Air–Sea Coupling Enhances the East Asian Winter Climate Response to the Atlantic Multidecadal Oscillation

A simple air-sea coupled model, the atmospheric general circulation model （AGCM） of the National Centers for Environmental Prediction coupled to a mixed-layer slab ocean model, is employed to investigate the impact of air-sea coupling on the signals of the Atlantic Multidecadal Oscillation （AMO）. A regional coupling strategy is applied, in which coupling is switched off in the extratropical North Atlantic Ocean but switched on in the open oceans elsewhere. The coupled model is forced with warm-phase AMO SST anomalies, and the modeled responses are compared with those from parallel uncoupled AGCM experiments with the same SST forcing. The results suggest that the regionally coupled responses not only resemble the AGCM simulation, but also have a stronger intensity. In comparison, the coupled responses bear greater similarity to the observational composite anomaly. Thus, air-sea coupling enhances the responses of the East Asian winter climate to the AMO. To determine the mechanism responsible for the coupling amplification, an additional set of AGCM experiments, forced with the AMO-induced tropical SST anomalies, is conducted. The SST anomalies are extracted from the simulated AMO-induced SST response in the regionally coupled model. The results suggest that the SST anomalies contribute to the coupling amplification. Thus, tropical air-sea coupling feedback tends to enhance the responses of the East Asian winter climate to the AMO.

Arctic Sea Ice and Eurasian Climate:A Review

The Arctic plays a fundamental role in the climate system and has shown significant climate change in recent decades,including the Arctic warming and decline of Arctic sea-ice extent and thickness. In contrast to the Arctic warming and reduction of Arctic sea ice, Europe, East Asia and North America have experienced anomalously cold conditions, with record snowfall during recent years. In this paper, we review current understanding of the sea-ice impacts on the Eurasian climate.Paleo, observational and modelling studies are covered to summarize several major themes, including： the variability of Arctic sea ice and its controls; the likely causes and apparent impacts of the Arctic sea-ice decline during the satellite era,as well as past and projected future impacts and trends; the links and feedback mechanisms between the Arctic sea ice and the Arctic Oscillation/North Atlantic Oscillation, the recent Eurasian cooling, winter atmospheric circulation, summer precipitation in East Asia, spring snowfall over Eurasia, East Asian winter monsoon, and midlatitude extreme weather; and the remote climate response（e.g., atmospheric circulation, air temperature） to changes in Arctic sea ice. We conclude with a brief summary and suggestions for future research.

A New Method for Predicting the Decadal Component of Global SST

A simple approach that considers both internal decadal variability and the effect of anthropogenic forcing is developed to predict the decadal components of global sea surface temperatures (SSTs) for the three decades 2011-2040. The internal decadal component is derived by harmonic wave expansion analyses based on the quasiperiodic evolution of the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO), as obtained from observational SST datasets. Furthermore, the external decadal component induced by anthropogenic forcing is assessed with a second-order fit based on the ensemble of projected SSTs in the experiments with multiple coupled climate models associated with the third Coupled Model Intercomparison Project (CMIP3) under the Intergovernmental Panels on Climate Change (IPCC) Special Reports on Emissions Scenario (SRES) A1B. A validation for the years from 2002 to 2010 based on a comparison of the predicted and the observed SST and their spatial correlation, as well as the root mean square error (RMSE), suggests that the approach is reasonable overall. In addition, the predicted results over the 50°S-50°N global band, the Indian Ocean, the western Pacific Ocean, the tropical eastern Pacific Ocean, and the North and the South Atlantic Ocean are presented.

Mapping Spatial and Temporal Variations of Leaf Area Index for Winter Wheat in North China

Leaf area index （LAI） is an important parameter in a number of models related to ecosystem functioning, carbon budgets, climate, hydrology, and crop growth simulation. Mapping and monitoring the spatial and temporal variations of LAI are necessary for understanding crop growth and development at regional level. In this study, the relationships between LAI of winter wheat and Landsat TM spectral vegetation indices （SVIs） were analyzed by using the curve estimation procedure in North China Plain. The series of LAI maps retrieved by the best regression model were used to assess the spatial and temporal variations of winter wheat LAI. The results indicated that the general relationships between LAI and SVIs were curvilinear, and that the exponential model gave a better fit than the linear model or other nonlinear models for most SVIs. The best regression model was constructed using an exponential model between surface-reflectance-derived difference vegetation index （DVI） and LAI, with the adjusted R2 （0.82） and the RMSE （0.77）. The TM LAI maps retrieved from DVILAI model showed the significant spatial and temporal variations. The mean TM LAI value （30 m） for winter wheat of the study area increased from 1.29 （March 7, 2004） to 3.43 （April 8, 2004）, with standard deviations of 0.22 and 1.17, respectively. In conclusion, spectral vegetation indices from multi-temporal Landsat TM images can be used to produce fine-resolution LAI maps for winter wheat in North China Plain.

A High Resolution Nonhydrostatic Tropical Atmospheric Model and Its Performance

A high resolution nonhydrostatic tropical atmospheric model is developed by using a ready-made regional atmospheric modeling system. The motivation is to investigate the convective activities associated with the tropical intraseasonal oscillation (ISO) through a cloud resolving calculation. Due to limitations in computing resources, a

Implementation of a One-Dimensional Enthalpy Sea-Ice Model in a Simple Pycnocline Prediction Model for Sea-Ice Data Assimilation Studies

To further explore enthalpy-based sea-ice assimilation, a one-dimensional （1D） enthalpy sea-ice model is implemented into a simple pycnocline prediction model. The 1D enthalpy sea-ice model includes the physical processes such as brine expulsion, flushing, and salt diffusion. After being coupled with the atmosphere and ocean components, the enthalpy sea-ice model can be integrated stably and serves as an important modulator of model variability. Results from a twin experiment show that the sea-ice data assimilation in the enthalpy space can produce smaller root-mean-square errors of model variables than the traditional scheme that assimilates the observations of ice concentration, especially for slow-varying states. This study provides some insights into the improvement of sea-ice data assimilation in a coupled general circulation model.

The seasonal foot printing mechanism of spring Arctic sea ice in the Bergen climate models

The influence of spring Arctic sea ice variability on the Pacific Decadal Oscillation （PDO） like sea surface temperature （SST） variability is established and investigated using an Atmosphere Ocean General Circulation Model （AOGCM） of the Bergen Climate Model version 2 （BCM2）. The spring Arctic sea ice variability affects the mid-latitudes and tropics through the propagation of the anomalous Eliassen-Palm （E-P） flux from the polar region to mid- and low-latitudes during boreal spring. The pathway includes anomalous upward wave activity, which propagates to the high troposphere from near the surface of the polar region, turns southward between 500 hPa and 200 hPa and extends downward between 50~N and 70~N, influencing the near surface atmospheric circulation. The alteration of the near surface atmospheric circulation then causes anomalous surface ocean circulation. These circulation changes consequently leads to the SST anomalies in the North Pacific which may persist until the following summer, named seasonal ＂foot printing＂ mechanism （SFPM）.

Skill Assessment of North American Multi-Models Ensemble (NMME) for June-September (JJAS) Seasonal Rainfall over Ethiopia

In recent years, there has been increasing demand for high-resolution seasonal climate forecasts at sufficient lead times to allow response planning from users in agriculture, hydrology, disaster risk management, and health, among others. This paper examines the forecasting skill of the North American Multi-model Ensemble (NMME) over Ethiopia during the June to September (JJAS) season. The NMME, one of the multi-model seasonal forecasting systems, regularly generates monthly seasonal rainfall forecasts over the globe with 0.5 - 11.5 months lead time. The skill and predictability of seasonal rainfall are assessed using 28 years of hindcast data from the NMME models. The forecast skill is quantified using canonical correlation analysis (CCA) and root mean square error. The results show that the NMME models capture the JJAS seasonal rainfall over central, northern, and northeastern parts of Ethiopia while exhibiting weak or limited skill across western and southwestern Ethiopia. The performance of each model in predicting the JJAS seasonal rainfall is variable, showing greater skill in predicting dry conditions. Overall, the performance of the multi-model ensemble was not consistently better than any single ensemble member. The correlation of observed and predicted seasonal rainfall for the better performing models—GFDL-CM2p5-FLOR-A06, CMC2-CanCM4, GFDL-CM2p5-FLOR-B01 and NASA-GMAO-062012—is 0.68, 0.58, 0.52, and 0.5, respectively. The COLA-RSMAS-CCSM4, CMC1- CanCM3 and NCEP-CFSv2 models exhibit less skill, with correlations less than 0.4. In general, the NMME offers promising skill to predict seasonal rainfall over Ethiopia during the June-September (JJAS) season, motivating further work to assess its performance at longer lead times.

Contribution of the tropical sea surface temperature in the Arctic tropospheric warming during 1979-2013

本文通过CAM6-Nor大气环流模式模拟实验研究热带海表温度在1979-2013年北极对流层增温中的贡献.分析结果表明热带海表温度变化可以解释历史模拟实验中的秋季和1月增温的30%-40%.这意味着热带海表温度可能是1979年至2013年间北极冬季对流层变暖的主要驱动因素之一.除了1月份850hPa以下的北极增温,热带海表温度的影响通常来自热带中东部太平洋,热带印度-西太平洋和热带大西洋的联合作用.对1月份850hPa以下的北极增温的影响主要来自热带印度-西太平洋.

Performance of climate reanalyses in the determination of pan-Arctic terrestrial rain-on-snow events

Rain-on-snow(ROS)events can cause rapid snowmelt,leading to flooding and avalanches in the pan-Arctic and can also lead to starvation and the death of massive ungulates.Reanalysis products(e.g.,ERA-I,ERA5-land,JRA55,MERRA2)are the primary source data for the research about ROS events in the large-scale region.However,the accuracy and reliability of reanalyses have never been evaluated with respect to the determination of terrestrial ROS events.The present study aims to statistically evaluate the performance of reanalysis datasets in identifying ROS events with different criteria based on in-situ rainfall data and MODIS snow cover product.The results show that all reanalysis datasets exhibit poor performance(Recall≤0.16,Kappa coefficient≤0.26,F-score≤0.42,MCC≤0.33)in all criteria in the pan-Arctic,mainly due to the low accuracy of rainfall data(r≤0.56).Nevertheless,the spatial distribution pattern and hot spots of ROS from all reanalysis datasets are essentially close.The hot spots of ROS are mainly located on the coast of Alaska,Norway,and Greenland.All reanalyses demonstrate an increase in rainy days,but there is little overall change in ROS events due to the reduction in snow cover days.This work suggests that none of the current reanalyses are reliable in the determination of ROS events due to the poor representation of the rainfall parameterization scheme.The development of alternative strategies that can investigate ROS events at large-scale is urgently needed in a changing Arctic under rapid warming.

层结稳定性“豁口”与北太平洋副热带中部模态水形成机制

本文首先指出北太平洋副热带中部模态水(简称中部模态水)的形成具有显著的"局地"特征,其形成海区在(165°E^160°W,38°N^42°N)区间.海气通量分析表明单纯的外部大气强迫场(太阳短波辐射、净热通量和风应力旋度)不能解释中部模态水形成海区的"局地"性;进一步对上层海洋层结季节变化特征的分析发现秋季(9~10月)在北太平洋中部上层海洋(<75m)(165°E^160°W,38°N^42°N)区间存在特殊的浮力频率低值区——层结稳定性"豁口".该层结稳定性"豁口"作为"预条件(Precondition Mechanism)"机制对中部模态水形成的"局地"特征给出了合理的解释.在上述研究的基础上,基于一个上层海洋混合层热平衡方程,通过诊断分析揭示该层结稳定性"豁口"是由海表热力强迫、垂向挟卷、Ekman平流和地转平流效应共同导致的,"豁口"东、西边界的确定直接或间接地取决于海表热力强迫、Ekman冷平流和地转暖平流的纬向分布差异.

与北极涛动或北半球环状模相关的纬向对称的正规模态

作为一个全球气候变率的重要模态,北极涛动或北半球环状模态(AO/NAM)一般由北半球海平面气压变率的EOF1来表示。但是通常认为EOF1仅有统计学意义而并不能够表明它是否是一种实际的物理模态。另一方面,现有的波-流相互作用理论也仅能给出纬向风的纬向平均状况,而不能够保证沿着某一纬圈的变化,所以它并不意味着一个具有半球尺度的纬向对称的相干结构能够组织起来。因此AO/NAM的形成机制仍然是一个具有争议的问题。文章提出纬向对称的或环状的正规模态与基本气候态附近的线性(进而非线性)动力学之间有着最直接的联系,因而可能在AO/NAM动力学的理解中扮演重要角色。为了深入探讨AO/NAM的动力学本质,使用p-坐标球面原始方程计算了冬季北半球及全球气候基本态附近的线性动力系统纬向对称的正规模态,发现在半球尺度上的纬向对称正规模态具有AO/NAM的经向偶极子及垂直方向近似正压的空间结构特征。而这些纬向对称正规模的时间变化尺度取决于与其他非纬向对称正规模间的相互作用。从而说明了AO/NAM可能是半球尺度上纬向对称正规模态的动力学行为的反映。

The Response of the East Asian Summer Monsoon to Strong Tropical Volcanic Eruptions

A 600-year integration performed with the Bergen Climate Model and National Centers for Environmental Prediction/National Center for Atmospheric Research （NCEP/NCAR） reanalysis data were used to investigate the impact of strong tropical volcanic eruptions on the East Asian summer monsoon （EASM） and EASM rainfall.Both the simulation and NCEP/NCAR reanalysis data show a weakening of the EASM in strong eruption years.The model simulation suggests that North and South China experience droughts and the Yangtze-Huaihe River Valley experiences floods during eruption years.In response to strong tropical volcanic eruptions,the meridional air temperature gradient in the upper troposphere is enhanced,which leads to a southward shift and an increase of the East Asian subtropical westerly jet stream （EASWJ）.At the same time,the land-sea thermal contrast between the Asian land mass and Northwest Pacific Ocean is weakened.The southward shift and increase of the EASWJ and reduction of the land-sea thermal contrast all contribute to a weakening of the EASM and EASM rainfall anomaly.

The East Pacific Wavetrain:Its Variability and Impact on the Atmospheric Circulation in the Boreal Winter

The East Pacific wavetrain （EPW） refers to here the intense stationary wave activity detected in the troposphere over the East Pacific and North America in 45 northern winters from 1958 to 2002. The EPW is generated in the lower troposphere over the East Pacific, propagating predominantly eastward into North America and slightly upward then eventually into the stratosphere. The intensity of the EPW varies from year to year and exhibits apparent decadal variability. For the period 1958-1964, the EPW was in its second maximum, and it was weakest for the period 1965-1975, then it was strongest for the period 1976-1987. After 1987, the EPW weakened again. The intensity and position of the members （i.e., the Aleutian low, the North American trough, and the North American ridge） of the EPW oscillate from time to time. For an active EPW versus a weak EPW, the Aleutian low deepens abnormally and shifts its center from the west to the east of the date line, in the middle and upper troposphere the East Asian trough extends eastward, and the Canadian ridge intensifies at the same time. The opposite is true for a weak EPW. Even in the lower stratosphere, significant changes in the stationary wave pattern are also observed. Interestingly the spatial variability of the EPW assumes a Pacific-North American （PNA）-like telecon- nection pattern. It is likely that the PNA low-frequency oscillation is a reflection of the oscillations of intensity and position of the members of the EPW in horizontal direction.

Potential Correlation between the Decadal East Asian Summer Monsoon Variability and the Pacific Decadal Oscillation

This study discusses the potential contribution of the Pacific decadal oscillation(PDO)to the weakening of the East Asian summer monsoon(EASM)and the evident correlation between the positive PDO and"Southern flood and Northern drought(SFND)"summer rainfall pattern over East China.The mechanism behind this contribution is also discussed.

Association of Indian Ocean ITCZ Variations with the Arctic Oscillation during Boreal Winter

In this study,the authors analyzed the associations between the Arctic Oscillation(AO)and the tropical Indian Ocean(TIO)intertropical convergence zone(ITCZ)in boreal winter for the period 1979–2009.A statistically significant AO-TIO ITCZ linkage was found.The ITCZ vertical air motion is significantly associated with the AO,with upward(downward)air motion corresponding to the positive(negative)AO phase.The Arabian Sea anticyclone plays a crucial role in linking the AO and the TIO ITCZ.The Arabian Sea vorticity is strongly linked to high-latitude disturbances in conjunction with jet stream waveguide effects of disturbance trapping and energy dispersion.During positive(negative)AO years,the Arabian Sea anticyclone tends to be stronger(weaker).The mean vorticity over the Arabian Sea,averaged from 850hPa to 200 hPa,has a significant negative correlation with AO(r=0.63).The anomalous anticyclone over the Arabian Sea brings stronger northeastern winds,which enhance the ITCZ after crossing the equator and result in greater-than-normal precipitation and minimum outgoing long-wave radiation.

Influence of solar wind energy flux on the interannual variability of ENSO in the subsequent year

作为地球系统的主要能量来源,进入地磁系统的太阳风能量通量(E_(in))一直难以估算。因此,E_(in)对气候的影响也没有得到广泛的研究。基于三维磁流体动力模拟估算的E_(in),本文指出,太阳风能量通量不仅存在准11年周期的年代际变率,同时还存在2-4年周期的年际变率。与以往主要关注太阳活动在年代际尺度上的气候效应的研究不同,本文揭示出太阳风能量通量与次年ENSO年际变率存在显著的联系。

A Sensitivity Study of Arctic Ice-Ocean Heat Exchange to the Three-Equation Boundary Condition Parametrization in CICE6

In this study,we perform a stand-alone sensitivity study using the Los Alamos Sea ice model version 6(CICE6)to investigate the model sensitivity to two Ice-Ocean(IO)boundary condition approaches.One is the two-equation approach that treats the freezing temperature as a function of the ocean mixed layer(ML)salinity,using two equations to parametrize the IO heat exchanges.Another approach uses the salinity of the IO interface to define the actual freezing temperature,so an equation describing the salt flux at the IO interface is added to the two-equation approach,forming the so-called three-equation approach.We focus on the impact of the three-equation boundary condition on the IO heat exchange and associated basal melt/growth of the sea ice in the Arctic Ocean.Compared with the two-equation simulation,our three-equation simulation shows a reduced oceanic turbulent heat flux,weakened basal melt,increased ice thickness,and reduced sea surface temperature(SST)in the Arctic.These impacts occur mainly at the ice edge regions and manifest themselves in summer.Furthermore,in August,we observed a downward turbulent heat flux from the ice to the ocean ML in two of our three-equation sensitivity runs with a constant heat transfer coefficient(0.006),which caused heat divergence and congelation at the ice bottom.Additionally,the influence of different combinations of heat/salt transfer coefficients and thermal conductivity in the three-equation approach on the model simulated results is assessed.The results presented in this study can provide insight into sea ice model sensitivity to the three-equation IO boundary condition for coupling the CICE6 to climate models.

Simulating Tropical Instability Waves in the Equatorial Eastern Pacific with a Coupled General Circulation Model

Satellite observations of SSTs have revealed the existence of unstable waves in the equatorial eastern Pacific and Atlantic oceans. These waves have a 20-40-day periodicity with westward phase speeds of 0.4-0.6 m s^-1 and wavelengths of 1000-2000 km during boreal summer and fall. They are generally called tropical instability waves （TIWs）. This study investigates TIWs simulated by a high-resolution coupled atmosphere-ocean general circulation model （AOGCM）. The horizontal resolution of the model is 120 km in the atmosphere, and 30 km longitude by 20 km latitude in the ocean. Model simulations show good agreement with the observed main features associated with TIWs. The results of energetics analysis reveal that barotropic energy conversion is responsible for providing the main energy source for TIWs by extracting energy from the meridional shear of the climatological-mean equatorial currents in the mixed layer. This deeper and northward-extended wave activity appears to gain its energy through baroclinic conversion via buoyancy work, which further contributes to the asymmetric distribution of TIWs. It is estimated that the strong cooling effect induced by equatorial upwelling is partially （-30%-40%） offset by the equatorward heat flux due to TIWs in the eastern tropical Pacific during the seasons when TIWs are active. The atmospheric mixed layer just above the sea surface responds to the waves with enhanced or reduced vertical mixing. Furthermore, the changes in turbulent mixing feed back to sea surface evaporation, favoring the westward propagation of TIWs. The atmosphere to the south of the Equator also responds to TIWs in a similar way, although TIWs are much weaker south of the Equator.