the model is capable of capturing the impact of ISV on the seasonal mean by its shift in the probability of phases. Furthermore, the interannual relationships of seasonal mean, SAISV, and seasonally averaged long-wave variability （SALWV; i.e., the part with periods longer than the intraseasonal scale） of the WNPSM and ISM with SST and low-level circulation are examined. The observed seasonal mean, SAISV, and SALWV show similar correlation patterns with SST and atmospheric circulation, but with different details. However, the model presents these correlation distributions with unrealistically small differences among different scales, and it somewhat overestimates the teleconnection between monsoon and tropical central-eastern Pacific SST for the ISM, but underestimates it for the WNPSM, the latter of which is partially related to the too-rapid decrease in the impact of E1 Nifio-Southern Oscillation with forecast time in the model.     

A fast version of LASG/IAP climate system model and its 1000-year control integration

A fast version of the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geo- physical Fluid Dynamics(LASG)/Institute of Atmospheric Physics(IAP)climate system model is briefly documented.The fast coupled model employs a low resolution version of the atmospheric component Grid Atmospheric Model of IAP/LASG(GAMIL),with the other parts of the model,namely an oceanic component LASG/IAP Climate Ocean Model(LICOM),land component Common Land Model(CLM),and sea ice component from National Center for Atmospheric Research Community Climate System Model (NCAR CCSM2),as the same as in the standard version of LASG/IAP Flexible Global Ocean Atmosphere Land System model(FGOALS g).The parameterizations of physical and dynamical processes of the at- mospheric component in the fast version are identical to the standard version,although some parameter values are different.However,by virtue of reduced horizontal resolution and increased time-step of the most time-consuming atmospheric component,it runs faster by a factor of 3 and can serve as a useful tool for long- term and large-ensemble integrations.A 1000-year control simulation of the present-day climate has been completed without flux adjustments.The final 600 years of this simulation has virtually no trends in global mean sea surface temperatures and is recommended for internal variability studies.Several aspects of the control simulation’s mean climate and variability are evaluated against the observational or reanalysis data. The strengths and weaknesses of the control simulation are evaluated.The mean atmospheric circulation is well simulated,except in high latitudes.The Asian-Australian monsoonal meridional cell shows realistic features,however,an artificial rainfall center is located to the eastern periphery of the Tibetan Plateau persists throughout the year.The mean bias of SST resembles that of the standard version,appearing as a"double ITCZ"(Inter-Tropical Convergence Zone)associated with a westward extension of the equatorial eastern Pacific cold tongue.The sea ice extent is acceptable but has a higher concentration.The strength of Atlantic meridional overturning is 27.5 Sv.Evidence from the 600-year simulation suggests a modulation of internal variability on ENSO frequency,since both regular and irregular oscillations of ENSO are found during the different time periods of the long-term simulation.     

The Shallow Meridional Overturning Circulation in the Northern Indian Ocean and Its Interannual Variability

The shallow meridional overturning circulation (upper 1000 m) in the northern Indian Ocean and its interannual variability are studied, based on a global ocean circulation model (MOM2) with an integration of 10 years (1987-1996). It is shown that the shallow meridional overturning circulation has a prominent seasonal reversal characteristic. In winter, the flow is northward in the upper layer and returns southward at great depth. In summer, the deep northward inflow upwells north of the equator and returns southward in the Ekman layer. In the annual mean, the northward inflow returns through two branches: one is a southward flow in the Ekman layer, the other is a flow that sinks near 10°N and returns southward between 500 m and 1000 m. There is significant interannual variability in the shallow meridional overturning circulation, with a stronger (weaker) one in 1989 (1991) and with a period of about four years. The interannual variability of the shallow meridional overturning circulation is intimately r     

The Role of the Kuroshio in the Winter North Pacific Ocean-Atmosphere Interaction： Comparison of a Coupled Model and Observations

A comparative study between the output of the Flexible Global Climate Model Version 1.0 (FGCM- 1.0) and the observations is performed. At 500 hPa, the geopotential height of FGCM is similar to the observations, but in the North Pacific the model gives lower values, and the differences are most significant over the northern boundary of the Pacific. In a net heat flux comparison, the spatial patterns of the two are similar in winter, but more heat loss appears to the east of Japan in FGCM than in COADS. On the interannual timescale, strong (weak) Kuroshio transports to the east of Taiwan lead the increasing (decreasing) net heat flux, which is centered over the Kuroshio Extension region, by 1–2 months, with low (high) pressure anomaly responses appearing at 500 hPa over the North Pacific (north of 25N) in winter. The northward heat transport of the Kuroshio is one of the important heat sources to support the warming of the atmosphere by the ocean and the formation of the low pressure anomaly at 500 hPa over the North Pacific in winter.     

SIMULATION OF SUMMER MONSOON CLIMATE OVER EASTERN CHINA USING A REGIONAL SPECTRAL MODEL

In this paper, we evaluate the characteristics of the surface air temperature and the precipitation of summer monsoon, using the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) for 20 years (1984-2003). The RSM model was designated over the eastern China with a horizontal grid spacing of approximately 30 km. The model is driven by the NCEP/NCAR reanalysis data and runs from May 21 to September 1 for each of the 20 years. The distribution and variation patterns of the 20-year summer mean surface air temperature and precipitation are reproduced by the RSM and the differences between the simulation and observation are small. However, the model overestimates the interannual variability of summer precipitation in eastern China. The correlation coefficients of the 20-year averaging summer precipitation over the whole region and the sub-domains are above 0.8. The simulated probability distributions of daily maximum and minimum temperatures are similar to the observations. Days of different precipitation intensities in the simulations are generally consistent with the observations: the simulated days of light rain, moderate rain, heavy rain and torrential rain closely resemble the observations, but the simulated maximum centers of the distribution are north of the observed ones.     

A Numerical Study on the Winter Monsoon and Cold Surge over East Asia

By using the improved regional climate model （RegCM_NCC）, a numerical study has been undertaken for the East Asia region over a period of 5 years （1998-2002） in an effort to evaluate the model＇s ability to reproduce the winter monsoon conditions that were observed. The results showed that the model can successfully simulate the basic characteristics of the winter monsoon circulations, including the location and intensity of the cold-surface, high-pressure system, as well as the wind patterns and the intensity of the winter monsoon. The simulated occurrence frequency and regions of the cold surge were consistent with the observations. The simulated rainfall distribution over China was consistent with the observations collected in South China. The features of the simulated moisture transport were also in good agreement with the observations that were derived from the NCEP reanalysis data, indicating that moisture transport coming from the Bay of Bengal trough plays a crucial role in supplying moisture needed for precipitation in South China. In addition, the moisture transport coming from the near-equatorial west-Pacific was also important. These two branches of moisture transport converged in South China, as a prerequisite for occurrence of the precipitation that was observed there. Heat budgets have shown that the development of a heat sink over the East Asian continent was remarkable and its thermal contrast relative to the neighboring seas was the important forcing factor for the winter monsoon activity. The simulation also indicated that the significant differences in circulation patterns and rainfalls during the winters of 1997/98 and 1998/99 were affected by cold and warm ENSO events, respectively. The above analysis demonstrated the model＇s ability to simulate the East Asian winter monsoon.     

华南春季降水和水汽输送的年代际变化特征

The characteristics of spring precipitation and water vapor transport in South China were analyzed by using observational data and the National Centers for Environmental Prediction （NCEP） reanalysis data. The results show that, during the spring, each component of the water cycle （precipitation, wind field, specific humidity, water vapor transport, etc.） in South China exhibits a notable interdecadal variability. An abrupt increase in spring precipitation occurred in the early 1970s. During the dry period from 1958 to 1971, a water vapor flux divergence （positive divQ） existed in South China, which may have led to the deficiency in rainfall. However, during the wet period from 1973 to 1989, there was a remarkable water vapor flux convergence （negative divQ） in South China, which may have resulted in the higher rainfall. The interdecadal variability of water vapor transport is closely related to the interdecadal variability of wind fields, although the interdecadal variability of specific humidity also plays a role to some extent, and the interdecadal variability of the zonal water vapor transport contributes much more to the interdecadal variability of spring precipitation than the meridional water vapor transport.     

Simulations of the East Asian subtropical westerly jet by LASG/IAP AGCMs

Performances of two LASG/IAP(State Key Laboratory of Numerical Modeling for Atmospheric Sci- ences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics)Atmospheric General Circulation Models(AGCMs),namely GAMIL and SAMIL,in simulating the major characteristics of the East Asian subtropical westerly jet(EASWJ)in the upper troposphere are examined in this paper.The mean vertical and horizontal structures and the correspondence of the EASWJ location to the meridional temperature gradient in the upper troposphere are well simulated by two models.However,both models underestimate the EASWJ intensity in winter and summer,and are unable to simulate the bimodal distribution of the ma- jor EASWJ centers in mid-summer,relative to the observation,especially for the SAMIL model.The biases in the simulated EASWJ intensity are found to be associated with the biases of the meridional temperature gradients in the troposphere,and furthermore with the surface sensible heat flux and condensation latent heating.The models capture the major characteristics of the seasonal evolution of the diabatic heating rate averaged between 30°-45°N,and its association with the westerly jet.However,the simulated maximum diabatic heating rate in summer is located westward in comparison with the observed position,with a rela- tively strong diabatic heating intensity,especially in GAMIL.The biases in simulating the diabatic heating fields lead to the biases in simulating the temperature distribution in the upper troposphere,which may further affect the EASWJ simulations.Therefore,it is necessary to improve the simulation of the meridional temperature gradient as well as the diabatic heating field in the troposphere for the improvement of the EASWJ simulation by GAMIL and SAMIL models.